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Effect of repeated firings on biaxial flexural strength of different CAD/CAM lithium disilicate reinforced materials in two different thicknesses
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Effect of repeated firings on biaxial flexural strength of different CAD/CAM lithium disilicate reinforced materials in two different thicknesses
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Content
Effect of Repeated Firings on Biaxial Flexural Strength of Different CAD/CAM Lithium
Disilicate Reinforced Materials in Two Different Thicknesses
By
Sarah Alsaleh, BDS
A Thesis Presented to the
FACULTY OF THE USC HERMAN OSTROW SCHOOL OF DENTISTRY UNIVERSITY OF
SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirement for the Degree
MASTER OF SCIENCE
(BIOMATERIALS AND DIGITAL DENTISTRY)
December 2022
Copyright 2022 Sarah Alsaleh
ii
Dedication
To my parents, Ebtesam Alsaloum and Essam Alsaleh, for their unconditional support, love, and
their gift of life. To my sisters, Deema and Dana, and my brothers, Saad and Anas, who always
cheered me. To my husband, Mohammed, who has been my source of bravery, inspiration, and
passion throughout the journey. To have you as my partner is indeed a blessing. To everyone
who has inspired me to excel in my life. I have an everlasting debt of gratitude to all of you.
iii
Acknowledgments
First and foremost, I am thankful to Allah, for the blessings and strength he awarded me
throughout my life. Then, to my home country, the Kingdom of Saudi Arabia, and the Saudi
Ministry of Education for supporting me.
I owe my deepest appreciation to my advisor, Dr. Jin-Ho Phark, for all the help and support
he offered me to complete this thesis and for the time you contributed. Your extensive knowledge
and direction were vital in helping me complete my thesis. I am grateful for all of your efforts and
insights to make my learning experience meaningful.
To my co-advisor, Dr. Sillas Duarte, my deep gratitude for your support. I sincerely
appreciate your helpful instruction and the engaging intellectual discussions. Thank you for
advancing my education, clinically and theoretically, in digital and adhesive restorative dentistry
and for your guidance throughout the course of the program.
My sincerest gratitude to my co-advisor, Dr. Fabiana Varjao, thank you for your kind and
unconditional support and for taking the time to read my thesis. I am pleased to continue working
with you and looking forward to learning more from you.
I would like to thank my faculty members throughout my training in the Advanced
Operative and Adhesive Dentistry program, Dr. Alena Knezevic, Dr. Eddie Sheh, and Dr. Jenny
Son. I am thankful for your great support, guidance, and encouragement throughout my training.
I would like to acknowledge the program specialist, Ms. Karen Guillen, for going above
and beyond to accommodate my requests.
In the end, I want to express my gratitude to my fellow residents, Nazanin Forghani and
Andrea Ramirez, for their continuous supportive words as we work through this journey together.
iv
Table of Contents
Dedication ...................................................................................................................................................................... ii
Acknowledgments ......................................................................................................................................................... iii
List of Tables................................................................................................................................................................. vi
List of Figures ................................................................................................................................................................ x
Abstract ....................................................................................................................................................................... xiii
Chapter 1: Introduction .................................................................................................................................................. 1
1.1. History of Dental Ceramics: .............................................................................................................................. 1
1.2. Ceramics Classification: .................................................................................................................................... 4
1.2.1. Classification by composition: ................................................................................................................... 4
1.2.2. Classification by processing method: ....................................................................................................... 17
1.3. Ceramic Strength Properties: .......................................................................................................................... 23
I. Role of cracks in glass-matrix ceramics:......................................................................................................... 24
II. Effect of firing ............................................................................................................................................... 25
III. Effect of thickness ........................................................................................................................................ 26
1.4. Strength Testing Methods:................................................................................................................................ 28
I. Compressive strength ...................................................................................................................................... 28
II. Tensile strength .............................................................................................................................................. 28
III. Shear strength ............................................................................................................................................... 29
IV. Flexural strength: .......................................................................................................................................... 29
1.5. Weibull Distribution: ........................................................................................................................................ 32
1.5. Objective: ......................................................................................................................................................... 35
1.6. Aim: .................................................................................................................................................................. 35
1.7. Null Hypothesis: ............................................................................................................................................... 35
Chapter 2: Materials and methods................................................................................................................................ 36
2.1. Study Design: ................................................................................................................................................... 37
2.2. Preparation of Specimens: ............................................................................................................................... 40
I. ISO 6872-2015: ............................................................................................................................................... 41
II. Cylinder design: ............................................................................................................................................. 42
III. Milling cylinders: ......................................................................................................................................... 43
IV. Preparation of sectioning: ............................................................................................................................. 44
V. Sectioning: ..................................................................................................................................................... 45
VI. Polishing: ...................................................................................................................................................... 46
VII. Group distribution: ...................................................................................................................................... 48
2.4. Firing of Specimens: ........................................................................................................................................ 50
I. Firing protocol:................................................................................................................................................ 52
II. After firings: .................................................................................................................................................. 56
2.5. Biaxial Flexural Strength Testing (BFS): ........................................................................................................ 57
I. ISO 6872-2015: ............................................................................................................................................... 58
II. Piston-on-three-ball: ...................................................................................................................................... 59
III. Positioning device: ....................................................................................................................................... 60
v
IV. Testing protocol: .......................................................................................................................................... 61
V. Calculating the biaxial flexural strength: ....................................................................................................... 63
2.6. Statistical Analysis: .......................................................................................................................................... 64
I. Biaxial flexural strength data: ......................................................................................................................... 64
II. Weibull analysis: ............................................................................................................................................ 65
Chapter 3: Results ........................................................................................................................................................ 67
3.1. Descriptive Analysis of the Biaxial Flexural Strength: .................................................................................... 67
3.2. Data Normality Analysis and Equality of Variances: ...................................................................................... 70
3.3 Analysis of Variance (ANOVA): ........................................................................................................................ 72
Material ............................................................................................................................................................... 73
Firing: ................................................................................................................................................................. 86
Thickness: ........................................................................................................................................................... 99
4.4 Weibull Analysis: ............................................................................................................................................. 104
Material: ........................................................................................................................................................... 108
Firing: ............................................................................................................................................................... 120
Thickness: ......................................................................................................................................................... 132
4.5. Results Summary: ........................................................................................................................................... 133
Chapter 4: Discussion ................................................................................................................................................ 136
4.1. Material: ......................................................................................................................................................... 136
ISO 6872-2015: ................................................................................................................................................ 136
Crystal size: ...................................................................................................................................................... 137
Crystalline phase: ............................................................................................................................................. 139
4.2. Firing: ............................................................................................................................................................ 144
Effect of repeated firings: ................................................................................................................................. 145
Effect of glaze: ................................................................................................................................................. 148
4.3. Thickness: ....................................................................................................................................................... 151
4.4. Weibull Analysis ............................................................................................................................................. 154
4.5. Strength Testing: ............................................................................................................................................ 156
Chapter 5: Conclusions .............................................................................................................................................. 158
References: ................................................................................................................................................................. 159
vi
List of Tables
Table 1: Product name, manufacturer, composition, shade, LOT number, and dimensions of
CAD/CAM blocks used ................................................................................................................ 38
Table 2: Groups distribution ........................................................................................................ 49
Table 3: Firing parameter of selected lithium disilicate CAD/CAM blocks ............................... 51
Table 4: Overall biaxial flexural strength (MPa) of IPS e.max CAD, Amber Mill, Initial LiSi,
and n!ce ......................................................................................................................................... 69
Table 5: Shapiro-Wilk test (normality) ........................................................................................ 70
Table 6: Levene’s test (equality of variances) ............................................................................. 71
Table 7: Mauchly's Test of Sphericity ......................................................................................... 71
Table 8: Summary of analysis of variance of (material, firing, and thickness) and their
interactions .................................................................................................................................... 72
Table 9: Estimated Marginal Means (Material) ........................................................................... 73
Table 10: Group-wise comparisons (Material) ............................................................................ 73
Table 11: Mean biaxial flexural strength and standard deviation of IPS e.max CAD ................. 74
Table 12: One-way ANOVA of IPS e.max CAD (0.5 mm) ........................................................ 75
Table 13: One-way ANOVA of IPS e.max CAD (1.00 mm) ...................................................... 75
Table 14: Group-wise comparisons of IPS e.max CAD (0.5 mm) .............................................. 75
Table 15: Group-wise comparisons of IPS e.max CAD (1.00 mm) ............................................ 76
Table 16: Mean biaxial flexural strength and standard deviation of Amber Mill........................ 77
Table 17: One-way ANOVA of Amber Mill (0.5 mm) ............................................................... 78
Table 18: One-way ANOVA of Amber Mill (1.00 mm) ............................................................. 78
Table 19: Group-wise comparisons of Amber Mill (0.5 mm) ..................................................... 78
Table 20: Group-wise comparisons of Amber Mill (1.00 mm) ................................................... 79
Table 21: Mean biaxial flexural strength and standard deviation of Initial LiSi ......................... 80
Table 22: One-way ANOVA of Initial LiSi (0.5 mm) ................................................................. 81
vii
Table 23: One-way ANOVA of Initial LiSi (1.00 mm) ............................................................... 81
Table 24: Group-wise comparisons of Initial LiSi (0.5 mm) ....................................................... 81
Table 25: Group-wise comparisons of Initial LiSi (1.00 mm) ..................................................... 82
Table 26: Mean biaxial flexural strength and standard deviation of n!ce .................................... 83
Table 27: One-way ANOVA of n!ce (0.5 mm) ........................................................................... 84
Table 28: One-way ANOVA of n!ce (1.00 mm) ......................................................................... 84
Table 29: Group-wise comparisons of n!ce (0.5 mm) ................................................................. 84
Table 30: Group-wise comparisons of n!ce (1.00 mm) ............................................................... 85
Table 31: Estimated Marginal Means (Firing) ............................................................................. 86
Table 32: Group-wise comparisons (Firings) .............................................................................. 86
Table 33: Mean biaxial flexural strength and standard deviation at baseline .............................. 87
Table 34: One-way ANOVA of tested materials at baseline (0.5 mm) ....................................... 88
Table 35: One-way ANOVA of tested materials at baseline (1.00 mm) ..................................... 88
Table 36: Group-wise comparisons at baseline (0.5 mm) ........................................................... 88
Table 37: Group-wise comparisons at baseline (1.00 mm) ......................................................... 89
Table 38: Mean biaxial flexural strength and standard deviation at one firing ........................... 90
Table 39: One-way ANOVA of tested materials at one firing (0.5 mm)..................................... 91
Table 40: One-way ANOVA of tested materials at one firing (1.00 mm)................................... 91
Table 41: Group-wise comparisons at one firing (0.5 mm) ......................................................... 91
Table 42: Group-wise comparisons at one firing (1.00 mm) ....................................................... 92
Table 43: Mean biaxial flexural strength and standard deviation at three firings........................ 93
Table 44: One-way ANOVA of tested materials at three firings (0.5 mm) ................................. 94
Table 45: One-way ANOVA of tested materials at three firings (1.00 mm) ............................... 94
Table 46: Group-wise comparisons at three firings (0.5 mm) ..................................................... 94
Table 47: Group-wise comparisons at three firings (1.00 mm) ................................................... 95
viii
Table 48: Mean biaxial flexural strength and standard deviation at five firings ......................... 96
Table 49: One-way ANOVA of tested materials at five firings (0.5 mm) ................................... 97
Table 50: One-way ANOVA of tested materials at five firings (1.00 mm) ................................. 97
Table 51: Group-wise comparisons at five firings (0.5 mm) ....................................................... 97
Table 52: Group-wise comparisons at five firings (1.00 mm) ..................................................... 98
Table 53: Estimated Marginal Means (Firing) ............................................................................. 99
Table 54: Groupwise comparisons (Firing) ................................................................................. 99
Table 55: Mean biaxial flexural strength and standard deviation at 0.5 mm (MPa).................. 100
Table 56: Mean biaxial flexural strength and standard deviation at 1.00 mm (MPa)................ 102
Table 57: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce. ....................................... 105
Table 58: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of IPS e.max CAD ................................................................................................ 108
Table 59: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of Amber Mill ....................................................................................................... 111
Table 60: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of Initial LiSi ......................................................................................................... 114
Table 61: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of n!ce ................................................................................................................... 117
Table 62: Weibull distribution (Two-parameters) Characteristic Strength ( 𝜎𝑂 ) and Weibull
Modulus (m) of Baseline ............................................................................................................ 120
Table 63: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of One firing .......................................................................................................... 123
Table 64: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of Three firings ..................................................................................................... 126
Table 65: Weibull distribution (Two-parameters) Characteristic Strength (𝜎𝑂 ) and Weibull
Modulus (m) of Five firings ........................................................................................................ 129
Table 66: Results summary of the group-wise comparisons (BFS) and the Weibull distribution
(Two-parameters) based on the material..................................................................................... 134
ix
Table 67: Results summary of the group-wise comparisons (BFS) and the Weibull distribution
(Two-parameters) based on the firing ......................................................................................... 135
Table 68: ISO 6872-2015
(151)
recommended clinical indications according to the flexural
strength ........................................................................................................................................ 137
Table 69: Summary of previous studies investigating the flexural strength of lithium disilicate
..................................................................................................................................................... 142
Table 1: Product name, manufacturer, composition, shade, LOT number, and dimensions of
CAD/CAM blocks usedf lithium disilicate ................................................................................. 142
x
List of Figures
Figure 1: Gracis et al.
(37)
ceramics classification based on their composition. ............................. 5
Figure 2: Silicon-oxygen-silicon bonds form the 3-D structure bridges of feldspathic glasses and
are occasionally broken up by altering cations such as sodium and potassium to provide the non-
bridging oxygen atoms a charge balance ........................................................................................ 7
Figure 3: Testing methods include compression, shear, tensile, and flexural strength ............... 28
Figure 4: Flexural strength testing ............................................................................................... 30
Figure 5: Weibull plot .................................................................................................................. 33
Figure 6: Likelihood contour plot ................................................................................................ 34
Figure 7: The process of testing biaxial flexural strength ........................................................... 36
Figure 8: Lithium disilicate reinforced glass-matrix ceramics: (A) IPS e.max CAD, (B) Amber
Mill, (C) GC Initial LiSi, and (D) n!ce ......................................................................................... 37
Figure 9: Study design ................................................................................................................. 39
Figure 10: Preparation of specimens ........................................................................................... 40
Figure 11: ISO6872-2015 specimen requirements ...................................................................... 41
Figure 12: A cylinder designed using Meshmixer (12mmX17mm) ............................................ 42
Figure 13: Positioning cylinder in the inLab CAM SW 18.1 ...................................................... 43
Figure 14: Milled cylinders from CAD/CAM blocks .................................................................. 43
Figure 15: Estimation of specimens per block (0.5 mm) ............................................................. 44
Figure 16: Estimation of specimens per block (1.00 mm) ........................................................... 44
Figure 17: Cylinder mounted on a precision saw (IsoMet 1000, Buehler, Lake Buff, IL, USA) 45
Figure 18: (A) Disc-shape specimen after slicing, (B) Slow-speed handpiece with coarse Soflex
discs, and (C) Disc-shape specimen after polishing. .................................................................... 46
Figure 19: Disc sanding machine (Mini Disc Sander 6-Inch Electric Sanding Belt Machine,
MXBAOHENG, USA) ................................................................................................................. 47
Figure 20: Verification of specimen final dimensions: 1.00 mm thickness and 12.00 mm
diameter......................................................................................................................................... 47
xi
Figure 21: Disc-shaped specimens placed on a crystallization tray (IPS e.max CAD
Crystallization Tray, Ivoclar Vivadent Schaan, Liechtenstein) .................................................... 50
Figure 22: IPS e.max CAD ready for baseline firing furnace (Programat SC3, Ivoclar Vivadent,
Schaan, Liechtenstein) .................................................................................................................. 50
Figure 23: Lab-side materials firing protocol .............................................................................. 52
Figure 24: Chair-side materials firing protocol ........................................................................... 54
Figure 25: Disc-shaped specimens prior to biaxial flexural strength testing ............................... 56
Figure 26: Biaxial flexural strength testing ................................................................................. 57
Figure 27: Universal testing machine (Instron 5965, Canton, MA, USA) .................................. 58
Figure 28: Piston-on-three-ball (Biaxial Flexion-ISO, Odeme Dental Research, Luzerna SC,
Brazil) ........................................................................................................................................... 58
Figure 29: The piston-on-three-ball fixture with 3 steel balls 3.2 mm in diameter, set 120° across
on a 10 mm support circle ............................................................................................................. 59
Figure 30: 3D printed positioning device (A) After printing (B) Verification of specimen fit ... 60
Figure 31: (A) The positioning device is placed inside the biaxial flexural strength testing
fixture, (B) The specimen is placed inside the positioning device, (C) Placing a plastic
polyethylene sheet on the specimen, and (D) The positioning device is removed, and the loading
piston is lowered ........................................................................................................................... 61
Figure 32: Disc-shaped specimens after biaxial flexural strength testing ................................... 62
Figure 33: Overall mean biaxial flexural strength (MPa) ............................................................ 67
Figure 34: Biaxial flexural strength of IPS e.max CAD .............................................................. 74
Figure 35: Biaxial flexural strength of Amber Mill ..................................................................... 77
Figure 36: Biaxial flexural strength of Initial LiSi ...................................................................... 80
Figure 37: Biaxial flexural strength of n!ce ................................................................................. 83
Figure 38: Biaxial flexural strength at baseline ........................................................................... 87
Figure 39: Biaxial flexural strength at one firing ........................................................................ 90
Figure 40: Biaxial flexural strength at three firings ..................................................................... 93
Figure 41: Biaxial flexural strength at five firings ...................................................................... 96
xii
Figure 42: Change of biaxial flexural strength with firing and material (0.5 mm) ................... 100
Figure 43: Change of biaxial flexural strength with firing and material (1.00 mm) ................. 102
Figure 44: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of IPS
e.max CAD ................................................................................................................................. 109
Figure 45: Likelihood contour plot (IPS e.max CAD)............................................................... 110
Figure 46: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of
Amber Mill.................................................................................................................................. 112
Figure 47: Likelihood contour plot (Amber Mill) ..................................................................... 113
Figure 48: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of Initial
LiSi .............................................................................................................................................. 115
Figure 49: Likelihood contour plot (Initial LiSi) ....................................................................... 116
Figure 50: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of n!ce
..................................................................................................................................................... 118
Figure 51: Likelihood contour plot (n!ce) ................................................................................. 119
Figure 52: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at
baseline ....................................................................................................................................... 121
Figure 53: Likelihood contour plot (Baseline) .......................................................................... 122
Figure 54: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at one
firing ............................................................................................................................................ 124
Figure 55: Likelihood contour plot (One Firing) ....................................................................... 125
Figure 56: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at three
firings .......................................................................................................................................... 127
Figure 57: Likelihood contour plot (Three Firings) ................................................................... 128
Figure 58: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at five
firings .......................................................................................................................................... 130
Figure 59: Likelihood contour plot (Five Firings) ..................................................................... 131
Figure 60: Interlocking effect in glass–ceramics ....................................................................... 138
Figure 61: The ratio of the glass matrix to the crystalline phase of the tested materials as well as
lithium disilicate crystal content and size ................................................................................... 140
xiii
Abstract
Purpose: To evaluate the biaxial flexural strength of four CAD/CAM lithium disilicate
reinforced glass ceramic; IPS e.max CAD (EX) and Amber Mill (AM) as “lab-side”, and Initial
LiSi Block (LS) and n!ce (NC) as “chair-side” according to the effect of two thicknesses and
repeated firings.
Material and methods: CAD/CAM blocks were prepared according to ISO 6872-2015.
Four hundred and eighty (n=480) discs were prepared in total with a diameter of 12.00 mm (±0.02).
Each selected material (n=120) was divided into two thicknesses: 1.00 mm (±0.03) and 0.5 mm
(±0.02), having 60 discs for each thickness in each material. The specimens were subdivided
according to firings: baseline (BL), one firing (1F), three firings (3F), and five firings (5F). The
firings cycles were performed according to the manufacturers’ instructions. The biaxial flexural
strength test (piston-on-three-ball) was performed according to ISO 6872-2015 using a universal
testing machine.
Biaxial flexural strength data were analyzed using parametric tests: three-way and one-way
ANOVA (α=0.05) with Bonferroni post-hoc test. Weibull analysis was used to calculate the
Weibull Modulus and Characteristic Strength to create Weibull plots and likelihood contour plots.
Results: The biaxial flexural strength of the materials differed from each other
(EX=AM>NC>LS). A significant difference was found between the firings, regardless of the
thickness, and the general ranking of firings was (3F>5F>1F>BL). Higher thickness (1.00 mm)
presented a higher biaxial flexural strength value. Higher Weibull modulus and characteristic
strength values were observed with lab-side vs. chair-side materials.
xiv
Conclusions: Repeated firings significantly affected the biaxial flexural strength of EX,
AM, LS, and NC CAD/CAM lithium disilicate materials. The biaxial flexural strength increased
with increased thickness. Lab-side materials (EX and AM) have a lower probability of failure than
chair-side materials (LS and NC).
Chapter 1: Introduction
1.1. History of Dental Ceramics:
When dental technology advanced significantly in the 18th century, there was a growing
need for esthetic replacements for missing natural teeth.
(24)
Dental technology was present at the
beginning of the Roman century BC, but it was mainly overlooked until the 17th century.
(24, 25)
Throughout the 18th century, a wide variety of materials were recognized to substitute teeth,
including human teeth, animal teeth that had been carved and modified to resemble human teeth,
ivory, and porcelain teeth.
(24)
Pierre Fauchard was the first to see the potential of porcelain in the
dentistry.
(25)
In 1723, Pierre Fauchard discovered that denture metal bases could be coated to
resemble natural teeth and gingiva.
(25)
The first porcelain dentures were produced in 1774 by a Parisian pharmacist named Alexis
Duchateau and a Parisian dentist named Nicholas Dubois de Chemant.
(25, 26)
In 1808 in Paris,
Giuseppangelo Fonzi made the first set of custom-made porcelain teeth incorporating platinum
pins.
(27)
These teeth, which Fonzi labeled "terrametallic incorruptible," transformed prosthetic
dentistry with their mechanical and esthetic adaptability.
(26)
In 1886, Charles Land combined polished platinum foil as a substrate with the extreme
heat of a gas furnace to establish the first fused feldspathic porcelain inlays and crowns.
(24, 26)
The
all-porcelain crown system had desirable qualities.
(28, 29)
However, it wasn't widely recognized
until alumina was developed as a reinforcing phase in dental porcelain.
(28, 29)
Full crowns and fixed
partial dentures (FPDs) were made possible by adding leucite to porcelain formulae in the 1950s
because it increased the coefficient of thermal expansion and allowed them to fuse with gold alloys.
(24, 30)
2
In the late 1950s, created porcelain-fused-to-metal (PFM) crown was to reduce the
possibility of internal microcracking throughout the cooling phase of production.
(31)
Stress cracks
were avoided from growing by the bond of the metal and porcelain.
(31)
PFM crowns have a lower
porcelain failure, but adding an opaque layer to mask the metal in these restorations reduced their
aesthetic appearance.
(31)
All-ceramic restorations were returned in 1965 when industrial
aluminous porcelain (more than 50%) was added to the manufacturing of feldspathic porcelain.
(31)
This innovative design of porcelain jacket crown, created by W. McLean and T.H. Hughes,
featured an inner core of aluminous porcelain that contained 40% to 50% alumina crystals.
(24)
However, the main disadvantage was its increased opacity.
(32)
Dental ceramics research has progressively emphasized advancements in metal-ceramic
systems, contributing to superior alloys, porcelain metal bonding, and porcelains.
(24)
More options
for achieving esthetic results were available in the 1980s with the development of "shrink-free"
all-ceramic crowns systems like Cerestore (Coors Biomedical, Lakewood, CO) and castable glass-
ceramic crown systems like Dicor (Dentsply/York Division, York, PA).
(24)
This prompted interest
in all-ceramic prostheses and introduced improved ceramics with novel processing techniques.
(24)
The In-Ceram ceramic system is another all-ceramic system from the same period (VITA
Zahnfabrik, Bad Saeckingen, Germany).
(24)
This was produced using the slip-casting process that
was used to make an alumina coping.
(24)
After being partially sintered, the alumina particles were
next penetrated by a low-viscosity glass.
(24)
Later, efforts were undertaken to improve the system's
translucency by replacing magnesium aluminate spinell with aluminum oxide.
(24)
Consequently,
despite having higher optical qualities, the In-Ceram Spinell system exhibited lower strength.
(24)
Another method that attempted to address the issue of porcelain shrinking after firing was
the Empress system (Ivoclar Vivadent, Schaan, Liechtenstein).
(33)
This process is based on
3
pressing a high-temperature investment mold made using the lost wax technique with a leucite-
reinforced glass-ceramic formulation.
(33)
The subsequent generation of this technology, Empress
II (Ivoclar Vivadent), was developed based on a lithium disilicate glass-ceramic with superior
mechanical properties.
(33)
A current development with this lithium disilicate glass-ceramic is
e.max (Ivoclar Vivadent), which is supplied as customized ingots for pressing as well as blocks
for CAD/CAM use.
(33)
CAD/CAM technology was simultaneously introduced to dentistry, enabling the use of
materials like zirconia that can only be produced using this technology.
(34)
Additionally,
CAD/CAM block versions of both conventional and novel materials, including feldspathic
porcelain as well as lithium disilicate reinforced glass-ceramics, are now widely accessible.
(34)
4
1.2. Ceramics Classification:
Dental ceramics are classified using several methods. One of these methods is based on
ceramic composition.
(35)
This type of classification guides dentists in material selection in each
case. Moreover, ceramics can be classified according to the method of processing. This
classification is significant as it defines the processes used to produce dental restorations.
1.2.1. Classification by composition:
a) Kelly and Benetti classification:
According to the commonly used Kelly and Benetti classification system, there are three types
of ceramic materials: (1) predominantly glassy materials, (2) particle-filled glasses, and (3)
polycrystalline ceramics, which lack glass.
(36)
Dentists may need more coherence in estimating
the glass phase content required for the ceramic to be classified as either predominantly glassy or
particle-filled glasses under this system of glass content classification.
(37)
In addition, this classification does not represent major advancements made in ceramic
technology.
(37)
Advancements such as materials developed by resin with high ceramic content
(resin-matrix) are now produced using ceramics that are synthetically created rather than using
naturally occurring components (such as feldspar).
(37)
As a result, these materials' standardization
and quality control were improved.
(37)
5
b) Gracis et al. classification:
Dental ceramics were classified into three categories by Gracis et al.: (I) glass-matrix ceramics,
(II) polycrystalline ceramics, (III) and resin-matrix ceramics (Figure 1)
(37)
This classification
included subclassification for each category to clarify their composition as well as allowing the
novel materials to be classified.
(37)
These ceramics are differentiated according to the phase of
their structure.
(37)
Accordingly, ceramics are divided into three categories according to if a glass-matrix phase is
present (glass-matrix ceramics), if it is absent (polycrystalline ceramics), or if it has an organic
matrix that is highly infused filled with ceramic particles (resin-matrix ceramics).
(37)
The
following section will focus on lithium disilicate in glass-matrix ceramics and briefly define other
subcategories of glass-matrix ceramics, polycrystalline ceramics, and resin-matrix ceramics.
Figure 1: Gracis et al.
(37)
ceramics classification based on their composition.
6
I. Glass-matrix ceramics
Glass-matrix ceramics are nonmetallic, inorganic, and have a glass phase finely dispersed
crystalline phases.
(37, 38)
Glass-matrix ceramics are manufactured by carefully controlling the glass
crystallization, nucleating it evenly through the glass phase, or embedding one or more crystals in
the structure.
(38, 39)
The crystalline phase can take up 0.5% to 100% of the composition but usually
takes about 30% to 70% of the final composition.
(39)
One of the critical determinants of the mechanical and esthetic qualities of the final product,
such as its toughness and translucency, is the type, size, and volume fraction of the crystalline
phase, as well as its distribution within the glass matrix.
(39)
Furthermore, the inclusion of a
crystalline phase increases the mechanical properties of the final product by preventing the
propagation and growth of cracks.
(40)
The mechanical properties of the glassy phase are improved
by filling the grain boundaries.
(40)
Glass matrix ceramics provide several advantages over
traditional ceramics, such as simple synthesis methods, less shrinkage, and increased translucency
due to less internal light scattering.
(41, 42)
The glass matrix ceramics are divided into three subcategories: A) Feldspathic ceramics, B)
synthetic materials such as lithium disilicate, and C) glass-infiltrated.
(A) Natural feldspathic:
Feldspathic porcelain is an aluminosilicate glass derivative.
(37, 38)
It consists of pure natural
feldspars (orthoclase KAlSi3O8 and albite NaAlSi3O8), kaolin (Al2Si2O5(OH)4), quartz (SiO2), as
well as metal oxide additives.
(37, 38)
Typically, the composite of feldspathic porcelain consists of
52–62wt.% SiO2, 11–16wt.% Al2O3, 9–11wt.% K2O, 5–7wt.% Na2O, and additives.
(43)
The feldspar components are melted to produce the glassy structure of feldspathic
porcelain.
(37)
Its amorphous structure is composed of a disorganized network of chains resembling
7
silica polymers linked by sharing the same oxygen atoms.
(37)
The chemical structure of the glass
affects how long the chains are.
(37)
These chains can be modified by changing the initial mineral
ratios and using metal oxide modifiers that facilitate silica chain breakdown during the melting.
(36, 44-46)
Shortened Si-O chains increase the fluidity of the melt and reduce the fusing temperature.
(37)
As per Figure 2, feldspathic glasses include sodium and potassium, which can alter
important properties of the glass, for example, decreasing firing temperatures or rising thermal
expansion.
(37)
Figure 2: Silicon-oxygen-silicon bonds form the 3-D structure bridges of feldspathic glasses and are occasionally broken up by
altering cations such as sodium and potassium to provide the non-bridging oxygen atoms a charge balance
Feldspathic is present in several materials: IPS Empress Esthetic, IPS Empress CAD, IPS
Classic (Ivoclar Vivadent, Schaan, Liechtenstein), Vitadur, Vita VMK 68, and Vitablocs (VITA
Zahnfabrik, Bad Säckingen, Germany).
(37)
Various methods have been proposed to reinforce feldspathic ceramics, such as the addition of
supporting particles
(47, 48)
and a compressive surface layer that is developed through chemical or
thermal tempering.
(49, 50)
8
The addition of gadolinium aluminate/alumina fibers (GdAlO3/Al2O3) and zirconia-silica
(O4SiZr) has been suggested to increase the mechanical properties of felspathic porcelain.
(47, 48)
Nevertheless, it has been observed that after reinforcing, their optical behavior changed due to an
increased porosity.
(51)
Additionally, because of gradual fracture propagation, in which the size of
the defects increases, this porosity may reduce the strength of feldspathic porcelains.
(52)
Thermal tempering of feldspathic ceramics is also used to increase mechanical properties.
(53)
The ceramic temperature is raised to a value that is slightly above the glass transition but below its
softening point.
(53)
As a result, a compressive layer is generated, making temperature gradients
between the surface and the bulk of the ceramic.
(53)
Therefore, the exterior of the glass produces
compression on the outer surface while the inside is in a condition of tension.
(53)
Additionally, in-
vitro research on this method showed encouraging results in terms of an increase in flexural
strength.
(49)
Chemical tempering is another method used to strengthen felspathic ceramics.
(54)
This
technique involves ion exchange at the glass surface, which results in forming a compressive layer
as larger ions replace smaller ions.
(54)
In this method, pure feldspathic porcelain is submerged in
a molten salt bath using potassium nitrate (KNO3) below the glass transition temperature, roughly
500 °C. K+ ions from the KNO3 bath often replace Na+ or Li+ ions.
(54)
Applying this technique to dental porcelains is achieved by strengthening complex
geometries and thin components (up to 100 µm in thickness), the high magnitude of surface
compression, and the absence of optical distortion.
(55)
It appears promising that ion exchange for
dental ceramics will improve the mechanical properties of dental porcelains to withstand fast crack
propagation.
(56)
9
(B) Synthetic:
Synthetic glass matrix ceramic includes three sub-categories leucite-based, lithium
disilicate, and fluorapatite-based.
B.1. Leucite-based glass-matrix ceramics:
Leucite-based glass-matrix ceramics are made by the same phase system (SiO2–Al2O3–K2O)
as the natural feldspathic porcelains but with a greater content K2O of at least 12wt.%.
(37, 38)
Leucite glass-ceramics consist of a 35–45% volume fraction of randomly shaped 1–5 μm crystals
evenly and densely distributed in the glassy matrix and arranged like strings of beads along the
glass grain boundaries.
(37, 38)
A large quantity of leucite crystals reinforces the glass by defecting and arresting the
propagation of cracks.
(37, 38)
Leucite glass-ceramics present excellent esthetics thanks to a high
and adjustable translucency and the possibility of coloring the glass in natural tooth shades through
addition of metal oxide pigments.
(37, 38)
Although the strength of leucite glass ceramics is as much
as twice the strength of conventional feldspathic porcelains; it is still insufficient for posterior fixed
dental prosthetics.
(37, 38)
Their applications span from resin-bonded laminate veneers; to inlays and
onlays; and anterior and posterior crowns.
(37, 38)
Leucite-based glass-matrix ceramics is present in several materials: IPS d.Sing (Ivoclar
Vivadent, Schaan, Liechtenstein), VM7, VM9, VM13 (VITA Zahnfabrik, Bad Säckingen,
Germany), Noritake EX-3, Cerabien, Cerabien ZR (Kuraray Noritake Okayama, Japan)
(36)
.
B.2 Lithium disilicate and derivatives
Lithium disilicate (LDS) glass-matrix ceramics are derived from the SiO2–Li2O system
that Stookey first explored at in the 1950s.
(57, 58)
It was first introduced to dentistry by Beall &
Echeverria and commercialized by Ivoclar Vivadent in 1998.
(57, 58)
LDS consists of up to 70% fine
10
rod-like entangled lithium disilicate (Li2Si2O5) crystals and lithium orthophosphate (Li3PO4)
crystals (in minor amounts) that are randomly oriented and uniformly dispersed in a glassy matrix.
(57, 58)
Microstructure:
Natural felspathic (purely glass-based) systems have lower mechanical properties as they
do not provide enough resistance against defects and potential crack propagation.
(59)
Nevertheless,
the progression of cracks can be slowed down or prevented by dispersing crystals into the glass
matrix, such as alumina, zirconia, leucite, or lithium disilicate.
(35, 36)
The effect of these crystals
reinforces the glass matrix, leading to dispersion strengthening to improve the mechanical
properties compared to unreinforced materials
(59)
.
Phosphorus pentoxide (P2O5) acts as a heterogeneous nucleating agent and promotes the
nucleation of the lithium silicate phases.
(57, 58)
To enhance the optical and mechanical, other
powders are added to the base glass matrix ,such as aluminum oxide (Al2O3), aluminum
metaphosphate (Al[PO3]3), potassium oxide (K2O), zinc oxide (ZnO), zirconium dioxide (ZrO2),
Calcium oxide (CaO), or cerium dioxide (CeO2).
(38)
Lithium disilicate ceramics must undergo the additional processing procedures of milling
and post-mill crystallization to be suitable for oral use.
(60, 61)
Initially, the heat treatment aid in
enlarging the crystals.
(60, 61)
Those enlarged crystals interlock with each other.
(43)
This results in
a meshwork-like structure.
(43)
Post-milling crystallization results in dendritic (tree-like or sheaf-
like with significant branching) or spherulitic (subparallel needle-like crystallites are radiating
from the center and forming spherical mass) morphology.
(43)
High strength and fracture toughness
are achieved by these structures in glass ceramics.
(43)
Ceramic machinability while milling
11
prevents low-accuracy milling, which can lead to marginal discrepancies in the restoration and
clinical failure.
(62, 63)
The microstructure and morphology of the interlocked crystals in LDS allow high
mechanical strength.
(64)
They are characterized by high crystallinity, which promotes bridging and
prevent crack propagation.
(65-71)
These materials' crystallinity is influenced by various factors,
including their chemical composition, additives, and nucleating agents.
(72-74)
The crystal’s
morphology is affected by the amount of nucleating agent and heat and time treatment.
(65, 71, 75)
The binary SiO2-Li2O glass-ceramics have limited chemical durability.
(43)
Al2O3 and K2O are
additives to increase the chemical durability and, as a result, the suitability of restorations.
(43)
For the chemical composition, lithium metasilicate is considered the first crystallized phase
occurring at low temperatures and acts as a prosecutor for the following crystallized phase, lithium
disilicate.
(76)
The lithium metasilicate phase is preferred for milling in the CAD/CAM technology
due to its softness.
(76)
However, lithium metasilicate is strong enough to be efficiently milled
without severe wear.
(43, 77)
The metastable lithium metasilicate phase is transformed into the
desired high-strength lithium disilicate phase through suitable heat treatment.
(76)
Crystallization:
The crystallization mechanism starts with phase transition by nucleation (crystals
formation) followed by the growth of these crystals through the glass matrix.
(38)
Lithium disilicate
crystallization is heterogeneous and can be achieved using either a two-stage or three-stage
processing method ,depending on the fabrication method.
(38)
CAD/CAM LDS blocks undergo a
three-stage process, while heat-pressed LDS have a two-stage process.
(38)
The manufacturer performs the first step in both processes, which compacts or pressure
casts the glass base powders into molds.
(38)
The poured melt is transferred into a pre-heated
12
furnace at 450-550°C before cooling to room temperature to relax the glass block and avoid stress
generation in the glass.
(38, 43)
At this point, the glass blocks are kept in the furnace at the same
temperature for about 1 hour to initiate nucleation of the lithium silica phases in the form of nano-
lithium ortho-phosphate nuclei before proceeding with the crystallization heat treatments.
(38, 43)
After nucleation (formation of crystals), crystal growth improves physical and mechanical
properties by maximizing the generation of crystals and compression stress around the crystals.
(57,
78)
Three-stage crystallization:
A three-stage crystallization procedure is used to manufacture restorations from
CAD/CAM blocks.
(38)
The second step is to heat the block at 690–710°C for 10 to 30 minutes to
develop the lithium metasilicate (Li2SiO3) crystals.
(43)
At this stage, the block consist of a 40%
intermediate metasilicate phase containing crystals that are equally scattered, platelet-shaped
consists of a 40% intermediate metasilicate phase containing equally scattered, platelet-shaped
crystals, and 0.2–1.0 μm long.
(43)
Consequently, the material is considered machinable for the appropriate dental restoration
by the end user, either in the dental laboratory or clinically.
(38)
The third step in this process is a
final heat treatment for the restoration at 850°C for 20 to 31 minutes.
(43)
This heat treatment leads
the lithium metasilicate (Li2SiO3) crystals to develop lithium disilicate (Li2Si2O5) as well as tiny
quantities of lithium orthophosphate crystals.
(38)
The lithium disilicate crystals occupy 70% of the
phase, are rod-like, and are 1.5 μm long.
(38, 43)
Two-stage crystallization:
Using the lost wax hot-pressing technique, a two-stage crystallization process is used to
manufacture restorations from glass ingot.
(38)
In the first stage, a single heat treatment at 750-
13
850°C for approximately two hours crystallizes lithium disilicate from the glass ingot containing
nuclei produced in the cooling or pre-heating processes.
(38, 43)
The second stage is completed in the dental laboratory.
(38)
The crystallized ingot is heat-
pressed at 920 °C for 5 to 15 minutes to flow viscously into the lost wax mold used to fabricate
the desired restoration.
(38)
The lithium disilicate crystals occupy 70% of the phase, are needle-like,
and are 3 to 6 μm long.
(38)
Products:
IPS Empress 2 (Ivoclar Vivadent) is fabricated using a heat-pressing technology based on
the lost-wax technique.
(44, 79)
It is recommended for anterior and posterior restorations and has
improved flexural strength (360 MPa), which is more than twice that of leucite-based IPS Empress.
(44, 79)
IPS e.max Press (Ivoclar Vivadent) was introduced in 2005 with improved flexural strength
(400 MPa) compared to the former IPS Empress 2.
(80, 81)
Refined crystal size is observed with IPS
e.max, which enhances the physical properties and translucency through a different firing process.
(82)
Despite having a high crystalline content, this material exhibits high translucency because the
lithium-disilicate crystals have a comparatively low refractive index.
(24)
IPS e.max Press can be
used as monolithic for posterior restorations or as core material for anterior restorations.
(80)
IPS e.max CAD (Ivoclar Vivadent) is a machinable lithium-disilicate block that was
established to be used with CAD/CAM processing technology.
(83)
As mentioned earlier, these
blocks are subjected to a three-stage crystallization process.
(83)
In the second stage, the flexural
strength is between 130 to 150 MPa, allowing machining.
(83)
After the third stage of processing,
final crystallization, the flexural strength is 360 MPa, indicated for anterior and posterior units.
(79,
14
80, 84)
It is considered a “lab-side” material because the end user is required to apply heat treatment
to fully crystallize these blocks as those come in non-tooth colored blocks (blue).
(82)
Amber Mill (HassBio) was introduced in 2019 after the expiration of the IPS e.max CAD
patent.
(4, 85)
contains dense and cross-linked lithium disilicate (Li 2O 5Si 2) crystals.
(4)
The flexural
strength is 450 MPa after the end user fully crystallizes it.
(4)
Similar to IPS e.max CAD, it is
considered a “lab-side” material since it comes in amber glass color.
(4)
CEREC Tessera (Dentsply Sirona) was introduced in 2021 and contains both lithium
disilicate (Li 2O 5Si 2) and virgilite (LiAlSi2O6) crystals.
(86)
Virgilite is a lithium aluminum silicate
(LAS) type of crystal formed at a nanoscale during the firing process which, adds strength and
contributes to improved esthetics.
(86)
The manufacturer's specifications claim flexural strength of
700 MPa.
(86)
Even though the manufacturer claims that the Tessera block is fully crystalized, it
still requires an approximately 4 minutes heat treatment which considers this material “lab-side.”
(86)
n!ce (Straumann) contains lithium disilicate (Li 2O 5Si 2) and spodumene (LiAlSi2O6),
making it a lithium aluminosilicate disilicate.
(87)
The manufacturer does not require heat treatment
for these blocks as they are offered tooth-colored and referred to as ”chair-side.”
(87)
The
manufacturer claims a flexural strength of 350 ±50 MPa.
(87)
Initial LiSi Block (GC) is another “chair-side material that does not require heat treatment.
(88)
The composition of the Initial LiSi Block was not disclosed by the manufacturer. However, a
study found that it contains 29% lithium disilicate (Li 2O 5Si 2), 9.5% lithium phosphate (Li3PO4),
and 38.5% glass phase.
(89)
The manufacturer claims a flexural strength of 400 MPa.
(88)
Currently, Amber Mill, Initial LiSi Block, and n!ce have very limited research regarding
their microstructure and none on their mechanical strength.
15
B.3. Fluorapatite-based glass-matrix ceramics:
Fluorapatite-based glass-matrix ceramics crystallize via a controlled volume crystallization
mechanism, producing a variety of concentrations of nano-fluorapatite crystals with a diameter of
100 nm and a length of less than 300 nm, as well as micro-fluorapatite crystals with a diameter of
300 nm and a length of 2–5 µm.
(43, 90)
The amount of fluorapatite in the glass-ceramic is between 19 and 23 wt.%, and each of
the two crystal types has an adjustable volume fraction that enables the regulation of translucency
(the number of microcrystals), brightness, and opalescence (amount of nano-crystals).
(43, 90)
Fluorapatite glass-matrix ceramics is available in the market as IPS e.max Zir-Press, and
IPS e.max Ceram (Ivoclar Vivadent, Schaan, Liechtenstein).
(36, 37)
(C) Glass-infiltrated
Alumina, magnesium, or zirconia particles are infiltrated with glass.
(37, 38)
This ceramic type
is known as In-Ceram ceramics and consists of a sintered mass infiltrated with low-viscosity glass.
(37, 38)
Glass-infiltrated ceramics are available as VITA In-Ceram Spinell, VITA In-Ceram
Alumina, and VITA In-Ceram Zirconia (VITA Zahnfabrik, Bad Säckingen, Germany).
(37)
II. Polycrystalline ceramics
Polycrystalline ceramics are nonmetallic, inorganic, and do not have a glass phase.
(37)
High-strength polycrystalline ceramics were created due to crystalline materials being used more
frequently and the glass phase being used less and less until it was completely absent from the
16
material microstructure.
(37)
These ceramics are formed through directly sintering crystals,
producing a dense, polycrystalline structure free of glass without using an intermediary matrix.
(44)
In contrast to glasses, which have a less dense and irregular network, these solid-sintered
monophase ceramics are differentiated by regular arrays where the atoms are densely packed,
producing a solid and resistant material that is difficult to crack.
(36, 91)
Processing polycrystalline ceramics is more challenging than processing glassy ceramics.
(37)
Dental cores and frameworks with solid-sintered aluminous oxide (alumina, AlO) or zirconium
oxide (ZrO) were developed by the availability of computer-aided manufacturing.
(36, 82)
When
compared to glassy ceramics, these higher-strength materials are typically more opaque, and
certain polycrystalline ceramics exhibit cast alloy-like opacity.
(37)
They are generally indicated as
substructure materials that glassy ceramics are veneered over to produce esthetically pleasing
outcomes.
(82)
The polycrystalline ceramics are divided into four subcategories: alumina, stabilized
zirconia, zirconia toughened alumina, and alumina toughened zirconia.
(37)
Procera AllCeram (Nobel Biocare, Zu ̈rich, Switzerland) and In-Ceram AL (VITA
Zahnfabrik, Bad Säckingen, Germany) are representatives of the alumina type of polycrystalline
ceramic.
(37)
Cercon (Dentsply-Sirona, USA), Lava (3M ESPE, St. Paul, MN, USA), and IPS e.max
ZirCAD (Ivoclar Vivadent, Schaan, Liechtenstein) represent the stabilized zirconia in the
polycrystalline ceramic.
(37)
III. Resin-matrix ceramics
A resin-matrix ceramic is generally defined as a hybrid material made of an organic matrix
that is densely filled with inorganic particles such as ceramics.
(92)
The purpose of the polymer
17
matrix is to provide a support structure to reinforce the inorganic network of the hybrid material.
(93)
The resin-matrix ceramics used to be excluded from the ceramic classification.
(37)
However,
in 2013 The, American Dental Association Code on Dental Procedures and Nomenclature defined
ceramics as “pressed, fired, polished, or milled materials containing predominantly inorganic
refractory compounds including porcelains, glasses, ceramics, and glass-ceramics.”
(94)
Considering the resin matrix hybrids include more than 50% inorganic particles, and they are
classified as dental ceramics.
(37)
The resin matrix ceramics are divided into three subcategories: resin nanoceramic, glass
ceramic in an interpenetrating resin matrix, and zirconia-silica ceramic in a resin interpenetrating
resin matrix.
(37)
Lava Ultimate(3M ESPE, St. Paul, MN, USA) represents resin nanoceramic,
Enamic (VITA Zahnfabrik, Bad Säckingen, Germany) represents interpenetrating resin matrix,
and Shofu Block HC (Shofu, Kyoto, Japan) is representative of zirconia-silica ceramic in an
interpenetrating resin matrix.
(37)
1.2.2. Classification by processing method:
Processing techniques novel to dentistry have been developed, such as (I) slip-casting, (II)
heat-pressing, and (III) Computer-Aided-Design Computer-Aided Machining (CAD/CAM).
(95)
I. Slip casting
A slip combines ceramic powder particles suspended in a low-viscosity fluid (usually
water).
(96)
Forming a mold or negative reproduction of the desired framework geometry is called
slip casting.
(97)
A slip is then poured into the mold. The slip contains powder particles that, when
compacted against the mold walls, form a thin layer of green ceramic that will serve as the
18
framework.
(97)
The mold is comprised of a substance (often gypsum) that drains some water from
the slip into the walls of the mold through capillary action.
(96)
After partial sintering, the
framework can be withdrawn from the mold since it has become sufficiently strong to support its
weight and the residual slip is then discarded.
(97)
The resulting ceramic is highly porous and needs to be thoroughly sintered or penetrated
with molten glass before veneering porcelain can be used.
(96)
Because the reinforcing crystalline
particles create a continuous network throughout the framework, ceramics made via slip casting
can have stronger fracture resistance than powder condensation.
(97)
The method's complex set of
processes, which make it difficult to get an exact fit and could lead to internal flaws that weaken
the material from insufficient glass infiltration, are probably to blame for slip casting's limited use
in dentistry.
(98-100)
II. Hot pressing
Pressable dental ceramics are made using molds that are made using the lost wax technique.
(96)
Manufacturers sell prefabricated ingots of pressable ceramics composed of crystalline particles
dispersed throughout a glassy substance.
(96)
However, pressable ceramics do not have much
porosity and can have a more significant crystalline content since the ingots are made from
nonporous glass ingots by performing a heat treatment that causes some of the glass to crystallize.
(96)
The microstructure is similar to that of powder porcelains.
(101)
One can anticipate that this
method will result in a well-controlled and homogeneous material.
(101)
The pressable ingots are progressively pressed into the lost wax mold in the dental
laboratory after being heated to a temperature where they become a highly viscous liquid.
(96)
The
benefit of hot pressing is that dental technicians are already skilled at using the lost wax procedure
with metal alloys to achieve good fit precision.
(99, 100)
Contrary to popular belief, the lack of
19
porosity and higher crystalline content do not increase fracture resistance or reduce strength
variability.
(96)
Typically, pressable ceramics are exclusively used as core and framework materials.
(102)
Pressable veneering materials are available; nevertheless, the depth of layered esthetics may
be constrained when employing pressable ceramics for veneering materials.
(96)
The popularity of heat-pressed ceramics relies on the ability to use the lost-wax technique
to produce dental ceramic restorations.
(101)
Dental technicians are usually familiar with this
technique, commonly used to cast dental alloys.
(101)
In addition, the equipment needed to heat-
press dental ceramics is relatively inexpensive.
(83, 101)
The first generation of heat-pressed dental
ceramics contains leucite as a reinforcing crystalline phase.
(101)
The second generation is lithium
disilicate-based.
(83)
III. Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM):
Computer-aided design (CAD) and computer-aided manufacturing (CAM) technology
introduced a new era in modern dentistry.
(103, 104)
Francois Duret first presented CAD/CAM
technologies in 1971.
(103, 105)
These technologies were created to increase the durability and natural
appearance of dental restorations and to accomplish the same results more quickly and easily
without sacrificing precision.
(103, 105)
High-accuracy machining of materials like titanium and high-
performance ceramics has been made possible by CAD/CAM technologies.
(106)
As early as 1971, Duret was the pioneer to establish dental CAD/CAM technology,
fabricating crowns using an optical impression of a tooth and a computer-aided milling machine.
(107)
In 1983, he created the first dental CAD/CAM restoration, and in November 1985, he showed
his technique by making a posterior crown restoration for his wife in less than an hour at the French
Dental Association's international convention.
(108)
Then, Duret created the Sopha system.
(108)
20
In 1983, Andersson et al. developed the Procera method for manufacturing high-precision
dental crowns (now recognized as Nobel Procera, Nobel Biocare, Zurich, Switzerland).
(109)
Andersson et al. was the first to use CAD/CAM for composite veneered restorations .
(109)
The first commercial CAD/CAM system was developed by Mormann who consulted with
Brandestini, an electrical engineer, about the concept of using optics to scan the teeth.
(110)
By
1985, the combination of an optical scanner as well as a milling machine was performed to
complete the first chair-side inlay.
(110)
CEREC, an abbreviation for computer-assisted ceramic
reconstruction, was the name given to the device.
(110)
In the mid-1980s, Rekow et al. developed a dental CAD/CAM system that used
photographs as well as a high-resolution scanner to acquire data and then mill restorations using a
5-axis machine.
(111)
CAD/CAM manufacturing includes two types: (A) subtractive manufacturing and (B)
additive manufacturing.
(A) Subtractive manufacturing:
Subtractive manufacturing (SM) is extensively used in dentistry for the production of
dental pieces such as crowns and bridges.
(112, 113)
SM was initially intended for fully sintered
ceramic blocs (hard machining), it has now been expanded to partially sintered ceramics (soft
machining), which are later fully heat treated to ensure adequate sintering.
(114)
A.1. Hard machining
Currently, fully sintered ceramics based on feldspar, leucite, and lithium disilicate are available
for CAD/CAM hard machining of dental restorations.
(64)
With the introduction of partially
crystallized ceramics in the lithium silicate system, a sophisticated method for CAD/CAM
machining of fully sintered ceramics was proposed.
(64)
21
Both lithium metasilicate (Li2SiO3) and lithium disilicate (Li2Si2O5) crystal nuclei can be
found in the partially crystallized ceramic blocks.
(115)
The ceramic is machinable in this state.
(115)
There are two levels of translucency that can be obtained for ceramic blocks depending on their
crystallization pre-treatment.
(115)
The lithium metasilicate crystals of the high translucency (HT)
material are fewer and larger in the pre-crystallized state, while smaller crystals with higher density
are present in materials with low translucency (LT).
(115)
A.2. Soft machining
Partially sintered zirconia ceramic blocks were proposed for soft-machining using
CAD/CAM technology to create dental restorations.
(116, 117)
The concept compensates for the
approximately 25% volume shrinkage that will take place during the sintering of the zirconia
blocks.
(64)
The partially sintered blocks are easy to mill, resulting in significant time savings and
reduced tool wear.
(64)
(B) Additive manufacturing:
The main disadvantage of the CAD-CAM methodology, despite the fact that it is now well-
established in dentistry, is the significant material waste that occurs during machining because it
is a subtractive manufacturing process.
(118)
In some cases, the waste amounts to about 90% of the
prefabricated block, and the remainder is typically not reusable.
(118)
A potential solution to this
issue is the additive manufacturing (AM) process, often known as 3D printing.
(118)
Similar to subtractive manufacturing, additive manufacturing begins with intraoral
scanning and acquisition before using CAD software to create a 3D digital model.
(119, 120)
The 3D
digital model is then converted into a digital file and sent to a 3D printer.
(119, 120)
After that, each
material layer is sequentially deposited inside the 3D printer on top of the other to create a three-
22
dimensional part.
(119, 120)
The completed dental prosthesis requires several post-processing
activities, such as support removal, washing, and heat treatment.
(119, 120)
Dental prostheses can be produced using a variety of AM techniques, including direct
inkjet printing (DIP), selective laser melting (SLM), and stereolithography (SLA).
(119)
23
1.3. Ceramic Strength Properties:
A material's behavior to loading is described by its mechanical characteristics.
(121, 122)
The
resistance of a substance toward deformation, crack development, or fracture and the resulting
stress are measured by all mechanical properties.
(121)
Strength is a material's mechanical feature that ensures the prosthesis performs safely over
time which is a crucial element in the manufacturing of a dental prosthesis.
(121, 123)
Dental
applications of mechanical properties include hardness, fracture toughness, ductility, brittleness,
elasticity, and strength.
(121)
Strength is the maximum amount of stress that can be given to a material before it fractures
and fails structurally.
(66)
It measures forces across a specific area of a stressed structure instead
of the attraction between atoms like the elastic modulus does.
(59)
It is a crucial mechanical property
that determines the performance of brittle materials.
(124, 125)
Nevertheless, processing defects and
microcracks that naturally develop during thermal or mechanical processes can considerably
impact the strength measurement.
(126, 127)
Consequently, strength is considered a conditional
property.
(127)
Furthermore, several factors have a significant influence on the assessment of
strength such as testing design, specimen geometry, polishing procedures, and testing
environments.
(128, 129)
Brittle materials such as dental ceramics are much stronger in compression compared to
tension.
(59)
This is because they are unable to reduce tensile stresses at the tip of microstructural
defects, such as cracks, through an inelastic deformation.
(59)
Flexural testing is typically used to
evaluate the strength of brittle materials because they are more convenient to use than pure tensile
approaches.
(130)
24
I. Role of cracks in glass-matrix ceramics:
In several clinical situations, structural flaws or defects are associated with the failure of
brittle materials.
(131)
Accordingly, it is occasionally vital to examine the structural elements and
any defects when investigating the properties of these materials.
(131)
This makes it possible to
comprehend how defects, structural elements, and properties relate to one another.
(131)
A greater
comprehension of the characteristics of materials is achievable after this is established.
(131)
Usually, macroscopic, or microscopic levels are used for structural research.
(131)
Understanding
how microscopic flaws or defects, such as cracks, impact the behavior of most structural materials
under load is essential.
(131)
The mechanical behavior of brittle materials can be significantly influenced by crack
initiation, orientation, and propagation.
(132, 133)
In fact, the majority of dental ceramics have at least
two types of defects: intrinsic and surface defects.
(134)
Hypothetically, the size of the defect's tip
is comparable to the distance between atoms in the structural material.
(134)
The buildup of stress
at the flaw tip leads to localized stresses.
(134)
As a result, this leads to crack propagation through the material, if the crack isn’t opposed,
deflected, or arrested by a hindrance.
(135)
Consequently, if the crack is not opposed, deflected, or
arrested by a barrier, it will propagate throughout the material.
(135)
It should be emphasized that
in such a case, the type of stress is very important; in a brittle material, tensile stresses close to the
crack tip cannot be reduced.
(135)
As a result, cracks could propagate and eventually fracture under
a low-average tensile stress.
(135)
This demonstrates the major impact of crack size, distribution,
and orientation.
(135)
The crack size has a significant impact in this regard.
(134)
The concept of stress
concentration can be used to explain this.
(134)
The largest cracks propagate to fracture by the
25
influence of their orientation as well as the amount of the most unfavorable forces, tensile stresses.
(134)
The so-called slow crack growth phenomenon is one of the essential properties of critical
cracks in brittle materials.
(133)
Cracks may develop gradually over time.
(134)
Even under less-than-
critical stress conditions, this is still possible.
(133)
It occurs in a favorable setting, such as the oral
cavity, where liquids such as saliva or water are available.
(133)
It is hypothesized that this happens
when water molecules interact with the crack tip, breaking the metal-oxygen link and creating
hydroxides.
(133)
When subjected to mechanical forces, a crack may develop gradually until it reaches a
critical size, at which point it is sufficient to result in a fracture.
(134)
If this is the case, the strength
may gradually decrease over time, shortening the lifespan of the dental restoration.
(134)
The
mechanical strength of dental ceramics is thus highly dependent on variables like crack size
distribution, shape, and orientation.
(134)
II. Effect of firing
The fabrication of glass ceramic restorations frequently also includes the application of
stain oxides and glazing for finishing, which is essential for mimicking tooth structure and
appearance.
(136)
The inability of manufacturer-recommended finishing procedures, such as glaze
firings, to reduce the hard-machining damage indicates that significant flaws produced by grinding
during the milling stage may become fracture origins.
(136)
Some studies show certain heat treatments' ability to reduce the glass matrix's viscosity,
causing a decrease in the depth of fissures, crack–tip blunting Field (115, 116), and even sealing
defects.
(137, 138)
Healing seems to be favored by materials containing silicon (Si), which when
26
subjected to the proper heat treatment, go through an oxidation reaction and generate viscous SiO2-
based products (crystalline or amorphous), which tend to seal the defects.
(139)
Ideally, firings should refrain from interfering with the desired effect of the glaze or the
optical characteristics of the restoration.
(140)
Heat treatments must also meet the requirements for
a structural balance of the glassy and crystalline phases, as slight variations in the microstructure
resulting from firing may influence the mechanical, chemical, or physical properties of a specific
material.
(140)
However, the limit of firings that affects the mechanical properties was not clear.
Therefore, in this study, we aim to evaluate the effect of repeated firings on lithium disilicate.
III. Effect of thickness
Laboratory investigation of restorative material strength prior to clinical use is essential to
decide if it can be regarded as a reliable treatment option. In bonded ceramics, several factors affect
the mechanical behavior of both restoration/tooth complexes including restoration thickness and
quality of the adhesive interface.
(141)
Thus, the strength of the tooth, restoration, and adhesive
system contributes to the performance of indirect restorations.
(142, 143)
On the other hand, scientific evidence on the minimum acceptable thickness for glass
ceramic occlusal veneers is still scarce. A study addressed the significance of decreasing the
preparation depth between 1.00 mm and 0.5 mm.
(144)
This study demonstrated that reducing the
preparation depth to 0.5 mm had no significant influence on the pressable lithium disilicate ceramic
onlay restorations' ability to resist fracture.
(144)
According to other research, even in the presence
of minimum thicknesses, dental tissue preservation and restoration bonding on enamel ensure
considerable resistance to restoration.
(145-147)
27
The effect of material thickness on the strength of glass-matrix ceramics is very limited.
Therefore, this study emphasizes on the effect of repeated firings of lithium disilicate reinforced
glass-matrix ceramics in varying thicknesses.
28
1.4. Strength Testing Methods:
Several testing methods are possible to get information on the ceramic's strength behavior.
(148)
These testing methods include compression, shear, tensile, and flexural strength. (Figure 3)
(148)
Figure 3: Testing methods include compression, shear, tensile, and flexural strength
I. Compressive strength
Materials that are expected to sustain occlusal loading typically undergo compression
testing.
(149)
Since the majority of mastication forces are compressive in nature, it is crucial to
investigate materials in this situation.
(149)
Two axial sets of forces are applied to a specimen in
opposite directions to evaluate a material's compressive strength in order to simulate the molecular
structure of the material (Figure 3).
(149)
II. Tensile strength
Materials that are subjected to axial forces along a straight line and in opposite directions,
result in tension (Figure 3).
(149)
Tensile strength describes the material's resistance to this load.
(149)
Elongation is the length change caused by the application of a tensile force to a body prior to
its failure.
(149)
Tensile stresses are generated by loads that stretch or elongate a material.
(149)
29
III. Shear strength
The maximum stress a material can withstand prior to failing in a shear mode of loading is
known as the shear strength.
(149)
Shearing is caused by two sets of forces that are parallel to one
another but not in a straight line
(149)
, as shown in Figure 3. It is crucial when investigating the
interfaces of two materials, such as the ceramic-metal or implant-bone interface.
(149)
The punch
or push-out method, in which an axial load is applied to push one material through another, is one
way to assess the shear strength of dental materials.
(149)
IV. Flexural strength:
A material's flexural strength determines its ability to bend without breaking.
(149)
It is
achieved when a material reaches its maximum flexibility before reaching its proportional limit.
(149)
Many materials' bending or flexural properties are frequently more crucial than their tensile
or compressive properties.
(150)
This assesses the strength of restorative material that is supported at both ends and
subjected to static load.
(150)
Stresses on the top surface are compressive, while those on the lower
surface are considered tensile, as in Figure 3
(150)
It is possible to consider that this test combines
aspects of compressive and tensile testing.
(150)
Dental materials must tolerate repeated flexing and
bending since flexural forces result from forces created in clinical conditions.
(150)
High flexural
strength is desirable once these materials are subjected to chewing stress, resulting in irreversible
deformation.
(149)
The International Organization for Standardization (ISO) has standardized flexural
strength testing methods by following ISO 6872-2015
(151)
to include (A) three-point bending, (B)
four-point bending, and (C) biaxial flexural.
30
Figure 4: Flexural strength testing
(A) Three-point bending
According to ISO 6872-2015
(151)
, the three-point bending test requires bar-shaped
specimens. In the three-point bend test, the specimen is positioned on two support pins and the
load is applied in the middle (Figure 4).
(149)
Maximum bending occurs below the support pins.
(149)
The test is known as a non-uniform central stress field.
(149)
This is due to the small area of
stress under the loading pins causing stress concentration.
(149)
The test is significantly dependent on the surface finish of the specimen edges.
(149)
Since
this is where the most stress is present, cracks and defects might initiate leading to relatively lower
flexural strength.
(152)
(B) Four-point bending
The four-point bending test requires bar-shaped specimens as well according to ISO 6872-
2015.
(151)
In the four-point bend test, the specimen is positioned on two supports and the load is
applied symmetrically at two sites (Figure 4).
(149)
Maximum bending occurs in between the
loading pins.
(149)
The test is known as a uniform central stress field.
(149)
Compared to the three-
point test, this is due to a wider area of stress between the loading pins causing a stress distribution.
(149)
31
Similar to the three-point bending test, the specimen edges are significantly dependent on
the surface finish which might initiate cracks and defects initiate leading to relatively lower
flexural strength.
(149, 152)
(C) Biaxial flexural
Biaxial flexural tests have several advantages compared to uniaxial tests including
multiaxial stress being produced as well as edge failures being eliminated.
(153)
Uniaxial tests
display defects in their bar-shaped specimens developed by the bending process.
(154)
However,
these defects are not found in either disc-shaped specimens used for biaxial flexural strength
testing or clinical crowns.
(154)
Consequently, biaxial flexural strength has been widely used to
evaluate the strength of the dental ceramics.
(3, 6, 12, 14, 18-23)
In dentistry, biaxial flexural strength has various arrangements such as piston-on-ring and
piston-on-three-ball.
(155)
Ring-on-ring applies symmetrical and constant stress within the loading
ring. However, it required flat specimens.
(156)
The piston-on-three-ball (Figure 4) was developed
primarily to address out-of-flatness of specimens, which will dependably locate readily on three
points.
(156)
However, in ISO 6782-2015, the piston-on-three-ball test was selected. Yet, the ISO
6872 biaxial strength formulas were initially developed for piston-on-ring testing.
(157)
While the focus of this study is to evaluate the strength of lithium disilicate glass-ceramic, the
biaxial flexural strength test (piston-on-three-ball) was selected to investigate following the ISO
6872-2015.
(151)
Previous studies have investigated the flexural strength of lithium disilicate have
been reported to range from 189.19 MPa to 529.46 MPa in the literature.
(2, 3, 6, 8, 9, 12, 14-17, 19-23)
The
ceramic strength data are usually not normally distributed, therefore, the data are further analyzed
using a probability distribution such as the Two-parameter Weibull distribution as indicated by
ISO 6872-2015.
(151)
32
1.5. Weibull Distribution:
The mean strength and standard deviation serve as the statistical measure of strength.
(158)
Nevertheless, the strength of ceramics typically varies greatly due to the impact of different sizes
of internal processing flaws, which can occasionally lead to unpredictable results.
(158)
Therefore,
it is debatable if the brittle ceramic materials' mean strength is accurate and represents their true
strength.
(158, 159)
The size and shape of a large crack within the area of maximal tensile stresses is the most
crucial factor in the fracture.
(126)
The larger the defect is, the more likely it fails at lower tensile
strength.
(126, 160)
The size and shape of the specimens are the other two important parameters that
must be taken into account when performing brittle testing.
(126)
For instance, larger defects are
more likely to be present in large specimens than in small specimens.
(126)
Consequently, the
specimen fails at lower stresses and the largest flaw is more prone to propagate.
(126)
The Weibull
distribution may therefore provide a useful explanation of the underlying statistical variation in the
fracture stress behavior of the ceramic.
(161)
Weibull analysis has been used to evaluate the reliability of such materials and to predict
strength variability during clinical applications
(20, 21, 162-164)
for different specimen geometries (bar
(162)
, disk
(20, 21, 165)
, and crown
(163, 166)
); and mechanical properties (flexural strength,
(20, 21, 162-165)
failure load
(163, 166)
, and hardness.
(21, 167)
).
The Weibull distribution describes the survival and failure rates of brittle materials.
(168)
By
analyzing failure probability using the Weibull distribution as opposed to relying solely on material
strength, it may be possible to better predict the life of the materials.
(169)
The Weibull distribution includes two parameters: Weibull modulus and characteristic
strength.
(168)
The Weibull modulus is considered the shape parameter representing the
33
distribution of flaws within a brittle material.
(168)
Higher Weibull modulus values indicate a
material's higher level of homogeneity and more reliability as a structural material.
(161)
A low
Weibull modulus indicates a wide variation of measured strength values and a higher probability
of existing flaws weakening brittle materials.
(161)
The Weibull modulus of lithium disilicate has
been reported to range from 5.71 to 24.36 in the literature.
(2, 9, 12, 14, 19-21)
The Weibull characteristic strength is the Weibull scale parameter representing the
failure probability of approximately 62.3%.
(168)
With higher characteristic strength values a
material represents less probability of failure compared to a material with lower characteristic
strength value.
(161)
The Weibull characteristic strength of lithium disilicate has been reported to
range from 189 to 631.1 MPa in the literature.
(2, 9, 12, 14, 19-21)
Figure 5: Weibull plot
For graphical analysis, Weibull plots are used to represent the biaxial flexural strength data
against the probability of failure in percentage.
(170)
In the plot (Figure 5), the biaxial flexural
strength data (MPa) is on the x-axis, and the probability of failure (%) is on the y-axis. The
horizontal dotted line is the Weibull characteristic strength representing the point at which 62.32%
of specimens will have failed. Weibull modulus affects the steepness of each group in the plot
34
according to the standard deviation of the biaxial flexural strength value. For example, in this plot
(Figure 5), the shallow line for material #3 (blue) represent a low Weibull modulus due to the
scattered data and high standard deviation compared to the steep lines for materials #1 (black) and
#2 (red).
Figure 6: Likelihood contour plot
The statistical significance of the differences between the Two-parameter Weibull
distributions is assessed using a likelihood contour plot.
(171)
The plot (Figure 6) represents the
Weibull modulus on the y-axis and the characteristic strength on the x-axis.
(171)
The plot shows
contour/oval shapes of the Two-parameters, if these shapes intersect/overlap with one another, the
Weibull parameters are not significant.
(171)
For example, in the plot, the contour shapes of material
#1 (black) and material #2 (red) intersects. However, material #3 (blue) does not intersect with
material #1 and #1 indicates that material #3 is statistically significantly different than material #1
and material #2.
35
1.5. Objective:
An abundant amount of research investigated the mechanical properties of IPS e.max CAD.
However, the mechanical properties for the new materials Amber Mill, Initial LiSi, and n!ce were
not investigated. Therefore, we are targeting at biaxial flexural strength of those materials in
different thicknesses (0.5 mm and 1.00 mm) as well as the effect of repeated firings (baseline, one
firing, three firings, and five firings).
1.6. Aim:
The purpose of this in vitro study is to evaluate the effect of varying thickness and repeated
firings on the biaxial flexural strength of “lab-side” materials: IPS e.max CAD and Amber Mill as
well as “chair-side” materials: Initial LiSi and n!ce.
1.7. Null Hypothesis:
• Different types of CAD/CAM lithium disilicate materials do not affect the biaxial flexural
strength.
• Repeated firings of different types of CAD/CAM lithium disilicate materials do not affect
the biaxial flexural strength.
• Different thicknesses of different types of CAD/CAM lithium disilicate materials do not
affect the biaxial flexural strength.
36
Chapter 2: Materials and methods
An overview of the testing process of the biaxial flexural strength is presented in Figure
7. Starting with the study design which follows the ISO 6872-2015 specification and selecting the
tested materials. Then, to prepare the specimens cylinders were designed and milled. After that,
the cylinders were sectioned into disc-shaped specimens and polished to desired thickness.
Consequently, the specimens were fired. Then, the specimens were subjected to biaxial flexural
strength testing followed by statistical analysis of the BFS data and Weibull analysis.
Figure 7: The process of testing biaxial flexural strength
37
2.1. Study Design:
Four machinable lithium disilicate CAD/CAM blocks were selected for this study: IPS
e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein) and Amber Mill (AM, Hassbio, Kangreung,
Korea) as lab-side groups. GC Initial LiSi Block (GC Corp, Tokyo, Japan) and n!ce (Straumann,
Freiburg, Germany) as chair-side groups (Figure 8.). The CAD/CAM blocks used in this in vitro
study are described in Table 1.
Figure 8: Lithium disilicate reinforced glass-matrix ceramics: (A) IPS e.max CAD, (B) Amber Mill, (C) GC Initial LiSi, and (D)
n!ce
38
Table 1: Product name, manufacturer, composition, shade, LOT number, and dimensions of CAD/CAM blocks used
Products Manufacturer Composition (wt.%) Shade LOT Number Dimensions (mm)
IPS e.max CAD
(1)
Ivoclar
Vivadent,
Schaan,
Liechtenstein
SiO2: 57.0-80 .0
Li2O: 11.0-19.0
K2O: 0.0-13.0
P2O5: 0.0-11.0
ZrO2: 0.0-8.0
ZnO: 0.0-8.0
Coloring oxides: 0.0-12.0
A1 HT #Z02MRY 12.40X14.50X18.00
Amber Mill
(4)
HassBio,
Kangreung,
Korea
SiO2: <78 %
Li2O: <12 %
Coloring oxides: <12 %
HT #EBE05NC2601 12.00X14.00X18.00
Initial LiSi
(5)
GC, Tokyo,
Japan
SiO2: 81%
P2O5: 8.1%
Na2O: 5.9%
Al2O3: 3.8%
TiO2: 0.5%
CeO2: 0.6%
(10)
A1 HT #2201241 12.70X14.70X18.00
n!ce
(7)
Straumann
Freiburg,
Germany
SiO2: 64-70
Li2O: 10.5-12.5
Al2O3: 10.5-11.5
Na2O: 1-3
K2O: 0-3
P2O5: 3-8
ZrO2: 0-0.5
CaO: 1-2
Coloring oxides: 0-9
A1 HT #JEX10 12.40X14.50X18.00
39
This in-vitro study investigated a total of (n=480) disc-shaped specimens and (n=120) for
each CAD/CAM block. Each material group was subdivided into two thicknesses: (1) 1.00 mm,
(2) 0.5 mm. Then each CAD/CAM material group was investigated following four conditions: (1)
baseline, (2) one firing, (3) three firings, and (4) five firings. All specimens were subjected to
biaxial flexural strength in the universal testing machine following ISO 6872:2015 (Figure 9).
(151)
Figure 9: Study design
40
2.2. Preparation of Specimens:
A summary of the specimens’ preparation is presented in Figure 10. Following ISO 6872-
2015, the dimensions of the specimen were considered. After that, a cylinder was designed to fit
CAD/CAM blocks. The rectangular blocks were milled into cylinders. Then, an estimation of
specimens per block was performed. Next, the cylinders were sectioned into two thicknesses, 0.5
mm and 1.00 mm. After that, the specimens were polished to desired thickness.
Figure 10: Preparation of specimens
41
I. ISO 6872-2015:
To prepare specimens for biaxial flexural strength testing, the ISO 6872-2015
(151)
is
followed. The standard requires the specimens to be disc-shaped with a thickness of 1.2 ±0.2 mm
and a diameter of 14.00 ±2.00 mm (Figure 11).
Therefore, the rectangular-shaped CAD/CAM blocks needed to change to cylinders in
order to allow for disc-shaped sectioning. The dimension of each block size C14 (Table 1) was
accounted for to ensure the fit of a cylinder with a diameter of 12 mm and a height of 17 mm.
Figure 11: ISO6872-2015 specimen requirements
42
II. Cylinder design:
After measuring the dimensions of each CAD/CAM block, a cylinder was designed using
Meshmixer (Meshmixer Version 3.5.474, Autodesk, San Rafael, California, USA). The dimension
of the cylinder was 12.00 mm in diameter and 17.00 mm in height (Figure 12) and exported as a
Standard Triangle Language file (.STL).
Figure 12: A cylinder designed using Meshmixer (12mmX17mm)
43
III. Milling cylinders:
The (.STL) file of the cylinder shape was imported into inLab CAM software (inLab CAM
SW 18.1, Dentsply-Sirona, Bensheim, Germany). The cylinder was positioned in the software
(Figure 13). Then, a set of burs, Step bur 12S, and Cylinder pointed bur 12S, were used to mill
the blocks into cylindrical form (Figure 14) using a milling machine (Sirona inLab MC XL,
Dentsply-Sirona, Bensheim, Germany).
Figure 13: Positioning cylinder in the inLab CAM SW 18.1
Figure 14: Milled cylinders from CAD/CAM blocks
44
IV. Preparation of sectioning:
Prior to specimen sectioning, two thicknesses were determined for testing 0.5 mm and 1.00
mm. The dimensions of each block CAD/CAM block were accounted for to estimate the number
of specimens per block according to the thickness required. The total number of specimens (n=480)
was estimated.
For the 0.5 mm thickness specimens, each block was sectioned into 19 specimens (Figure 15).
Also, for the 1.00 mm thickness specimens, each block was sectioned into 12 specimens (Figure
16). The first slice was intended to calibrate the precision saw (IsoMet 1000, Buehler, Lake Buff,
IL, USA).
Figure 15: Estimation of specimens per block (0.5 mm)
Figure 16: Estimation of specimens per block (1.00 mm)
45
V. Sectioning:
Then, the cylinders were mounted on a precision saw (Figure 17) (IsoMet 1000, Buehler, Lake
Buff, IL, USA) and sliced horizontally into discs using with a diamond blade (102 mm diameter,
0.3 mm thickness; IsoMet Blade 15LCA, Buehler, Lake Buff, IL, USA) to the designated
thicknesses of 0.5 mm and 1.00 mm under distilled water-cooling (Arrowhead Mountain Spring
Water, BlueTriton, San Bernardino, CA, USA).
Figure 17: Cylinder mounted on a precision saw (IsoMet 1000, Buehler, Lake Buff, IL, USA)
46
VI. Polishing:
After slicing, a slow-speed handpiece was used with coarse Soflex discs (Soflex Discs 2381
Coarse Red 1/2", 3M/ESPE, St. Paul, MN, USA) to reduce the irregularities on specimens (Figure
18).
Figure 18: (A) Disc-shape specimen after slicing, (B) Slow-speed handpiece with coarse Soflex discs, and (C) Disc-shape
specimen after polishing.
The disc-shaped specimens that were not at the desired thickness were polished manually on a
disc sanding machine (Mini Disc Sander 6-Inch Electric Sanding Belt Machine, MXBAOHENG,
USA) (Figure 19) under distilled water-cooling (Arrowhead Mountain Spring Water, BlueTriton,
San Bernardino, CA, USA).
The top and bottom surfaces of the discs were polished using 320 grit (CarbiMet Abrasive
Sheet, 320 SiC, Buehler, Lake Buff, IL, USA) to reduce the thickness to the desired thickness
followed by the use of 400, 600, 800, and 1200 grit silicon carbide paper for all specimens
(CarbiMet Abrasive Sheet, 400, 600, 800, and 1200 SiC, Buehler, Lake Buff, IL, USA) to polish
the surfaces (Figure 19).
47
Figure 19: Disc sanding machine (Mini Disc Sander 6-Inch Electric Sanding Belt Machine, MXBAOHENG, USA)
The polishing was done to eliminate irregularities caused by machining and slicing and to
confirm the final thickness with digital caliper (Mitutoyo digital caliper; Mitutoyo Corp,
Kawasaki, Japan) with a 0.001-mm accuracy (Figure 20).
Figure 20: Verification of specimen final dimensions: 1.00 mm thickness and 12.00 mm diameter
48
VII. Group distribution:
After polishing, the discs-shaped specimens were placed in an ultrasonic bath (Ultrasonic
Cleaning Systems, Quantrex, Kearny, NJ, USA) and immersed in distilled water (Arrowhead
Mountain Spring Water, BlueTriton, San Bernardino, CA, USA) for 10 minutes.
Four hundred and eighty discs were randomly divided into four groups according to the
selected CAD/CAM blocks (n=120) and subdivided based on the thickness (0.5 mm and 1.00)
(n=60) and the number of firings (baseline, one firing, three firings, and five firings) (n=15).
The specimens were air dried and kept in plastic containers (Clear plastic storage box 10 Grids
5 Inch x 2.5 Inch, Yansanido, China) according to their assigned groups in Table 2.
49
Table 2: Groups distribution
Material
Thickness
(mm)
Firing Group name Group Number Abbreviation Specimens #
IPS e.max CAD
( n=120)
1.00 mm
(n=60)
Baseline (n=15) 0.5 mm baseline (1) 5EB 1-15
One Firing (n=15) 0.5 mm one firing (2) 5E1 16-30
Three Firings (n=15) 0.5 mm three firings (3) 5E3 31-45
Five Firings (n=15) 0.5 mm five firings (4) 5E5 46-60
0.5 mm
(n=60)
Baseline (n=15) 1 mm baseline (5) 1EB 61-75
One Firing(n=15) 1 mm one firing (6) 1E1 76-90
Three Firings(n=15) 1 mm three firings (7) 1E3 91-105
Five Firings (n=15) 1 mm five firings (8) 1E5 106-120
Amber Mill
( n=120)
1.00 mm
(n=60)
Baseline (n=15) 0.5 mm baseline (9) 5AB 121-135
One Firing (n=15) 0.5 mm one firing (10) 5A1 136-150
Three Firings (n=15) 0.5 mm three firings (11) 5A3 151-165
Five Firings (n=15) 0.5 mm five firings (12) 5A5 166-180
0.5 mm
(n=60)
Baseline (n=15) 1 mm baseline (13) 1AB 181-195
One Firing(n=15) 1 mm one firing (14) 1A1 196-210
Three Firings(n=15) 1 mm three firings (15) 1A3 211-225
Five Firings (n=15) 1 mm five firings (16) 1A5 226-240
Initial LiSi
( n=120)
1.00 mm
(n=60)
Baseline (n=15) 0.5 mm baseline (17) 5LB 241-255
One Firing (n=15) 0.5 mm one firing (18) 5L1 256-270
Three Firings (n=15) 0.5 mm three firings (19) 5L3 270-285
Five Firings (n=15) 0.5 mm five firings (20) 5L5 286-300
0.5 mm
(n=60)
Baseline (n=15) 1 mm baseline (21) 1LB 301-315
One Firing(n=15) 1 mm one firing (22) 1L1 316-330
Three Firings(n=15) 1 mm three firings (23) 1L3 331-345
Five Firings (n=15) 1 mm five firings (24) 1L5 346-360
n!ce
( n=120)
1.00 mm
(n=60)
Baseline (n=15) 0.5 mm baseline (25) 5NB 361-375
One Firing (n=15) 0.5 mm one firing (26) 5N1 376-390
Three Firings (n=15) 0.5 mm three firings (27) 5N3 391-405
Five Firings (n=15) 0.5 mm five firings (28) 5N5 406-420
0.5 mm
(n=60)
Baseline (n=15) 1 mm baseline (29) 1NB 421-435
One Firing(n=15) 1 mm one firing (30) 1N1 536-450
Three Firings(n=15) 1 mm three firings (31) 1N3 451-465
Five Firings (n=15) 1 mm five firings (32) 1N5 466-480
50
2.4. Firing of Specimens:
The specimens of both thickness, 0.5 mm and 1.00, in the lithium disilicate CAD/CAM block
were divided into 4 groups according to the firings (baseline, 1 firing, 3 firings, and 5 firings)
(n=15/group). The specimens were placed on a crystallization tray (IPS e.max CAD Crystallization
Tray, Ivoclar Vivadent Schaan, Liechtenstein) (Figure 21). The firings were performed in a
furnace (Programat SC3, Ivoclar Vivadent, Schaan, Liechtenstein) (Figure 22) according to the
manufacturer’s firing parameters (Table 3).
Figure 21: Disc-shaped specimens placed on a crystallization tray (IPS e.max CAD Crystallization Tray, Ivoclar Vivadent
Schaan, Liechtenstein)
Figure 22: IPS e.max CAD ready for baseline firing furnace (Programat SC3, Ivoclar Vivadent, Schaan, Liechtenstein)
51
Table 3: Firing parameter of selected lithium disilicate CAD/CAM blocks
Product
IPS e.max CAD
HT A1
(1)
Amber Mill
HT
(4)
Initial LiSi
HT A1
(5)
n!ce
HT A1
(7)
Firing parameter Crystallization Firing Crystallization Firing Firing Firing
Stand-by temperature (B): 403°C 403°C 400°C 400°C 450°C 400°C
Closing Time (S): [min] 6:00 6:00 3:00 3:00 2:00 2:00
Heating rate (t1): [°C/min] 90 90 60 60 45 60
Firing Temperature: (T1) 820°C 820°C 815°C 815°C 730-750°C 770-800°C
Holding Time (H1): [min:sec] 0:10 0:10 15:00 15:00 1:00 0:10
Heating rate (t2): [°C/min] 30 30
Firing Temperature (T2): 840°C 840°C
Holding Time (H2): [min:sec] 7:00 3:00
Vacuum 1: 11-12 [°C] 550-770°C 550-770°C 550°C 550°C
Vacuum 2: 21-22 [°C] 820-850°C 820-850°C 815°C 815°C
Long-term cooling (L): 700°C 700°C 690°C 690°C 0 400°C
Cooling rate (tl): [°C/min] 0 0 0 0 0 25
52
I. Firing protocol:
(A) Lab-side materials
The firing protocol of the lab-side materials, IPS e.max CAD, and Amber Mill is
summarized in Figure 23.
Figure 23: Lab-side materials firing protocol
53
The firing protocol of the lab-side materials is as the following:
Pre-crystallization: IPS e.max CAD and Amber Mill, are partially crystallized lithium
disilicate CAD/CAM blocks. All specimens (n=120/material) were subjected to a crystallization
firing.
Baseline The specimens were removed from the furnace and placed at room temperature
for 10 minutes. The baseline groups (n=15/group) for each material were kept in their perspective
containers.
One firing: One firing cycle was performed for the remaining specimens (n=90/material).
The specimens were kept at room temperature for 10 minutes. The one firing groups (n=15/group)
for each material were kept in their assigned containers.
Three firings: Two more firing cycles were completed for the remaining specimens
(n=60/material). After each firing, the specimens were kept at room temperature for 10 minutes
before the next firing. The three firings’ groups (n=15/group) for each material were kept in their
given containers.
Five firings: Additional two firing cycles were performed for the remaining specimens
(n=30/material). following each firing, the specimens were kept at room temperature for 10
minutes before the next firing. The five firings’ groups (n=15/group) for each material were kept
in their perspective containers.
54
(B) Chair-side materials:
The firing protocol of the lab-side materials, Initial LiSi, and n!ce is summarized in
Figure 24.
Figure 24: Chair-side materials firing protocol
55
The firing protocol of the chair-side materials is as the following:
Baseline: GC Initial LiSi and Straumann n!ce are fully crystalized reinforced lithium
disilicate CAD/CAM blocks. The manufacturers do not require firing but give the clinician the
choice to stain and glaze. The baseline groups (n=15/group) were kept as is after polishing the
disc-shaped specimens without being subjected to heat treatment.
One firing: One firing cycle was performed for the remaining specimens (n=90/material).
The specimens were kept at room temperature for 10 minutes. The one firing groups (n=15/group)
for each material were kept in their assigned containers.
Three firings: Two more firing cycles were completed for the remaining specimens
(n=60/material). After each firing, the specimens were kept at room temperature for 10 minutes
before the next firing. The three firings’ groups (n=15/group) for each material were kept in their
given containers.
Five firings: Additional two firing cycles were performed for the remaining specimens
(n=30/material). following each firing, the specimens were kept at room temperature for 10
minutes before the next firing. The five firings’ groups (n=15/group) for each material were kept
in their perspective containers.
56
II. After firings:
Once all specimens completed the firings, the specimens were numbered from 1 to 480
according to Table 2 on one side using a black marker (Sharpie ultrafine, Newell, Atlanta, GA,
USA). The other side of the specimens had the thickness and diameter written using a mechanical
pencil (BIC mechanical pencil 0.7, BIC Milford, Connecticut).
Photos were taken for all specimens. (Nikon Z5, Nikon, Tokyo, Japan) (Figure 25). Then, all
specimens were kept in plastic containers (Clear plastic storage box 10 Grids 5 Inch x 2.5 Inch,
Yansanido, China) according to the assigned groups in Table 2.
Figure 25: Disc-shaped specimens prior to biaxial flexural strength testing
57
2.5. Biaxial Flexural Strength Testing (BFS):
The biaxial flexural strength testing is summarized in Figure 26.
Figure 26: Biaxial flexural strength testing
58
I. ISO 6872-2015:
ISO 6872-2015
(151)
was used to test the biaxial flexural strength (piston-on-three-ball test)
using the universal testing machine (Instron 5965, Canton, MA, USA) (Figure 27).
The piston-on-three-ball testing fixture (Biaxial Flexion-ISO, Odeme Dental Research,
Luzerna SC, Brazil) was mounted to the universal mechanical testing machine (Instron 5965,
Canton, MA, USA) (Figure 28).
Figure 27: Universal testing machine (Instron 5965, Canton, MA, USA)
Figure 28: Piston-on-three-ball (Biaxial Flexion-ISO, Odeme Dental Research, Luzerna SC, Brazil)
59
II. Piston-on-three-ball:
The testing fixture (piston-on-three-ball), has 3 steel balls 3.2 mm in diameter, set 120°
across on a 10 mm support circle (Figure 29). Prior to testing, the thickness, diameter, and group
number of each specimen were recoded into the testing software Bluehill 3 (Instron, Norwood,
MA, V3.04, Bluehill 3, USA).
Figure 29: The piston-on-three-ball fixture with 3 steel balls 3.2 mm in diameter, set 120° across on a 10 mm support circle
60
III. Positioning device:
The specimens were placed into the piston-on-three-ball using a positioning device to
ensure that specimens were placed in the center of the specimen holder. The positioning device
was designed in Meshmixer (Meshmixer Version 3.5.474, Autodesk, San Rafael, California, USA)
and 3D printed using Photon Mono X (Photon Mono X, Anycubic, Shenzhen, China) (Figure 30).
Figure 30: 3D printed positioning device (A) After printing (B) Verification of specimen fit
61
IV. Testing protocol:
After placing the positioning device (Figure 31-A), a specimen was placed within the
specimen holder on top of the 3 balls (Figure 31-B) Then, a plastic polyethylene sheet with a
thickness of 0.05 mm (LDPR poly bags 3 IN X 3IN X 2MIL, Dental subdues) was placed on the
specimen to enable even load delivery (Figure 31-C). After that, the positioning device was
removed, and the loading piston, which has a 1.4 mm diameter flat tip, was lowered, and centered
to the specimen (Figure 31-D).
The biaxial flexural strength test was conducted at a crosshead speed of 0.5 mm/min in air
at room temperature till the specimen’s failure using universal testing machine (Instron 5965,
Canton, MA, USA).The maximum load (N) of each specimen was recorded into the testing
software Bluehill 3 (Instron, Norwood, MA, V3.04, Bluehill 3, USA). After testing, fractured
specimens were collected and kept using tape (Scotch tape, Scotch, St. Paul, Minnesota, USA).
Photos were taken for all specimens. (Nikon Z5, Nikon, Tokyo, Japan) (Figure 32).
Figure 31: (A) The positioning device is placed inside the biaxial flexural strength testing fixture, (B) The specimen is placed
inside the positioning device, (C) Placing a plastic polyethylene sheet on the specimen, and (D) The positioning device is
removed, and the loading piston is lowered
62
Figure 32: Disc-shaped specimens after biaxial flexural strength testing
63
V. Calculating the biaxial flexural strength:
The maximum load (N) data for each specimen was exported from the testing software
Bluehill 3 (Instron, Norwood, MA, V3.04, Bluehill 3, USA). Then, imported it into 2022 Microsoft
Excel (Excel version 16.59, Microsoft Office, Redmond, WA, USA) and organized.
To calculate the biaxial flexural strength (σ), ISO 6872
(151)
was followed to convert the
maximum load (N) to (MPa) for each specimen based on the following equation:
Equation 1:
σ =
−0.2387 𝑃 (𝑋 − 𝑌 )
𝑏 2
The biaxial flexural strength (σ) is the maximum center tensile stress measured in
megapascals (MPa). The maximum load (𝑷 ) is the total load producing fracture measured in
newtons (N). The thickness of the specimen at the fracture origin (𝒃 ) for each specimen was
individually measured in millimeters (mm). (𝑿 ) and (𝒀 ) are determined as follows:
Equation 2:
𝑋 = (1 + 𝑣 )𝐼𝑛 (
𝑟 2
𝑟 3
)
2
+
(1 − 𝑣 )
2
(
𝑟 2
𝑟 3
)
2
Equation 3:
𝑌 = (1 + 𝑣 ) [1 + 𝐼𝑛 (
𝑟 1
𝑟 3
)
2
] + (1 − 𝑣 ) (
𝑟 1
𝑟 3
)
2
Where (𝒗 ) is Poisson’s ratio which is 0.25
(172)
. The radius of support circle (𝒓 𝟏 ) is measured in
millimeters (mm); which is 5.00 mm. The radius of loaded area/loading piston (𝒓 𝟐 ) is measured
in millimeters (mm); which is 0.7 mm. The radius of specimen (𝒓 𝟑 ) is measured in millimeters
(mm).
64
2.6. Statistical Analysis:
I. Biaxial flexural strength data:
The raw data of the were exported from Bluehill 3 software (Instron, Norwood, MA, V3.04,
Bluehill, USA), imported into 2022 Microsoft Excel (Excel version 16.59, Microsoft Office,
Redmond, WA, USA), and organized. SPSS 28.0.0 software (IBM SPSS Statistics 28.0.0, SPSS
Inc., Chicago, IL, USA) was used to analyze the data.
Quantitative variables are described by the Mean, Standard Deviation (SD), range
(Minimum–Maximum), Standard Error (SE), and 95% confidence interval of the mean were
performed in SPSS. Shapiro-Wilk test was used to test the normality hypothesis of the biaxial
flexural strength data. Mostly, parametric tests are used for normally distributed data. Levene’s
test was used to check the homogeneity of variances.
SPSS General Linear Model (GLM) repeated measures ANOVA was used to analyze
the main variables (material, firing, and thickness) and their interactions. Mauchly's test of
Sphericity was used to test whether or not the assumption of sphericity is met in a repeated
measures ANOVA. For multiple comparisons post-hock test, Bonferroni Method is applied.
Bonferroni Correction also known as Bonferroni type adjustment is important when conducting
multiple analyses on the same variable as the chance of committing a Type I error increases, thus
increasing the likelihood of coming about a significant result by pure chance. To correct for this,
or protect from Type I error, a Bonferroni correction is conducted. The significance level is
considered at ⍺=0.05.
SPSS offers Bonferroni-adjusted significance tests for pairwise comparisons. This
adjustment is available as an option for post hoc tests and for the estimated marginal means feature.
First, it divides the desired alpha-level by the number of comparisons. Second, it uses the number
65
calculated as the p-value for determining significance. So, for example, with alpha set at 0.05, and
three comparisons, the p-value required for significance would be 0.05/3 = .0167. This is an
unadjusted p-value.
To obtain the corrected p-value, SPSS multiplies the unadjusted p-value of .016 by 3, which
equals to 0.048. Since this value is less than 0.05, it would conclude that the difference was
significant.
II. Weibull analysis:
Further statistical analysis was performed using Weibull analysis according to ISO 6872-
2015.
(151)
The Weibull Two-parameter distribution function correlates the cumulative probability of
failure (𝑃 𝑓 ) of an area under tensile stress to two-parameter estimates: Weibull modulus (𝑚 ) that
represents the shape parameter and Weibull characteristic strength (σ
𝑂 ) which represents the scale
parameter. The two-parameters Weibull distribution was found using the following equation:
Equation 4:
𝑃 𝑓 = 1 − exp [− (
σ
σ
𝑂 )
𝑚 ]
(𝑷 𝒇 ) is the probability of failure and (𝛔 ) is the failure stress, which is the biaxial flexural
strength data. (𝛔 𝑶 ) is the Weibull characteristic strength, and (𝒎 ) is the Weibull modulus.
To calculate the Weibull Two-parameter distribution, the specimens (N) in each group
(n=15) were ranked in an ascending order based on the BFS values and numbered (i) according to
their order (1-15). From this assigned numerical order, 𝑃 𝑓 was calculated as the following:
Equation 5:
𝑃 𝑓 = 1 − (
𝑖 − 0.5
𝑁 )
66
Then, the natural logarithms of each specimen were calculated (ln σ), and double natural
logarithm of 𝑃 𝑓 as in Equation 6. Next, the ln σ was plotted in the y-axis and lnln[1/(1 − 𝑃 𝑓 )] on
the x-axis.
Equation 6:
lnln[1/(1 − 𝑃 𝑓 )]
The linear regression equation (Equation 7) gives the shape parameter (𝒎 ), which is the
Weibull modulus. Thus, the Weibull modulus, (𝒎 ), is equal to the slope of the linear regression
fit.
Equation 7:
𝑌 = 𝑚𝑥 + 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
The scale parameter, or Weibull characteristic strength (σ
𝑂 ) (the point at which 𝑃 𝑓 = 62.30%
failure and σ =0), is gained by dividing the intercept (𝑏 ) by 𝑚 , and reconverting the data by
applying the exponential function. (Equation 8)
Equation 8:
σ
𝑂 = exp (
−𝑏 𝑚 )
Weibull analysis of the data was performed using R (v4.2.1; R Core Team 2022, PBC,
Boston, MA, USA). The "fitdist" package was used to fit Weibull plots to the data to calculate the
Weibull Characteristic Strength and Weibull Modulus of each group and generate Weibull plots
and likelihood contour plots.
67
Chapter 3: Results
3.1. Descriptive Analysis of the Biaxial Flexural Strength:
A summary of the descriptive statistical analysis of the results showing the mean biaxial
flexural strength and the standard deviation for the tested materials is shown in Table 4.
Overall, the highest mean biaxial flexural strength value was seen in Amber Mill 1.00
(251.08 ±83.56 MPa) after three firings. While the lowest mean biaxial flexural strength value was
observed in Initial LiSi 0.5 mm (42.10 ±16.14 MPa) as baseline (Figure 33).
Figure 33: Overall mean biaxial flexural strength (MPa)
IPS e.max CAD presented higher biaxial flexural strength values after one firing in the 1.0
mm thickness (242.57 ±27.84 MPa). Whereas higher biaxial flexural strength was found after the
baseline crystallization in the 0.5 mm thickness (67.63 ±13.48 MPa).
Amber Mill in the 1.00 mm thickness showed the highest overall biaxial flexural strength
values after three firings (251.08 ±83.56 MPa). Although in the 0.5 mm thickness the highest mean
biaxial flexural strength value was seen after one firing (63.18 ±19.34 MPa).
0.00
50.00
100.00
150.00
200.00
250.00
300.00
0.5 mm 1.00 mm 0.5 mm 1.00 mm 0.5 mm 1.00 mm 0.5 mm 1.00 mm
IPS e.max CAD Amber Mill LiSi n!ce
Baseline
One Firing
Three Firings
Five Firings
68
Initial LiSi exhibited higher biaxial flexural strength values after five fairings for both
thicknesses, 0.5 mm and 1.00 mm, (63.67 ±22.41 MPa) and ( 154.42 ±32.44 MPa), respectively.
n!ce showed the highest mean biaxial flexural strength values after five firings in the 1.00
mm thickness (199.23 ±59.61 MPa). However, in the 0.5 mm higher biaxial flexural strength was
observed after three firings (73.75 ±31.47 MPa).
69
Table 4: Overall biaxial flexural strength (MPa) of IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce
Material Group
Group
No. &
Abv.
σ Mean σ SD Std. Error
95% Confidence Interval
for Mean
Maximum Minimum
σ Lower
bound
σ Upper
bound
IPS e.max CAD
0.5 mm baseline (1) 5EB 67.63 ±49.57 12.79 226.78 281.68 93.30 48.46
0.5 mm one firing (2) 5E1 61.11 ±12.97 12.67 197.95 252.31 84.34 37.86
0.5 mm three firings (3) 5E3 57.69 ±48.35 12.48 188.22 241.77 81.22 35.3
0.5 mm five firings (4) 1E5 57.54 ±59.62 15.39 183.24 249.28 94.06 32.14
1 mm baseline (5) 1EB 215.66 ±39.00 10.06 194.06 237.25 276.16 150.33
1 mm one firing (6) 1E1 242.57 ±27.84 7.18 227.57 257.99 289.74 180.35
1 mm three firings (7) 1E3 234.79 ±42.86 11.06 211.78 258.52 309.99 164.57
1 mm five firings (8) 1E5 202.09 ±35.99 9.29 182.16 222.01 268.76 137.15
Amber Mill
0.5 mm baseline (9) 5EB 53.16 ±13.21 12.18 174.26 226.53 81.76 32.92
0.5 mm one firing (10) 5A1 63.18 ±19.34 19.27 205.95 288.64 114.54 35.03
0.5 mm three firings (11) 5A3 62.67 ±19.04 18.75 199.37 279.82 103.98 35.44
0.5 mm five firings (12) 5A5 59.81 ±16.25 16.99 195.06 267.97 83.08 34.49
1 mm baseline (13) 1AB 209.24 ±80.18 19.21 208.73 249.95 446.03 125.14
1 mm one firing (14) 1A1 187.49 ±43.82 11.44 164.47 213.57 268.64 112.74
1 mm three firings (15) 1A3 251.08 ±83.56 19.24 206.71 289.27 443.09 146.19
1 mm five firings (16) 1A5 207.92 ±66.95 16.74 173.45 245.27 309.09 96.07
Initial LiSi
0.5 mm baseline (17) 5LB 42.10 ±16.14 15.79 128.36 196.13 80.29 27.95
0.5 mm one firing (18) 5L1 54.98 ±27.26 25.23 149.94 258.20 125.98 21.19
0.5 mm three firings (19) 5L3 56.91 ±23.61 21.78 210.22 163.49 98.99 21.49
0.5 mm five firings (20) 5L5 63.67 ±22.41 21.78 190.73 284.16 107.97 32.01
1 mm baseline (21) 1LB 119.69 ±23.23 5.43 108.96 132.28 168.80 86.72
1 mm one firing (22) 1L1 123.77 ±14.21 3.28 115.75 129.75 143.88 103.51
1 mm three firings (23) 1L3 121.62 ±23.09 5.70 106.92 131.40 179.76 90.70
1 mm five firings (24) 1L5 154.42 ±34.41 8.40 134.89 170.94 212.51 102.48
n!ce
0.5 mm baseline (25) 5NB 45.80 ±14.12 12.98 144.69 200.40 78.13 28.36
0.5 mm one firing (26) 5N1 63.98 ±22.40 19.38 195.14 278.28 113.88 35.11
0.5 mm three firings (27) 5N3 73.75 ±31.47 30.33 214.42 344.52 144.08 35.46
0.5 mm five firings (28) 5N5 56.24 ±20.32 20.21 166.97 253.69 82.45 19.93
1 mm baseline (29) 1NB 126.73 ±22.94 6.27 114.77 141.69 177.67 94.40
1 mm one firing (30) 1N1 157.90 ±30.27 8.58 141.63 178.44 213.92 113.82
1 mm three firings (31) 1N3 164.80 ±39.13 10.19 143.82 187.55 233.87 124.27
1 mm five firings (32) 1N5 199.23 ±57.44 14.58 167.28 229.84 370.45 143.80
70
3.2. Data Normality Analysis and Equality of Variances:
The Shapiro-Wilk test was used to test the normality of the biaxial flexural strength
values. The test revealed that the data were not normally distributed, only 9 out of the 32 groups
deviated from the normality assumption (p<0.05) (Table 5).
Table 5: Shapiro-Wilk test (normality)
Material Group
Group
No. & Abv.
Shapiro-Wilk
Statistic df Sig.
IPS e.max CAD
0.5 mm baseline (1) 5EB .962 15 .733
0.5 mm one firing (2) 5E1 .978 15 .952
0.5 mm three firings (3) 5E3 .924 15 .223
0.5 mm five firings (4) 1E5 .963 15 .736
1 mm baseline (5) 1EB .930 15 .277
1 mm one firing (6) 1E1 .968 15 .833
1 mm three firings (7) 1E3 .967 15 .805
1 mm five firings (8) 1E5 .985 15 .993
Amber Mill
0.5 mm baseline (9) 5EB .969 15 .850
0.5 mm one firing (10) 5A1 .841 15 .013
*
0.5 mm three firings (11) 5A3 .892 15 .071
0.5 mm five firings (12) 5A5 .940 15 .378
1 mm baseline (13) 1AB .815 15 .006
*
1 mm one firing (14) 1A1 .961 15 .705
1 mm three firings (15) 1A3 .930 15 .271
1 mm five firings (16) 1A5 .945 15 .445
Initial LiSi
0.5 mm baseline (17) 5LB .835 15 .011
*
0.5 mm one firing (18) 5L1 .854 15 .020
*
0.5 mm three firings (19) 5L3 .953 15 .578
0.5 mm five firings (20) 5L5 .943 15 .422
1 mm baseline (21) 1LB .943 15 .419
1 mm one firing (22) 1L1 .928 15 .256
1 mm three firings (23) 1L3 .922 15 .210
1 mm five firings (24) 1L5 .943 15 .420
n!ce
0.5 mm baseline (25) 5NB .828 15 .009
*
0.5 mm one firing (26) 5N1 .853 15 .019
*
0.5 mm three firings (27) 5N3 .877 15 .043
*
0.5 mm five firings (28) 5N5 .929 15 .265
1 mm baseline (29) 1NB .940 15 .381
1 mm one firing (30) 1N1 .954 15 .586
1 mm three firings (31) 1N3 .870 15 .034
*
1 mm five firings (32) 1N5 .787 15 .002
*
The significance level is .050
* Not normally distributed groups. (p=<0.05)
71
Levene’s test was used to evaluate the homogeneity of the data and it showed that the
variances are not homogeneous (p<0.001) (Table 6).
Table 6: Levene’s test (equality of variances)
Levene Statistic df1 df2 Sig.
σ (MPa)
Based on Mean 4.001 31 448 <.001
Based on Median 2.701 31 448 <.001
Based on Median and with adjusted df 2.701 31 215.85 <.001
Based on trimmed mean 3.729 31 448 <.001
Parametric tests such as analysis of variance (ANOVA) have four requirements including
equal sample size, independent variable, normal distribution, and homogeneity of variance. The
first two (equal sample size and independent variable) were met. However, parametric test such as
Generalized Linear Mixed Model (GLMM) repeated measured ANOVA was applied. The 9
groups that were not normally distributed reveal that the deviation is relatively slight.
Analysis of variance (ANOVA) is quite robust against violations of the normality and
heterogeneity assumptions.
(173-175)
The repeated measured ANOVA allows the analysis of the
main variables (material, firing, and thickness) and their interactions.
(175)
Mauchly's Test of Sphericity was used to assess the assumption of the sphericity to allow
the use of repeated measures ANOVA (Table 7). It revealed that there is no statistically significant
violation of the sphericity assumption (p=0.278).
Table 7: Mauchly's Test of Sphericity
Within Subjects Effect
Mauchly's
W
Approx.
Chi-
Square
df P Value
Epsilon
b
Greenhouse-
Geisser
Huynh-
Feldt
Lower-
bound
firing .945 6.307 5 .278 .961 1.000 .333
Tests the null hypothesis that the error covariance matrix of the orthonormalized transformed dependent variables
is proportional to an identity matrix.
a. Design: Intercept + mat + th + mat * th
Within Subjects Design: firing
b. May be used to adjust the degrees of freedom for the averaged tests of significance. Corrected tests are
displayed in the Tests of Within-Subjects Effects table.
72
3.3 Analysis of Variance (ANOVA):
Repeated measures 3-way ANOVA was used to evaluate BFS values with material and
thickness as between-subject factors and firing being within-subject, including their interactions.
Group-wise comparisons (post-hoc) were performed separately for each group using the
Bonferroni correction due to multiple comparisons (α=0.05).
The analysis of variance (ANOVA) of the Biaxial Flexural Strength has assessed the
overall interaction between IPS e.max CAD (EX), Amber Mill (AM), Initial LiSi (LS), and n!ce
(NC) of different thicknesses and repeated firings (Table 8).
Table 8: Summary of analysis of variance of (material, firing, and thickness) and their interactions
Source
Type III Sum
of Squares
df Mean Square F P Value
Intercept 7058219.63 1 7058219.63 6248.57 0.000
*
Material 179174.43 3 59724.81 52.87 0.000
*
Thickness 1796379.41 1 1796379.41 1590.32 0.000
*
Firing 19677.47 3 7162.60 5.21 0.002
*
Material * thickness 173377.76 3 57792.59 51.16 0.000
*
Material * Firing 41166.76 8 4994.90 3.63 0.000
*
Firing * Thickness 8681.36 3 3160.02 2.30 0.083
Firing * Material * Thickness 38907.16 8 4720.74 3.44 0.000
*
Error 126512.30 112 1129.57
Table 8 presents the ANOVA between (material, firing, and thickness) and their
interactions. A statistically significant difference was found, except for the interaction between
firing and thickness that was not statistically significantly different (p=0.083).
73
Material
Overall:
The biaxial flexural strength data showed that IPS e.max CAD had the highest BFS values,
followed by Amber Mill, then n!ce, and lastly Initial LiSi according to the estimated marginal
means based on the materials tested (EX>AM>NC>LS) (Table 9).
Table 9: Estimated Marginal Means (Material)
Material Mean Std. Error
95% Confidence Interval
Lower
Bound
Upper
Bound
IPS e.max CAD 142.39 3.07 136.31 148.46
Amber Mill 136.82 3.07 130.74 142.90
Initial LiSi 94.79 3.07 88.71 100.87
n!ce 111.06 3.07 104.98 117.13
The group-wise comparisons between the tested materials (IPS e.max CAD, Amber Mill,
Initial LiSi, and n!ce) presented that all the materials were generally statistically significantly
different (p<0.05) from each other (Table 10) with the exception of the group-wise comparison
between IPS e.max CAD and Amber Mill which were not statistically significantly different
(p=1.000).
Table 10: Group-wise comparisons (Material)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill 5.57 4.34 1.000 -6.09 17.22
Initial LiSi 47.59 4.34 0.000
*
35.94 59.25
n!ce 31.33 4.34 0.000
*
19.68 42.98
Amber Mill
Initial LiSi 25.76 4.34 0.000
*
14.11 37.42
n!ce -16.26
*
4.34 0.001
*
-27.92 -4.61
Initial LiSi n!ce 31.33 4.34 0.000
*
19.68 42.98
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
74
IPS e.max CAD:
The mean biaxial flexural strength and standard deviation of IPS e.max CAD for the
repeated firings (baseline, one firing, three firings, and five firings) for both thicknesses 0.5 mm
and 1.00 mm are presented in Table 11 (Figure 34).
Table 11: Mean biaxial flexural strength and standard deviation of IPS e.max CAD
Firing 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
Baseline 67.63 ±49.57 215.66 ±39.00
One firing 61.11 ±12.97 242.57 ±27.84
Three firings 57.69 ±48.35 234.79 ±42.86
Five firings 57.54 ±59.62 202.09 ±35.99
Figure 34: Biaxial flexural strength of IPS e.max CAD
The highest overall mean biaxial flexural strength value in IPS e.max CAD was observed
at one firing 1.00 mm (242.13 ±26.79 MPa); while the lowest was at five firings 0.5 mm (57.54
±15.81 MPa).
0.5 mm: The BFS value was highest at baseline (67.63 ±49.57 MPa), followed by one
firing (61.11 ±12.97 MPa), the three firings (57.69 ±48.35 MPa), and lastly five firings (57.54
±59.62 MPa). (BL>1F>3F>5F)
67.63
61.11
57.69
57.54
215.66
242.57
234.79
201.09
0
50
100
150
200
250
300
350
Baseline One Firing Three Firings Five Firings
MPa
0.5 mm
1.00 mm
75
1.00 mm: The highest BFS value was seen at one firing (242.13 ±26.79 MPa), followed
by three firings (234.79 ±42.86 MPa), then baseline (215.66 ±39.00 MPa), and lastly five firings
(202.09 ±35.99 MPa). (1F>3F>BL>5F)
ANOVA (IPS e.max CAD):
One-way ANOVA was used to evaluate the significance of BFS values between the firings
in each thickness (Table 12, Table 13). There was a statistically significant difference (p=0.016)
between the firings in the 1.00 mm thickness of IPS e.max CAD.
Table 12: One-way ANOVA of IPS e.max CAD (0.5 mm)
Firing N F P Value
Baseline 15
1.65 0.187
One firing 15
Three firings 15
Five firings 15
Total 60
Table 13: One-way ANOVA of IPS e.max CAD (1.00 mm)
Firing N F P Value
Baseline 15
3.72 0.016
*
One firing 15
Three firings 15
Five firings 15
Total 60
Group-wise comparisons were performed separately for both thicknesses of IPS e.max
CAD (Table 14, Table 15) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 14: Group-wise comparisons of IPS e.max CAD (0.5 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing 6.52 5.20 1.000 -7.70 20.74
Three firings 9.94 5.20 0.365 -4.28 24.16
Five firings 10.10 5.20 0.342 -4.12 24.32
One firing
Three firings 3.42 5.20 1.000 -10.80 17.64
Five firings 3.58 5.20 1.000 -10.64 17.80
Three firings Five firings 0.15 5.20 1.000 -14.07 14.37
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 14 presents the group-wise comparisons for 0.5 mm thickness firings of IPS e.max
CAD. No statistically significant difference was found between the firings.
76
Table 15: Group-wise comparisons of IPS e.max CAD (1.00 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -26.91 13.45 0.301 -63.70 9.88
Three firings -19.12 13.45 0.963 -55.92 17.67
Five firings 13.57 13.45 1.000 -23.22 50.36
One firing
Three firings 7.79 13.45 1.000 -29.00 44.58
Five firings 40.48 13.45 0.023
*
3.69 77.27
Three firings Five firings 32.70 13.45 0.109 -4.10 69.49
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 15 presents the group-wise comparisons for 1.00 mm thickness firings of IPS e.max
CAD. It revealed a statistically significant difference between one firing and five firings (p=0.023).
77
Amber Mill:
The mean biaxial flexural strength and standard deviation of Amber Mill for the repeated
firings (baseline, one firing, three firings, and five firings) for both thicknesses 0.5 mm and 1.00
mm are presented in Table 16 (Figure 35).
Table 16: Mean biaxial flexural strength and standard deviation of Amber Mill
Firing 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
Baseline 53.16 ±13.21 209.24 ±80.18
One firing 63.18 ±19.34 187.49 ±43.82
Three firings 62.67 ±19.04 251.08 ±83.56
Five firings 59.81 ±16.25 207.92 ±66.95
Figure 35: Biaxial flexural strength of Amber Mill
The highest overall mean biaxial flexural strength value in Amber Mill was observed at
three firings 1.00 mm (251.08 ±83.56 MPa); while the lowest was at baseline 0.5 mm (53.16
±13.21 MPa).
0.5 mm: The BFS value was highest at three firings (62.67 ±19.04 MPa), followed by one
firing (63.18 ±19.34 MPa), the five firings (59.81 ±16.25 MPa), and lastly baseline (53.16 ±13.21
MPa). (3F>1F>5F>BL)
53.16
63.18
62.67
59.81
209.24
187.49
251.08
207.92
0
50
100
150
200
250
300
350
Baseline One Firing Three Firings Five Firings
MPa
0.5 mm
1.00 mm
78
1.00 mm: The highest BFS value was seen at three firings (251.08 ±83.56 MPa), followed
by baseline (209.24 ±80.18 MPa), then five firings (207.92 ±66.95 MPa), and lastly one firing
(187.49 ±43.82 MPa). (3F>BL>5F>1F)
ANOVA (Amber Mill):
One-way ANOVA was used to evaluate the significance of BFS values between the firings
in each thickness (Table 17, Table 18). There was no statistically significant difference found
between the firings in both thicknesses (p>0.05).
Table 17: One-way ANOVA of Amber Mill (0.5 mm)
Firing N F P Value
Baseline 15
1.08 0.363
One firing 15
Three firings 15
Five firings 15
Total 60
Table 18: One-way ANOVA of Amber Mill (1.00 mm)
Firing N F P Value
Baseline 15
2.16 0.103
One firing 15
Three firings 15
Five firings 15
Total 60
Group-wise comparisons were performed separately for both thicknesses of Amber Mill
(Table 19, Table 20) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 19: Group-wise comparisons of Amber Mill (0.5 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -10.02 6.26 0.691 -27.13 7.10
Three firings -9.51 6.26 0.805 -26.63 7.61
Five firings -6.65 6.26 1.000 -23.76 10.47
One firing
Three firings 0.50 6.26 1.000 -16.61 17.62
Five firings 3.37 6.26 1.000 -13.75 20.49
Three firings Five firings 2.87 6.26 1.000 -14.25 19.98
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 19 presents the group-wise comparisons for 0.5 mm thickness firings of Amber Mill.
No statistically significant difference was found between the firings.
79
Table 20: Group-wise comparisons of Amber Mill (1.00 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing 21.76 25.70 1.000 -48.54 92.05
Three firings -41.84 25.70 0.654 -112.13 28.46
Five firings 1.32 25.70 1.000 -68.97 71.62
One firing
Three firings -63.59 25.70 0.098 -133.89 6.70
Five firings -20.43 25.70 1.000 -90.73 49.86
Three firings Five firings 43.16 25.70 0.591 -27.13 113.45
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 20 presents the group-wise comparisons for 1.00 mm thickness firings of Amber
Mill. No statistically significant difference was found between the firings.
80
Initial LiSi:
The mean biaxial flexural strength and standard deviation of Initial LiSi for the repeated
firings (baseline, one firing, three firings, and five firings) for both thicknesses 0.5 mm and 1.00
mm are presented in Table 21 (Figure 36).
Table 21: Mean biaxial flexural strength and standard deviation of Initial LiSi
Firing 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
Baseline 42.10 ±16.14 119.69 ±23.23
One firing 54.98 ±27.26 123.77 ±14.21
Three firings 56.91 ±23.61 121.62 ±23.09
Five firings 63.67 ±22.41 154.42 ±34.41
Figure 36: Biaxial flexural strength of Initial LiSi
The highest overall mean biaxial flexural strength value in Initial LiSi was observed at five
firings 1.00 mm (154.42 ±34.41 MPa); while the lowest was at baseline 0.5 mm (42.10 ±16.14
MPa).
0.5 mm: The BFS value was highest at five firings (63.67 ±22.41 MPa), followed by three
firings (56.91 ±23.61 MPa), the one firing (54.98 ±27.26 MPa), and lastly baseline (42.10 ±16.14
MPa). (5F>3F>1F>BL)
42.1
54.98
56.91
63.67
119.69
123.77
121.62
154.42
0
50
100
150
200
Baseline One Firing Three Firings Five Firings
MPa
0.5 mm
1.00 mm
81
1.00 mm: The highest BFS value was seen at five firings (154.42 ±34.41 MPa), followed
by one firing (123.77 ±14.21 MPa), the three firings (121.62 ±23.09 MPa), and lastly baseline
(119.69 ±23.23 MPa). (5F>1F>3F>BL)
ANOVA (Initial LiSi):
One-way ANOVA was used to evaluate the significance of BFS values between the firings
in each thickness (Table 22, Table 23). There was a statistically significant difference (p=0.000)
between the firings in the 1.00 mm thickness of Initial LiSi.
Table 22: One-way ANOVA of Initial LiSi (0.5 mm)
Firing N F P Value
Baseline 15
2.36 0.080
One firing 15
Three firings 15
Five firings 15
Total 60
Table 23: One-way ANOVA of Initial LiSi (1.00 mm)
Firing N F P Value
Baseline 15
6.60 0.000
*
One firing 15
Three firings 15
Five firings 15
Total 60
Group-wise comparisons were performed separately for both thicknesses of Initial LiSi
(Table 24, Table 24 presents the group-wise comparisons for 0.5 mm thickness firings of Initial
LiSi. No statistically significant difference was found between the firings.
Table 25) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 24: Group-wise comparisons of Initial LiSi (0.5 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -12.89 8.29 0.755 -35.57 9.80
Three firings -14.81 8.29 0.477 -37.49 7.88
Five firings -21.57 8.29 0.071 -44.26 1.11
One firing
Three firings -1.92 8.29 1.000 -24.61 20.76
Five firings -8.69 8.29 1.000 -31.37 14.00
Three firings Five firings -6.76 8.29 1.000 -29.45 15.92
*. The mean difference is significant at the .05 level.
82
a. Adjustment for multiple comparisons: Bonferroni.
Table 24 presents the group-wise comparisons for 0.5 mm thickness firings of Initial LiSi.
No statistically significant difference was found between the firings.
Table 25: Group-wise comparisons of Initial LiSi (1.00 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -4.08 9.05 1.000 -28.83 20.68
Three firings -1.93 9.05 1.000 -26.69 22.83
Five firings -34.72933
*
9.05 0.001
*
-59.49 -9.97
One firing
Three firings 2.15 9.05 1.000 -22.61 26.91
Five firings -30.65400
*
9.05 0.007
*
-55.41 -5.89
Three firings Five firings -32.80067
*
9.05 0.003
*
-57.56 -8.04
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 25 presents the group-wise comparisons for 1.00 mm thickness firings of Initial
LiSi. It revealed a statistically significant difference of five firings with baseline, one firing, and
three firings (p<0.05).
83
n!ce:
The mean biaxial flexural strength and standard deviation of n!ce for the repeated firings
(baseline, one firing, three firings, and five firings) for both thicknesses 0.5 mm and 1.00 mm are
presented in Table 26 (Figure 37).
Table 26: Mean biaxial flexural strength and standard deviation of n!ce
Firing 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
Baseline 45.80 ±14.12 126.73 ±22.94
One firing 63.98 ±22.40 157.90 ±30.27
Three firings 73.75 ±31.47 164.80 ±39.13
Five firings 56.24 ±20.32 199.23 ±57.44
Figure 37: Biaxial flexural strength of n!ce
The highest overall mean biaxial flexural strength value in n!ce was observed at five firings
1.00 mm (199.22 ±57.44 MPa); while the lowest was at baseline 0.5 mm (45.81 ±14.12 MPa).
0.5 mm: The BFS value was highest at three firings (73.75 ±31.47 MPa), followed by one
firing (63.98 ±22.40 MPa), the five firings (56.24 ±20.32 MPa), and lastly baseline (45.81 ±14.12
MPa). (3F>1F>5F>BL)
1.00 mm: The highest BFS value was seen at five firings (199.22 ±57.44 MPa), followed
by three firings (164.80 ±39.13 MPa), the one firing (157.90 ±30.27 MPa), and lastly baseline
(126.73 ±22.94 MPa). (5F>3F>1F>BL)
45.8
63.98
73.75
56.24
126.73
157.9
164.8
199.23
0
50
100
150
200
250
300
Baseline One Firing Three Firings Five Firings
MPa
0.5 mm
1.00 mm
84
ANOVA (n!ce):
One-way ANOVA was used to evaluate the significance of BFS values between the firings
in each thickness (Table 27, Table 28). There was a statistically significant difference (p<0.05)
between the firings in both thicknesses of n!ce.
Table 27: One-way ANOVA of n!ce (0.5 mm)
Firing N F P Value
Baseline 15
4.00 0.011
*
One firing 15
Three firings 15
Five firings 15
Total 60
Table 28: One-way ANOVA of n!ce (1.00 mm)
Firing N F P Value
Baseline 15
8.46 0.000
*
One firing 15
Three firings 15
Five firings 15
Total 60
Group-wise comparisons were performed separately for both thicknesses of n!ce (Table
29, Table 30) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 29: Group-wise comparisons of n!ce (0.5 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -18.17 8.38 0.205 -41.08 4.74
Three firings -27.94800
*
8.38 0.009
*
-50.86 -5.04
Five firings -10.44 8.38 1.000 -33.35 12.47
One firing
Three firings -9.77 8.38 1.000 -32.68 13.14
Five firings 7.74 8.38 1.000 -15.17 30.65
Three firings Five firings 17.51 8.38 0.246 -5.40 40.42
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 29 presents the group-wise comparisons for 0.5 mm thickness firings of n!ce. It
revealed a statistically significant difference between baseline and three firings (p=0.009).
85
Table 30: Group-wise comparisons of n!ce (1.00 mm)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One firing -31.17 14.46 0.212 -70.72 8.38
Three firings -38.07 14.46 0.065 -77.62 1.48
Five firings -72.49333
*
14.46 0.000
*
-112.04 -32.94
One firing
Three firings -6.90 14.46 1.000 -46.45 32.66
Five firings -41.32200
*
14.46 0.035
*
-80.87 -1.77
Three firings Five firings -34.43 14.46 0.124 -73.98 5.12
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 30 presents the group-wise comparisons for 1.00 mm thickness firings of n!ce. It
revealed a statistically significant difference between baseline and five firings (p=0.000) as well
as one firing and five firings (p=0.035).
86
Firing:
Overall:
The biaxial flexural strength data revealed that the highest values were observed after three
firings, then five firings, followed by one firing, and lastly baseline according to the estimated
marginal means based on the firings (3F>5F>1F>BL) (Table 31).
Table 31: Estimated Marginal Means (Firing)
Firing Mean Std. Error
95% Confidence Interval
Lower
Bound
Upper
Bound
Baseline 133.33 3.55 126.29 140.37
One Firing 139.88 2.86 134.21 145.54
Three Firings 147.58 3.97 139.71 155.44
Five Firings 144.96 3.95 137.12 152.79
The group-wise comparisons between the firings (baseline, one firing, three firings, and
five firings) are presented in Table 32. A statistically significant difference was found for the
group-wise comparison between baseline and three firings (p=0.001) as well as baseline and five
firings (p=0.031).
Table 32: Group-wise comparisons (Firings)
(I) Firing
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
Baseline
One Firing -9.01 4.10 0.179 -20.02 1.99
Three Firing -17.25 4.59 0.001
*
-29.57 -4.92
Five Firing -13.25 4.66 0.031
*
-25.75 -0.74
One firing
Three Firing -8.24 4.56 0.439 -20.47 4.00
Five Firing -4.23 4.06 1.000 -15.14 6.67
Three firings Five Firing 4.00 5.39 1.000 -10.48 18.49
*. The mean difference is significant at the .05 level.
87
Baseline:
The mean biaxial flexural strength and standard deviation at baseline for the tested
materials (IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) for both thicknesses 0.5 mm and
1.00 mm are presented in Table 33 (Figure 38).
Table 33: Mean biaxial flexural strength and standard deviation at baseline
Material 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
IPS e.max CAD 67.63 ±49.57 215.66 ±39.00
Amber Mill 53.16 ±13.21 209.24 ±80.18
Initial LiSi 42.10 ±16.14 119.69 ±23.23
n!ce 45.81 ±14.12 126.73 ±22.94
Figure 38: Biaxial flexural strength at baseline
The highest overall mean biaxial flexural strength value at baseline was observed in IPS
e.max CAD 1.00 mm (215.27 ±39.00 MPa); while the lowest was Initial LiSi 0.5 mm (42.10 ±16.14
MPa).
0.5 mm: The BFS value was highest in IPS e.max CAD (67.63 ±49.57 MPa), followed by
Amber Mill (53.16 ±13.21 MPa), the n!ce (45.81 ±14.12 MPa), and lastly Initial LiSi (42.10
±16.14 MPa). (EX>AM>NC>LS)
67.6
53.2
42.1
45.8
215.7
209.2
119.7
126.7
0
50
100
150
200
250
300
350
IPS e.max CAD Amber Mill Initial LiSi n!ce
MPa
0.5 mm
1.00 mm
88
1.00 mm: The highest BFS value was seen in IPS e.max CAD (215.66 ±39.00 MPa),
followed by Amber Mill (209.24 ±80.18MPa), then n!ce (126.73 ±22.94 MPa), and lastly Initial
LiSi (119.69 ±23.23 MPa). (EX >AM >NC>LS)
ANOVA (baseline):
One-way ANOVA was used to evaluate the significance of BFS values between the
materials in each thickness (Table 34, Table 35). Overall, a statistically significant difference
(p=0.000) between the materials in both thicknesses at baseline.
Table 34: One-way ANOVA of tested materials at baseline (0.5 mm)
Material N F P Value
IPS e.max CAD 15
6.87 0.000
*
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Table 35: One-way ANOVA of tested materials at baseline (1.00 mm)
Material N F P Value
IPS e.max CAD 15
17.77 0.000
*
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Group-wise comparisons were performed separately for both thicknesses at baseline
(Table 36, Table 37) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 36: Group-wise comparisons at baseline (0.5 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill 14.47
*
4.93 0.03
*
0.98 27.96
Initial LiSi 14.47
*
4.93 0.03
*
0.98 27.96
n!ce 21.82
*
4.93 0.00
*
8.33 35.32
Amber Mill
Initial LiSi 0.00 4.93 1.00 -13.49 13.49
n!ce 7.36 4.93 0.85 -6.14 20.85
Initial LiSi n!ce 7.36 4.93 0.85 -6.14 20.85
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
89
Table 36 presents the group-wise comparisons for 0.5 mm thickness materials at baseline.
It revealed a statistically significantly different (p<0.05) between IPS max CAD with Amber Mil,
Initial LiSi, and n!ce.
Table 37: Group-wise comparisons at baseline (1.00 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill 6.42 17.34 1.000 -41.00 53.84
Initial LiSi 95.96
*
17.34 0.000
*
48.55 143.39
n!ce 88.93
*
17.34 0.000
*
41.51 136.35
Amber Mill
Initial LiSi 89.55
*
17.34 0.000
*
42.13 136.97
n!ce 82.51
*
17.34 0.000
*
35.10 129.93
Initial LiSi n!ce -7.04 17.34 1.000 -54.46 40.38
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 37 presents the group-wise comparisons for 1.00 mm thickness materials at baseline.
It revealed a statistically significantly different (p=0.000) of IPS e.max CAD with Initial LiSi and
n!ce as well as Amber Mill with Initial LiSi and n!ce.
90
One Firing:
The mean biaxial flexural strength and standard deviation at one firing for the tested
materials (IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) for both thicknesses 0.5 mm and
1.00 mm are presented in Table 38 (Figure 39).
Table 38: Mean biaxial flexural strength and standard deviation at one firing
Material 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
IPS e.max CAD 61.11 ±12.97 242.57 ±27.84
Amber Mill 63.18 ±19.34 187.49 ±43.82
Initial LiSi 54.98 ±27.26 123.77 ±14.21
n!ce 63.98 ±22.40 157.90 ±30.27
Figure 39: Biaxial flexural strength at one firing
The highest overall mean biaxial flexural strength value at one firing was observed in IPS
e.max CAD 1.00 mm (242.13 ±27.84 MPa); while the lowest was Initial LiSi 0.5 mm (54.98 ±27.26
MPa).
0.5 mm: The BFS value was highest in n!ce (63.98 ±22.40 MPa), followed by Amber Mill
(63.18 ±19.34 MPa), then IPS e.max CAD (61.11 ±12.97 MPa), and lastly Initial LiSi (54.98
±27.26 MPa). (NC>AM>EX>LS)
61.11
63.18
54.98
63.98
242.57
187.49
123.77
157.90
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
IPS e.max CAD Amber Mill Initial LiSi n!ce
0.5 mm
1.00 mm
91
1.00 mm: The highest BFS value was observed in IPS e.max CAD (242.57 ±27.84 MPa),
followed by Amber Mill (187.49 ±43.82 MPa), then n!ce (157.90 ±30.27 MPa), and lastly Initial
LiSi (123.77 ±14.21 MPa). (EX>AM>NC>LS)
ANOVA (one firing):
One-way ANOVA was used to evaluate the significance of BFS values between the
materials in each thickness (Table 39, Table 40). A statistically significant difference (p=0.000)
was found between the materials in 1.00 mm at one firing.
Table 39: One-way ANOVA of tested materials at one firing (0.5 mm)
Material N F P Value
IPS e.max CAD 15
0.06 0.978
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Table 40: One-way ANOVA of tested materials at one firing (1.00 mm)
Material N F P Value
IPS e.max CAD 15
39.89 0.000
*
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Group-wise comparisons were performed separately for both thicknesses at one firing
(Table 41, Table 42) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 41: Group-wise comparisons at one firing (0.5 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill -2.07 6.88 1.000 -20.87 16.74
Initial LiSi -2.07 6.88 1.000 -20.87 16.74
n!ce -2.87 6.88 1.000 -21.68 15.94
Amber Mill
Initial LiSi 0.00 6.88 1.000 -18.81 18.81
n!ce -0.80 6.88 1.000 -19.61 18.00
Initial LiSi n!ce -0.80 6.88 1.000 -19.61 18.00
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
92
Table 41 presents the group-wise comparisons for 0.5 mm thickness materials at one
firing. No statistically significant difference between each of the tested materials.
Table 42: Group-wise comparisons at one firing (1.00 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill 55.08
*
11.27 0.000
*
24.25 85.92
Initial LiSi 118.80
*
11.27 0.000
*
87.97 149.64
n!ce 84.67
*
11.27 0.000
*
53.84 115.51
Amber Mill
Initial LiSi 63.71
*
11.27 0.000
*
32.88 94.55
n!ce 29.58 11.27 0.067 -1.25 60.42
Initial LiSi n!ce -34.13
*
11.27 0.022
*
-64.97 -3.30
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 42 presents the group-wise comparisons for 1.00 mm thickness materials at one
firing. It revealed that all the materials were generally statistically significantly different (p<0.05)
from each other with the exception of the group-wise comparison between (Amber Mill and n!ce)
that were not statistically significantly different (p=0.067).
93
Three Firings:
The mean biaxial flexural strength and standard deviation at three firings for the tested
materials (IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) for both thicknesses 0.5 mm and
1.00 mm are presented in Table 43 (Figure 40).
Table 43: Mean biaxial flexural strength and standard deviation at three firings
Material 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
IPS e.max CAD 57.69 ±48.35 234.79 ±42.86
Amber Mill 62.67 ±19.04 251.08 ±83.56
Initial LiSi 56.91 ±23.61 121.62 ±23.09
n!ce 73.75 ±31.47 164.80 ±39.13
Figure 40: Biaxial flexural strength at three firings
The highest overall mean biaxial flexural strength value at three firings was observed in
Amber Mill 1.00 mm (251.08 ±83.56 MPa); while the lowest was IPS e.max CAD 0.5 mm (57.69
±14.52 MPa).
0.5 mm: The BFS value was highest in n!ce (73.75 ±31.47 MPa), followed by Amber Mill
(62.67 ±19.04 MPa), then IPS e.max CAD (57.69 ±48.35 MPa), and lastly Initial LiSi (56.91
±23.61 MPa). (NC >AM >EX>LS)
57.69
62.67
56.91
73.75
234.79
251.08
121.62
164.80
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
400.00
IPS e.max CAD Amber Mill Initial LiSi n!ce
MPa
0.5 mm
1mm
94
1.00 mm: The highest BFS value was observed in Amber Mill (251.08 ±83.56 MPa),
followed by IPS e.max CAD (234.79 ±42.86 MPa), then n!ce (164.80 ±39.13 MPa), and lastly
Initial LiSi (121.62 ±23.09 MPa). (AM>EX>NC>LS)
ANOVA (three firings):
One-way ANOVA was used to evaluate the significance of BFS values between the
materials in each thickness (Table 44, Table 45). A statistically significant difference (p=0.000)
was found between the materials in 1.00 mm at three firings.
Table 44: One-way ANOVA of tested materials at three firings (0.5 mm)
Material N F P Value
IPS e.max CAD 15
1.44 0.241
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Table 45: One-way ANOVA of tested materials at three firings (1.00 mm)
Material N F P Value
IPS e.max CAD 15
20.23 0.000
*
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Group-wise comparisons were performed separately for both thicknesses at three firings
(Table 46, Table 47) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 46: Group-wise comparisons at three firings (0.5 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill -4.98 8.01 1.000 -26.90 16.94
Initial LiSi -4.98 8.01 1.000 -26.90 16.94
n!ce -16.07 8.01 0.299 -37.98 5.85
Amber Mill
Initial LiSi 0.00 8.01 1.000 -21.92 21.92
n!ce -11.08 8.01 1.000 -33.00 10.84
Initial LiSi n!ce -11.08 8.01 1.000 -33.00 10.84
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 46 presents the group-wise comparisons for 0.5 mm thickness materials at three
firings. No statistically significant difference was found between each of the tested materials.
95
Table 47: Group-wise comparisons at three firings (1.00 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill -16.29 19.05 1.000 -68.39 35.80
Initial LiSi 113.16
*
19.05 0.000
*
61.07 165.26
n!ce 69.98
*
19.05 0.003
*
17.89 122.08
Amber Mill
Initial LiSi 129.45
*
19.05 0.000
*
77.36 181.56
n!ce 86.28
*
19.05 0.000
*
34.19 138.38
Initial LiSi n!ce -43.18 19.05 0.163 -95.27 8.92
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 47 presents the group-wise comparisons for 1.00 mm thickness materials at three
firings. A statistically significant difference (p<0.05) was found between IPS e.max CAD with
Initial LiSi and n!ce, as well as between Amber Mill with Initial LiSi and n!ce.
96
Five Firings:
The mean biaxial flexural strength and standard deviation at five firings for the tested
materials (IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) for both thicknesses 1.00 mm and
0.5 mm is presented in Table 48 (Figure 41).
Table 48: Mean biaxial flexural strength and standard deviation at five firings
Material 0.5 mm SD 0.5mm 1.00 mm SD 1.00 mm
IPS e.max CAD 57.54 ±59.62 202.09 ±35.99
Amber Mill 59.81 ±16.25 207.92 ±66.95
Initial LiSi 63.67 ±22.41 154.42 ±34.41
n!ce 56.24 ±20.32 199.22 ±57.44
Figure 41: Biaxial flexural strength at five firings
The highest overall mean biaxial flexural strength value at five firings was observed in
Amber Mill 1.00 mm (207.92 ±66.95 MPa); while the lowest was n!ce 0.5 mm (56.24 ±20.32 MPa).
0.5 mm: The BFS value was highest in Initial LiSi (63.67 ±22.41 MPa), followed by
Amber Mill (59.81 ±16.25 MPa), then IPS e.max CAD (57.54 ±59.62 MPa), and lastly n!ce (56.24
±20.32 MPa). (LS >AM >EX >NC)
1.00 mm: The highest Characteristic Strength was observed in Amber Mill (207.92 ±66.95
MPa), followed by IPS e.max CAD (202.09 ±35.99 MPa), then n!ce (199.22 ±57.44 MPa), and
lastly Initial LiSi (154.42 ±34.41 MPa). (AM>EX>NC>LS)
57.53
59.81
63.67
56.24
202.09
207.92
154.42
199.22
0.00
50.00
100.00
150.00
200.00
250.00
300.00
350.00
IPS e.max CAD Amber Mill Initial LiSi n!ce
MPa
0.5 mm
1mm
97
ANOVA (five firings):
One-way ANOVA was used to evaluate the significance of BFS values between the
materials in each thickness (Table 49, Table 50). A statistically significant difference (p=0.000)
was found between the materials in 1.00 mm at five firings.
Table 49: One-way ANOVA of tested materials at five firings (0.5 mm)
Material N F P Value
IPS e.max CAD 15
0.16 0.924
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Table 50: One-way ANOVA of tested materials at five firings (1.00 mm)
Material N F P Value
IPS e.max CAD 15
3.54 0.000
*
Amber Mill 15
Initial LiSi 15
n!ce 15
Total 60
Group-wise comparisons were performed separately for both thicknesses at five firings
(Table 51, Table 52) using Bonferroni correction due to multiple comparisons (α=0.05).
Table 51: Group-wise comparisons at five firings (0.5 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill -4.98 8.01 1.000 -26.90 16.94
Initial LiSi -4.98 8.01 1.000 -26.90 16.94
n!ce -16.07 8.01 0.299 -37.98 5.85
Amber Mill
Initial LiSi 0.00 8.01 1.000 -21.92 21.92
n!ce -11.08 8.01 1.000 -33.00 10.84
Initial LiSi n!ce -11.08 8.01 1.000 -33.00 10.84
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 51 presents the group-wise comparisons for 0.5 mm thickness materials at five
firings. No statistically significant difference (p=1.000) was found between each of the tested
materials.
98
Table 52: Group-wise comparisons at five firings (1.00 mm)
(I) Material
Mean
Difference (I-
J)
Std. Error P Value.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
IPS e.max CAD
Amber Mill -5.83 18.49 1.000 -56.41 44.75
Initial LiSi 47.67 18.49 0.075 -2.92 98.25
n!ce 2.87 18.49 1.000 -47.72 53.45
Amber Mill
Initial LiSi 53.49
*
18.49 0.032
*
2.91 104.08
n!ce 8.70 18.49 1.000 -41.89 59.28
Initial LiSi n!ce -44.80 18.49 0.112 -95.39 5.78
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
Table 52 presents the group-wise comparisons for 1.00 mm thickness materials at five
firings. It revealed that a statistically significant difference (p=0.032) was found only between
(Amber Mill and LiSi). For the group-wise comparison between other materials there was not a
statistically significantly different (p>0.05).
99
Thickness:
Overall:
The biaxial flexural strength data revealed that the higher values were seen in the 1.00 mm
thickness compared to 0.5 mm according to the estimated marginal means based on the thickness
(1.00 mm>0.5 mm) (Error! Not a valid bookmark self-reference.).
Table 53: Estimated Marginal Means (Firing)
Thickness Mean Std. Error
95% Confidence Interval
Lower
Bound
Upper
Bound
0.5 mm 60.09 2.17 55.79 64.39
1.00 mm 182.44 2.17 178.14 186.74
The groupwise comparison between the thicknesses (0.5 mm and 1.00 mm) is presented in
Table 54. A statistically significant difference was found between both thicknesses (p=0.000).
Table 54: Groupwise comparisons (Firing)
(I) Thickness
Mean
Difference (I-
J)
Std. Error Sig.
a
95% Confidence Interval for
Difference
a
Lower Bound Upper Bound
0.5 mm 1.00 mm 82.007 3.414 .000
*
75.243 88.772
Based on estimated marginal means
*. The mean difference is significant at the .05 level.
a. Adjustment for multiple comparisons: Bonferroni.
100
0.5 mm
The mean biaxial flexural strength and standard deviation at 0.5 mm for the tested materials
(IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) based on the firing (baseline, one firing, three
firings, and five firings) are presented in Table 55 (Figure 42).
Table 55: Mean biaxial flexural strength and standard deviation at 0.5 mm (MPa)
Firing IPS e.max CAD Amber Mill Initial LiSi n!ce
Baseline 67.63 ±49.57 53.16 ±13.21 42.10 ±16.14 45.81 ±14.12
One Firing 61.11 ±12.97 63.18 ±19.34 54.98 ±27.26 63.98 ±22.40
Three Firing 57.69 ±48.35 62.67 ±19.04 56.91 ±23.61 73.75 ±31.47
Five Firing 57.54 ±59.62 59.81 ±16.25 63.67 ±22.41 56.24 ±20.32
Figure 42: Change of biaxial flexural strength with firing and material (0.5 mm)
The highest overall mean biaxial flexural strength value in 0.5 mm thickness was observed
in n!ce at three firings (73.75 ±31.47 MPa); while the lowest was Initial LiSi at baseline (42.10
±16.14 MPa).
Baseline: The highest mean BFS value in the of the material tested in the 0.5 mm thickness
was IPS e.max CAD (67.63 ±49.57 MPa), followed by Amber Mill (53.16 ±13.21 MPa), then n!ce
(45.81 ±14.12 MPa), and lastly Initial LiSi (42.10 ±16.14 MPa).(EX>AM>NC>LS)
0
10
20
30
40
50
60
70
80
B A S E L I N E O N E F I R I N G T H R E E F I R I N G F I V E F I R I N G
MPa
IPS e.max CAD
Amber Mill
Initial LiSi
n!ce
101
One Firing: As for the mean BFS after one firing the highest value was n!ce (63.98 ±22.40
MPa), followed by Amber Mill (63.18 ±19.34 MPa), then IPS e.max CAD (61.11 ±12.97 MPa),
and lastly Initial LiSi (54.98 ±27.26MPa). (NC>AM>EX>LS)
Three Firings: like the one firing 0.5 mm thickness, the highest mean BFS was seen in
n!ce (73.75 ±31.47 MPa), followed by Amber Mill (62.67 ±19.04 MPa), then IPS e.max CAD
(57.69 ±48.35 MPa), and lastly Initial LiSi (56.91 ±23.61 MPa). (NC>AM>EX>LS)
Five Firings: Initial LiSi had the highest mean BFS value after five firings (63.67 ±22.41
MPa), followed by Amber Mill (59.18 ±16.25 MPa), then IPS e.max CAD (57.54 ±59.62 MPa),
and lastly n!ce (56.24 ±20.32 MPa).(LS>AM>EX>NC)
102
1.00 mm
The mean biaxial flexural strength and standard deviation at 1.00 mm for the tested
materials (IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce) based on the firing (baseline, one
firing, three firings, and five firings) are presented in Table 56 (Figure 43).
Table 56: Mean biaxial flexural strength and standard deviation at 1.00 mm (MPa)
Firing IPS e.max CAD Amber Mill Initial LiSi n!ce
Baseline 215.66 ±39.00 209.24 ±80.18 119.69 ±23.23 126.73 ±22.94
One Firing 242.57 ±27.84 187.49 ±43.82 123.77 ±14.21 157.90 ±30.27
Three Firing 234.79 ±42.86 251.08 ±83.56 121.62 ±23.09 164.80 ±39.13
Five Firing 202.09 ±35.99 207.92 ±66.95 154.42 ±34.41 199.22 ±57.44
Figure 43: Change of biaxial flexural strength with firing and material (1.00 mm)
The highest overall mean biaxial flexural strength value in 1.00 mm thickness was
observed in Amber Mill at three firings (251.08 ±83.56 MPa); while the lowest was Initial LiSi at
baseline (119.69 ±23.23 MPa).
Baseline: The highest mean BFS in the of the material tested in the 1.00 mm thickness was
IPS e.max CAD (215.66 ±39.00 MPa), followed by Amber Mill (209.24 ±80.18 MPa), then n!ce
(126.73 ±22.94 MPa), and lastly Initial LiSi (119.69 ± 23.23 MPa). (EX>AM>NC>LS)
0
50
100
150
200
250
300
B A S E L I N E O N E F I R I N G T H R E E F I R I N G F I V E F I R I N G
MPa
IPS e.max CAD
Amber Mill
Initial LiSi
n!ce
103
One Firing: As for the mean BFS after one firing the highest value was IPS e.max CAD
(242.57 ±27.84 MPa), followed by Amber Mill (187.49 ±43.82 MPa), then n!ce (157.90 ±30.27
MPa), and lastly Initial LiSi (123.77 ±14.21 MPa). (EX>AM>NC>LS)
Three Firings: The highest mean BFS value was seen in Amber Mill (251.08 ±83.56
MPa), followed by IPS e.max CAD (234.79 ±42.86 MPa), then n!ce (164.80 ±39.13 MPa), and
Initial LiSi (121.62 ±23.09 MPa). (AM>EX>NC>LS)
Five Firings: Amber Mill had the highest mean BFS value after five firings (207.92 ±66.95
MPa), followed by IPS e.max CAD (202.09 ±35.99 MPa), then n!ce (199.22 ±57.44 MPa), and
lastly Initial LiSi (154.42 ±34.41 MPa). (AM>EX>NC>LS)
104
4.4 Weibull Analysis:
The Two-parameter Weibull distribution is described using a shape (Weibull modulus)
parameter as well as a scaling (characteristic strength) parameter. They are estimated from the
biaxial flexural strength data.
The Weibull modulus signifies the shape parameter that affects the slope of the data on the
Weibull plot. Also, the Weibull plot represents the Weibull Characteristic Strength that indicates
the scale parameter and affects the spread of the distribution.
Weibull statistics were performed according to ISO 6872-2015
(151)
to analyze the biaxial
flexural strength data. R (v4.2.1; R Core Team 2022, PBC, Boston, MA, USA) was used to
calculate the Weibull Modulus (𝑚 ) and Weibull Characteristic Strength (σ
𝑂 ) as well as obtain
Weibull plots and likelihood contour plots.
The Weibull distribution Two-parameters of the biaxial flexural strength data of IPS e.max
CAD, Amber Mill, LiSi Initial, and n!ce, are presented in Table 57.
105
Table 57: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of IPS e.max CAD, Amber Mill, Initial LiSi, and n!ce.
Material Group
Group
No. &
Abv.
Weibull parameters
Weibull Characteristic Strength (σ
𝑂 ) Weibull Modulus (𝑚 )
σ
𝑂 Mean σ
𝑂 SD
σ
𝑂
Lower
bound
σ
𝑂 Upper
bound
𝑚 Mean 𝑚 SD
𝑚 Lower
bound
𝑚 Upper
bound
IPS e.max CAD
0.5 mm baseline (1) 5EB 64.16 ±6.43 51.55 76.77 5.57 ±1.09 3.44 7.71
0.5 mm one firing (2) 5E1 66.22 ±3.27 59.81 72.64 5.52 ±1.11 3.34 7.70
0.5 mm three firings (3) 5E3 63.26 ±3.89 55.63 70.88 4.45 ±0.88 2.72 6.17
0.5 mm five firings (4) 1E5 63.38 ±4.37 54.82 71.94 3.97 ±0.75 2.49 5.45
1 mm baseline (5) 1EB 234.78 ±9.72 212.61 250.73 6.50 ±1.33 3.89 9.12
1 mm one firing (6) 1E1 254.38 ±6.68 241.30 267.47 10.39 ±2.07 6.33 14.45
1 mm three firings (7) 1E3 252.23 ±10.91 230.85 273.61 6.31 ±1.26 3.85 8.78
1 mm five firings (8) 1E5 220.18 ±17.65 185.60 254.77 6.48 ±1.28 3.96 8.99
Amber Mill
0.5 mm baseline (9) 5EB 58.19 ±3.58 51.17 65.21 4.44 ±0.85 2.77 6.11
0.5 mm one firing (10) 5A1 70.01 ±5.58 59.06 80.95 3.43 ±0.63 2.21 4.66
0.5 mm three firings (11) 5A3 69.51 ±5.46 58.81 80.21 3.49 ±0.65 2.22 4.77
0.5 mm five firings (12) 5A5 65.79 ±4.03 57.89 73.70 4.44 ±0.93 2.62 6.26
1 mm baseline (13) 1AB 231.67 ±23.60 188.53 281.03 2.73 ±0.49 1.78 3.68
1 mm one firing (14) 1A1 204.56 ±11.67 181.68 227.44 4.79 ±0.94 2.95 6.64
1 mm three firings (15) 1A3 280.04 ±23.52 233.94 326.15 3.26 ±0.62 2.05 4.48
1 mm five firings (16) 1A5 230.93 ±16.76 198.08 263.78 3.73 ±0.79 2.19 5.28
Initial LiSi
0.5 mm baseline (17) 5LB 47.34 ±4.61 38.30 56.37 2.82 ±0.53 1.79 3.86
0.5 mm one firing (18) 5L1 62.36 ±7.71 47.26 77.46 2.22 ±0.41 1.41 3.03
0.5 mm three firings (19) 5L3 62.36 ±4.59 50.62 68.88 2.72 ±0.55 1.64 3.80
0.5 mm five firings (20) 5L5 71.23 ±6.16 59.17 83.30 3.17 ±0.62 1.95 4.38
1 mm baseline (21) 1LB 129.22 ±6.49 116.50 141.94 5.46 ±1.03 3.44 7.48
1 mm one firing (22) 1L1 129.92 ±3.38 123.29 136.54 5.48 ±2.16 3.24 7.73
1 mm three firings (23) 1L3 131.12 ±6.70 117.99 144.25 5.37 ±0.98 3.46 7.29
1 mm five firings (24) 1L5 168.05 ±9.14 150.13 185.97 5.03 ±1.00 3.08 6.98
n!ce
0.5 mm baseline (25) 5NB 50.88 ±4.09 42.86 58.90 3.42 ±0.63 2.17 4.66
0.5 mm one firing (26) 5N1 71.58 ±6.41 59.01 84.15 3.07 ±0.57 1.94 4.19
0.5 mm three firings (27) 5N3 83.34 ±8.85 65.99 100.69 2.58 ±0.49 1.61 3.55
0.5 mm five firings (28) 5N5 62.88 ±5.17 52.74 73.02 3.29 ±0.71 1.91 4.68
1 mm baseline (29) 1NB 136.31 ±6.55 123.48 149.15 5.71 ±1.06 3.63 7.80
1 mm one firing (30) 1N1 170.25 ±8.02 154.53 185.97 5.81 ±1.13 3.59 8.03
1 mm three firings (31) 1N3 180.16 ±10.62 159.35 200.97 4.65 ±0.91 2.86 6.44
1 mm five firings (32) 1N5 216.76 ±9.14 198.85 234.67 3.43 ±0.60 2.26 4.61
106
Weibull Characteristic Strength (𝝈 𝑶 ):
The Weibull characteristic strength is defined as the point at which 62.32% of specimens
will have failed. High characteristic strength values indicate low failure probability in comparison
with low characteristic strength values that indicate high failure probability.
Weibull modulus (𝒎 ):
The Weibull modulus (𝑚 ) presents the reliability of the ceramic based on the biaxial flexural
strength data. A high Weibull modulus indicates that the data are close and that the "standard
deviation" is low. In contrast, a low Weibull modulus indicates that the data are scattered and that
the "standard deviation" is significant. Low Weibull modulus materials will not be as reliable as
those with a high modulus and will have a broad distribution of failure.
Probability of failure:
The Weibull analysis includes the presentation of a Weibull plot (Figure 5) which is used to
graph the biaxial flexural strength data (MPa) on the x-axis and the probability of failure (%) on
the y-axis. The plot shows the Weibull characteristic strength as a horizontal line to represent the
63.2% failure probability. The Weibull modulus affects the steepness of each group based on the
standard deviation of the biaxial flexural strength values.
Also, to determine whether the Weibull distribution Two-parameters are statistically
significantly different from one another, the likelihood contour plot (Figure 6) was used. The x-
axis presents the characteristic strength, and the y-axis presents the Weibull modulus which both
use 95 % confidence bounds.
107
In the likelihood contour plot, the two parameters appear as oval/circle shapes. The width of
these shapes is determined by the standard deviation of the characteristic strength. While the length
of these shapes is controlled by the standard deviation of the Weibull modulus. Once these shapes
intersect/overlap with one another, the two parameters are not statistically significant from each
other.
108
Material:
IPS e.max CAD:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) of IPS e.max
CAD for the repeated firings in 0.5 mm and 1.00 mm are presented in Table 58.
Table 58: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of IPS e.max CAD
Firing Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
Baseline
0.5 mm 64.16 ±6.43 5.57 ±1.09
1.00 mm 234.78 ±9.72 6.50 ±1.33
One firing
0.5 mm 66.22 ±3.27 5.52 ±1.11
1.00 mm 254.38 ±6.68 10.39 ±2.07
Three firings
0.5 mm 63.26 ±3.89 4.45 ±0.88
1.00 mm 252.23 ±10.91 6.31 ±1.26
Five firings
0.5 mm 63.38 ±4.37 3.97 ±0.75
1.00 mm 220.18 ±17.65 6.48 ±1.28
Overall, the highest Characteristic Strength in IPS e.max CAD was found at one firing
1.00 mm (254.38 ±6.68 MPa). Moreover, the highest Weibull Modulus was also found at one firing
1.00 mm (10.39 ±2.07).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest at one firing (66.22 ±3.27 MPa),
followed by baseline (64.16 ±6.43 MPa), then five firings (63.38 ±4.37 MPa), and lastly three
firings (63.26 ±3.89 MPa). (1F>BL>5F>3F)
1.00 mm: The highest Characteristic Strength value was seen at one firing (254.38 ±6.68
MPa), followed by three firings (252.23 ±10.91 MPa), then baseline (234.78 ±9.72 MPa), and
lastly five firings (220.18 ±17.65 MPa). (1F>3F>BL>5F)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value was observed at baseline (5.57 ±1.09),
followed by one firing (5.52 ±1.11), then three firings (4.45 ±0.88), and lastly five firings (3.97
±0.75). (BL>1F>3F>5F)
109
1.00 mm: The maximum Weibull Modulus value was seen at one firing (10.39 ±2.07),
followed by baseline (6.50 ±1.33), then five firings (6.48 ±1.28), and lastly three firings (6.31
±1.26). (1F>BL>5F>3F)
Probability of Failure
Weibull plot:
The Weibull plot of the repeated firings for both thicknesses of IPS e.max CAD with the
Weibull characteristic strength at the failure probability of 63.2% is shown in Figure 44.
Figure 44: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of IPS e.max CAD
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with one firing, followed by three firings, then baseline, and lastly five firings. Whereas the
0.5 mm thickness firings were lower with one firing, followed by baseline, then five firings, and
lastly three firings. This indicates a higher failure probability of three firings 0.5 mm compared to
one firing 1.00 mm of IPS e.max CAD. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
110
Contour plot:
The likelihood contour plot of the repeated firings for both thicknesses of IPS e.max CAD
is shown in Figure 45.
Figure 45: Likelihood contour plot (IPS e.max CAD)
On the left side of the plot, the 0.5 mm firings all intersect, indicating no statistically
significant difference. On the right side of the plot, 1.00 mm firings do not intersect between one
firing and five firings indicating a statistically significant difference.
111
Amber Mill:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) of Amber
Mill for the repeated firings in 0.5 mm and 1.00 mm are presented in Table 59.
Table 59: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of Amber Mill
Firing Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
Baseline
0.5 mm 58.19 ±3.58 4.44 ±0.85
1.00 mm 231.67 ±23.60 2.73 ±0.49
One firing
0.5 mm 70.01 ±5.58 3.43 ±0.63
1.00 mm 204.56 ±11.67 4.79 ±0.94
Three firings
0.5 mm 69.51 ±5.46 3.49 ±0.65
1.00 mm 280.04 ±23.52 3.26 ±0.62
Five firings
0.5 mm 65.79 ±4.03 4.44 ±0.93
1.00 mm 230.93 ±16.76 3.73 ±0.79
Overall, the highest Characteristic Strength in Amber Mill was found at three firings 1.00
mm (280.04 ±23.52 MPa). Moreover, the highest Weibull Modulus was found at one firing 1.00
mm (4.79 ±0.94).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest at one firing (70.01 ±5.58 MPa),
followed by three firings (69.51 ±5.46 MPa), then five firings (65.79 ±4.03 MPa), and lastly
baseline (58.19 ±3.58 MPa). (1F>3F>5F>BL)
1.00 mm: The highest Characteristic Strength value was seen at three firings (280.04
±23.52 MPa), followed by baseline (231.67 ±23.60 MPa), then five firings (230.93 ±16.76 MPa),
and lastly one firing (204.56 ±11.67 MPa). (3F>BL>5F>1F)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value was observed at five firings (4.44 ±0.93),
followed by baseline (4.44 ±0.85), then three firings (3.49 ±0.65), and lastly one firing (3.43
±0.63). (5F>BL>3F>1F)
112
1.00 mm: The maximum Weibull Modulus value was seen at one firing (4.79 ±0.94),
followed by five firings (3.73 ±0.79), then three firings (3.26 ±0.62), and lastly baseline (2.73
±0.49). (1F>5F>3F>BL)
Probability of Failure
Weibull plot:
The Weibull plot of the repeated firings for both thicknesses of Amber Mill with the
Weibull characteristic strength at the failure probability of 63.2% is shown in Figure 46.
Figure 46: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of Amber Mill
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with three firings, followed by baseline, then five firings, and lastly one firing. Whereas the
0.5 mm thickness firings were lower with one firing, followed by three firings, then five firings,
and lastly baseline. This indicates a higher failure probability of baseline 0.5 mm compared to
three firings 1.00 mm of Amber Mill. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
113
Contour plot:
The likelihood contour plot of the repeated firings for both thicknesses Amber Mill is
shown in Figure 47.
Figure 47: Likelihood contour plot (Amber Mill)
On the left side of the plot, the 0.5 mm firings all intersect, indicating no statistically
significant difference. On the right side of the plot, 1.00 mm firings do not intersect between one
firing and three firings indicating a statistically significant difference.
114
Initial LiSi:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) of Initial LiSi
for the repeated firings in 0.5 mm and 1.00 mm are presented in Table 60.
Table 60: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of Initial LiSi
Firing Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
Baseline
0.5 mm 47.34 ±4.61 2.82 ±0.53
1.00 mm 129.22 ±6.49 5.46 ±1.03
One firing
0.5 mm 62.36 ±7.71 2.22 ±0.41
1.00 mm 129.92 ±3.38 5.48 ±2.16
Three firings
0.5 mm 63.26 ±3.89 2.72 ±0.55
1.00 mm 131.12 ±6.70 5.37 ±0.98
Five firings
0.5 mm 71.23 ±6.16 3.17 ±0.62
1.00 mm 168.05 ±9.14 5.03 ±1.00
Overall, the highest Characteristic Strength in Initial LiSi was found at five firings 1.00
mm (168.05 ±9.14 MPa). Moreover, the highest Weibull Modulus was found at one firing 1.00 mm
(5.48 ±2.16).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest at five firings (71.23 ±6.16 MPa),
followed by three firings (63.26 ±3.89 MPa),then one firing (62.36 ±7.71 MPa), and lastly baseline
(47.34 ±4.61 MPa). (5F>3F>1F>BL)
1.00 mm: The highest Characteristic Strength value was seen at five firings (168.05 ±9.14
MPa), followed by three firings (131.12 ±6.70 MPa), then one firing (129.92 ±3.38 MPa), and
lastly baseline (129.22 ±6.49 MPa). (5F>3F>1F>BL)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value was observed at five firings (3.42 ±0.63),
followed by baseline (3.17 ±0.62), then three firings (2.72 ±0.55), and lastly one firing (2.22
±0.41). (5F>BL>3F>1F)
115
1.00 mm: The maximum Weibull Modulus value was seen at one firing (5.48 ±2.16),
followed by baseline (5.46 ±1.03), then three firings (5.37 ±0.98), and lastly five firings (5.03
±1.00). (1F>BL>3F>5F)
Probability of Failure
Weibull plot:
The Weibull plot of the repeated firings for both thicknesses of Initial LiSi with the Weibull
characteristic strength at the failure probability of 63.2% is shown in Figure 48.
Figure 48: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of Initial LiSi
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with five firings, followed by three firings, then one firing, and lastly baseline. Whereas the
0.5 mm thickness firings were lower with five firings, followed by three firings, then one firing,
and lastly baseline. This indicates a higher failure probability of baseline 0.5 mm compared to five
firings 1.00 mm of LiSi Initial. The higher standard deviation of the BFS values impacted the shape
of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
116
Contour plot:
The likelihood contour plot of the repeated firings for both thicknesses of Initial LiSi is
shown in Figure 49
Figure 49: Likelihood contour plot (Initial LiSi)
On the left side of the plot, the 0.5 mm firings do not intersect between baseline and five
firings indicating a statistically significant difference. On the right side of the plot, 1.00 mm firings
do not intersect between five firings with baseline, one firing, and three firings indicating a
statistically significant difference.
117
n!ce:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) of n!ce for
the repeated firings in 0.5 mm and 1.00 mm are presented in Table 61.
Table 61: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of n!ce
Firing Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
Baseline
0.5 mm 50.88 ±4.09 3.42 ±0.63
1.00 mm 136.31 ±6.55 5.71 ±1.06
One firing
0.5 mm 71.58 ±6.41 3.07 ±0.57
1.00 mm 170.25 ±8.02 5.81 ±1.13
Three firings
0.5 mm 83.34 ±8.85 2.58 ±0.49
1.00 mm 180.16 ±10.62 4.65 ±0.91
Five firings
0.5 mm 62.88 ±5.17 3.29 ±0.71
1.00 mm 216.76 ±9.14 3.43 ±0.60
Overall, the highest Characteristic Strength in n!ce was found at five firings 1.00 mm
(216.76 ±9.14 MPa). Moreover, the highest Weibull Modulus was found at one firing 1.00 mm
(5.81 ±1.13).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest at three firings (83.34 ±8.85 MPa),
followed by one firing (71.58 ±6.41 MPa), then five firings (62.88 ±5.17 MPa), and lastly baseline
(50.88 ±4.09 MPa). (3F>1F>5F>BL)
1.00 mm: The highest Characteristic Strength value was seen at five firings (216.76 ±9.14
MPa), followed by three firings (180.16 ±10.62 MPa), then one firing (170.25 ±8.02 MPa), and
lastly baseline (136.31 ±6.55 MPa). (5F>3F>1F>BL)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value was observed at baseline (3.42 ±0.63),
followed by five firings (3.29 ±0.71), then one firing (3.07 ±0.57), and lastly three firings (2.58
±0.49). (BL>5F>1F>3F)
118
1.00 mm: The maximum Weibull Modulus value was seen at one firing (5.81 ±1.13),
followed by baseline (5.71 ±1.06), then three firings (4.65 ±0.91), and lastly five firings (3.43
±0.60). (1F>BL>3F>5F)
Probability of Failure
Weibull plot:
The Weibull plot of the repeated firings for both thicknesses of n!ce with the Weibull
characteristic strength at the failure probability of 63.2% is shown in Figure 50.
Figure 50: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) of n!ce
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with five firings, followed by three firings, then one firing, and lastly baseline. Whereas the
0.5 mm thickness firings were lower with three firings, followed by one firing, then five firings,
and lastly baseline. This indicates a higher failure probability of baseline 0.5 mm compared to five
firings 1.00 mm of n!ce. The higher standard deviation of the BFS values impacted the shape of
the slope (Weibull modulus) leading to scattered data causing a shallow slope.
119
Contour plot:
The likelihood contour plot of the repeated firings for both thicknesses of n!ce is shown in
Figure 51.
Figure 51: Likelihood contour plot (n!ce)
On the left side of the plot, the 0.5 mm firings do not intersect between baseline and three
firings indicating a statistically significant difference. On the right side of the plot, 1.00 mm firings
do not intersect between baseline with one firing, three firings, and five firings as well as between
one firing and three firings indicating a statistically significant difference.
120
Firing:
Baseline:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) at baseline
of the tested martials in 0.5 mm and 1.00 mm are presented in Table 62.
Table 62: Weibull distribution (Two-parameters) Characteristic Strength ( 𝜎 𝑂 ) and Weibull Modulus (m) of Baseline
Material Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
IPS e.max CAD
0.5 mm 64.16 ±6.43 5.57 ±1.09
1.00 mm 234.78 ±9.72 6.50 ±1.33
Amber Mill
0.5 mm 58.19 ±3.58 4.44 ±0.85
1.00 mm 231.67 ±23.60 2.73 ±0.49
Initial LiSi
0.5 mm 47.34 ±4.61 2.82 ±0.53
1.00 mm 129.22 ±6.49 5.46 ±1.03
n!ce
0.5 mm 50.88 ±4.09 3.42 ±0.63
1.00 mm 136.31 ±6.55 5.71 ±1.06
Overall, the highest Characteristic Strength in the baseline data was found in IPS e.max
CAD 1.00 mm (234.78 ±23.60 MPa). Moreover, the highest Weibull Modulus was also found in
IPS e.max CAD 1.00 mm (6.50 ±1.33 MPa).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest in IPS e.max CAD (64.16 ±6.43
MPa), followed by Amber Mill (58.19 ±3.58 MPa), then n!ce (50.88 ±4.09 MPa), and lastly Initial
LiSi (62.36 ±7.71 MPa). (EX>AM>NC>LS)
1.00 mm: The highest Characteristic Strength value was seen in IPS e.max CAD (234.78
±23.60 MPa), followed by Amber Mill (231.67 ±9.72 MPa), then n!ce (136.31 ±6.55 MPa), and
lastly Initial LiSi (129.22 ±6.49 MPa). (AM >EX >NC>LS)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value for 0.5 mm was observed IPS e.max CAD
(5.57 ±1.09), followed by Amber Mill (4.44 ±0.85), then n!ce (3.42 ±0.63), and lastly Initial LiSi
(2.82 ±0.53). (EX>AM>NC>LS)
121
1.00 mm: The maximum Weibull Modulus value for 1.00 mm was seen in IPS e.max CAD
(6.50 ±1.33), followed by n!ce (5.71 ±1.06), then Initial LiSi (5.46 ±1.03), and lastly Amber Mill
(2.73 ±0.49). (EX>NC>LS>AM)
Probability of Failure
Weibull plot:
The Weibull plot for the tested martials in both thicknesses at baseline with the Weibull
characteristic strength at the failure probability of 63.2% is shown in Figure 52
Figure 52: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at baseline
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with IPS e.max CAD, followed by Amber Mill, then n!ce, and lastly Initial LiSi. Whereas
the 0.5 mm thickness firings were lower with IPS e.max CAD, followed by Amber Mill, then n!ce,
and lastly Initial LiSi. This indicates a higher failure probability of Initial LiSi 0.5 mm compared
to IPS e.max CAD 1.00 mm at baseline. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
122
Contour plot:
The likelihood contour plot for the tested materials in both thicknesses at baseline is shown
in Figure 53.
Figure 53: Likelihood contour plot (Baseline)
On the left side of the plot, the 0.5 mm firings do not intersect between IPS e.max CAD
with Initial LiSi and n!ce indicating a statistically significant difference. On the right side of the
plot, 1.00 mm firings do not intersect between IPS e.max CAD with Amber Mill, three firings,
Initial LiSi, and n!ce as well as between Amber Mill with Initial LiSi and n!ce indicating a
statistically significant difference.
123
One Firing:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) at one firing
of the tested materials in 0.5 mm and 1.00 mm are presented in Table 63.
Table 63: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of One firing
Material Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
IPS e.max CAD
0.5 mm 66.22 ±3.27 5.52 ±1.11
1.00 mm 254.38 ±6.68 10.39 ±2.07
Amber Mill
0.5 mm 70.01 ±5.58 3.43 ±0.63
1.00 mm 204.56 ±11.67 4.79 ±0.94
Initial LiSi
0.5 mm 62.36 ±7.71 2.22 ±0.41
1.00 mm 129.92 ±3.38 5.48 ±2.16
n!ce
0.5 mm 71.58 ±6.41 3.07 ±0.57
1.00 mm 170.25 ±8.02 5.81 ±1.13
Overall, the highest Characteristic Strength in the one firing data was found in IPS e.max
CAD 1.00 mm (254.38 ±6.68 MPa). Moreover, the highest Weibull Modulus was also found in
IPS e.max CAD 1.00 mm (10.39 ±2.07).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest in n!ce (71.58 ±6.41 MPa),
followed by Amber Mill (70.01 ±5.58 MPa), then IPS e.max CAD (66.22 ±3.27 MPa), and lastly
Initial LiSi (62.36 ±7.71 MPa). (NC>AM>EX>LS)
1.00 mm: The highest Characteristic Strength value was observed in IPS e.max CAD
(254.38 ±6.68 MPa), followed by Amber Mill (204.56 ±11.67 MPa), then n!ce (170.25 ±8.02
MPa), and lastly Initial LiSi (129.92 ±3.38 MPa). (EX>AM>NC>LS)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value for 0.5 mm was observed in IPS e.max CAD
(5.52 ±1.11), followed by Amber Mill (3.43 ±0.63), then n!ce (3.07 ±0.57), and lastly Initial LiSi
(2.22 ±0.41). (EX>AM>NC>LS)
124
1.00 mm: The maximum Weibull Modulus value for 1.00 mm was seen in IPS e.max CAD
(10.39 ±2.07), followed by n!ce (5.81 ±1.13), then Initial LiSi (5.48 ±2.16), and lastly Amber Mill
(4.79 ±0.94). (EX>NC>LS>AM)
Probability of Failure
Weibull plot:
The Weibull plot for the tested materials in both thicknesses at one firing with the Weibull
characteristic strength at the failure probability of 63.2% is shown in Figure 54.
Figure 54: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at one firing
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with IPS e.max CAD, followed by Amber Mill, then n!ce, and lastly Initial LiSi. Whereas
the 0.5 mm thickness firings were lower with n!ce, followed by Amber Mill, then IPS e.max CAD,
and lastly Initial LiSi. This indicates a higher failure probability of Initial LiSi 0.5 mm compared
to IPS e.max CAD 1.00 mm at one firing. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
125
Contour plot:
The likelihood contour plot for the tested materials in both thicknesses at one firing is
shown in Figure 55
Figure 55: Likelihood contour plot (One Firing)
On the left side of the plot, the 0.5 mm firings do not intersect between IPS e.max CAD
and Initial LiSi indicating a statistically significant difference. On the right side of the plot, 1.00
mm firings do not intersect between IPS e.max CAD with Amber Mill, Initial LiSi, and n!ce as
well as between Amber Mill and Initial LiSi and between Initial LiSi and n!ce indicating a
statistically significant difference.
126
Three Firings:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) at three
firings of the tested materials in 0.5 mm and 1.00 mm are given in Table 64.
Table 64: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of Three firings
Material Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
IPS e.max CAD
0.5 mm 63.26 ±3.89 4.45 ±0.88
1.00 mm 252.23 ±10.91 6.31 ±1.26
Amber Mill
0.5 mm 69.51 ±5.46 3.49 ±0.65
1.00 mm 280.04 ±23.52 3.26 ±0.62
Initial LiSi
0.5 mm 62.36 ±4.59 2.72 ±0.55
1.00 mm 131.12 ±6.70 5.37 ±0.98
n!ce
0.5 mm 83.34 ±8.85 2.58 ±0.49
1.00 mm 180.16 ±10.62 4.65 ±0.91
Overall, the highest Characteristic Strength in the three firings data was found in Amber
Mill 1.00 mm (280.04 ±23.52 MPa). Meanwhile, the highest Weibull Modulus was found in IPS
e.max CAD 1.00 mm (6.31 ±1.26).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength was highest in n!ce (83.34 ±8.85 MPa), followed by
Amber Mill (69.51 ±5.46 MPa), then IPS e.max CAD (63.26 ±3.89 MPa), and lastly Initial LiSi
(62.36 ±4.59 MPa). (NC >AM >EX>LS)
1.00 mm: The highest Characteristic Strength was observed in Amber Mill (280.04 ±23.52
MPa), followed by IPS e.max CAD (252.23 ±10.91 MPa), then n!ce (180.16 ±10.62 MPa), and
lastly Initial LiSi (131.12 ±6.70 MPa). (AM>EX>NC>LS)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus for 0.5 mm was observed in IPS e.max CAD (4.45
±0.88), followed by Amber Mill (3.49 ±0.65), then Initial LiSi (2.72 ±0.55), and lastly n!ce (2.58
±0.49). (EX>AM>LS>NC)
127
1.00 mm: The maximum Weibull Modulus for 1.00 mm was seen in IPS e.max CAD (6.31
±1.26), followed by Initial LiSi (5.37 ±0.98), then n!ce (4.65 ±0.91), and lastly Amber Mill (3.26
±0.62). (EX> LS> NC >AM)
Probability of Failure
Weibull plot:
The Weibull plot for the tested materials in both thicknesses at three firings with the
Weibull characteristic strength at the failure probability of 63.2% is shown in Figure 56.
Figure 56: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at three firings
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with Amber Mill, followed by IPS e.max CAD, then n!ce, and lastly Initial LiSi. Whereas
the 0.5 mm thickness firings were lower with n!ce, followed by Amber Mill, then IPS e.max CAD,
and lastly Initial LiSi. This indicates a higher failure probability of Initial LiSi 0.5 mm compared
to Amber Mill 1.00 mm at three firings. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
128
Contour plot:
The likelihood contour plot for the tested materials in both thicknesses at three firings is
shown in Figure 57.
Figure 57: Likelihood contour plot (Three Firings)
On the left side of the plot, the 0.5 mm firings do not intersect between IPS e.max CAD
and n!ce indicating a statistically significant difference. On the right side of the plot, 1.00 mm
firings do not intersect between each of the materials indicating a statistically significant difference
between all 1.00 mm tested materials.
129
Five Firings:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ) at five firings
of the tested materials in 0.5 mm and 1.00 mm are presented in Table 65.
Table 65: Weibull distribution (Two-parameters) Characteristic Strength (𝜎 𝑂 ) and Weibull Modulus (m) of Five firings
Material Thickness 𝛔 𝑶 Mean 𝛔 𝑶 SD 𝒎 Mean 𝒎 SD
IPS e.max CAD
0.5 mm 63.38 ±4.37 3.97 ±0.75
1.00 mm 220.18 ±17.65 6.48 ±1.28
Amber Mill
0.5 mm 65.79 ±4.03 4.44 ±0.93
1.00 mm 230.93 ±16.76 3.73 ±0.79
Initial LiSi
0.5 mm 71.23 ±6.16 3.17 ±0.62
1.00 mm 168.05 ±9.14 5.03 ±1.00
n!ce
0.5 mm 62.88 ±5.17 3.29 ±0.71
1.00 mm 216.76 ±9.14 3.43 ±0.60
Overall, the highest Characteristic Strength in the five firings data was found in Amber Mill
1.00 mm (230.93 ±16.76 MPa). Meanwhile, the highest Weibull Modulus was found in IPS e.max
CAD 1.00 mm (6.31 ±1.26).
Weibull Characteristic Strength (σ
𝑂 )
0.5 mm: The Characteristic Strength value was highest in Initial LiSi (60.23 ±6.16 MPa),
followed by Amber Mill (65.79 ±4.03 MPa), then IPS e.max CAD (63.38 ±4.37 MPa), and lastly
n!ce (62.88 ±5.17 MPa). (LS >AM >EX >NC)
1.00 mm: The highest Characteristic Strength value was observed in Amber Mill (230.93
±16.76 MPa), followed by IPS e.max CAD (220.18 ±17.65 MPa), then n!ce (216.76 ±9.14 MPa),
and lastly Initial LiSi (168.05 ±9.14 MPa). (AM>EX>NC>LS)
Weibull Modulus (𝑚 )
0.5 mm: The highest Weibull Modulus value for 0.5 mm was observed in Amber Mill
(4.44 ±0.93), followed by IPS e.max CAD (3.97 ±0.75), then n!ce (3.29 ±0.71), and lastly Initial
LiSi (3.17 ±0.62). (AM>EX>NC>LS)
130
1.00 mm: The maximum Weibull Modulus for 1.00 mm was seen in IPS e.max CAD (6.31
±1.26), followed by Initial LiSi (5.37 ±0.98), then n!ce (4.65 ±0.91), and lastly Amber Mill (3.26
±0.62). (EX> LS> NC >AM)
Probability of Failure
Weibull plot:
The Weibull plot for the tested materials in both thicknesses at five firings with the Weibull
characteristic strength at the failure probability of 63.2% is shown in Figure 58.
Figure 58: Weibull plot (Plots of failure probabilities against biaxial flexural strengths) at five firings
In the plot, the 1.00 mm thickness firings displayed the highest characteristic strength
values with Amber Mill, followed by IPS e.max CAD, then n!ce, and lastly Initial LiSi. Whereas
the 0.5 mm thickness firings were lower with Initial LiSi, followed by Amber Mill, then IPS e.max
CAD, and lastly n!ce. This indicates a higher failure probability of Initial LiSi 0.5 mm compared
to Amber Mill 1.00 mm at five firings. The higher standard deviation of the BFS values impacted
the shape of the slope (Weibull modulus) leading to scattered data causing a shallow slope.
131
Contour plot:
The likelihood contour plot for the tested materials in both thicknesses at five firings is
shown in Figure 59.
Figure 59: Likelihood contour plot (Five Firings)
On the left side of the plot, the 0.5 mm all intersect, indicating no statistically significant
difference. On the right side of the plot, 1.00 mm firings do not intersect between Initial LiSi with
IPS e.max CAD, Amber Mill, and n!ce indicating a statistically significant difference .
132
Thickness:
Overall:
The Two-parameters, Characteristic Strength (σ
𝑂 ) and Weibull Modulus (𝑚 ), presented
higher values in 1.00 mm compared to 0.5 mm (Table 55). The likelihood contour plots that were
used to determine the statistically significant differences between the two parameters did not
intersect between 0.5 mm and 1.00 mm in all the plots based on material and firing (Figure 45,
Figure 47, Figure 49, Figure 51, Figure 53, Figure 55, Figure 57, Figure 59). This indicate that
0.5 mm and 1.00 are statistically significantly different.
133
4.5. Results Summary:
A summary of the results is presented in Table 66 showing the statistically significant
differences seen based on the material in the group-wise comparisons of the biaxial flexural
strength data as well as the statistically significant difference seen in the likelihood contour plot of
the Two-parameter Weibull distribution (Weibull modulus and Characteristic strength). Similarly,
a summary of the results showing the statically significant differences based on the firings is shown
in Table 67.
134
Table 66: Results summary of the group-wise comparisons (BFS) and the Weibull distribution (Two-parameters) based on the
material
Material Thickness Firing
Biaxial flexural
strength
Weibull distribution
IPS e.max CAD
0.5 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Not significant Not significant
One firing
Three firings Not significant Not significant
Five firings Not significant Not significant
Three firings Five firings Not significant Not significant
1.00 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Not significant Not significant
One firing
Three firings Not significant Not significant
Five firings Significant Significant
Three firings Five firings Not significant Not significant
Amber Mill
0.5 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Not significant Not significant
One firing
Three firings Not significant Not significant
Five firings Not significant Not significant
Three firings Five firings Not significant Not significant
1.00 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Not significant Not significant
One firing
Three firings Not significant Significant
Five firings Not significant Not significant
Three firings Five firings Not significant Not significant
Initial LiSi
0.5 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Not significant Significant
One firing
Three firings Not significant Not significant
Five firings Not significant Not significant
Three firings Five firings Not significant Not significant
1.00 mm
Baseline
One firing Not significant Not significant
Three firings Not significant Not significant
Five firings Significant Significant
One firing
Three firings Not significant Not significant
Five firings Significant Significant
Three firings Five firings Significant Significant
n!ce
0.5 mm
Baseline
One firing Not significant Not significant
Three firings Significant Not significant
Five firings Not significant Not significant
One firing
Three firings Not significant Not significant
Five firings Not significant Not significant
Three firings Five firings Not significant Not significant
1.00 mm
Baseline
One firing Not significant Significant
Three firings Not significant Significant
Five firings Significant Significant
One firing
Three firings Not significant Significant
Five firings Significant Not significant
Three firings Five firings Not significant Not significant
135
Table 67: Results summary of the group-wise comparisons (BFS) and the Weibull distribution (Two-parameters) based on the
firing
Firing Thickness Material
Biaxial flexural
strength
Weibull distribution
Baseline
0.5 mm
IPS e.max CAD
Amber Mill Significant Not significant
Initial LiSi Significant Significant
n!ce Significant Significant
Amber Mill
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Initial LiSi n!ce Not significant Not significant
1.00 mm
IPS e.max CAD
Amber Mill Not significant Significant
Initial LiSi Significant Significant
n!ce Significant Significant
Amber Mill
Initial LiSi Significant Significant
n!ce Significant Significant
Initial LiSi n!ce Not significant Not significant
One Firing
0.5 mm
IPS e.max CAD
Amber Mill Not significant Not significant
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Amber Mill
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Initial LiSi n!ce Not significant Significant
1.00 mm
IPS e.max CAD
Amber Mill Significant Significant
Initial LiSi Significant Significant
n!ce Significant Significant
Amber Mill
Initial LiSi Significant Significant
n!ce Not significant Not significant
Initial LiSi n!ce Significant Significant
Three Firings
0.5 mm
IPS e.max CAD
Amber Mill Not significant Not significant
Initial LiSi Not significant Not significant
n!ce Not significant Significant
Amber Mill
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Initial LiSi n!ce Not significant Not significant
1.00 mm
IPS e.max CAD
Amber Mill Not significant Not significant
Initial LiSi Significant Not significant
n!ce Significant Not significant
Amber Mill
Initial LiSi Significant Not significant
n!ce Significant Not significant
Initial LiSi n!ce Not significant Not significant
Five Firings
0.5 mm
IPS e.max CAD
Amber Mill Not significant Not significant
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Amber Mill
Initial LiSi Not significant Not significant
n!ce Not significant Not significant
Initial LiSi n!ce Not significant Not significant
1.00 mm
IPS e.max CAD
Amber Mill Not significant Not significant
Initial LiSi Not significant Significant
n!ce Not significant Not significant
Amber Mill
Initial LiSi Significant Significant
n!ce Not significant Not significant
Initial LiSi n!ce Not significant Significant
136
Chapter 4: Discussion
The objective of this study was to observe the effect of firings on the biaxial flexural
strength properties of four CAD/CAM lithium disilicate products with two different thicknesses.
The data indicate that the biaxial flexural strength values of the tested materials vary at different
thicknesses and different firings, thus rejecting all null hypotheses.
4.1. Material:
Our results revealed that a statistically significant difference was found between the four
different lithium disilicate materials in regard to the mean biaxial flexural strength values.
Therefore, the first null hypothesis was rejected.
In this study, IPS e.max CAD had the highest mean biaxial flexural strength, followed by
Amber Mill and n!ce compared to Initial LiSi which had the lowest overall mean biaxial flexural
strength.
ISO 6872-2015:
Currently, several types of lithium disilicate glass-ceramic materials have been utilized for
lab-side and chair-side fabrication of all-ceramic restorations using CAD/CAM technology.
Lithium disilicate glass-ceramic maintains a relatively high strength, which is high enough for full-
coverage crowns in the posterior area. The flexural strength requirements for dental ceramics,
according to their clinical indications, were classified by ISO 6872-2015
(151)
based on the mean
value Table 68. Our data for 1.00 mm has fallen within the recommended indications of adhesively
bonded anterior and posterior single units. While for our 0.5 mm data, it is recommended to be
used for adhesively bonded anterior units.
137
Table 68: ISO 6872-2015
(151)
recommended clinical indications according to the flexural strength
Class Clinical indication
Flexural
strength (MPa)
1 Veneering and adhesively bonded anterior single units 50
2 Adhesively bonded single unit anterior and posterior 100
3 Non-adhesively bonded single unit anterior and posterior 300
4
Non-adhesive substructure for single and multi-unit anterior and
posterior (premolar)
300
5 Substructure for three-unit posterior 500
6 Substructure for four-unit posterior 800
The mechanical strength of dental materials determines the clinical success of dental
restorations. Dental restorations are subjected to compressive, tensile, and shear stresses under
clinical conditions.
(176)
Differences in mechanical properties for materials with similar chemical
compositions are most likely related to varying microstructures.
(43)
Glass-matrix ceramics have two types of interfaces: between crystalline phases and
between the crystalline phase and the glassy matrix.
(78)
By adjusting the refractive indices of the
crystalline phase and the glassy matrix, as well as the light path at the interface between these two
phases, one can modify the translucency of a lithium disilicate ceramic.
(43)
The ceramic's strength
may vary depending on its crystalline phases.
(59)
Discrete structural planes in crystals induce
cracks to deflect, branch, or split.
(59)
Flexural strength and fracture toughness of glass- ceramics
are controlled by crystal size and the ratio of the glass matrix to the crystalline phase.
(43)
Crystal size:
The crystal size performs dual effects on the flexural strength of the glass-ceramic: the
“interlocking effect” caused by the larger-sized crystals, as well as a “micro residual stress effect”
related to the balancing tensile stresses in the glass matrix.
(71)
The interlocking effect of crystals
is a mechanism strengthening for glass ceramics.
(76, 177)
The irregular rod-like lithium disilicate
138
crystals in the glass-ceramics forms interlocking microstructures, which could retard crack
progression in the glass-ceramics, resulting in effective strengthening.
(78)
Figure 60 demonstrates the strengthening mechanism, in which in most cases the crack
propagates along with the crystals/glass phase interface, rather than traverse the dispersed crystals.
(178)
In other words, the crack occurs along the crystal's boundaries rather than through the crystals.
Figure 60: Interlocking effect in glass–ceramics
The crystal size of the lithium disilicate depended on the crystal size of the lithium
metasilicate crystals. Larger lithium metasilicate crystals produced finer lithium disilicate crystals
because these lithium metasilicate crystals provided more nucleation sites for lithium disilicate
crystals.
(179)
Crystals with a larger size may provide a stronger mechanical strength through the
development of a more organized microstructure.
(180)
In this study, the crystal size of lithium disilicate for the selected materials has been
reported previously as the following: IPS e.max CAD (1–1.5 μm), n!ce (1 µm), Initial LiSi (0.3
μm), and Amber Mill (0.2 μm).
(181, 182)
Even though our results indicate that the highest flexural
strength was observed in IPS e.max CAD, followed by Amber Mill, then n!ce, and lastly Initial
LiSi; other factors influence the interlocking effect such as the ratio of the glass matrix to the
crystalline phase.
139
Crystalline phase:
The ratio of the glass matrix to the crystalline phase of the tested materials has been
reported previously as the following: IPS e.max CAD contains about 30% glass phase and 70%
crystalline phase.
(183)
While Amber Mill contains 47.7% glass phase and 52.3% crystalline phase.
(183)
For Initial LiSi, the glass phase is about 61.5% while the crystalline phase is 38.5%.
(183)
whereas n!ce contains about 20% glass phase and 80% crystalline phase.
(183)
The higher the reinforcing crystalline components, the more improved the mechanical
properties and vice versa.
(184)
This explains the significant difference in the values of biaxial
flexural strength values among the tested materials.
However, the amount of lithium disilicate Li2Si2O5 crystals between the tested materials is
an indicator of the observed results. IPS e.max CAD has 62.6% LD crystals, Amber Mill has 46.1%
LD crystals, Initial LiSi has 29%LD crystals, and n!ce has 28.5% LD crystals.
The amount and size of lithium disilicate crystals validate our biaxial flexural strength data
(Figure 61). The large size (1-1.5 µm) and amount (62.6%) of LD crystals for IPS e.max CAD
justify its high biaxial flexural strength values. For Amber Mill, the size of the small LD crystals
(0.2 µm) is compensated by the amount of LD crystals (46.1%) dispersed within the glassy matrix.
On the other hand, Initial LiSi had a small LD crystal size (0.4 µm) and a low amount of LD
crystals (29%) dispersed in a relatively high glass matrix (61.5%). However, for n!ce, the large
size of crystals (1 µm) did not compensate for the low amount of LD crystals (28.5%).
140
Figure 61: The ratio of the glass matrix to the crystalline phase of the tested materials as well as lithium disilicate crystal content
and size
Our biaxial flexural strength data obtained for IPS e.max CAD, Amber Mill, Initial LiSi,
and n!ce were relatively lower than the values claimed by the manufacturers. IPS e.max CAD has
been reported to have a mean flexural strength value of 530 MPa
(185)
, while in this study it was
234.79 ±42.86 MPa after three firings. Amber Mill as well was disclosed by the manufacturer to
have a mean flexural strength value of 450 MPa
(4)
, while out results indicated that it was 251.08
±83.56 MPa after three firings. Initial LiSi also was reported by the manufacturer to have a mean
flexural strength value of approximately 400 MPa
(88)
, whereas in our data it was 121.62 ±23.09
MPa after three firings. Similarly, n!ce was stated by the manufacturer to have a mean flexural
strength value of 350 ±50 MPa
(87)
, however, the results of this study showed a value of 164.80
±39.13 MPa after three firings.
Multiple factors influence these differences seen between the reported flexural strength
values from the manufacturers and our data. These include the testing method, specimen
dimensions, test environment, polishing procedures, stress rates, and stress area.
(186)
In general, the flexural strength of the lithium disilicate glass ceramics evaluated in the
present study fell within the ranges from 189.19 to 529.46 MPa as reported previously in the
literature.
(2, 3, 6, 8, 9, 12, 14-17, 19-23)
141
A summary of previous studies investigating the flexural strength of lithium disilicate
according to the test method, thickness, variable, and reported flexural strength, Weibull modulus,
and characteristic strength is given in Table 69.
142
Table 69: Summary of previous studies investigating the flexural strength of lithium disilicate
Authors Material Test method
Thickness
(mm)
variable Flexural strength (MPa)
Weibull
modulus
Characteristic
strength
(MPa)
Material Comparison
Sedda, 2014
(2)
IPS e.max CAD Three-point bending test 1.2±0.2
Comparing FS of lithium
disilicate to other types of
ceramics
336.06 ±40.09 9.0 355.27
Lien, 2015
(8)
IPS e.max CAD Three-point bending test 1.2±0.2
Comparing FS of lithium
disilicate to experiments
lithium disilicate
367 ±44
Leung, 2015
(9)
IPS e.max CAD Three-point bending test 2.00
Comparing FS of lithium
disilicate to other types of
ceramics
341.88 ±40.25 10.01 359.17
Fonzar, 2017
(12)
IPS e.max CAD
IPS e.max Press
Biaxial flexural strength 1.2±0.2
Comparing BFS of CAD/CAM
and heat-pressed
345.74 ±68.00
344.35 ±65.94
9.21
6.39
359.12
340.32
Yin, 2019
(14)
Amber Mill Biaxial flexural strength 1.2
Comparing BFS of lithium
disilicate
529.46 ±63.67 8.63 631.1
Wang, 2019
(16)
IPS e.max CAD
IPS e.max Press
Biaxial flexural strength 1.2±0.2
Comparing BFS of CAD/CAM
and heat-pressed
295.8 ±48.9
321.5 ±34.5
Longhini, 2019
(18)
IPS e.max CAD Biaxial flexural strength 1.2
Comparing BFS of lithium
disilicate to other types of
ceramics
248.6 ±6 37
Stawarczyk, 2021
(21)
IPS e.max CAD
Amber Mill
n!ce
Biaxial flexural strength 0.95
Comparing BFS of lithium
disilicate
418 ±57.6
308 ±57.2
206 ±27.7
6.54
5.71
7.55
442
365
189
Mangla, 2022
(22)
IPS e.max CAD
Amber Mill
CEREC Tessera
Biaxial flexural strength
1.00 ±
0.05
Comparing BFS of lithium
disilicate
459.72 ±52.39
286.07 ±75.93
344.87 ±49.19
Demirel, 2022
(23)
IPS e.max CAD
CEREC Tessera
Biaxial flexural strength 1.2
Comparing BFS of lithium
disilicate based on coffee
thermocycling
424.3 ±52.26
463.22 ±48.55
143
Firing
Kang, 2013
(3)
IPS e.max CAD
Rosetta SM
Biaxial flexural strength 1.2
Comparing BFS before firing
and after
B:234.0 ±49.5 A:408.3 ±85.9
B:204.2 ±47.0 A:443.5 ±64.3
Miranda, 2020
(6)
IPS e.max CAD Biaxial flexural strength 1.2±0.2
Comparing BFS of lithium
disilicate based on repeated
firings
One firing 208.94 ±31.05
Three firings 241.58 ±49.39
Five firings 247.24 ±21.41
Subaşı, 2022
(11)
IPS e.max CAD Three-point bending test 1.2
Comparing FS of lithium
disilicate based on repeated
firings
Baseline: 145.04 ±42.58
One firing: 139.92 ±49.54
Three firings: 188.99 ±77.62
Thickness
Hussien, 2022
(13)
IPS e.max CAD
Rosetta SM
Biaxial flexural strength
1.00
0.5
Comparing BFS of lithium
disilicate based on thickness
after thermocycling
0.5: 209.06 / 1.00: 480.91
0.5: 198.30/ 1.00: 461.95
Surface treatment
Zogheib, 2011
(15)
IPS e.max CAD Three-point bending test 2.00
Comparing FS based on the
effect of hydrofluoric acid
etching duration (control, 20s,
60s, 90s, 180s)
Control: 417 ±55
20s: 367 ±68
60s:363 ±84
90s: 329 ±70
120s: 314 ±62
Menees, 2014
(17)
IPS e.max CAD Three-point bending test 2.50
Comparing FS based on the
effect of particle abrasion and
hydrofluoric acid etching
Control: 376.5
30 µm Al 2O 3 at 300 kPa: 241.9
9.5% HF 120s: 353.5
Tribst, 2020
(19)
IPS e.max CAD Biaxial flexural strength 1.2
Comparing BFS of lithium
disilicate based on effect of
surface treatment(10% HF,
self-etching primer)
10% HF:491.8 ±47
Self-etching primer:499 ±17
13.36
24.36
517.69
506.73
Lima, 2021
(20)
IPS e.max CAD Biaxial flexural strength 1.2±0.2
Comparing BFS of lithium
disilicate based on effect of
surface treatment (control,
10% HF, 50 µm Al 2O 3,
silicatization, self-etching
primer)
Control: 236.21 ±34.63
10% HF : 289.30 ±40.11
50 µm Al 2O 3: 201.31 ±12.44
Silicatization: 189.19 ± 31.49
Self-etching primer: 298.87
±53.29
8.9
9.37
19.3
7.17
6.4
248.4
303.7
206.6
201.3
320.1
144
4.2. Firing:
Due to the influence of firing/heat treatment on the mean biaxial flexural strength, the
second null hypothesis was rejected.
The results of this study showed that the mean biaxial flexural strength values of the lab-
side materials are higher compared to the baseline chair-side materials, for both thicknesses, and
in all firing conditions. In addition, the peak means biaxial flexural strength was seen after three
firings for all materials, followed by five firings, then one firing, and lastly baseline. Overall, the
difference between the BFS values among the firings was statistically significant.
Several studies have investigated the effects of temperature or firing conditions on various
dental porcelain systems.
(138, 187-189)
It has been reported previously that the firing parameters
including cycle, temperature, heating rate, holding time, and cooling time all affect the distribution
of the glass and crystal phases in the microstructure.
(188, 190)
Subsequently, there have been
numerous studies and reports on the effects of temperature or firing techniques on ceramic systems.
(138, 187-189)
In IPS e.max CAD, repeated firings did not affect the BFS significantly in 0.5 mm.
However, repeated firings significantly decreased the BFS after five firings compared to one firing
in 1.00 mm. This indicates that repeated firings of IPS e.max CAD decreased the BFS values only
after five firings.
Amber Mill did not reveal a significant difference in repeated firings with one-way
ANOVA in both thicknesses. However, by using the contour plot, repeated firings significantly
increased after three firings compared to one firing in 1.00 mm. This indicates that repeated firings
of Amber Mill increased the BFS values after three firings.
145
Initial LiSi did not reveal a significant difference in repeated firings with one-way
ANOVA in 0.5 mm. However, by using the contour plot, repeated firings significantly increased
the BFS after five firings compared to the baseline. In 1.00 mm, three firings significantly
increased the BFS compared to one firing. Also, five firings were significantly increased compared
to the baseline and one firing. This indicates that repeated firings increased the BFS value of Initial
LiSi.
For n!ce, ANOVA revealed a significant increase in the BFS after three firings compared
to the baseline of 0.5 mm. In 1.00 mm, ANOVA revealed that three firings significantly increased
the BFS compared to the baseline. Also, five firings were significantly increased compared to one
firing. However, by using the contour plot, repeated firings significantly increased the BFS after
one, three, and five firings compared to the baseline. Also, repeated firings significantly increased
the BFS after three firings compared to one firing. This indicates that repeated firings increased
the BFS value of n!ce.
Effect of repeated firings:
Although the manufacturers of “lab-side” lithium disilicate glass-ceramic claim that only
a single firing is needed, additional correction firings may be required in clinical practice. Ceramic
restorations may need sometimes need to be fired more than normal due to several reasons such as
the change of the color, optimizing the esthetics, and the requirement of ceramic addition.
(191)
During these operations, the material is exposed to high temperatures and hot-cold cycles.
(191)
At the end of each firing cycle, the restoration is allowed to cool to room temperature.
(192)
The
thermal behavior during the heating and cooling cycles could stress the restoration.
(193)
146
Changes in the morphology of the crystalline structure of lithium disilicate glass-ceramics
after heat treatments at different temperatures and times have previously been reported. The
repeated firing had no significant effect on the composition of the crystalline phases
(8)
but affected
crystal size
(194, 195)
and flexural strength
(195)
significantly.
A study evaluated the effect of repeated firings on the hardness and LD crystal size of IPS
e.max Press.
(194)
They had four groups (one firing, two firings, three firings, and four firings).
(194)
They found that the degree of hardness increased with repeated firings, however, they exhibit
greater abrasion resistance and are difficult to polish, and are difficult to roughen the surface during
surface treatment.
(194)
Interestingly, they found that the crystal size increased from one firing (60.7 nm) to two
firings (66.1 nm), then increased in three firings (65.6 nm), but then decreased in four firings (33.2
nm).
(194)
In this case, this might be an explanation for lower mean BFS values in our data after the
third firing. Their SEM analysis revealed that fully crystallized specimens that received one to four
firing cycles showed a highly interlocking microstructure consisting of long-needle-shaped lithium
disilicate crystals that were exposed by the dissolving of the glass matrix after etching with HF
acid.
(194)
Another study evaluated the effect of firing times for an experimental non-heat-pressed
LDS using a two-stage crystallization process and changing the time of firing from 1 hour up to 8
hours.
(195)
They found that the length and width of the crystals increased continuously from 2.60
± 0.62 μm to 4.70 ± 0.87 μm and from 0.49 ± 0.10 μm to 0.64 ± 0.17 μm, respectively, with the
firing time.
(195)
The flexural strength demonstrated a significant increase from 1 hour to 6 hours
but decreased afterward.
(195)
147
Other studies have reported the effects of firing on the microstructure of lithium disilicate-
glass ceramic in which the ceramic became denser, the voids in the microstructure disappeared
after sintering
(195, 196)
, and the porosity increased after heat-pressing.
(197)
However, it was also
reported in another study that a homogeneous surface consisted of a dense union between glassy
and crystalline phases without voids or gaps.
(198)
Therefore, to address whether multiple firing
cycles would influence the microstructure of lithium disilicate glass-ceramic, which in turn would
potentially impact the mechanical properties, SEM was used to observe the microstructure and
morphology of the ceramic surface.
The effects of multiple firings were evaluated in another study
(199)
, according to the
number of firings: baseline, one firing, two firing, and three firings on the flexural strength of
lithium disilicate ceramics (IPS e.max Press and IPS e.max CAD) using three-point bend test in 2
mm thickness. In this study
(199)
, compared the repeated firings following the full crystallization
firing cycle. IPS e.max Press showed a higher mean BF (318.02 MPa) at two firings.
(199)
Whereas
the lowest was (278.53 MPa) at two firings.
(199)
IPS e.max CAD showed the highest mean BF
(307.82 MPa) at baseline.
(199)
While the lowest was 275.23 MPa at two firings.
(199)
They
determined that the repeated firing processes did not affect the flexural strength of the specimens
in either group
(199)
. The differences between our data are related to the use of a different testing
method and increased specimen thickness.
Repeated firings have been reported in an investigation comparing IPS e.max CAD and
VITA Suprinity (one firing, four firings, and six firings).
(200)
The repeated firings did not affect
the flexural strength of zirconia-reinforced lithium silicate glass-ceramic; however, for lithium
disilicate glass-ceramic, the flexural strength increased significantly after one firing.
(200)
After one
148
firing, IPS e.max CAD had a mean BFS of 259.39 MPa.
(200)
However, it was 277.97 MPa and
318.96 MPa after four and six firings, respectively.
(200)
In studies on the repeated firing effect of glass-ceramics, firing periods were limited to between
one and nine times.
(138, 187-189)
The first firing helps to eliminate microcracks and release the
stresses caused by the grinding and polishing procedures, as recommended by the manufacturers.
(138, 187-189)
The second and third firings are thought to be critical to producing the staining or
layering technique for the restoration, prior to placing it inside the patient’s mouth. (136, 186-188)
The fourth and subsequent firings are required when further shape and color corrections are
needed. After the third firing, it is expected that the restoration is ready to be placed inside the
patient’s mouth.
(138, 187-189)
The final four firings, after the third firing, were supposed to be
necessary only in case further shape and color adjustments for the ceramic restoration are needed.
(138, 187-189)
Effect of glaze:
Dental restorations with glaze finishes have been thought to have a strengthening effect by
reducing the sharpness or depth of surface flaws.
(188)
Nevertheless, according to several studies,
glazing did not affect strength. This may be because internal microstructure alone does not always
dictate strength; instead, strength is determined by the surface condition combined with the interior
microstructure.
(201)
Furthermore, it was reported that leucite-reinforced feldspathic porcelains might not
become stronger due to the glazing impact of heat treatments.
(202)
The specimens in one research
were carefully polished with 1 m diamond paste.
(202)
Their results indicate that there is no
additional advantage from a short thermal glaze cycle over-polishing, suggesting that there may
not have been significant smoothness benefits with glazing with such a flawless surface.
(202)
They
149
conclude that while glazing alone does not give the material any more strength, a smooth finish
will achieve a similar strengthening effect.
(202)
A study evaluated the effect of multiple firings (one firing, three firings, and five firings)
on the biaxial flexural strength of IPS e.max CAD with 1.2 mm thickness according to the glaze
application.
(203)
They found that the mean BFS for the control group (no glaze) was (208.94 MPa)
at one firing, (241.58 MPa) at three firings, and (247.24 MPa) at five firings.
(203)
The mean BFS
was for the single glaze (181.07 MPa) at one firing, (185.71 MPa) at three firings, and (207.23
MPa) at five firings.
(203)
However, the double-glaze group was (126.88 MPa) at one firing, (125.02
MPa) at three firings, and (137.53 MPa) at five firings.
(203)
Their SEM analysis concluded that
stains cause a large degree of porosity on the surface by creating an amorphous glass layer on the
lithium disilicate ceramic, which is responsible for lowering lower biaxial flexural strength,
regardless of the application technique.
(203)
Extended glaze firing has been proposed which significantly increased the flexural strength
of leucite-reinforced ceramic after machining compared with manufacturer-recommended glaze
firing.
(136, 204)
In addition, it demonstrated that crack healing was greater in densely sintered
feldspathic, leucite-reinforced, and lithium disilicate glass ceramics compared to conventional
manufacturer-recommended glaze firing.
(136, 204)
A study compared the effect of adding glaze to IPS e.max CAD with thermocycling for
500,000 cycles to 1.4 mm thickness specimens.
(205)
For the extended glaze group, the same initial
temperature, preheating time, and temperature increase rate as those used for the conventional
glaze firing.
(205)
However, the dwell time was set to 15 minutes, and slow cooling was performed
by keeping the furnace closed until it reached 200 ºC.
(205)
They found that IPS e.max CAD had a
150
mean BFS of (134.7 MPa) at baseline without the application of glaze.
(205)
The BFS value
increased to (147.0 MPa) after the application of extended glaze.
(205)
Based on these findings, in our study, we did not utilize the application of glaze as its
application is subjective and we aimed to understand the effect of firing solely on the tested
materials. Clinically speaking, ceramics cannot be finished as well as polished to have only
micron-sized flaws; in these cases, glazing fills bigger, possibly sharp-edged flaws.
151
4.3. Thickness:
The thickness affected the mean biaxial flexural strength. Thus, the third null hypothesis
was rejected. Our results showed that increasing the thickness increased the mean biaxial flexural
strength.
Manufacturers of lithium disilicate reinforced glass-ceramic recommend an occlusal
thickness of at least 1.5 mm for monolithic as well as conventionally luted full crowns, while only
minimally invasive full crowns, as well as partial/tabletop posterior restorations appear to have an
occlusal thickness of 1.0 mm.
(1, 4, 87, 88, 185)
Lithium disilicate occlusal veneer designs that are ultrathin (less invasive than
manufacturer recommendations) have been used even when there is an occlusal clearance between
0.4 and 0.6 mm at the central grooves and 1.0 mm on the cusp tips.
(206, 207)
Our data presented a
similar effect of thickness with previous literature to the strength of CAD/CAM lithium disilicate
material.
(208-211)
A study found that crown material and thickness were the most important parameters
influencing fracture probability.
(208)
Another study found that the excellent durability of lithium
disilicate crowns was primarily due to the specimens' thickness, which reached 2.00 mm at the
occlusal surface, and the ability of CAD/CAM technology to produce homogenous blocks with
few flaws and microstructural defects.
(209)
They concluded that it is reasonable to expect a rapid
decrease in the load needed to cause a bulk fracture in CAD/CAM LD when the thickness is
decreased.
(209)
In another study
(210)
compared the biaxial flexural strength of disc-shaped specimens of
IPS e.max CAD in two thicknesses, 1.00 mm and 0.5 mm. Firing cycles were performed with the
152
application of a thin layer of glaze, then subjected to thermocycling of 5000 cycles
(210)
. They
found that the mean biaxial flexural strength values increase with the increase in thickness
(210)
.
Additionally, one study
(211)
evaluated four groups of ceramic thicknesses produced by
milling CAD/CAM lithium disilicate IPS e.max CAD blocks (0.5 mm, 1 mm, 1.5 mm, and 2 mm).
Their findings demonstrated that the mean load-to-fracture for 1.5 mm was much higher than for
1 mm or 0.5 mm, and they advised milled lithium disilicate crowns with a minimum thickness of
1.5 mm for single posterior restoration.
(211)
According to the study, there is a gradual increase in
the load to fracture across four different occlusal crown thicknesses (0.5 mm, 1 mm, 1.5 mm, and
2 mm).
(211)
In our study, we evaluate the biaxial flexural strength in 0.5 mm as minimal thickness for
ceramics clinically. However, because glass ceramic is brittle, bonding must be done with resin
cement, and the presence of an enamel substrate can improve the predictability of ceramic
restorations.
(212, 213)
The type of surface conditioning and bonding surface has an impact on how well ceramic
adheres to the tooth structure. The total-etch technique allowed restorations to be bonded to teeth
with a bond strength of up to 28 MPa in the enamel and 13 to 20 MPa in the dentin (depending on
the adhesive system used).
(214, 215)
Self-etching primers are now widely available from manufacturers, and they are meant to
make the adhesive bonding process simpler. Regarding these self-etching primers, various bond
strength values have been described in the literature. Regardless of the manufacturer, ceramic
bonded to enamel had a bond strength of roughly 26 MPa, but ceramic bonded to dentin had a
bond strength ranging from 15 to 29 MPa.
(215-217)
153
After each firing, the microstructures of the tested lithium disilicate reinforced glass-
ceramics should be analyzed in order to better understand if the flexural strength values increased
or decreased. To understand more about the long-term effectiveness of the evaluated material-
firing-thickness combinations, clinical studies should be conducted.
154
4.4. Weibull Analysis
A ceramic material's resistance, strength, and structural dependability are shown through
Weibull analysis.
(133)
. The mean biaxial flexural strength values do not reveal the spread or the
shape of the distribution of strength values. The Weibull modulus helps in understanding this.
Weibull modulus, which represents data scattering and is generally applied for the reliability of
strength data for ceramic materials, is related to the probability of failure.
(218)
The Weibull modulus of the multiple firing was observed to generally drop or stay within
a similar Weibull modulus, except in Initial LiSi 0.5 where the Weibull modulus was seen to
increase with repeated firings.
During firing, flaws or microcracks may be deflected due to the interlocking effect. Due to
the brittle nature of ceramic, these defects can produce significant differences in strength, typically
in the 10% range.
(219)
A material's tendency to fail predictably is estimated using the Weibull
modulus.
(219)
As ceramic is a brittle material, such flaws can cause wide variations in strength
results, commonly in the range of 10%.
(219)
The Weibull modulus is used as a forecaster of the
reliability of a material to fail in a predictable fashion.
The non-uniform distribution of flaws with a highly variable crack length and a wide
variation of strength may be the cause of a lower Weibull modulus, which indicates increased
variability in the strength data or unreliability in the material.
(137)
A low Weibull modulus reflects
a high variation in measured strength values and an increase in the probability that flaws will
interact to weaken a brittle material. The use of these materials for dental restorations will result
in a greater variation in fracture force and decreased reliability. Chair-side materials presented low
Weibull modulus in comparison with lab-side materials. Initial LiSi exhibited the lowest Weibull
modulus among all tested materials.
155
The Weibull distribution is based on several hypotheses, including the ones that all crack
angles are equally probable
(220)
and that ceramic fails at the "weakest link."
(218)
The distribution
of failures may deviate from a Weibull distribution pattern because some ceramics may fail quicker
from two nearby defects or fractures connecting rather than the greatest (theoretically weakest)
flaw. However, these assumptions may not always be accurate.
(221)
Initial fractographic analysis
of specimens prior to testing would determine the site and precise mode of fracture, however, it
was outside the scope of this research.
Aside from that, some authors have suggested that when comparing mean strength or mean
failure values, dentists would like to learn when 5% of failures have occurred as opposed to 50%
(mean strength) or even 62.3% (characteristic strength) of failures have happened.
(222, 223)
It is proposed to change this to a 5% predicted probability of failure to make it more
applicable to clinical situations.
(223)
Statistical analysis around the 5% failure cut-off will need
even more specimens to achieve statistical power. According to ISO 6872-2015, we chose to use
characteristic strength (63.2% failure rate) for statistical comparisons between groups, and we
recommend using the 5% failure rate threshold for clinical studies of ceramics rather than for
applications involving materials testing, which is what this study was not intended for.
The statistical significance of the difference between the two Weibull distributions is
assessed using a likelihood contour method
(224)
. Materials with 0.5 mm thickness do not appear to
be affected by the firing as much as materials with 1.00 mm. In our data, we observed an increased
in the Weibull modulus in IPS e.max CAD, LiSi, and n!ce with the increase in thickness. However,
Amber Mill resulted in a lower Weibull modulus with the increase in thickness. This has been
justified previously in the literature because the thickness of the specimen does not contribute to
the results of the Weibull distribution.
(225)
156
4.5. Strength Testing:
All-ceramic dental restorations have been widely employed to replace missing teeth due to
their superior esthetic appearance, durability, as well as biocompatibility. Ceramics generally have
high compressive strength relative to low tensile strength.
(153)
Tensile strength, thus, plays a
significant role in the clinical effectiveness of dental restorations.
(153)
Tensile stresses caused by compressive occlusal forces can cause cracks to spread from
surface flaws and porosities down the inner surface of ceramic crowns.
(132)
Early restoration
failures may be caused by surface flaws typically introduced during the manufacturing process.
(132)
A given ceramic material's distribution of fracture sizes, shapes, and orientation varies from
specimen to specimen, and the strength of that material is statistically distributed in accordance
with the distribution of defect sizes.
(132)
Strength has frequently been the first measure investigated to determine the clinical
potential and limits of dental ceramics, even though the relationship between the mechanical
properties and clinical performance of ceramic material may be influenced by several factors. The
flexural strength of ceramic materials can be evaluated using various testing techniques. The three-
point-bend, biaxial flexural testing, and four-point bend are the three main strength-testing
methods used in dentistry today (ISO 6872-2015).
(151)
Since a greater volume of material is loaded
during four-point bending than during three-point testing, it is more representative.
(149)
The benefit of the biaxial flexural method is that the specimen is relatively insensitive to
specimen geometry and defect orientation.
(129)
It is independent of edge flaws, which are typical
in bar testing and can affect the strength outcome.
(187, 201)
This is significant since defect size is
known to impact strength, and whether striations in bar specimens are lateral or longitudinal during
grinding and polishing influences strength outcomes favorably or unfavorably.
(155)
157
Piston-on-ring and piston-on-three-ball testing are the two types of biaxial testing; the latter
is more optimal because it incorporates less friction.
(155)
Therefore, the piston-on-three-ball biaxial
flexural strength test of ceramics is an established standard materials test (ISO 6872-2008)
(151)
and
we utilized this test to investigate the effect of multiple firing cycles on the strength of CAD/CAM
lithium disilicate reinforced materials in two different thicknesses.
Although the ISO standard (ISO 6872-2015)
(151)
defines the testing criteria, there is some
leeway for interpretation. As a result, many researchers have set up their testing apparatus slightly
differently. The loading rate (crosshead speed) range specified by the ISO standard is 1.0 ±0.5
mm/min, and the greater the rate (1.5 mm/min), the better the strength outcomes in general.
(201)
The piston diameter is another factor that has a direct relationship with results and has been
suggested to influence the results with smaller piston diameter higher strength results may be seen.
(200) In ISO 6872, the piston diameter is defined as 1.4±0.2mm.
(151)
There is always a chance of
misalignment when using a guided piston instead of a self-aligning contact, and damage might
occur to the piston tip during fracture. Also, we have employed a 3D-printed positioning device to
guide the specimens in the center of the three balls.
The strength of a material suggests its dependability under force over-time to the casual
observer. The measured strength value, however, was obtained under specific experimental
conditions rather than being an intrinsic feature of the material. Asking for the correct strength has
no value because it relies not only on the material but also on the manufacturer, the production
process, and numerous other factors.
Strength values are usually applied as structural performance indicators. When it comes to
dental ceramics, understanding the surface and internal microstructure, processing history, testing
methods, and environment are necessary to put these strength statistics into context.
158
Chapter 5: Conclusions
Under the conditions and limitations of the present study and based on the findings presented,
the following conclusions can be drawn:
• Although multiple firings changed the biaxial flexural strength of IPS e.max CAD, Amber
Mill, LiSi Initial, and n!ce, they can still be considered clinically acceptable.
• Lab-side materials presented higher biaxial flexural strength compared to chair-side
materials.
• The biaxial flexural strength values for the CAD/CAM lithium disilicate reinforced
material was: IPS e.max CAD=Amber Mill >n!ce> Initial LiSi.
• The repeated firings of IPS e.max CAD decreased the biaxial flexural strength values only
after five firings in 0.5 mm and 1.00 mm.
• The repeated firings of Amber Mill increased the biaxial flexural strength values after three
firings in 1.00 mm. However, the repeated firings was not affected in 0.5 mm.
• The repeated firings of Initial LiSi and n!ce increased the biaxial flexural strength values.
• Generally, higher biaxial flexural strength values were obtained after three firings,
followed by five firings, then one firing, and lastly baseline.
• The firings of the tested materials indicated a significant difference in the biaxial flexural
strength, regardless of the thickness.
• Higher biaxial flexural strength values were obtained in 1.00 mm specimens than 0.5 mm
specimens.
• Higher Weibull modulus and Weibull characteristic strength values were observed with
lab-side materials than in chair-side materials indicating a higher failure probability for
chair-side materials.
159
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Abstract (if available)
Abstract
Purpose: To evaluate the biaxial flexural strength of four CAD/CAM lithium disilicate reinforced glass ceramic; IPS e.max CAD (EX) and Amber Mill (AM) as “lab-side”, and Initial LiSi Block (LS) and n!ce (NC) as “chair-side” according to the effect of two thicknesses and repeated firings.
Material and methods: CAD/CAM blocks were prepared according to ISO 6872-2015. Four hundred and eighty (n=480) discs were prepared in total with a diameter of 12.00 mm (±0.02). Each selected material (n=120) was divided into two thicknesses: 1.00 mm (±0.03) and 0.5 mm (±0.02), having 60 discs for each thickness in each material. The specimens were subdivided according to firings: baseline (BL), one firing (1F), three firings (3F), and five firings (5F). The firings cycles were performed according to the manufacturers’ instructions. The biaxial flexural strength test (piston-on-three-ball) was performed according to ISO 6872-2015 using a universal testing machine.
Biaxial flexural strength data were analyzed using parametric tests: three-way and one-way ANOVA (α=0.05) with Bonferroni post-hoc test. Weibull analysis was used to calculate the Weibull Modulus and Characteristic Strength to create Weibull plots and likelihood contour plots.
Results: The biaxial flexural strength of the materials differed from each other (EX=AM>NC>LS). A significant difference was found between the firings, regardless of the thickness, and the general ranking of firings was (3F>5F>1F>BL). Higher thickness (1.00 mm) presented a higher biaxial flexural strength value. Higher Weibull modulus and characteristic strength values were observed with lab-side vs. chair-side materials.
Conclusions: Repeated firings significantly affected the biaxial flexural strength of EX, AM, LS, and NC CAD/CAM lithium disilicate materials. The biaxial flexural strength increased with increased thickness. Lab-side materials (EX and AM) have a lower probability of failure than chair-side materials (LS and NC).
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Alsaleh, Sarah
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Effect of repeated firings on biaxial flexural strength of different CAD/CAM lithium disilicate reinforced materials in two different thicknesses
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School of Dentistry
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Biomaterials and Digital Dentistry
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2022-12
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biaxial flexural strength
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