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Comparison of color stability in CAD/CAM and conventional complete denture materials

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Comparison of color stability in CAD/CAM and conventional complete denture materials

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Comparison of Color Stability in CAD/CAM and Conventional Complete Denture
Materials
by
Yun-Chu Chen, DDS

A Thesis Presented to the
FACULTY OF THE USC HERMAN OSTROW SCHOOL OF DENTISTRY
UNIVERSITY OF SOUTHERN CALIFRONIA
In Partial Fulfillment of the
Requirement for the Degree
MASTER OF SCIENCE
BIOMATERIALS AND DIGITAL DENTISTRY

December 2020

Copyright 2020 Yun-Chu Chen
ii
Acknowledgements
I would like to express my sincere appreciation to my advisors, Dr. Jin-Ho Phark and Dr.
Sillas Duarte. This research project would not have been accomplished without their suggestions
and help. Besides my advisors, I would like to thank Dr. Cheryl Park, a member of my thesis
committee, for her encouragement and insightful comments. The contribution of our statistician
Dr. Xiao-Ying Deng is truly appreciated in conducting the statistical analysis for this research.
Last but not least, I would like to thank my family, especially my parents, for their love and support
throughout my life.

iii
Table of Contents
Acknowledgements ......................................................................................................................... ii
List of Tables ................................................................................................................................. vi
List of Figures .............................................................................................................................. viii
Abbreviations .................................................................................................................................. x
Abstract .......................................................................................................................................... xi
Introduction ..................................................................................................................................... 1
Importance of complete dentures .............................................................................................................. 1
Conventional fabrication method for complete dentures .......................................................................... 1
CAD/CAM fabrication methods for complete dentures ........................................................................... 2
Denture base materials .............................................................................................................................. 4
Color science in dentistry ......................................................................................................................... 6
Color stability of denture base materials .................................................................................................. 9
Comparison of Color stability of different denture base materials ......................................................... 11
Aims ........................................................................................................................................................ 12
Materials & Methods .................................................................................................................... 13
Conventional PMMA denture base material (Group A) ......................................................................... 14
Milled PMMA denture base (Group B) .................................................................................................. 20
3D-printed PMMA denture base (Group C) ........................................................................................... 21
First color measurement – Baseline ........................................................................................................ 22
iv
Immersion of the specimens ................................................................................................................... 24
pH value measurement ........................................................................................................................... 26
Second color measurement – After immersion, before re-polishing ...................................................... 27
Third color measurement – After immersion, after re-polishing ............................................................ 27
Data management and statistical analysis ............................................................................................... 28
Results ........................................................................................................................................... 29
CIEL*a*b* color value ........................................................................................................................... 29
CIEDE2000 Color difference of denture base materials ........................................................................ 36
Discussion ..................................................................................................................................... 44
Color Stability of different denture base materials ................................................................................. 44
Staining ability of different immersing mediums ................................................................................... 45
Finishing and polishing of denture bases ................................................................................................ 46
Determining color change ....................................................................................................................... 48
Perceptibility and acceptability thresholds ............................................................................................. 49
Environmental factors ............................................................................................................................. 49
Limitations of the study .......................................................................................................................... 50
Conclusions ................................................................................................................................... 52
References ..................................................................................................................................... 53
Conflicts of Interest ....................................................................................................................... 64
Funding ......................................................................................................................................... 64
v
IRB ................................................................................................................................................ 64

vi
List of Tables
Table 1. Denture base materials (manufacturer data for group B and C) ..................................... 13
Table 2. Staining solutions in this study ....................................................................................... 27
Table 3. Baseline CIEL*a*b* color value (E1) ............................................................................ 29
Table 4. CIEL*a*b* color value after immersion, before re-polishing (E2) ................................ 30
Table 5. CIEL*a*b* color value after re-polishing (E3) .............................................................. 30
Table 6. Pictures of representative specimens from each group for three color measurements ... 32
Table 7. L* differences for each group, ∆L
1
: difference of L1 and L2, ∆L
2
: difference of L1 and
L3, ∆L
3
: difference of L1 and L3. ................................................................................................. 33
Table 8. a* differences for each group, ∆a
1
: difference of a1 and a2, ∆a
2
: difference of a1 and a3,
∆a
3
: difference of a1 and a3 .......................................................................................................... 34
Table 9. b* differences for each group, ∆b
1
: difference of b1 and b2, ∆b
2
: difference of b1 and
b3, ∆b
3
: difference of b1 and b3 ................................................................................................... 35
Table 9. CIEDE2000 color difference results ............................................................................... 36
Table 10. Results of Levene's test ................................................................................................. 37
Table 11. Three-way ANOVA results .......................................................................................... 38
Table 12. Independent t-test for material A and B ........................................................................ 38
Table 13. Independent t-test for material B and C ........................................................................ 39
Table 14. Independent t-test for material A and C ........................................................................ 39
Table 15. Independent t-test for beverage 1 and 2 ........................................................................ 39
Table 16. Independent t-test for beverage 2 and 3 ........................................................................ 39
Table 17. Independent t-test for beverage 1 and 3 ........................................................................ 40
Table 18. Independent t-test, material A, subgroup 1 and 2 ......................................................... 40
vii
Table 19. Independent t-test, material A, subgroup 2 and 3 ......................................................... 40
Table 20. Independent t-test, material A, subgroup 1 and 3 ......................................................... 41
Table 21. Independent t-test, material B, subgroup 1 and 2 ......................................................... 41
Table 22. Independent t-test, material B, subgroup 2 and 3 ......................................................... 42
Table 23. Independent t-test, material B, subgroup 1 and 3 ......................................................... 42
Table 24. Independent t-test, material C, subgroup 1 and 2 ......................................................... 43
Table 25. Independent t-test, material C, subgroup 1 and 3 ......................................................... 43
Table 26. Independent t-test, material C, subgroup 2 and 3 ......................................................... 43

viii
List of Figures
Figure 1. Polymerization process of MMA. ................................................................................... 5
Figure 2. Munsell color solid .......................................................................................................... 7
Figure 3. CIELAB color space ........................................................................................................ 7
Figure 4. CAD/CAM block used for creating the mold ................................................................ 15
Figure 5. Mold made from duplicating silicone materials ............................................................ 15
Figure 6. Wax pattern created from the mold ............................................................................... 15
Figure 7. The bottom layer of the flask filled with plaster; two wax patterns were placed. ......... 16
Figure 8. Acrylic resin dough packed into the microstone in the flask. ....................................... 18
Figure 9. A digital caliper was used to confirm dimension of the specimens. ............................. 19
Figure 10. Specimen number marked at the corner of one surface of each specimen. ................. 20
Figure 11. Milled PMMA block stabilized for sectioning. ........................................................... 20
Figure 12. Milled PMMA blocks cut into rectangular pieces. ...................................................... 21
Figure 13. Three-dimensional object created by SketchUp Pro ................................................... 22
Figure 14. An example of image captured by the spectrophotometer in this study. Outline of the
square specimen was selected to measure the color value at the center of the surface. ............... 23
Figure 15. Color data analysis using Crystaleye software. L*, a*, b* value of the target area at
the center of the milled denture base specimen were obtained from the software. ...................... 24
Figure 16. Subgroup 1 specimens immersed in coffee medium in the water bath device. ........... 25
Figure 17. Subgroup 2 specimens spread out in the container and 120 ml red wine was poured for
each immersion. ............................................................................................................................ 25
Figure 18. Plastic containers used to store specimens in the laboratory incubator. ...................... 26
Figure 19. ∆L for each group ........................................................................................................ 34
ix
Figure 20. ∆a for each group ......................................................................................................... 35
Figure 21. ∆b for each group ........................................................................................................ 36
Figure 22. CIEDE2000 color difference of each group ................................................................ 37

x
Abbreviations
3D Three-dimensional
ANOVA analysis of variance
AT acceptability threshold
CAD/CAM Computer-aided design/computer-aided manufacturing
CAM Computer-aided manufacturing
CIE Commission International de I’Eclairage
CIEDE Commission International de I’Eclairage Delta E
CNC computer numerical control
DC degree of conversion
FDM fused deposition modeling
PMMA polymethyl methacrylate
PT perceptibility threshold
SLA stereolithography
SLM selective laser melting
SLS selective laser sintering
VPS vinyl polysiloxane

xi
Abstract
Objectives: The purpose of this in vitro study is to compare the color stability of subtractively and
additively manufactured denture bases against that of conventionally heat-activated polymethyl
methacrylate (PMMA) complete denture bases.
Methods: A total of 135 complete denture base specimens (dimension of 14±0.1 x 14±0.1 x 2±0.1
mm) were fabricated using three different methods (n=45 per group): A. conventional heat-
activated, B. milled, and C. 3D-printed. After immersing in 37°C distilled water for 24 hours,
specimens were dried with filter paper. Baseline color measurement using Commission
International de I’Eclairage (CIE) color parameters L*, a*, b* was repeated three times for each
specimen by using an imaging spectrophotometer (Crystaleye® Spectrophotometer, Olympus,
Tokyo, Japan) on a white background. The specimens were immersed in three different solutions:
60°C coffee (VIA Pike Place Roast, Starbucks Coffee Company, Seattle, WA, USA) (Subgroup
1), 18°C room temperature red wine (2017 Cabernet Sauvignon, Charles Shaw, CA, USA),
(Subgroup 2) and 37°C distilled water (Subgroup 3); fifteen specimens from each group were
tested for each solution. Specimens from group A and B were completely immersed into the
designated solution for 15 minutes twice per day for 30 days; the solutions were refreshed for
every immersion. All specimens were kept in a laboratory incubator at 37°C between each
immersion. Specimens from group C were immersed into distilled water and stored in 37°C
laboratory incubator, the water was refreshed twice per day. After 30 days, all specimens were
removed from the medium and the second color measurement was performed before re-polishing.
All specimens were then polished with a wet rag wheel and ultra-fine pumice before the third
measurement was performed. For standardization, all measurements were conducted in the same
manner as described above.
xii
Statistical Analysis: All statistical analyses were computed using Microsoft Excel software (Excel;
Microsoft Corp, Redmond, WA, USA). The CIEDE2000 color difference (ΔE00) between the color
coordinates was calculated by applying the CIEDE2000 color difference formula. (1) This allowed
for comparison of color values between the three different measurements described previously. A
three-way ANOVA was conducted among three variables at 95% confidence level (α=.05).
Independent t-tests were performed to determine the statistical difference between different
materials, different staining solutions (α=.05) and within each group.
Results: There was significantly less color change in conventional and milled denture base
materials compared to the 3D-printed denture base resin (p<0.001) with no significant difference
between conventional and milled PMMA (p>0.05). There was no significant difference in extrinsic
and intrinsic color change among all groups of materials. Overall, coffee produced more color
change when compared to red wine and distilled water after 30 days of immersion period (p<0.05).
No significant difference in color change was detected between red wine and distilled water.
Conclusion: Within the limitation of this study, the color stability of conventional and milled
PMMA was superior to 3D-printed denture base materials. The color stability of milled denture
bases was comparable to traditional PMMA denture base, both were within the
perceptibility/acceptability thresholds of 1.71/4.00 ΔE00. Specimens stained with coffee had more
color difference when compared to red wine and water after 30 days of immersion period. No
significant color difference in extrinsic and intrinsic color changes was found for all the materials
in this study.
Clinical Significance: Good color stability of denture materials is essential to meet patients’
esthetic requirements. Change in color may also indicate aging of the denture materials, which
may compromise their mechanical strength. This in vitro study compared the color stability of
xiii
different denture base materials after exposure to different mediums, a factor important for
complete denture patients with high esthetics demand.
Keywords: 3D-Printing; additive manufacturing; milling; subtractive manufacturing; complete
denture; color stability
1
Introduction
Importance of complete dentures
Implant treatment has gained popularity in the treatment of edentulous patients in the past
decades to meet the esthetic and functional needs of patients. Long-term studies have shown high
survival rates and high patient satisfaction with implant-supported fixed dental prostheses. (2)
However, the option of fabricating complete dentures is still critical for fully edentulous patients
with anatomical, physical, or financial limitations. (3) Complete dentures can also be utilized as
interim prostheses during the transitional stage of treatments, or as a valuable diagnostic tool when
planning implant-based restorative treatment.
Conventional fabrication method for complete dentures
Conventionally, complete dentures are fabricated in four or more clinical appointments.
This usually starts with preliminary impressions, followed by a second visit to capture the borders
and final impressions of the oral tissues. (4) Baseplates and wax-rims are then made on stone casts
poured from the final impression and another appointment is required to register the maxillary and
mandibular jaw relation at the desired vertical dimension of occlusion. (4) For esthetic evaluation,
the artificial maxillary anterior teeth are usually set on the wax-rim and they are tried in the
patient’s mouth to evaluate the occlusal plane, midline, extraoral profile, and phonetics. (4) A
facebow transfer is then used to mount the casts in a semi-adjustable articulator to set the remaining
artificial teeth with the desired occlusal scheme. (4) Generally, the definitive complete dentures
will be delivered at the fourth or fifth visit. Finally, further adjustment or reline of the dentures
may be needed during follow-up appointments, depending on the clinical situation, the patient’s
ability to adapt to the new prostheses, and maintenance. (4)
2

CAD/CAM fabrication methods for complete dentures
Computer-aided design/computer-aided manufacturing (CAD/CAM) technology has been
widely applied to the field of dentistry, especially in the fabrication of dental prostheses.
Computer-aided manufacturing (CAM) can be classified into two categories, subtractive
manufacturing and additive manufacturing. Subtractive manufacturing is normally referred to as
“milling”, this technique utilizes computer numerical control (CNC) to cut prefabricated material
blocks into 3D products. In contrast, additive manufacturing constructs 3D objects by deposition
of the material, layer by layer. (5) There are several types of additive manufacturing techniques,
the most common of which is stereolithography (SLA), which utilizes the photopolymerization
method, and is normally referred to as “rapid prototyping”. Other types of additive manufacturing
methods including fused deposition modeling (FDM) (depositing 3D objects via material
extrusion), powder bed fusion (including selective laser sintering (SLS) and selective laser melting
(SLM)) and jet printing techniques (such as binder jetting and material jetting). (6)
3D-printing of removable prostheses, such as complete dentures and removable partial
dentures is one of the common applications of digital dentistry. The first scientific literature
reporting the use of rapid prototyping in the fabrication of digital dentures was published in 1994.
(7) In 1997, another group of researchers introduced a digital method to duplicate existing dentures
and mill new dentures utilizing CNC technology. (8) There are several reported advantages
regarding the integration of a digital workflow into dental restorative procedures. They are
considered to be more time and cost efficient, and more comfortable for the patient. (9) In addition,
the existing problem of 6 to 7% volumetric shrinkage from the polymerization process of
conventional heat-processed complete dentures is eliminated when fabricating digital dentures
using either manufacturing technique. (10)
3
As mentioned previously, delivery of a conventional complete denture generally requires
more than four visits, while digital dentures in the current dental market generally require less than
that. (10) (11) There are several companies in the market that provide services for the fabrication
of digital dentures. The most common ones include Ivoclar Vivadent (Ivoclar Vivadent Inc., NY,
USA), AvaDent (Global Dental Science LLC, Scottsdale, AZ, USA), and Dentca (Dentca Inc.,
Torrance, CA, USA). Some companies propose delivery of complete dentures in as few as two
appointments, without try-in, or three appointments, including an appointment for trying of trial
dentures. (12) The first clinical appointment consists of procedures for data acquisition. Different
methods have been developed by different manufacturers; most of them have developed
instruments and materials for making final impression at this visit without the need for a
preliminary impression. The information needed for traditional dentures such as maxillary and
mandibular jaw relation, desired vertical dimension, lip support, position of incisal edge, midline,
and occlusal plane can be established during the first patient visit as well. A final denture can be
delivered at the second patient visit if a trial denture visit is not attempted. (12) The digital denture
workflow has been described in a publication by Bonnet et al. (13) in 2017. The use of a patented
“Centric Tray” is utilized for making impressions at the desired vertical dimension of occlusion
and the “UTS CAD” device can be connected to the centric tray to check the occlusal plane at the
same visit to facilitate the treatment process.
In terms of accuracy, in vitro studies show that complete dentures made by milling
techniques are comparable to those made by traditional pack-and-press techniques. (14) (15) The
area of lowest accuracy on the intaglio surface was found in the posterior palatal seal area and
border seal area in a recent systematic review of in vitro studies. (15) Better accuracy and retention
of the denture base was found in milled dentures as compared to traditional heat-processed
4
dentures. (16) When comparing the adaptation of denture bases to the underlying oral tissues,
milled dentures were shown to have better adaptation compared to 3D-printed dentures in a recent
in vitro study. (17) In another similar study, conventional heat-activated dentures had better
accuracy and retention than 3D-printed dentures. (18)
When comparing the two different CAD/CAM manufacturing methods, additive
manufacturing has an additional advantage of reducing material waste in comparison to subtractive
manufacturing, since there is always excess material leftover after milling from prefabricated
blocks. (5) (19) Furthermore, fine detail reproduction from additive manufacturing may be
superior because of inherent limitations related to the cutting axes of existing milling machines.
As a result, 3D-printing has become more popular in the fabrication of prostheses and other fields
of dentistry.
Denture base materials
There are several requirements for an ideal denture base material. These include good
physical properties, dimensional stability, ease of repair, low water solubility, and color stability.
(4) Currently, the polymer that is used most commonly for fabricating denture bases is poly-methyl
methacrylate (PMMA) resin.
PMMA polymerization involves converting monomers into a polymer by free radical
polymerization. (Figure 1) The stages of polymerization include activation, initiation, propagation,
and termination. This process begins with initiation of the chain reaction by activators which
generates free radicles. The unpaired electron in these initiator radicals approach other monomers
and the propagation of the process occurs via repeated addition of the reactive center. Termination
of the polymerization process occurs by one of two different reactions, combination or
disproportionation. The termination by combination is the combination of two macromolecules
5
with free radicals to form a covalent bond. Conversely, disproportionation happens when a
hydrogen ion is transferred from one macromolecule to another and a double bond is formed. This
results in two different types of polymers as the end products. (20) At lower temperature,
termination occurs predominantly by the combination method while disproportionation happens
more at higher temperatures. (21) Finally, additional chemical bonds may be added during the
polymerization process between different polymerization chains. This form of interconnection is
called cross-linkage, and the degree to which it occurs also affects the physical properties of the
end product. (4)
The chemical formula of PMMA is (C5O2H8)n. PMMA is considered a lightweight polymer.
Its compressive strength is between 85 to 110 MPa and its tensile strength is reported as 30 to 50
MPa. (22) Heat is generated from the exothermic reaction of PMMA polymerization. The reported
volumetric shrinkage of PMMA is around 6-7% (23) and its maximum water absorption ratio is
reported as 0.3 to 0.4% by weight. (24) The degree of water sorption affects the material’s
mechanical properties as well.
(25)
Figure 1. Polymerization process of MMA.
PMMA can be heat-, microwave-, or light-activated. The most common method used for
denture fabrication is heat-activation. (26) This is an exothermic procedure (6) and good
temperature control is important. If the temperature exceeds the boiling point of un-reacted
monomers during processing, it may results in porosity in the end product. Increased porosity in
turn affects physical properties and susceptibility to discoloration. Porous denture surfaces result
6
in more staining, calculus deposition, and microorganisms’ attachment. (4) (27) Candida albicans
plays an important role in the etiology of denture stomatitis, and is found on acrylic resin surfaces
in vivo. Its adherence is affected by the surface roughness of the denture base material. (28)
Despite the advantages of using PMMA as denture base material, some drawbacks exist.
Polymerization shrinkage of the acrylic resin makes it susceptible to distortion, while porosity of
the material, water absorption, concerns regarding residual unreacted monomers and the
mechanical properties such as impact strength and flexural strength are other areas of concern. (29)
The additive manufacturing procedure of complete dentures is accomplished by layering
apposition of resin materials by curing with a laser, UV light, heat or visible light. (30) The
subtractive or CAD/CAM milling process, on the other hand, involves milling the complete
denture out of an industrially pre-polymerized PMMA blank. This blank is produced under high
pressure and some sources claim this results in superior mechanical properties and less residual
monomer compared to traditional PMMA. (31)
Color science in dentistry
It is the clinician’s goal to provide dental treatment not only to meet a patient’s functional
need, but also to satisfy a patient’s esthetics demand. Therefore, it is important that dentists
understand the theories and clinical applications in the field of color science in dentistry.
Several color space systems have been proposed and utilized in attempt to communicate
colors. The first quantitative color system for perceptions of color is the Munsell color system
proposed in 1905. (32) In this system, color perception can be described by three variables; hue,
which is the color name such as red, blue, and green; value represents the lightness of the color;
and chroma, which is the purity of the color. All perceptible colors stand a specific area of the
“Munsell color solid”. (Figure 2)
7

Figure 2. Munsell color solid
(https://munsell.com/about-munsell-color/how-color-notation-works/munsell-color-space-and-solid/) (33)
In 1931, a new color system was proposed by the Commission International de l’Éclairage
(CIE; International Commission on Illuminatio) to describe colors in X, Y, and Z tristimulus values,
based on the three primary colors: red, green, and blue. In 1976, the now-widely-used CIEL*a*b*
color space was proposed. In this space, L* indicates lightness of the color and a* and b* indicate
the red/green and blue/yellow chroma of the color, respectively, as shown in Figure 3. The
CIEL*a*b* color space contains all colors and is not mediated by human perception. Thus, it is
considered the most scientific color system used in material color testing. (34)

Figure 3. CIELAB color space
(https://sensing.konicaminolta.asia/what-is-cie-1976-lab-color-space/) (35)
The CIELab color difference (ΔE*ab) is calculated as follows: ΔE*ab = [(ΔL*)2 + (Δa*)2
+ (Δb*)2]1/2. (36) This formula has been widely used in different industries. However, in 2001,
an improved formula in calculating color difference was developed. This is now known as the
8
CIEDE2000 formula (ΔE00), which is reported to reflect the color differences perceived by the
human eye better than the previous CIELab formula (ΔE*ab). (37)
In terms of esthetics, it is important to match the color of fabricated prostheses with that
of surrounding oral structures. Shade guides such as the VITAPAN Classical and the VITAPAN
3D-Master shade guides (VITA Zahnfabrik, Bad Säckingen, Germany) are commonly used for
shade matching of teeth in dental clinics. (38) Fabrication of shade guides for denture base
mucosal colors were also reported in a previous study. (39) Besides the use of shade guides,
which is a subjective visual shade matching method, several objective methods have been
developed for color measurement in dentistry. These include spectrophotometers, colorimeters,
and digital cameras. (38)
Visual shade matching is affected by differences in human observers and the illumination
conditions in the surrounding environment. (40) Color measurement with a spectrophotometer as
an instrumental method is considered to be more accurate in shade matching compared to the
visual method. (41) In a clinical study, three dentist observers assessed shade and their accuracy
was found to be 33% less than a reference spectrophotometer. (42) Another method in color
measurement utilizes colorimeters that quantify color by measuring red, green, and blue values
according to the conditions of illumination. Colorimeters may potentially be less accurate than
spectrophotometers over time due to aging of the color filters. (43) The complexity and cost of
the instrumental color measurements of spectrophotometers and colorimeters limit their use in
the clinical setting; visual shade assessment is still the most widely used methods today.
To apply the technology of color quantification, it is essential to establish the clinical
parameters for visual significance. The perceptibility threshold is the tolerance of perceptibility
in color difference (44) (45), while the acceptability threshold is the tolerance of acceptability in
9
color change. (46) The perceptibility and acceptability threshold have been well studied in the
field of dental materials. For composite resins, the reported 50:50% acceptability threshold was
approximately 3.3 ΔE*ab units. This means that when the color difference was 3.3 ΔE*ab, there is
an equal probability for observers to accept or reject the color difference. (47) Another study
found that the 50:50% acceptability was 2.72 ΔE*ab units for dental restorations adjacent to a
tooth. (48) In a study using the newer CIEDE2000 color difference formula to investigate the
perceptibility and acceptability thresholds for acrylic denture base resins, the 50:50%
perceptibility and acceptability thresholds were found to be 1.71 ΔE00 and 4.00 ΔE00 respectively.
(49)
Color stability of denture base materials
Denture base resins should be color stable over the long-term and their color should match
that of the patient’s own oral tissues for optimal esthetic results. In addition, color stability is
important because it indicates the durability of the material. (4) Several factors have the potential
to effect color change in dental restorations, such as temperature and humidity of the oral
environment, food consumption and smoking habits. (50) The factors affecting the discoloration
of denture base materials and denture teeth can be classified as intrinsic or extrinsic factors. (51)
Extrinsic discoloration of acrylic resins occurs via penetration of colorants from exogenous
sources. This has been reported to occur with consumption of intensely-colored beverages such as
tea, coffee, and red wine. (52) On the other hand, intrinsic discoloration has root causes in the
material itself, including the type of material and processing method used. (45) Color change could
be an indicator of aging or damaged of materials in terms of clinical significance. (53) The color
stability of various different dental materials, including composite resins, interim restorative resin
materials, soft denture liners, and acrylic resin denture teeth have been investigated, multiple
10
factors including different beverages, accelerated aging, cleaning agents, and smoking are found
to be responsible in discolorations. (34) (54) (55)
Surface roughness of materials is influenced directly by the material’s structure, and the
finishing and polishing procedures employed. (56) This is in turn related to its susceptibility to
extrinsic staining. The degree of conversion and water sorption characteristics also play an
important role in the color stability of the resin materials. Color change in PMMA resin is greatly
influenced by the hydrophobicity of the monomers and their water sorption. (57) The water
sorption of acrylic denture resins is related to the polar properties of molecules, the diffusion of
water to the gaps between interpolymeric spaces, and the residual unreactive monomers. The size
and number of the interpolymeric gaps in the acrylic resins determine the amount of water
absorption. Materials strength is negatively influence by water sorption due to the plasticization
effect. (58) Plasticization happens when absorbed water molecules expand in the interpolymeric
gaps and separate the polymer chains, thus the staining solutions penetrate and lead to discoloration.
(59) The degree of water sorption can be reduced if the acrylic resin undergoes more cross-linking
during polymerization. (60) The acceptable amount of water sorption and water solubility for
PMMA resin should not exceed 32 and 1.6 μg/mm
3
, respectively according to ISO specification
No. 20795. (36)
The mechanisms of tooth discoloration were suggested by Eriksen et al. (55) Extrinsic
staining occurs via three major mechanisms: (a) production of colored components in plaque by
chromogenic bacteria; (b) retention of colored substances from dietary constituents passing
through the oral cavity; and (c) formation of colored products from the chemical transformation of
pellicle components. The ingredients in common beverages that are related to staining are
composed of polyphenols such as catechins and leucoanthocyanins in coffee and tea. Other factors
11
such as smoking may also cause tooth discoloration, as described in the literature. (61) Increased
risk for discoloration was found in red wine drinkers and cigarette smokers. (62)
Comparison of Color stability of different denture base materials
A previous study investigating the color stability of milled and conventionally made
denture base resins showed that CAD-CAM milled dentures had greater resistance to stain
accumulation at the tooth-denture base interface than conventionally processed dentures. (60) A
higher degree of discoloration was shown in additively manufactured denture resin compared to
milled and conventionally-processed resins in a recent in vitro study comparing the color stability
of CAD/CAM acrylic resin teeth to conventional acrylic resin. (63) Similarly, another study also
found that the color stability of 3D-printed resins were inferior to conventional composite resins
after aging. (64)
Most of the studies comparing color stability of additive manufacturing materials to
conventional materials were done with tooth-colored materials. Since limited studies were found
relating to the color stability of pink denture base resins, our aim is to compare the color stability
of additively manufactured denture base resins to conventionally processed and milled denture
base resins. Three null/hypotheses were proposed for this in vitro study:
1. There is no significant difference in the color stability of denture bases fabricated by
heat-activated polymerization (conventional method), milling, or 3D-printing.
2. There is no significant difference in the amount of color change caused by coffee, red
wine and distilled water.
3. There is no difference in color before and after re-polishing of the materials.

12
Aims
The color stability of complete denture bases made with additive manufacturing,
subtractive manufacturing and conventional processing was compared. Further comparison of the
staining effect of different beverages; coffee, red wine, and distilled water were made, and the
extrinsic and intrinsic color change of three different manufactured denture base materials were
investigated.

13
Materials & Methods
A total of 135 flat, square samples with dimensions of 14±0.1 mm x 14±0.1 mm x 2±0.1
mm were made out of three different denture base materials. The groups tested in this study
included conventional, heat-processed PMMA (n = 45, Group A), milled PMMA (n = 45, Group
B), and 3D-printed PMMA (n = 45, Group C). (Table 1)

Group A Group B Group C
Denture base materials Conventional PMMA Milled PMMA 3D-printed PMMA
Product name Lucitone 199 Denture Base
Resin, Dentsply Sirona, York,
PA, USA
IvoBase CAD Pink-V, 98.5-
30 mm, Ivoclar Vivadent
Inc., NY, USA
Dentca Denture Base II,
Dentca Inc., Torrance, CA,
USA
Shade Light red pink Pink-V Reddish pink
Composition -Powder:
Polymethylmethacrylate
-Liquid:
methyl methacrylate
ethylene dimethacrylate

Polymethyl methacrylate,
co-polymer for impact
toughness modification,
pigments.
Methacrylate monomer,
Diurethane dimethacrylate,
Trimethylolpropane,
trimethacrylate. Initiator
(proprietary), stabilizer
(proprietary), pigment
(proprietary).

Polymerization Heat-activated Industrial Light polymerization
Flexural strength 69.4 MPa (23) ≥ 65 MPa (65) ≥ 65 MPa (66)
Flexural modulus 2420 MPa (23) ≥ 2000 MPa (65) ≥ 2000 MPa (66)
Water sorption 6 μg/mm
3
(67) ≤ 32 μg/mm
3
(65) 14 μg/mm
3
(66)
Lot number Powder: 00001991
Liquid: 180627
Z002KK BE19F10R
Table 1. Denture base materials (manufacturer data for group B and C)

14
Each group was divided into three subgroups, based on the designated storage medium.
Fifteen samples from each group were immersed into 60°C coffee (Subgroup 1), another 15
samples were immersed into 18°C room temperature red wine (Subgroup 2), and the remaining 15
samples were immersed into 37°C distilled water as a control group (Subgroup 3). All samples
were immersed into designated media for 15 minutes, twice a day over 30 days. The color value
of each sample was detected using an imaging spectrophotometer (Crystaleye
Spectrophotometer®, Olympus, Tokyo, Japan) and color data analysis was performed on the
corresponding software (Crystaleye Application Master®, Olympus, Tokyo, Japan) at (1) baseline,
(2) after immersion for 30 days, and (3) after repolishing of the specimens. Changes in color were
calculated between the different measuring points.
Conventional PMMA denture base material (Group A)
A conventional PMMA denture base material (Lucitone 199 Denture Base Resin, Dentsply
Sirona, York, PA, USA) in shade light red pink was used to fabricate samples using the heat-
activated processing method. A mold was created by positioning a CAD/CAM block (IPS e.max®
CAD, 14.5 x 14.5 x 32 mm, Ivoclar Vivadent Inc., NY, USA) (Figure 4) in the middle of an empty,
round, plastic container previously used for VPS putty (Extrude
®
VPS impression materials, Kerr
Corporation, Brea, CA, USA). Two-hundred ml of duplicating silicone material (Elite® Double
32 Fast, Zhermack SpA, Polesine, RO, Italy) was mixed, using equal amounts of catalyst and base,
and poured into the container until the CAD/CAM block was fully submerged. After 10 minutes,
the silicone mold was separated from both the plastic container and the CAD/CAM block (Figure
5). A total of 8 wax patterns were made out of baseplate wax (Base Plate Wax™, Kerr Corporation,
Brea, CA, USA) by melting the wax with a Bunsen burner (Adjustable Micro-Bunsen Burner with
Adjustable Gas Valve, Humboldt Mfg. Co., Elgin, IL, USA) and pouring it into the silicone mold.
15
After waiting for 30 minutes to allow complete cooling of the wax, the pattern was removed from
the mold by blowing air into the space between the mold and the wax pattern. (Figure 6)

Figure 4. CAD/CAM block used for creating the mold

Figure 5. Mold made from duplicating silicone materials

Figure 6. Wax pattern created from the mold

Four three-piece ejector-type flasks (Hanau™ Varsity Flasks – Ejector Type, Whip Mix,
Louisville, KY, USA) which contains three layers, the bottom, middle, and the top layer, were
utilized in the following flasking procedures for fabricating the conventional PMMA specimens.
16
Each flask was used to process two wax patterns – a total of four flasks were used to make eight
traditional heat-processing PMMA resin blocks.
The inner walls of the flask were lubricated manually by applying a layer of petroleum
jelly (CURAD Petroleum Jelly, Medline, Northfield, IL, USA). The bottom portion of the flask
was filled with plaster (Modern Materials® Lab Plaster Regular Set 50 lb, Kulzer North America,
South Bend, IN, USA) by mixing 150 g of plaster and 75 ml of water, and pouring it into the flask
under low speed vibration on a laboratory vibrator (Heavy Duty Vibrator, Ray Foster Dental
Equipment, Huntington beach, CA, USA). Two wax patterns were embedded approximately 2mm
deep in the plaster as it was setting. (Figure 7) After full setting, a layer of die-separating liquid
(Super-Sep™, Kerr Corporation, Brea, CA, USA) was applied on the plaster with a brush and air-
thinned. The middle portion of the flask was then assembled onto the bottom portion and filled
with microstone (Microstone Premium Golden, Whip Mix, Louisville, KY, USA) by mixing 280
g powder with 80 ml water in a vacuum mixer (Vac-U-Mixer Bowls 500 ml, Whip Mix, Louisville,
KY, USA) and pouring it into the flask under low speed vibration on a laboratory vibrator (Heavy
Duty Vibrator, Ray Foster Dental Equipment, Huntington beach, CA, USA). Finally, the top
portion of the flask was assembled to close the flask.

Figure 7. The bottom layer of the flask filled with plaster; two wax patterns were placed.

17
After the microstone achieved full set, the flask was placed in a plastic kettle (Elite Cuisine
32 oz White Electric Hot Pot Kettle, Maxi-Mati, Industry, CA, USA) and boiled for 5 minutes to
soften the wax. After removal, a lab knife (Buffalo knife, Buffalo Dental Mfg. Co. Inc., Syosset,
NY, USA) was used to separate the bottom and middle portions by prying them apart. The residual
wax was washed out by pouring a stream of boiling water from an electric kettle (Proctor Silex
Electric Tea Kettle, Proctor Silex, Glen Allen, VA, USA) into the flask. After air drying of the
flask, a layer of separating agent (Al-Cote® Separating Agent, Dentsply Sirona, York, PA, USA)
was applied with a brush inside the mold and on all stone surfaces.
The conventional PMMA resin was made by mixing 21g powder and 10 ml liquid using
powder / liquid ratio of 32 cc / 10 ml of denture base material (Lucitone 199 Denture Base Resin,
Dentsply Sirona, York, PA, USA) in a plastic container with lid (Daiso glass jar with handle cap,
Daiso Industries Co. Ltd., Higashihiroshima, Hiroshima, Japan). This prevented evaporation of
un-set resin monomer before the dough stage was reached. Two plastic sheets (DenSilk, Reliance
Dental Manufacturing LLC, Alsip, IL, USA) were placed on the top of the mold and the flask was
closed and compressed up to 2000 psi in a hydraulic press (Presses Denture Flask Hydraulic, Nevin
Laboratories Inc., Chicago, IL, USA). The flask was then opened, and excess resin was removed
with a no. 15 blade (15 Sterile Blade, Hu-Friedy Mfg. Co., Chicago, IL, USA). A second
compression/excess-removal cycle was performed with one plastic sheet and excess resin was
trimmed away in a similar fashion (Figure 8). The last compression cycle was performed without
any intermediate plastic sheets and the flask was transferred in the closed configuration to a spring
clamp (Hanau™ Flask Compress, Whip Mix, Louisville, KY, USA) for processing. The entire
flasking assembly was placed into a curing unit (Hanau 2-stage denture curing unit, Whip Mix,
18
Louisville, KY, USA) and heat-cured at 74°C for 8 hours. The flask was allowed to cool to room
temperature before deflasking.

Figure 8. Acrylic resin dough packed into the microstone in the flask.
After processing, the flask assembly was disassembled using a lab knife (Buffalo knife,
Buffalo Dental Mfg. Co. Inc., Syosset, NY, USA) to pry the bottom, middle, and top portions apart.
A diamond saw on a polishing lathe machine (Baldor polishing lathe, ABB Motors and Mechanical
Inc., Fort Smith, AR, USA) was utilized to make cuts on the plaster around the heat-polymerized
PMMA resin blocks and the surrounding stone was removed with a hammer (Estwing Hammer,
Estwing, Rockford, IL, USA). After deflasking, the polymerized resin blocks were placed into a
sealed plastic bag (Ziploc®, S. C. Johnson & Son, Inc., Racine, WI, USA) filled with a cleaning
solution made by mixing 4 g sodium citrate powder (J. T. Baker Sodium Citrate, Dihydrate,
Powder, USP, FCC, Avantor, Radnor Township, PA, USA) and 100 ml water. The bag was placed
into an ultrasonic device (SweepZone® 200 w/Timer & Drain, L&R Manufacturing, Kearny, NJ,
USA) for 15 minutes to remove the remaining stone on the resin surface. After removing the resin
blocks, they were washed under running tap water and dried with paper towels.
Each resin block was hand-polished for 30 seconds with sandpaper of 400-grit (CarbiMet®
2 Abrasive Papers 400-grit, Buehler Ltd., Lake Bluff, IL, USA) and 600-grit (CarbiMet® 2
Abrasive Papers 600-grit, Buehler Ltd., Lake Bluff, IL, USA) under running distilled water. The
19
resin blocks were sectioned into squares with a precision saw (Isomet 1000, Buehler Ltd., Lake
Bluff, IL, USA) using a diamond blade (Isomet diamond blade, 15LC, 3 in, Buehler Ltd., Lake
Bluff, IL, USA) at 900 rpm under distilled water. A total of 45 flat, square specimens with
dimensions of 14±0.1 x 14±0.1 x 2.1±0.1 mm were sectioned. The cut surfaces of each specimen
were hand-polished with sandpaper in ascending order from 600-grit, 800-grit and 1200-grit
(CarbiMet® 2 Abrasive Papers 600/800/1200-grit, Buehler Ltd., Lake Bluff, IL, USA) under
running distilled water. Each surface was polished at each grit level for 30 seconds. The sandpaper
was placed on a flat surface during the polishing procedure to avoid sloping caused by uneven
polishing. The final thickness of all specimens after polishing was confirmed to be 2.0±0.1 mm
using a digital caliper (Mitutoyo digital caliper; Mitutoyo Corp, Kawasaki, Japan; Figure 9) with
0.001-mm accuracy. A small carbide bur (1 HP round carbide bur, Brasseler USA, Savannah, GA,
USA) run under 20,000 rpm to mark each specimen with a unique number at the upper left corner
of one surface. (Figure 10)

Figure 9. A digital caliper was used to confirm dimension of the specimens.

20

Figure 10. Specimen number marked at the corner of one surface of each specimen.

Milled PMMA denture base (Group B)
A milled denture base block (IvoBase CAD Pink-V, 98.5-30 mm, Ivoclar Vivadent Inc.,
NY, USA) was cut using a diamond saw machine (Isomet, Buehler Ltd., Lake Bluff, IL, USA;
Figure 11) with a diamond blade (Isomet diamond blade, 15LC, 3 in, Buehler Ltd., Lake Bluff, IL,
USA) at 900 rpm. The block was first cut into long rectangular pieces (Figure 12), which were
then sliced into 45 specimens of 14±0.1 x 14±0.1 x 2.1±0.1 mm in dimension (Group B). The
polishing and numbering procedures were the same as Group A.

Figure 11. Milled PMMA block stabilized for sectioning.
21

Figure 12. Milled PMMA blocks cut into rectangular pieces.

3D-printed PMMA denture base (Group C)
A 3D object with dimension of 14.1 x 14.1 x 2.1 mm was created in a CAD software
(SketchUp Pro, Trimble Inc., Sunnyvale, CA, USA; Figure 13) and exported as a STL file. The
dimension of the 3D object was designed to be slightly larger than our desired final dimension of
specimens to allow for polishing after printing. The STL file was sent to Dentca Inc. (Torrance,
CA, USA) for printing of 45 specimens in a 3D-printed denture base resin (Dentca Denture Base
II, Dentca Inc., Torrance, CA, USA), shade reddish pink (Group C). The STL file was processed
in the printer operation software (SprintRay, Ver: 2.5.1.0, SprintRay, Los Angeles, CA, USA), and
the specimens were printed using a 3D-printer (SprintRay Pro 95, SprintRay, Los Angeles, CA,
USA). Following the completion of printing, the supporting structures were removed manually,
and the specimens were washed with isopropyl alcohol for 5 minutes in one container and 5
minutes in a second container. A curing unit (Dymax ECE 5000, Dymax corporation, Torrington,
22
CT, USA) was utilized for post-curing of the 3D-printed specimens in glycerin bath for a total of
20 minutes under UV light. The polishing and numbering procedures were conducted in the same
manner as for Groups A and B.

Figure 13. Three-dimensional object created by SketchUp Pro

First color measurement – Baseline
After preparing all specimens of the three groups, they were immersed into distilled water
in plastic containers (Lustroware Small Food Keepers, Lustroware, Inc., Torrance, CA, USA),
which were placed in a laboratory oven (National Appliance Co. 5510 800-Watt Oven, National
Appliance Co., Portland, OR, USA) with a temperature of 37°C for 24 hours. Then, all specimens
were rinsed with distilled water and dried with absorbent filter paper (Hario V60 Paper Coffee
Filters, Size 01, White, Untabbed, HARIO Co. Ltd., Tokyo, Japan) by dabbing the water gently
off from the surfaces of the specimens. Color measurement of the CIE L*a*b* data was performed
in front of a white background (ColorChecker® Grayscale, X-Rite, Grand Rapids, MI, USA) for
each specimen using an imaging spectrophotometer (Crystaleye® Spectrophotometer, Olympus,
Tokyo, Japan). The surface of the specimen with the marked number faced the white background
with the number in the upper right corner. This allowed the unmarked surface to be used for all
color measurements. The spectrophotometer was placed over the center of the specimen’s surface
23
to capture an image. This was repeated three times per specimen, and the images were transferred
into the corresponding computer software (Crystaleye® Application Software, Olympus, Tokyo,
Japan; Figure 14 and Figure 15) on a laptop (VAIO Computer, Sony Electronics Inc., Minato City,
Tokyo, Japan). The color analysis of each specimens was performed in the software to obtain the
L*, a*, b* value as the baseline color data (E1), and the data was entered into an excel spreadsheet
(Excel; Microsoft Corp, Redmond, WA, USA).

Figure 14. An example of image captured by the spectrophotometer in this study. Outline of the square specimen was selected to
measure the color value at the center of the surface.
24

Figure 15. Color data analysis using Crystaleye software. L*, a*, b* value of the target area at the center of the milled denture
base specimen were obtained from the software.

Immersion of the specimens
Each group of specimens were divided into three subgroups of fifteen specimens. In
subgroup 1, coffee was used as the immersion medium. In subgroup 2, red wine and in subgroup
3, distilled water. The temperature of all beverages was measured with a digital thermometer
(ThermoPro TP-02S Thermometer, ThermoPro, Toronto, Ontario, Canada).
Subgroup 1 specimens were placed into plastic containers (Lustroware Small Food Keepers,
Lustroware, Inc., Torrance, CA, USA) without overlap and the containers were placed into water
bath devices (Digital Water Bath, WhipMix, Louisville, KY, USA) with the temperature set to
60°C. The coffee solution was prepared by mixing one packet (3.3 g) powder of instant coffee
(VIA Pike Place Roast, Starbucks Coffee Company, Seattle, WA, USA) with 200 ml of 90°C hot
water. This solution was then cooled to 60°C and poured into the specimen containers for 15
minutes of immersion. (Figure 16)
25

Figure 16. Subgroup 1 specimens immersed in coffee medium in the water bath device.
The other staining solution used in this study was red wine (2017 Cabernet Sauvignon,
Charles Shaw, CA, USA). The specimens of subgroup 2 were placed in a plastic mesh sieve
without overlap. The sieve was then placed into a plastic container, and 120 mL of 18°C (room
temperature) red wine was poured in for 15 min of immersion. (Figure 17) All red wine bottles
were stored in a cool place at 18°C room temperature during the 30-day testing period.

Figure 17. Subgroup 2 specimens spread out in the container and 120 ml red wine was poured for each immersion.

Fifteen samples from each group were immersed into distilled water in plastic containers
and stored in 37˚C laboratory incubator as the control group (subgroup 3). The distilled water was
refreshed twice a day during the 30-day experimental period.
26

Figure 18. Plastic containers used to store specimens in the laboratory incubator.

The first immersion for the test specimens was conducted at around 7 AM and the second
at around 6 PM each day for a period of 30 days. The coffee solution was mixed fresh for each
immersion. Once a red wine bottle was uncorked, it was recorked after each pour. A fresh pour of
red wine was used for each immersion. One bottle contained enough red wine for 6 immersions (3
days). Immediately after immersion, all specimens were rinsed thoroughly with distilled water,
dried with filter paper, and kept in small plastic containers separated by groups. The containers
were filed with distilled water and kept at 37˚C in a laboratory incubator at all times.
pH value measurement
pH values of the three test solutions were measured with a pH meter (Digital pH meter,
VIVOSUN, Los Angeles, CA, USA). Before measuring, the pH meter was calibrated according to
the manufacturer’s instruction with provided reference buffering solutions of pH value 6.86, 4.00,
and 9.18 at 25°C sequentially. Each solution was mixed by completely dissolving one packet of
powder from the manufacturer in 250 ml distilled water. The probe of the pH meter was rinsed
and dried completely before immersion into the calibrating solutions. After calibrating, the pH
meter was placed into 18°C red wine, 60˚C coffee, and 37˚C distilled water separately to obtain
the pH value of each beverage. As with the buffering solutions, the probe rinsed and dried
27
completely between uses. The measurement was repeated three times for each beverage and the
mean value was recorded. (Table 2)
Second color measurement – After immersion, before re-polishing
After the 30-day immersion period, all the specimens were removed from the solution,
rinsed thoroughly with distilled water, and dried with filter paper. The color measurements of CIE
L*a*b* data were made using the same settings as those used during the baseline measurement.
Measurements were performed by the same operator using the spectrophotometer and repeated
three times per specimen. The mean value was then calculated and recorded as the second color
measurement data (E2).
Third color measurement – After immersion, after re-polishing
After the second color measurement, the unmarked surface of all specimens was polished
with a wet rag wheel on a polishing lathe machine (Baldor polishing lathe, ABB Motors and
Mechanical Inc., Fort Smith, AR, USA). First, the rag wheel was rinsed thoroughly with distilled
water and 50 mg of ultrafine pumice powder was mixed with 10 ml distilled water to form a slurry.
This was then applied on the specimen’s surface and polishing was carried out at 1,725 rpm for 10
seconds under light pressure. After polishing, all specimens were washed thoruougly with distilled
water, and dried with filter paper. The third color measurement was done for all specimen by the
same operator using the same setting as described previously. The measurements were repeated
three times for each specimen, and the mean value was calculated and recorded as the third color
measurement data (E3).
Subgroup Testing Solution Composition pH
1 Coffee Medium roast, Instant and microground coffee 3.3g each packet 4.8
2 Red wine 2017 Cabernet Sauvignon, alcohol content: 12.5% 3.8
3 Distilled water H 2O 6.5
Table 2. Staining solutions in this study
28
Data management and statistical analysis
The CIEL*a*b* color values of the baseline color measurement (E1), second color
measurement (E2), and the third color measurement (E3) obtained from the spectrophotometer
software were recorded in an Excel sheet (Excel; Microsoft Corp, Redmond, WA, USA) for all
specimens. The color change of each specimen was determined using the CIEDE2000 color
difference formula (1) to calculate the color difference between different measurements. The color
difference from the second color measurement (E2) to the baseline measurement (E1) was
calculated as ∆E00
1
; the color difference from the third color measurement (E3) to the baseline
measurement was calculated as ∆E00
2
; and the color difference from the third to second color
measurement was calculated as ∆E00
3
.
Statistical analysis was performed using the same software (Excel; Microsoft Corp,
Redmond, WA, USA) on separated sheets. A three-way analyses of variance (ANOVA) test was
performed and assessed at the 95% confidence level (α=.05) to determine statistically significant
differences among different materials, different staining solutions, and with different re-polishing
procedures. An independent t-test was performed to determine the statistical different between
different materials and different staining solutions pairs (α=.05).

29
Results
CIEL*a*b* color value
The CIEL*a*b* color values of the first color measurement (E1) are listed in Table 3.
Baseline color data shows that there is a consistency of CIEL*a*b* color value within each
group. Among the groups, the highest L* value was detected in the 3D-printed PMMA, followed
by the milled PMMA, and the least lightness was detected in the conventional PMMA
specimens. Color values of a* and b* show that conventional PMMA specimens are redder in
color, and the milled PMMA specimens are more toward yellow in the color space than the other
groups.

Group Subgroup L1* (mean±SD) a1* (mean±SD) b1* (mean±SD)
A, conventional
PMMA
1, coffee 60.86±0.66 41.11±1.16 17.20±0.48
2, red wine 60.92±0.84 41.01±1.19 16.94±0.34
3, distilled water 60.87±1.10 40.98±1.51 16.89±0.45
B, milled
PMMA
1, coffee 63.33±0.74 34.74±0.75 25.33±0.40
2, red wine 63.31±0.48 34.78±0.31 25.43±0.49
3, distilled water 63.30±0.52 34.88±0.46 25.40±0.37
C, 3d-printed
PMMA
1, coffee 70.19±0.28 22.48±0.29 12.19±0.40
2, red wine 70.51±0.32 22.19±0.33 11.70±0.40
3, distilled water 70.43±0.37 22.57±0.37 11.11±0.39
Table 3. Baseline CIEL*a*b* color value (E1)

The results of the second color measurement (E2) are shown in Table 4, and the results of
the third color measurement are shown in Table 5.

30
Group Subgroup L2* (mean±SD) a2* (mean±SD) b2* (mean±SD)
A, conventional
PMMA
1, coffee 59.74±0.84 39.51±1.27 17.23±0.41
2, red wine 60.62±0.80 40.42±1.12 16.62±0.44
3, distilled water 60.80±1.15 40.75±1.71 16.98±0.37
B, milled
PMMA
1, coffee 62.10±0.74 34.05±0.61 25.00±0.24
2, red wine 62.82±0.58 34.79±0.61 24.61±0.48
3, distilled water 63.15±0.39 35.01±0.56 24.79±0.37
C, 3d-printed
PMMA
1, coffee 67.41±0.62 21.34±0.20 13.99±0.63
2, red wine 69.89±0.57 22.33±0.41 8.58±0.15
3, distilled water 70.64±0.53 22.69±0.50 7.70±0.21
Table 4. CIEL*a*b* color value after immersion, before re-polishing (E2)

Group Subgroup L3* (mean±SD) a3* (mean±SD) b3* (mean±SD)
A, conventional
PMMA
1, coffee 60.22±0.99 39.09±1.06 15.68±0.59
2, red wine 60.49±0.62 39.80±1.45 15.88±0.98
3, distilled water 61.33±1.24 40.09±1.76 16.26±0.47
B, milled
PMMA
1, coffee 62.63±0.57 33.76±0.84 23.74±0.65
2, red wine 62.93±0.73 34.63±0.47 24.66±0.38
3, distilled water 63.57±0.64 34.77±0.76 24.44±0.72
C, 3d-printed
PMMA
1, coffee 68.00±0.99 20.66±0.44 11.31±1.03
2, red wine 70.43±0.65 21.88±0.49 8.36±0.33
3, distilled water 70.79±0.52 22.41±0.57 7.70±0.27
Table 5. CIEL*a*b* color value after re-polishing (E3)

Representative specimens of each material group before and after immersion in each
beverage as well as the specimens after polishing are displayed in Table 6.

31
E1 E2 E3
Group A
subgroup 1

Group A
subgroup 2

Group A
subgroup 3

Group B
subgroup 1

Group B
subgroup 2

32
Group B
subgroup 3

Group C
subgroup 1

Group C
subgroup 2

Group C
subgroup 3

Table 6. Pictures of representative specimens from each group for three color measurements

33
Table 7 showed the difference in lightness in all groups of specimens. The greatest
change in lightness was found in group C subgroup 1, which was 2.79±0.46 before re-polishing,
indicated the lightness change of 3D-printed specimens immersed in coffee medium after 30-day
immersion period was the greatest among all test groups. The least lightness difference was
found in conventional PMMA specimens that immersed in red wine after 30-day period, the
lightness change was 0.30±0.25 before re-polishing. Overall, specimens immersed in coffee
medium had the greatest difference in lightness for all three different denture base materials in
this study.
Group Subgroup ∆L
1
(mean±SD) ∆L
2
(mean±SD) ∆L
3
(mean±SD)
A, conventional
PMMA
1, coffee 1.11±0.33 0.74±0.44 0.55±0.40
2, red wine 0.30±0.25 0.60±0.47 0.53±0.38
3, distilled water 0.32±0.21 0.60±0.63 0.63±0.51
B, milled PMMA 1, coffee 1.23±0.37 0.79±0.51 0.65±0.52
2, red wine 0.49±0.38 0.61±0.35 0.39±0.31
3, distilled water 0.33±0.28 0.59±0.49 0.56±0.54
C, 3d-printed PMMA 1, coffee 2.79±0.46 2.28±0.89 0.84±0.54
2, red wine 0.62±0.43 0.56±0.27 0.71±0.45
3, distilled water 0.39±0.24 0.62±0.44 0.64±0.41
Table 7. L* differences for each group, ∆L
1
: difference of L1 and L2, ∆L
2
: difference of L1 and L3, ∆L
3
: difference of L1 and
L3.
34

Figure 19. ∆L for each group
The color change in a* value of each group is displayed in Table 8. The greatest change
in a* value was found in conventional PMMA specimens were that immersed in coffee medium,
the a* value difference from the 2
nd
color measurement to the baseline color data, ∆a
1
was
1.60±0.30 for this group. A bigger change in a* value was also found in 3D-printed PMMA
specimens immersed in coffee medium, the ∆a
1
was 1.13±0.25 for this group. The least change
of a* value after immersion and before re-polishing was found in 3D-printed PMMA immersed
in red wine.
Group Subgroup ∆a
1
(mean±SD) ∆a
2
(mean±SD) ∆a
3
(mean±SD)
A, conventional
PMMA
1, coffee 1.60±0.30 2.02±0.59 0.51±0.47
2, red wine 0.60±0.45 1.21±0.69 0.70±0.66
3, distilled water 0.44±0.54 0.94±0.89 0.79±0.57
B, milled PMMA 1, coffee 0.80±0.20 1.18±0.68 0.65±0.54
2, red wine 0.39±0.36 0.33±0.31 0.48±0.38
3, distilled water 0.42±0.18 0.53±0.25 0.64±0.41
C, 3d-printed PMMA 1, coffee 1.13±0.25 1.83±0.35 0.71±0.33
2, red wine 0.25±0.16 0.42±0.33 0.52±0.34
3, distilled water 0.35±0.24 0.43±0.31 0.33±0.28
Table 8. a* differences for each group, ∆a
1
: difference of a1 and a2, ∆a
2
: difference of a1 and a3, ∆a
3
: difference of a1 and a3
0
0.5
1
1.5
2
2.5
3
A1 A2 A3 B1 B2 B3 C1 C2 C3
L* differences for each group
∆L1 ∆L2 ∆L3
35

Figure 20. ∆a for each group

Table 9 displays the color change in b* value for each group of specimens. Among all
groups, 3D-printed PMMA specimens immersed in red wine and water showed the greatest
changing in b* value, while the least changing in b* value was found in conventional PMMA
immersed in distilled water for 30-day immersion period. For specimens immersed in coffee
medium, 3D-printed specimens also showed a greater change in b* value than conventional and
milled PMMA specimens.

Group Subgroup ∆b
1
(mean±SD) ∆b
2
(mean±SD) ∆b
3
(mean±SD)
A, conventional
PMMA
1, coffee 0.24±0.20 1.51±0.43 1.55±0.45
2, red wine 0.35±0.22 1.06±0.97 0.80±1.05
3, distilled water 0.13±0.10 0.67±0.41 0.76±0.36
B, milled PMMA 1, coffee 0.47±0.24 1.63±0.85 1.26±0.74
2, red wine 0.82±0.29 0.77±0.52 0.43±0.28
3, distilled water 0.61±0.30 0.97±0.74 0.74±0.45
C, 3d-printed PMMA 1, coffee 1.81±0.90 1.12±0.73 2.66±1.02
2, red wine 3.12±0.41 3.34±0.43 0.31±0.17
3, distilled water 3.41±0.41 3.41±0.43 0.16±0.09
Table 9. b* differences for each group, ∆b
1
: difference of b1 and b2, ∆b
2
: difference of b1 and b3, ∆b
3
: difference of b1 and b3
0
0.5
1
1.5
2
2.5
A1 A2 A3 B1 B2 B3 C1 C2 C3
a* differences for each group
∆a1 ∆a2 ∆a3
36

Figure 21. ∆b for each group

CIEDE2000 Color difference of denture base materials
The mean and standard deviation of color difference (∆E00) value of all groups calculated
with CIEDE2000 color difference formula is presented in Table 10.

Group Subgroup ∆E 00
1
(mean±SD) ∆E 00
2
(mean±SD) ∆E 00
3
(mean±SD)
A, conventional
PMMA
1, coffee 1.19±0.27 1.17±0.37 1.06±0.28
2, red wine 0.41±0.20 0.90±0.57 0.72±0.60
3, distilled water 0.36±0.26 0.71±0.60 0.74±0.44
B, milled PMMA 1, coffee 1.11±0.30 1.17±0.29 0.96±0.44
2, red wine 0.70±0.27 0.75±0.22 0.48±0.23
3, distilled water 0.54±0.18 0.81±0.43 0.65±0.44
C, 3d-printed PMMA 1, coffee 2.75±0.63 2.22±0.58 1.89±0.72
2, red wine 2.26±0.28 2.32±0.29 0.67±0.36
3, distilled water 2.40±0.26 2.41±0.27 0.56±0.32
Table 10. CIEDE2000 color difference results
0
0.5
1
1.5
2
2.5
3
3.5
A1 A2 A3 B1 B2 B3 C1 C2 C3
b* differences for each group
∆b1 ∆b2 ∆b3
37

Figure 22. CIEDE2000 color difference of each group

Levene’s test showed unequal variances of the data as listed in Table 11, and the three-way
ANOVA results are presented in Table 12. for three variables: the materials, solutions, and re-
polishing. Among the three factors, statistically significant difference was found in different
materials (p<0.001) and different staining solutions (p<0.001). The factor of re-polishing was not
significant in the result of this study (p=0.087).

Levene's Tests
type p-value
means 0.013911
medians 0.018024
trimmed 0.015736
Table 11. Results of Levene's test

0
0.5
1
1.5
2
2.5
3
A1 A2 A3 B1 B2 B3 C1 C2 C3
CIEDE2000 color difference
∆E001 ∆E002 ∆E003
38
Three Factor Anova

ANOVA

Alpha 0.05

SS df MS F p-value p eta-sq
Material 149.060 2 74.530 490.599 <0.001 0.796
Beverage 8.992 2 4.496 29.597 <0.001 0.190
Polishing 0.447 1 0.448 2.947 0.087 0.012
Material x Beverage 2.517 4 0.629 4.142 0.003 0.062
Material x Polishing 2.109 2 1.055 6.942 0.001 0.052
Beverage x Polishing 2.073 2 1.036 6.822 0.001 0.051
Material x Beverage x
Polishing
0.884 4 0.221 1.455 0.217 0.023
Within 38.283 252 0.152

Total 204.366 269 0.760

Table 12. Three-way ANOVA results

Overall group-wise comparisons conducted with independent t-tests (two-tail) showed that
there was no significant color changing (∆E00) when comparing conventional and milled PMMA
(p=0.441; Table 13). Both of them presented consistency in color value and with limited color
difference, while the 3D-printed denture base resin presented with more color changing after 30-
day immersion period comparing to conventional (p<0.001; Table 15) and milled denture materials
(p<0.001; Table 14).

T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.068 0.772 159.802 0.220 1.654

no
Two Tail 0.068 0.772 159.802 0.441 1.975 -0.187 0.082 no
Table 13. Independent t-test for material A and B

39
T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.062 25.079 171.267 0 1.654

yes
Two Tail 0.062 25.079 171.267 0 1.974 -1.671 -1.427 yes
Table 14. Independent t-test for material B and C
T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.074 21.602 172.145 0 1.654

yes
Two Tail 0.074 21.602 172.145 0 1.974 -1.747 -1.455 yes
Table 15. Independent t-test for material A and C

Overall comparison for three different beverages, specimens immersed in coffee had
significantly more color change than red wine (p=0.002; Table 16) and distilled water (p=0.002;
Table 18). There is no significant difference in color change between immersion in red wine and
distilled water (p=0.900; Table 17).

T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.121 3.131 177.020 0.001 1.654

yes
Two Tail 0.121 3.131 177.020 0.002 1.973 0.140 0.617 yes
Table 16. Independent t-test for beverage 1 and 2
T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.133 0.126 175.897 0.450 1.654

no
Two Tail 0.133 0.126 175.897 0.900 1.974 -0.245 0.279 no
Table 17. Independent t-test for beverage 2 and 3

40
T TEST: Unequal Variances

Alpha 0.05

std err t-stat df p-value t-crit lower upper sig
One Tail 0.129 3.073 172.276 0.001 1.654

yes
Two Tail 0.129 3.073 172.276 0.002 1.974 0.141 0.650 yes
Table 18. Independent t-test for beverage 1 and 3
For conventional PMMA group, results of two-tail independent t-tests showed specimens
immersed in coffee resulted in significantly more color change comparing to red wine (p<0.001;
Table 19) and distilled water (p<0.001; Table 21), while no significant difference in color change
was observed when comparing red wine and distilled water (p=0.365; Table 20).
t-Test: Two-Sample Assuming Unequal Variances (wine and coffee, material A)
Wine Coffee
Mean 0.658 1.151
Variance 0.250 0.156
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 55.000

t Stat -4.243

P(T<=t) one-tail 0.000

P(T<=t) two-tail 0.000

Table 19. Independent t-test, material A, subgroup 1 and 2

t-Test: Two-Sample Assuming Unequal Variances (wine and water, material A)
Wine Water
Mean 0.658 0.540
Variance 0.250 0.250
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 58.000

t Stat 0.913

P(T<=t) one-tail 0.183

P(T<=t) two-tail 0.365

Table 20. Independent t-test, material A, subgroup 2 and 3
41
t-Test: Two-Sample Assuming Unequal Variances (coffee and water, material A)
Coffee Water
Mean 1.151 0.540
Variance 0.156 0.250
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 55.000

t Stat 5.258

P(T<=t) one-tail 0.000

P(T<=t) two-tail 0.000

Table 21. Independent t-test, material A, subgroup 1 and 3

Similar results were found in the milled PMMA material: coffee created significant color
change as compared to red wine (p<0.001; Table 22) and distilled water (p<0.001; Table 24), while
no significant difference was found comparing red wine and distilled water (p=0.556; Table 23).

t-Test: Two-Sample Assuming Unequal Variances (Wine and coffee, material B)
Wine Coffee
Mean 0.721 1.141
Variance 0.062 0.092
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 56.000

t Stat -5.860

P(T<=t) one-tail 0.000

P(T<=t) two-tail 0.000

Table 22. Independent t-test, material B, subgroup 1 and 2

42
t-Test: Two-Sample Assuming Unequal Variances

Wine Water
Mean 0.721 0.674
Variance 0.062 0.132
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 51.000

t Stat 0.592

P(T<=t) one-tail 0.278

P(T<=t) two-tail 0.556

Table 23. Independent t-test, material B, subgroup 2 and 3

t-Test: Two-Sample Assuming Unequal Variances (coffee and water, material B)
coffee water
Mean 1.141 0.674
Variance 0.092 0.132
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 56.000

t Stat 5.399

P(T<=t) one-tail 0.000

P(T<=t) two-tail 0.000

Table 24. Independent t-test, material B, subgroup 1 and 3

On the other hand, there was no significant difference found for the 3D-printed PMMA
material immersed in three different beverages (Table 25, Table 26, Table 27).

43
t-Test: Two-Sample Assuming Unequal Variances (coffee and wine)
wine coffee
Mean 2.292 2.485
Variance 0.084 0.454
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 39.000

t Stat -1.443

P(T<=t) one-tail 0.079

P(T<=t) two-tail 0.157

Table 25. Independent t-test, material C, subgroup 1 and 2
t-Test: Two-Sample Assuming Unequal Variances (coffee and water)
water coffee
Mean 2.407 2.485
Variance 0.074 0.454
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 38.000

t Stat -0.587

P(T<=t) one-tail 0.280

P(T<=t) two-tail 0.561

Table 26. Independent t-test, material C, subgroup 1 and 3
t-Test: Two-Sample Assuming Unequal Variances (water and wine)
water wine
Mean 2.407 2.292
Variance 0.074 0.084
Observations 30.000 30.000
Hypothesized Mean Difference 0.000

df 58.000

t Stat 1.590

P(T<=t) one-tail 0.059

P(T<=t) two-tail 0.117

Table 27. Independent t-test, material C, subgroup 2 and 3
44
Discussion
The first and second hypothesis were rejected since significant color differences were
found among different denture base materials and among different staining beverages in the
results of present study. The third hypothesis was accepted as there is no significant interaction
between the color difference of re-polishing of the specimens and before re-polishing.
Color Stability of different denture base materials
Overall, significant differences in color stability were observed between milled, 3D-printed
and conventional PMMA denture base materials. Conventional and milled denture bases were not
significantly different from each other when immersed into all three staining solutions after 30
days, while the 3D-printed denture base material was found to have significantly more color
change than the other two materials. Similar outcomes were found in studies comparing milled
and conventional denture bases (60) (63) and denture teeth (60), showing milled denture materials
presented similar levels of color stability as conventional PMMA. In fact, based on the results from
another in vitro study, CAD/CAM milled denture bases were superior to conventional PMMA
dentures in terms of color stability. (68)
As previously mentioned, increased porosity from the heat polymerization process may
increase water sorption that affects the surface roughness and the color stability of acrylic resin.
The degree of conversion (DC) is the efficiency of polymerization, it is related to the amount of
cross-linking, and is expressed by the percentage of methacrylate groups that have been converted
after polymerization. The degree of conversion influences the physical properties of dental resins:
a lower degree of conversion may lead to a higher degree of water sorption and result in color
instability. (69) The degree of conversion for 3D-printed materials was comparatively low
compared to conventional PMMA in a previous study. (70) On the other hand, the milled denture
45
materials may present a higher degree of conversion due to industrial polymerization under high
temperature and pressure. (71) Industrial polymerization could result in CAD/CAM blocks
fabricated with better homogeneity and reduced porosity in comparison to conventional materials.
(72) The lower degree of conversion of the 3D-printed materials could be one of the possible
reasons for the reduced color stability when comparing to conventional heat-processed and
industrial milled PMMA denture base materials in the present study.
The plasticization effect from water sorption could negatively affect the material’s
mechanical properties. Formation of microcracks and degradation of the resin polymer may
potentially end up with link cleavage and gradual deterioration of the resin infrastructure. (73) This
sequence of events may be an explanation for the color instability of the 3D-printed denture base
in the distilled water observed in this study.
The shades of the three different denture base materials were different at baseline. The
conventional and milled denture base materials contained reddish fibers to mimic the veined
appearance of the vessels mucosa resulting in a more lively appearance but also in a non-
homogenous color. The 3D-printed denture base material was homogenous in color without
artificial veins. The difference in the characterization of fiber-like denture bases may create
variances in color change. However, there is no available study investigating the effects of fiber-
like components in denture bases.
Staining ability of different immersing mediums
The second hypothesis was rejected, as the coffee medium created more change in color in
the present study when compared to red wine and distilled water. Extracts from beverages such as
tea, coffee, and wine have been shown to increase stains on natural teeth and acrylic resin by their
secondary metabolites such as tartaric acid, tannins, caffeine, saponins and phenols. (50) (74) (75)
46
Staining solutions are considered as an extrinsic factor for discoloration because of the absorption
and adsorption of these stains. (56) The results of this study showed greater change in L* (Table
7), a* (Table 8), and b* (Table 9) values when specimens were immersed in coffee compared to
the other two media. This is in conflict with other studies reporting that red wine induced more
color change in denture teeth than coffee (50) (76) but is in agreement with one other study that
concluded coffee was the most chromogenic staining solution. (77) In addition, the concentration
of the coffee might decrease with the addition of artificial creamer, thus reducing the color change
observed in the materials. (78) The concentration of the beverage can be assumed to have an
influence on the degree of staining and may explain the difference in the results of different studies.
The change in color overall would have been greater if the specimens had been immersed
in staining solution for more than 30 days. However, a 30-day staining period with 15 minutes
twice a day was chosen based on the assumption that exposure to coffee and red wine is on average
15 to 30 min per day. This is a closer reproduction of real life, especially when compared to other
studies utilizing different staining protocols, such as continuous immersion of specimens for a
week. (52) (60) (79) The intrinsic color change measured after removing the extrinsic staining was
not significant in the present study. It is assumed that the environment for storing the specimens
between each immersion, and the total immersion time played a role in the result. The specimens
were all stored in the 37°C laboratory incubator without any light condition most of the time, with
only 15 minutes twice a day for staining, and the moisture change was not much as compared to
continuous immersion of the specimens in previous studies. (80) (81)
Finishing and polishing of denture bases
The cameo surface of the complete denture requires finishing and polishing after clinical
adjustments. (82) Traditional lathe polishing using a polishing paste or liquid polish is suggested
47
as it results in less surface roughness after polishing compared to polishing with tungsten carbide
burs. (83) The 3D-printed denture materials are treated with very similar polishing protocol as the
conventional complete dentures by polishing with a polishing paste without applying sealing
agents. Color change of 3D-printed provisional materials could be significant decreased with
surface polishing or protective coating agents that are light-polymerized. (65) In the present study,
the specimens were polished with wet rag wheel on a polishing lathe and with pumice slurry after
the 30-day immersion period, and the color difference were calculated from baseline to after re-
polishing. It was our intention to investigate the intrinsic color change of each materials by
removing the extrinsic staining from the surface of the specimens with polishing. The third null
hypothesis of this study was accepted, as the results of this study showed that there was no
significant difference in ∆E00
1
and ∆E00
2
, which represented the color difference of extrinsic and
intrinsic color change respectively.
Porous denture surfaces are more susceptible to staining and calculus deposition because
of increasing in water sorption. (4) (84) The controlling of temperature in heat-activated acrylic
resins is essential because excessive heat applied in the beginning of a polymerization cycle could
lead to external porosity while internal porosity may occur if too much heat was applied at the end
of the polymerization reaction. Temperature that is higher than the boiling point of the monomers
could lead to increasing porosity of the acrylic resin from boiling the monomers. (85) In the present
study, the conventional PMMA specimens were heat-cured at 74°C for 8 hours during the
polymerization process to reduce chances of porosity in the acrylic resin. The milled specimens
made from industrially-polymerized CAD/CAM blocks were considered to have low porosity as
well due to polymerization under high pressure. (72) Literature about porosity of 3D-printed
denture materials compared to conventional and milled materials is currently not available.
48
Determining color change
For determining color change in denture base materials, one can examine the materials with
visual examination, which is a subjective observation, or utilizing a spectrophotometer for an
objective result. Identification of minor changes in color could be detected by spectrophotometers
objectively, but the devices have to be calibrated before each use and should be used under
standard illuminating conditions. (86) It is reported that using spectrophotometers results in a 33%
increase in accuracy for color measurement when comparing to visual observation. (87) In this
study, the portable digital spectrophotometer was utilized to detect the color value of the specimens
in front of a white background to standardize the illuminating condition of all the color
measurements. The color was then evaluated and recorded using the CIE L*a*b* color system.
The L* value is a lightness of an object, the a* value is represented the redness and greenness of
the object’s color, the b* value is represented the yellowness and blueness of the color. By using
the CIE L*a*b* system, color differences may be expressed in units that facilitate the interpretation
of visual perception and clinical significance.
A previous formula in determine color change was ΔE*ab = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
which was proposed by Commission International de l’Éclairage (CIE) in 1978, and the attempt
of improving the CIELab formula was made and a new formula in calculating color difference was
proposed in 2001 as CIEDE2000 formula, studies have shown that the CIEDE2000 formula
reflects the minor color differences perceived by the human eye better than the previous CIELab
formula, and is more suitable in use for clinical interpretation in dental field. (37) (88) (89) (90)
Therefore, the CIEDE2000 color difference formula was selected for the present study to better
reflect the clinical results.
49
Perceptibility and acceptability thresholds
The color change of conventional denture base materials after immersion in coffee, red
wine, and distilled water were 1.17±0.37, 0.90±0.57, and 0.71±0.60 ∆E00, respectively. The
corresponding color change of milled denture base materials were 1.17±0.29, 0.75±0.22, and
0.81±0.43 ∆E00, which were all within the perceptibility and acceptability thresholds reported by
Ren et al. (89) for denture base materials. The 3D-printed denture base materials presented color
changes of 2.22±0.58, 2.32±0.29, and 2.41±0.27 ∆E00 for coffee, red wine, and distilled water after
30-day immersion period, which were clinically perceivable, but were still within the clinically
acceptable range.
Environmental factors
Considering daily use of the complete dentures, color change in a simulated environment
should be evaluated as dentures are exposed to varying temperature and pH conditions in clinical
circumstances. The natural phenolic pigments which are found in the extracts of black tea,
turmeric, and common grape vine, have been widely used for food coloration (91), and these
pigments could exhibit different colors. Phenolic pigments are reported to be the most common
chromogens in diet. (92) Extrinsic staining related to human saliva appears to be resulted from
the affinity interaction and the precipitation reaction with dietary chromogens. (93)
Patient’s oral hygiene could also play a role in discoloration of materials as the presence
of plaque and its metabolic end-products can degrade the organic matrix of resin materials. (94)
Studies showed that use of denture cleansers can be utilized in preventing color change for
traditional heat-processed dentures. (8) (95) However, no data is available about the effects of
denture cleansers on the color change in CAD/CAD denture materials to date.
50
Accelerating aging procedures have been used to show the effect of environmental
factors such as UV light, temperature, and moisture to the color change and physical properties
of materials. (96) Negative effect of accelerated aging on the color stability of denture liners and
resin-based restorative materials was shown in some studies. (80) (81) A higher degree of color
difference was found with PMMA materials treated with UV light exposure than coffee
immersion when comparing different provisional materials in restorative dentistry. (97) It was
suggested that the oxidation of residual unreacted carbon double bonds in the polymerized resins
could be facilitated under the exposure of UV light and in an environment that is oxygen-rich
because of the accelerated cleavage of carbon double bond. (76) (98) Significantly more
discoloration also happened in traditional denture PMMA after pre-aging by thermocycling
compared to samples without thermocycling. (44) (74) The lack of exposure to light and without
the pre-aging thermocycling procedure for the specimens in the present study might lead to
limited chemical change within the materials, thus preventing an intrinsic color change.
Limitations of the study
The present in vitro study lacks reproducibility of the clinical conditions in the human body.
Although the temperatures for staining and storing samples were controlled carefully, the absence
of salivary proteins may affect the clinical validity of the results of this study. A previous study
has showed an increase susceptibility of stain accumulation on tooth surface under the presence of
salivary proteins. (93) On the other hand, milled denture bases showed less color change compared
to conventional materials with the same staining protocol as the present study but using
thermocycling. (68) Such pre-aging of the materials with thermocycling procedures could be
considered in the future to simulate the long-term use of denture materials in the oral environment.
51
Furthermore, clinical evaluations are needed to verify the in vitro outcomes of the color stability
of CAD/CAM denture materials and traditional denture materials.
Currently, most dental labs are utilizing bonding procedures to bond prefabricated denture
teeth to the recess area of milled denture bases. The junction between denture teeth and the base
may be susceptible to stain accumulation, and milled dentures were reported to be more resistant
to the discoloration at this junction. (60) For 3D-printed dentures, bonding procedures are also
utilized to bond the 3D-printed denture teeth on the printed denture base, future studies are needed
to investigate the susceptibility of discoloration at the teeth-denture base interface of 3D-printed
denture materials.
52
Conclusions
Within the limitation of this study, color stability of denture base materials used for
subtractive milling technique was comparable to conventional heat-activated denture base
materials after the immersion period described here. Denture base materials stained with coffee
generated more discoloration than red wine and distilled water for conventional and milled
PMMA materials in the present study. Color differences were not perceivable and were within
acceptable limits for conventional and milled PMMA. Denture base materials made from
additive manufacturing technique had clinically perceivable discoloration after immersing into
all beverages, but within the clinically acceptable range. No significant difference in intrinsic
color change was found after immersion of three different denture base materials into coffee, red
wine, and distilled water in the present study.
53
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64
Conflicts of Interest
Conflicts of interest: none.

Funding
This study was supported by the BMDD program of the Herman Ostrow School of Dentistry of
USC.

IRB
No IRB necessary.

Abstract (if available)

Abstract Objectives: The purpose of this in vitro study is to compare the color stability of subtractively and additively manufactured denture bases against that of conventionally heat-activated polymethyl methacrylate (PMMA) complete denture bases. ❧ Methods: A total of 135 complete denture base specimens (dimension of 14±0.1 × 14±0.1 × 2±0.1 mm) were fabricated using three different methods (n=45 per group): A. conventional heat-activated, B. milled, and C. 3D-printed. After immersing in 37˚C distilled water for 24 hours, specimens were dried with filter paper. Baseline color measurement using Commission International de I’Eclairage (CIE) color parameters L*, a*, b* was repeated three times for each specimen by using an imaging spectrophotometer (Crystaleye® Spectrophotometer, Olympus, Tokyo, Japan) on a white background. The specimens were immersed in three different solutions: 60˚C coffee (VIA Pike Place Roast, Starbucks Coffee Company, Seattle, WA, USA) (Subgroup 1), 18˚C room temperature red wine (2017 Cabernet Sauvignon, Charles Shaw, CA, USA), (Subgroup 2) and 37˚C distilled water (Subgroup 3)

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Asset Metadata

Creator Chen, Yun-Chu (author)

Core Title Comparison of color stability in CAD/CAM and conventional complete denture materials

School School of Dentistry

Degree Master of Science

Degree Program Biomaterials and Digital Dentistry

Publication Date 11/29/2020

Defense Date 10/15/2020

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag 3D-printing,additive manufacturing,color stability,complete denture,milling,OAI-PMH Harvest,subtractive manufacturing

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Phark, Jin-Ho (committee chair), Duarte, Sillas (committee member), Park, Cheryl (committee member)

Creator Email judyczu@gmail.com,ychen@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-396955

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Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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