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Trueness evaluation of three-dimensionally (3D) printed provisional crowns by two digital light processing (DLP) printers
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Trueness evaluation of three-dimensionally (3D) printed provisional crowns by two digital light processing (DLP) printers
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Content
Trueness Evaluation of Three-Dimensionally (3D) Printed Provisional
Crowns by Two Digital Light Processing (DLP) Printers
By
Mohammed Alshanbari, BDS, MPH
A Thesis Presented to the
FACULTY OF THE USC HERMAN OSTROW SCHOOL OF DENTISTRY
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirement for the Degree
MASTER OF SCIENCE
BIOMATERIALS AND DIGITAL DENTISTRY
December 2020
Copyright 2020 Mohammed Alshanbari
ii
Dedication
I dedicate my dissertation work to my family and friends, who have encouraged me throughout this
journey. Also, to my country Saudi Arabia, for supporting my educational endeavors.
A special feeling of gratitude to my loving parents, Hussain and Dalal Alshanbari, whose prayers,
words of encouragement, and push for tenacity ring in my ears.
Sincere thanks to my lovely wife, Afnan Khalifah for her countless support, sacrifices, and words of
encouragement. Without her, I would not have accomplished this.
To my two kids, Hussain and Lina Alshanbari, who I missed their joys the most during the time of
COVID-19, while they are back to Saudi Arabia.
To my sisters Amal, Fatimah, Mashael, and brother Saleh, who never left my side and are very
special to my heart.
To my mother-in-law Fryal Bukhari, for taking care of my kids during difficult times and being
always there for my wife and kids and for me.
To my dear friends Dr. Abdulwadood Sharqawi, Dr. Hesahm Alhazmi, and Dr. Waleed
Abughramah, for their support, encouragement, and being there for me.
iii
Acknowledgements
It gives me great pleasure and privilege to have my second master's degree from the University of
Southern California and to thank everybody for giving me such wonderful and great opportunity,
even during this unprecedented time of COVID-19, we were able to complete my master thesis on
the determined time.
I would like to acknowledge all the committee members for their time, support, and guidance.
Foremost, a special thanks goes to my mentor, advisor, and committee chairman, Dr. Jin-Ho Phark,
thank you for your expertise and support throughout this learning process of my master thesis. Dr.
Sillas Duarte, my co-advisor and program director thank you very much for everything and for your
kindness, constant advice, support, and directions. Dr. Jenny Son, thank you for your valued
insights, encouragement, and clinical guidance.
I also thank the DENTCA, Inc. for providing the materials, instruments, and machines to be used
to conduct the research.
Also, I appreciated the help of my friend Dr. Ali Fahed Alqahtani, for his guidance in conducting
my statistical analysis.
Last but not the least, I thank my faculty Dr. Neimar Sartori, Mrs. Karen Guillen and co-residents at
the Advanced Operative and Adhesive Dentistry who have willingly supported me and helped me
during this journey.
iv
TABLE OF CONTENTS
Dedication ..................................................................................................................................................... ii
Acknowledgements ..................................................................................................................................... iii
List of Tables ................................................................................................................................................ v
List of Figures .............................................................................................................................................. vi
Abbreviations .............................................................................................................................................. vii
Abstract ...................................................................................................................................................... viii
1. Introduction .............................................................................................................................................. 1
2. Objective of the Study ............................................................................................................................ 8
3. Material and Methods ............................................................................................................................. 9
3.1. Study Design ................................................................................................................................................................. 9
3.2. Computer-Aided Additive Manufacturing ............................................................................................................. 11
3.2.1. 3D Printing .............................................................................................................................................................. 11
3.3. Scanning of Restorations .......................................................................................................................................... 13
3.4. 3D Trueness Analysis ................................................................................................................................................ 14
3.5. Statistical Analysis ...................................................................................................................................................... 15
4. Results ...................................................................................................................................................... 16
4.1 Numerical Findings .................................................................................................................................................... 16
4.2. Qualitative Findings ................................................................................................................................................... 20
5. Discussion ............................................................................................................................................... 22
5. Conclusion .............................................................................................................................................. 29
6. Conflicts of Interest ............................................................................................................................... 30
7. Funding ................................................................................................................................................... 31
References ................................................................................................................................................... 32
v
List of Tables
Table 1: Rationale and function of provisional restorations. ................................................................ 1
Table 2: Chemical composition and mechanical properties of materials for provisional restorations.
......................................................................................................................................................................... 4
Table 3: Technical specifications of the printers. .................................................................................. 11
Table 4: Technical information about the printing resin material. ..................................................... 11
Table 5: Technical specifications of the desktop scanner. ................................................................... 14
Table 6: Total numbers of printed crowns and success rates. ............................................................ 16
Table 7: Descriptive data: Mean absolute deviation values±SD (µm) and number of samples (N).18
Table 8: Results of the two-way ANOVA. ............................................................................................ 18
vi
List of Figures
Figure 1: Components of a DLP printer. ................................................................................................. 4
Figure 2: Flow chart of the experimental steps. .................................................................................... 10
Figure 3: Desktop scanner used for scanning the crowns. .................................................................. 13
Figure 4: Crown stabilized with putty for scanning. ............................................................................. 14
Figure 5: View of different surfaces of crowns printed with DLP technology Asiga MAX UV (Asiga,
Alexandria, Australia) and Cara Print 4.0 (Kulzer, Hanau, Germany). .............................................. 16
Figure 6: Distribution of untransformed data. ...................................................................................... 17
Figure 7: Distribution of transformed data. ........................................................................................... 17
Figure 8: Plot of the estimated marginal means showing no significant interaction between surfaces
and printers. ................................................................................................................................................ 19
Figure 9: Error bars for the CI of the two printers. ............................................................................. 20
Figure 10: Color difference map of the 3D deviation analysis for the external and intaglio surfaces
for all crowns printed with the Asiga MAX UV (Asiga, Alexandria, Australia): Green-Good fit; Red-
Positive error; Blue-Negative error. ........................................................................................................ 20
Figure 11: Color difference map of the 3D deviation analysis for the external and intaglio surfaces
for all crowns printed with the Cara Print 4.0 (Kulzer, Hanau, Germany): Green-Good fit; Red-
Positive error; Blue-Negative error. ........................................................................................................ 21
Figure 12: 3D color difference map of crowns from both printers Asiga MAX UV (Asiga,
Alexandria, Australia) and Cara Print 4.0 (Kulzer, Hanau, Germany). .............................................. 21
Figure 13: Virtual positioning of supporting structure in the Asiga printer software (Asiga Composer,
Asiga, Alexandria, Australia). ................................................................................................................... 24
Figure 14: Virtual alignment of the crowns with different designs of supporting structure: Asiga
printer software (Asiga Composer, Asiga, Alexandria, Australia)- uniformly cylindrical support; Cara
printer software (Cara Print Cam, Kulzer, Hanau, Germany) – tapered support. ........................... 25
Figure 15: Yield of printed crowns on building platform: Red circles indicating failed crowns during
printing. ........................................................................................................................................................ 25
vii
Abbreviations
AM: Additive Manufacturing
ANOVA: Analysis of Variance
CAD: Computer Aided Design
CI: Confident Interval
DMD: Digital Micromirror Device
DLP: Digital Light Processing
FDA: Food and Drug Administration
LED: Light-Emitting Diode
mm: Millimeter
µm: Micron or micrometer
MPa: Megapascal
PEMA: Polyethyl Methacrylate
PMMA: Polymethyl Methacrylate
RMSE: Root Mean Square Estimate
SD: Standard Deviation
SE: Standard Error
SLA: Stereolithography
STL: Standard Transformation Language, Surface Tessellation Language, or Standard Triangulation
Language
3D: Three-dimensional
UV: Ultraviolet
viii
Abstract
Trueness Evaluation of Three-Dimensionally (3D) Printed Provisional Crowns
by Two Digital Light Processing (DLP) Printers
Objective: The purpose of this in vitro study is to evaluate the trueness of three-dimensionally (3D)
printed provisional crowns using two high accuracy Digital Light Processing (DLP) printers.
Methods: A total of 20 provisional crowns were printed using UV-polymerized resin in two
different DLP printers, Cara Print 4.0 (Kulzer, Hanau, Germany) and Asiga MAX UV (Asiga,
Alexandria, Australia). These crowns were based on a reference crown obtained from a Standard
Tessellation Language (STL) file of a crown for a mandibular molar. Once the printing was
completed, the post-processing protocol followed the manufacturing instructions. Specimens were
placed in an alcohol bath using 99% isopropyl for two consecutive five-minute cycles, followed by
trimming of the supporting structures. Finally, printed crowns were soaked again for one minute in a
99% isopropyl alcohol bath. The additional light-cure step was not performed, which was the
purpose of this study to check for trueness of the printers without interference of additional light
polymerization. The external and intaglio surfaces of all 20 samples were scanned using a desktop
scanner (3Shape D710) to generate new STL files. To perform the trueness analysis, a surface
matching software (Geomagic 12) was used for superimposing the reference STL file with the new
STL files of the printed crowns and was used to perform a 3D comparison. Statistical analysis was
performed with two-way ANOVA with Dunnett’s T3 post-hoc for groupwise comparison at
α=0.05.
Results: There was a statistically significant difference in trueness between the two printers
(p<0.05). However, there was no significant difference between intaglio and external surfaces, and
ix
there was no interaction between the two variables, printers and surfaces, and they were found to be
not significant (p>0.05).
Conclusion: Within the limitations of this study, it can be concluded that trueness of 3D printed
provisional crowns using DLP printers is device dependent. While they print the external and
intaglio surfaces of the restorations equally well within the clinically acceptable range, one performed
significantly better than the other.
Clinical Significance: 3-dimensional (3D) printing technology is emerging in the dental field,
offering varieties of open opportunities to perform fast and reliable products. 3D printed
provisional crowns will help both the patient and clinician to decrease number of visits and deliver
durable provisional restoration in one visit without the need for multiple visits.
Keywords: 3D printed provisional restoration, Trueness, DLP technology, Surface matching
1
1. Introduction
Provisional restorations, also called interim restorations, are mostly required when
performing restorative procedures, for temporization or provisionalization of an on-going
procedure.
1
Provisionalization is necessary prior to placement of a final or definitive restoration.
2
Many authors have highlighted the rationale and the importance of provisional treatment as shown
in Table 1.
2-8
Table 1: Rationale and function of provisional restorations.
Comfort and
protection of pulpal
tissues and teeth
Cover exposed dentin and protect the prepared tooth from dentin
sensitivity and plaque accumulation on freshly prepared dentin with
open dentinal tubules which may cause subsequent pulpal pathology
Coronal seal In root-canal-filled teeth, maintenance of a good coronal seal can be
pivotal in a successful outcome to root canal treatment
Occlusion and
positional stability
Prevent migration of abutments by maintaining optimal proximal and
occlusal contacts
Esthetics An acceptable appearance, which will mimic the original tooth or
that of the final restoration, is essential and can help prevent
unwanted problems with tooth shape when fitting the final
restoration
Periodontal health Provide cleansable margins with appropriate emergence profile to
facilitate oral hygiene and stabilize gingival health as well as prevent
gingival overgrowth
Diagnosis Assess the effect of occlusal and aesthetic changes with the patient.
The provisional restorations can be altered as required until the
desired change is attained, and this can then be communicated to the
laboratory for incorporation into the final restoration. Also, they may
be used to help measure tooth reduction and assess a tooth’s long-
term prognosis.
Other Allow evaluation of vertical dimension, phonetics, and masticatory
function, provide anchorage for orthodontic brackets during tooth
movement, and evaluate and reinforce the patient’s oral home care
Provisional restorations have been fabricated from a variety of materials using mainly two
approaches: 1) chairside or directly
3546
in the clinical setting or 2) indirectly in the laboratory; a
combination of the two techniques is also possible.
3,9
For both approaches, the materials used can be
2
classified into chemically activated acrylic resins, heat-activated acrylic resins, light-activated acrylic
resins, and “dual” light and chemically activated acrylic resins.
3
Examples of these materials include
polymethyl methacrylate resins (PMMA), polyethyl methacrylate (PEMA), combinations of unfilled
methacrylate resins, and composite resins.
3
Nowadays, provisional restoration fabrication solely depends on conventional methods that
involve hand processing, either directly or indirectly. However, with technologic advancements,
fabrication of provisional restorations can be facilitated by subtractive or additive manufacturing
(AM), known as prototyping, for three-dimensional (3D) printing or milling of objects of various
shapes and sizes. These technologies have paved their way in applications in dentistry such as
printed surgical
10
and endodontic guides
11
, occlusal splints
12,13
and dental models.
14
In comparison to
AM, subtractive manufacturing involves increased material wastage, wear and tear of the milling
components, and they have limitations in terms of geometric design.
15,16
On the contrary, AM
technology is more favorable as less material is wasted and it allows more freedom in geometric
designs.
17
In term of files being used for such technology, utilization of Computer aided design
(CAD) software which enables the production of 3D design data. 3D CAD files usually come in the
format of many different names such as Stereolithography, Standard Transformation Language,
Surface Tessellation Language, or Standard Triangulation Language, also known as (STL) file, all of
which describe the surface of 3D bodies.
18
With advancements in digital dentistry for digital workflow, compared to the conventional
one, now it is possible to have everything done in one visit. According to Tahayeri et al.
19
once the
patient comes in, the dental provider starts by preparing the intended tooth, then scans the prepared
tooth, and designs the provisional digitally. The information is then sent to a chairside 3D printer
and processed within 20 minutes in addition to the time for post-processing procedures that include
cleaning and light-curing. The restoration is then finally ready for cementation.
19
3
AM utilizes printing technologies, which can be classified into four categories: (1) extrusion
printing, (2) inkjet printing, (3) laser-melting/sintering, and (4) lithography printing.
19
In dentistry,
most commonly lithography is used for 3D printing in the form of Stereolithography (SLA) and
Digital Light Processing (DLP).
18
SLA is the oldest and most frequently and commonly used technique for 3D printing in
dentistry.
18,28
This method utilizes a scanning laser to activate the monomer, which works by
structuring layers for objects constructed or printed by ultraviolet (UV)-sensitive liquid monomer
that gets polymerized and hardens by a laser scan.
18
SLA comes in two motions for the build
platform, one is top-down and the second is bottom-up approach.
The top-down approach building
platform is immersed in a liquid resin reservoir and then polymerized by ultraviolet (UV) single laser
beam that located under the resin reservoir.
18
For the approach of bottom-up, the laser beam scans
from the top of the resin reservoir, where the build platform moves down and a new layer of resin is
applied then comes back up to be cured by the laser beam.
18
In addition to the SLA, DLP is also becoming more common to be used in dentistry, in
which its light source comes from a projector that cures the entire layer at once.
30
The way it work is
just as the same approach of top-down, where the build platform moves vertically in z-axis.
31
When
compared to SLA, DLP uses a projector light source of high-power Light-Emitting Diode (LED) to
cure an entire layer, with a single shot. This is related to the unique digital micromirror device
(DMD) instead of curing each layer one after the other with precise laser light, that goes around the
layers to cure them,
18,28,29,31
which makes DLP printer faster.
18,30
Another difference that helps in
saving resin material is that the resin reservoir or vat for the DLP printer is relatively small
compared to the SLA printer.
30
The components of a DLP printer are illustrated in Figure 1.
4
Figure 1: Components of a DLP printer.
Printing materials for both SLA and DLP are made of photopolymerizing resin systems
that
become solid once light-cured.
22,23
These materials undergo a photo-chemical process of
photopolymerization at UV or violet range. This process involves linking small monomers to chain-
like polymers. To initiate the reaction, a catalyst is needed, which is the photo-initiator.
24
However, it seems that the classification of materials made for making provisional
restoration follows almost the same classification for conventional ones, either methacrylate or
composite resins.
20
Table 1 summarizes the available resin materials for provisional restorations
approved by the Food and Drug Administration (FDA).
20,42
Table 2: Chemical composition and mechanical properties of materials for provisional
restorations.
Properties
Envisiontec Nextdent
E-Dent 100 E-Dent 400 C&B C&B MFH
Chemical composition
Tetrahydrofurfuryl
methacrylate,
urethane
dimethacrilate,
Monomer based
on acrylic esters
Methacrylic
oligomers,
Phosphine
oxides
Not
provided
x-axis
5
phosphinoxide, and
multifunctional
acrylic resins
Tensile strength (MPa) 30 N/mm2 Not provided Not
provided
Not
provided
Flexural strength (MPa) 85-135 >100 85 85-100
Modulus of elasticity
(MPa)
>4500 2100 2300-2500 2400-2600
Maximum recommended
time in the intraoral
environment
1 year 1 year Not
provided
Not
provided
Minimum wall thickness,
occlusal (mm)
2 2 Not
provided
Not
provided
Minimum wall thickness,
circumferential (mm)
1.5 1.5 Not
provided
Not
provided
Although AM technology seems very promising, especially in the field of restorative
dentistry, factors involving the capabilities of the 3D printer may influence the mechanical and
physical properties of the printed product.
24
These factors include the thickness of the printed layer,
the degree and depth of polymerization, speed and intensity of the laser, shrinkage between layers,
amount of supportive material, the direction and angle of printing, as well as post-processing
procedures.
19,24,25
The printing quality can be influenced by the printer’s capability to produce a
suitable printed object, therefore, in terms of printing a provisional restoration, it requires marginal
and internal fit that is up to 125 µm.
20 ,27
Therefore, Alharbi et al. demonstrated that using a 3D
printer to print provisional crowns resulted in a printing accuracy between 30 µm up to 100 µm
compared to the original CAD designs.
26
To check the accuracy of 3D printers, this study measured
precision and trueness. Precision of the 3D printer refers to the capacity to make the same object
with the same dimension. On the other hand, trueness pertains to the discrepancy between the
actual dimensions of the desired object and the printed object.
20,25
Despite other factors, the overall
6
accuracy can be influenced primarily by the layer thickness; the higher the thickness of each layer,
the less accurate the printed object is.
26
Therefore, higher accuracy could be achieved by reducing
the thickness of each layer.
19
Also, printing time can be reduced by modifying the contact area with the support structure,
therefore reducing the manufacturing time to clean the printed object from the support structure.
28
Additionally, the build angle has also an effect on time and accuracy. Alharbi et al. explained that the
selection of the build angle will offer a printed crown the highest accuracy when having the smallest
necessary support structure and a self-supported geometry.
24
For a SLA type printer, a build-angle of
120° in combination with thin support structure showed to be ideal for accuracy.
26
On the other
hand, for a DLP printer, the recommended angle is found to be at 135°, although SLA printer
produced more accurate printed objects.
31
But it is also important to note that the provided values
are specifically related to the specifications and abilities of each printer.
31
However, different studies have tested 3D printed provisional crowns after completing the
post-processing steps, focusing on evaluating the effect of degree of conversion, layer thickness,
post-curing process,
29
degree of mechanical properties compared to conventional provisional crowns
and found them to have sufficient mechanical properties for intraoral use.
19
Other studies focused
on the effects of build direction on the mechanical properties
24,31
and another study was conducted
to evaluate factors influencing the dimensional accuracy.
26
However, all these studies reported results
in favor of 3D printed provisional crowns after being light-cured, stating that it eliminates any
printing defects. For example, one study conducted by Tahayeri et al, where they tested mechanical
properties of printed crowns using SLA printer, compared to bis-acrylic composite resin material
known as (Integrity
®
, Dentsply); and methyl methacrylate acrylic material known as Jet
®
, Lang
Dental Inc.). Interestingly what they found is that the printed restorations were found to have
comparable mechanical properties with these most common used materials even before being
7
additionally light-cured.
19
The effect of light-cure after printing contributes to higher degree of
conversion of the resin, improving the printed product in terms of biocompatibility, preventing the
release of residual monomer, and promotes both physical and mechanical properties.
21,29
This helps
in strengthening the product through the final polymerization effect and might also lead to better
printing accuracy.
24,26,31
However, there has been no study evaluating the trueness of printed
restorations using DLP printers without the effect of post-process light curing. Therefore, the
trueness of printing provisional crowns using DLP printer prior to being light cured is needed to be
investigated, and to evaluate the outcomes.
8
2. Objective of the Study
For the issue addressed above, this paper aims to evaluate the trueness of three-
dimensionally (3D) printed provisional crowns using two high accuracy Digital Light Processing
(DLP) printers prior to additional light curing the 3D printed provisional crowns. The null
hypotheses are that (1) there is no statistical difference between the two DLP printers in their
trueness among the tested groups and (2) that there is no difference between the intaglio and
external surface for the restorations printed with the same printers (p > 0.05).
9
3. Material and Methods
3.1. Study Design
A reference scan Standard Tessellation Language (STL) file for a designed full-coverage
crown for a mandibular molar was obtained and used with a 1 mm chamfer finish line. The STL file
was used for 3D-printing the crowns using Digital Light Processing (DLP) technology with two
different printers Cara Print 4.0 (Kulzer, Hanau, Germany) and Asiga MAX UV (Asiga, Alexandria,
Australia). The recommended angle for the building process with minimum numbers of supporting
structures was determined to be 120°.
24
Total sample size for the trueness analysis was 20 printed
crowns, 10 per each printer. The printed samples were scanned using a lab desktop scanner (3Shape
D710 Dental Scanner, 3Shape, Copenhagen, Denmark). The scans were then superimposed for
surface matching using a metrological software (Geomagic Qualify 12, 3D Systems, Rock Hill, SC,
USA) to compare them to the reference crown file for analysis of trueness of the DLP printers. See
the flow chart illustrating the experimental steps (Figure 2).
10
Figure 2: Flow chart of the experimental steps.
11
3.2. Computer-Aided Additive Manufacturing
3.2.1. 3D Printing
A total of 20 samples were printed using two DLP printers: 10 samples were printed using
the Asiga MAX UV (Asiga, Alexandria, Australia) and 10 samples using the Cara Print 4.0 (Kulzer,
Hanau, Germany). Table 3 provides information on technical specifications of the two printing
systems.
37,38
Table 3: Technical specifications of the printers.
These two printers work by using a digital projector with a combined light source in the UV
range for the Asiga MAX UV and violet range for the Cara Print 4.0, which polymerizes the liquid
resin layer-by-layer across the entire platform at once. The resin employed in this protocol is a
photo-polymerized resin (DENTCA Crown and Bridge, A2 Shade, DENTCA, Torrance, CA, USA).
Product details about the resin provided by the manufacture (DENTCA, Torrance, CA, USA) that
are displayed in Table 4. This resin can be cured by light in the visible as well as in the UV range.
Table 4: Technical information about the printing resin material.
Technical specification Cara Print 4.0 (Kulzer,
Hanau, Germany)
Asiga MAX UV (Asiga,
Alexandria, Australia)
CAM Software Cara Print CAM Asiga Composer
Building Platform Diameter
(X,Y,Z)
103x58x130 mm 119x67x75 mm
Light Source LED based on HD DLP UV-LED based on DLP
LED Wavelength 405 nm (violet) 385 nm (UV)
Resolution 53.6 µm 62 µm
Average Build Speed 50 mm/hour 60 mm/hour
Layer Thickness 30 – 100 µm 25-100 µm
Product Manufacturer Component Composition
DENTCA
Crown and
Bridge A2
Shade
DENTCA, Inc Methacrylate-based polymer
by radical polymerization
Urethane methacrylate oligomer,
methacrylate oligomers, and
monomers
12
The printing of the provisional crowns was based on the proprietary software of each printer
after transferring the STL file into each software (Cara Print Cam, Kulzer, Hanau, Germany; Asiga
Composer, Asiga, Alexandria, Australia). The printing parameters were standardized for both
printers. The printing layer thickness was set to 0.05 mm with a printing orientation of 120° angle,
with minimum supporting structure.
24
The supporting structures were placed on the buccal side of
the restoration as it had more surface area to accommodate them. The supporting structures were
placed automatically by each software programs. Supports near the margins of the crowns were
manually removed in the software before printing. Also, the automatically suggested dimensions of
the supporting structures were not modified. As the supporting structures were suggested differently
by each software, therefore, no standardization for both printers was possible.
Both printing machines have different dimensions for their building platform with the Asiga
MAX UV (Asiga, Alexandria, Australia) platform being larger than the one of Cara Print 4.0 (Kulzer,
Hanau, Germany) . Therefore, the building platform was utilized to print more than 10 crowns, in
order to check how successful each printer can print. The Asiga printer was able to accommodate 18
crowns, and Cara printer accommodated for 15 crowns. After printing the restorations, post-
processing procedures according to the resin manufacturing instructions were performed. First,
specimens were placed in a static alcohol bath using 99% isopropyl alcohol (IPA, Del Amo
Chemical, CA, USA) for two consecutive five-minute cycles. Specimens were then cleaned using a
brush to remove any residual resin, followed by manual trimming of the supporting structures using
a clipper (Kaisi KS-107, Kaisi Industry, Yongkang, China). Finally, they were soaked again for one
minute in 99% isopropyl alcohol (IPA, Del Amo Chemical, CA, USA) and air-dried. To check the
printing trueness only, the step of additional light-curing the specimens was eliminated and samples
were stored wrapped in aluminum foil (Kirkland Signature, Costco, Issaquah, WA, USA) to avoid
13
exposure to ambient or any other sources of light, which may polymerize the specimens further and
lead to dimensional changes.
3.3. Scanning of Restorations
A desktop scanner (3Shape D710 Dental Scanner, 3Shape, Copenhagen, Denmark) with an
accuracy of 10 µm was used to scan the samples (Figure 3).
39
Technical specification of the scanner
are listed in Table 5.
39
The scanner works by using two cameras and a red laser that have the ability
to effectively scan impressions, deep inlays and even full undercuts. The 3-axis motion system tilts,
rotates, and translates the object, facilitating scanning from any spherical viewpoint up to 350°. The
specimens were stabilized with putty material during the scanning process (Figure 4). Each sample
was scanned twice, once to capture the external and a second time to capture the intaglio surface of
the crown. For each scan a separate STL file was generated.
Figure 3: Desktop scanner used for scanning the crowns.
14
Table 5: Technical specifications of the desktop scanner.
Software Dental System Premium
Cameras 2 cameras with 3-axis motion system
Light technology Red laser
Accuracy for crown and bridge 10 µm
Scan time for single die – 3-unit bridge 25, 30 seconds – 80, 100 seconds
Figure 4: Crown stabilized with putty for scanning.
3.4. 3D Trueness Analysis
For performing the 3D-deviation analysis to determine trueness, the STL files were digitally
imported into a metrology software program (Geomagic Qualify 12, 3D Systems, Rock Hill, SC,
USA) and each reference STL file was virtually aligned with the newly obtained test STL file. The
scanned crowns were imported into the software as a new file named test file. Once the restoration
was trimmed, the reference STL file was imported into the software and marked as the reference
file. The two files were superimposed on each other to perform the surface matching process and
trueness analysis. Trueness refers to how close a test scan resembles the scan taken by a reference
15
scanner.
33
In this study, the trueness was analyzed by comparing the different test scans obtained
from the printed crowns of both printers with the reference scan of the provisional crown. For 3D
comparison, a maximum critical value of ±0.100 mm and a maximum nominal value of ± 0.03 mm
was set. The assignment of these threshold values facilitates the deviation measurement between the
reference crown and all 20 test crowns for their intaglio and external surfaces, separately. The
software analyzed and displayed positive and negative values in red, blue and green colors,
representing maximum and mean distances (positive and negative deviation) as well as the standard
deviation between the "capturing points" of the two digital crowns.
3.5. Statistical Analysis
The 3D analysis provided 3D comparison reports for each sample. To perform the trueness
analysis, average positive and negative deviations between the reference crown and test crown were
obtained to provide the Mean Absolute Deviation.
34,35
The formula to calculate the Mean Absolute
Deviation was:
!"#$!%# '()*+*"# ,#"*!+*(- |/| !"#$!%# -#%!+*"# ,#"*!+*(-
0
.
Data were tabulated in a spreadsheet program (Microsoft Excel, Microsoft, Redmond, WA, USA)
and then imported into a statistical software (SPSS Statistics V26.0, IBM, NY, USA). Data were then
statistically analyzed using a two-way Analysis of Variance (ANOVA) followed by Dunnett’s T3
post-hoc, since equal of variance assumption was violated. The dependent variable was the Mean
Absolute Deviation, the two factors as independent variables were the printers and the surfaces.
Differences below α=0.05 were considered to be significant.
16
4. Results
4.1 Numerical Findings
The total numbers of printed crowns for each printing device with the numbers of the
successfully printed crowns is depicted in Table 6.
Figure 5 shows an example of a printed crown using DLP printing technology from
different perspectives.
Table 6: Total numbers of printed crowns and success rates.
Printer
Total
N
Successfully Printed
N (%)
N Used for
Evaluation
Asiga 18 17 (94.4) 10
Cara 15 11 (73.3) 10
Figure 5: View of different surfaces of crowns printed with DLP technology Asiga MAX UV
(Asiga, Alexandria, Australia) and Cara Print 4.0 (Kulzer, Hanau, Germany).
Statistical analysis shows that the data for specimens obtained with the Cara printer was not
normally distributed as an extreme outlier was present; therefore, a logarithmic transformation
function of the data was performed for the dependent variable absolute mean deviation value.
Figure 6 shows the non-transformed data with the extreme outlier. Figure 7 shows the transformed
data with an outlier that is less extreme. The ANOVA is considered as a robust test that can deal
with such less extreme outlier to be included in the normal distribution assumption.
17
Figure 6: Distribution of untransformed data.
Figure 7: Distribution of transformed data.
18
After transformation, the data appeared normally distributed; with an overall therefore, the two-way
ANOVA was performed. Mean absolute deviation values are based on the pooled trueness analysis
for all 20 scans of the printed crowns. Table 7 displays the descriptive data of the study.
Table 7: Descriptive data: Mean absolute deviation values±SD (µm) and number of samples
(N).
Asiga
a
Cara
a
N
Overall 58±15 µm 69±21 µm 20
External
58±8 µm 70±13 µm 20
Intaglio 57±20 µm 68±27 µm 20
a
All
Values represent mean±SD except where indicated.
Two-way ANOVA test detected a significant difference for the mean absolute values between the
two DLP printers (p=0.048; Table 8).
Table 8: Results of the two-way ANOVA.
Mean Absolute Deviation
Values (trueness)
Asiga
a
Cara
a
Mean
Difference
P-value
CI
58±15 µm 69±21 µm 11 0.048 45-99
a
All
values represent mean±SD
There was no significant difference between the intaglio and external surfaces (p=0.438) and the
interaction between printer and surface was not significant (p=0.884; Figure 8).
19
Figure 8: Plot of the estimated marginal means showing no significant interaction between
surfaces and printers.
The mean for the Asiga MAX UV (Asiga, Alexandria, Australia) was 58±15 µm and for the Cara
Print 4.0 (Kulzer, Hanau, Germany) 69±2 µm. Based on the error bars graphs, the Confidence
Interval (CI) between the two printers shows that Asiga printer performed better (Figure 9). The
Cara printer has more deviation especially for the external surface. The Asiga printer has almost
similar values for both surfaces that are lower than compared to the Cara printer.
20
Figure 9: Error bars for the CI of the two printers.
4.2. Qualitative Findings
Figure 10 andFigure 11 show the trueness maps of the external and intaglio surfaces of
crowns manufactured with both printers. Green indicates values within the tolerance range based on
the qualitative analysis and this represents an accurate surface matching. Other colors indicate errors
in terms of trueness: cyan to blue indicates negative errors, which means there is a decrease in the
dimension of the printed crown, and yellow to red indicates a positive error due to increase in the
dimension of the printed crown.
Figure 10: Color difference map of the 3D deviation analysis for the external and intaglio
surfaces for all crowns printed with the Asiga MAX UV (Asiga, Alexandria, Australia):
Green-Good fit; Red-Positive error; Blue-Negative error.
21
Figure 11: Color difference map of the 3D deviation analysis for the external and intaglio
surfaces for all crowns printed with the Cara Print 4.0 (Kulzer, Hanau, Germany): Green-
Good fit; Red-Positive error; Blue-Negative error.
Trueness analysis was conducted for the external surface and the intaglio surface for each
sample from both printers. For both printers, the majority of the positive values are related to the
occlusal grooves and the buccal side where the supporting structures were placed. Also, the intaglio
surface of many samples has shown positive deviation as well. In terms of negative values, they were
observed on the buccal side around the areas where the supporting structure was placed, in addition
to the lingual side of the external surfaces (Figure 12). Few samples of the intaglio surface have
extreme negative values. Although both printers have shown positive and negative values, the Cara
Print 4.0 (Kulzer, Hanau, Germany) was showing more deviations.
Figure 12: 3D color difference map of crowns from both printers Asiga MAX UV (Asiga,
Alexandria, Australia) and Cara Print 4.0 (Kulzer, Hanau, Germany).
22
5. Discussion
This in vitro study evaluated the trueness of provisional crowns that were three-
dimensionally printed with DLP technique using UV-polymerized resin before post-processing
through light cure. Based on the results, the first null hypothesis was rejected, as the result showed a
statistically significant difference between the two printers used. However, the second null
hypothesis was accepted, as there was no statistically significant difference between the intaglio and
the external surface. A total of 20 crowns were 3D printed using two DLP printers, Cara Print 4.0
(Kulzer, Hanau, Germany) and Asiga MAX UV (Asiga, Alexandria, Australia) and evaluated before
being light-polymerized. To the author’s knowledge, the current study is the first study to evaluate
the trueness of 3D printed crowns by DLP printers before being additionally light-polymerized.
Additional light-curing after printing is used in general to enhance the mechanical and physical
properties, however it also can affect accuracy. Therefore, this step was excluded in this study. To
eliminate and/or minimize the handling and processing errors, one trained researcher performed the
fabrication and post-printing process, including the trueness analysis.
In terms of the trueness of the printed samples, a mean deviation of 58±15 µm was
observed for the Asiga MAX UV (Asiga, Alexandria, Australia) and 69±21 µm for the Cara Print 4.0
(Kulzer, Hanau, Germany). These printed crowns were printed at a 120° built-angle. When
compared to a study that used DLP technique for 3D printed crowns, the accuracy presented in
their study was 56 µm.
31
Another study that used SLA type printers revealed accuracy between 29-31
µm depending on the configuration of the supporting structures, where the first value is referred to
a thinner support structure, and the second value belongs to thicker type of support structure.
26
Kang et al. on the other hand, reported trueness values for printed provisional crowns using SLA
technology, however their printing angle was 180° with the support structure placed on the occlusal
surface. The reported trueness values ranged from 22.5±21 µm for the intaglio surface to 49.6±9.3
23
µm for the external surface.
36
The trueness values generated in the study by Kang and his colleagues,
were in part comparable to ours, but we observed less variation between both surfaces in our study.
E.g., for the printer that performed better in our study Asiga MAX UV (Asiga, Alexandria, Australia)
values ranged from 57±20 µm (intaglio) to 58±8 µm (external). Nonetheless, the presented absolute
mean values (from 58±15 µm to 69±21 µm) for both printers in our study are still considered within
the range of reported data, taking into the account theses samples were not light-cured once they
were cleaned after printing. Most of the studies that evaluated 3D printed crowns have photo-cured
their samples after printing between 10 – 30 minutes.
24,26,31,36
In fact, SLA printers have higher
resolution compared to DLP printer, that starts at 25 µm,
19
compared to the DLP printers used in
this study, Cara Print 4.0 (Kulzer, Hanau, Germany) has 53.6 µm of resolution
38
and Asiga MAX UV
(Asiga, Alexandria, Australia) is 62 µm.
37
Further analysis of the color map showed a positive deviation on the buccal surface close to the
support structure; this is in agreement with another study.
31
The authors explain this due to the
movement of the building platform in an upward motion during the fabrication process, therefore
causing the material to sag due to its weight,
31
(Figure 12). This was more observed in samples
printed by the Cara Print 4.0 (Kulzer, Hanau, Germany). In this essence, the support structure was
generated automatically by the software of both printers, however, any support structure that was
generated near to the margins of the crown was manually eliminated.
24
Figure 13 shows an example of the virtual positioning of supporting structures (violet) in the Asiga
printer software (Asiga Composer, Asiga, Alexandria, Australia).
.
Figure 13: Virtual positioning of supporting structure in the Asiga printer software (Asiga
Composer, Asiga, Alexandria, Australia).
Besides, the form and shape of the Cara Print 4.0 (Kulzer, Hanau, Germany) supporting
structures look different than the ones generated by the Asiga MAX UV (Asiga, Alexandria,
Australia). The Cara Print 4.0 (Kulzer, Hanau, Germany) supporting structures are partly tapered;
while starting with a wider diameter at the level of the build platform, they become gradually thinner
as they approach the object forming a cone shape. The supporting structure of Asiga MAX UV
(Asiga, Alexandria, Australia) are cylindrical and uniform (Figure 14). The dimeters of supporting
structures of both printers were not modified and used as suggested by the software.
25
Figure 14: Virtual alignment of the crowns with different designs of supporting structure:
Asiga printer software (Asiga Composer, Asiga, Alexandria, Australia)- uniformly cylindrical
support; Cara printer software (Cara Print Cam, Kulzer, Hanau, Germany) – tapered
support.
To assess the trueness, only 10 printed crowns were necessary in each group. However, the
investigators also wanted to assess the total yield of successfully printed crowns per build platform
for each printer. Both printers vary in the size of the build platform. While the Asiga MAX UV
(Asiga, Alexandria, Australia) can accommodate 18 crowns, the Cara Print 4.0 (Kulzer, Hanau,
Germany) can only accommodate 15 crowns. Out of the 18 crowns printed by the Asiga device, all
except one crown were successfully printed (94.4%). On the other hand, for the Cara printer only 11
out of 15 were successfully printed (73.3%). The crowns from both printers that did not complete
the printing process are depicted in Figure 15. This picture was taken immediately after printing,
before cleaning in the alcohol bath. On the other hand, the Cara Cara Print 4.0 (Kulzer, Hanau,
Germany) was faster in printing the crowns (25 minutes), while the Asiga MAX UV (Asiga,
Alexandria, Australia) needed one hour and two minutes.
Figure 15: Yield of printed crowns on building platform: Red circles indicating failed crowns
during printing.
Osman et al. have tested nine different angles of printed provisional crowns and placed all
crowns in the center of the build platform, as the DLP printer that was used has a LED light with a
narrow spectrum of 405.
31
In our study, the two printers have different wavelengths, Asiga MAX
26
UV (Asiga, Alexandria, Australia) has a UV light of 385nm, where Cara Print 4.0 (Kulzer, Hanau,
Germany) has a violet light of 405 nm. Interestingly, although Cara has a different source of light
and a longer wavelength, the number of crowns that failed is more than Asiga. All these crowns that
weren’t printed are on the corner on both printers. This is in accordance with a study that
highlighted the effect of the radiant energy that to be higher on the center of the build platform,
which leads to a better-printed object, compared to an object placed in a distance from the center
which caused ununiforme solidifying effect leading to less effective printing.
40
Another issue was found with crowns that were fabricated with the Cara Print 4.0 (Kulzer,
Hanau, Germany). Some of them sustained defects were in the areas where the supporting structures
had been placed. Once the support structure was removed some depressions were left, which
resulted in negative trueness values that were highlighted in blue. On the external surfaces, the Cara
printer exhibited higher means of 70±13 than the Asiga with 58±8, although these differences were
statistically not significant (p=0.438). Clinically, these defects in the crown surfaces might have made
a reprint of the restorations necessary. It is possible that all of these factors could contribute to the
difference in trueness values between the two printers.
Scanning of printed crowns was performed using the desktop scanner (3Shape D710 Dental
Scanner, 3Shape, Copenhagen, Denmark) which is an important step for providing new STL files to
conduct the 3D analysis. This scanner provides an accuracy of 10 µm.
39
This could influence our
results as this is not the best scanner available with high scanning accuracy. In a study that compared
different desktop scanners, among them was the same scanner used in this study, it was concluded
that a scanner with higher scanning accuracy is more useful to compare trueness and precision
analysis.
41
For the evaluation of trueness, a metrological software (Geomagic Qualify 12, 3D Systems,
Rock Hill, SC, USA) was used. To compare the reference with the test STL file, the software usually
27
automatically aligns both files. However, this automatic alignment does not always work. In such
cases, the alignment process has to be performed manually using three-point detection, introducing
the possibility of extremely deviating values.
It was evident from our study that the Asiga MAX UV (Asiga, Alexandria, Australia)
performed significantly better in terms of trueness, although the time needed for printing was almost
double the amount needed for printing crowns by the Cara Print 4.0 (Kulzer, Hanau, Germany).
Additionally, provisional crowns printed with the Asiga MAX UV (Asiga, Alexandria, Australia)
were not defective in relation to supporting structures, as observed in several crowns printed with
the Cara Print 4.0 (Kulzer, Hanau, Germany).
Based on our findings, there are different factors affecting the trueness analysis. However,
the trueness of the provisional crowns produced by the Cara Print 4.0 (Kulzer, Hanau, Germany)
was inferior to the Asiga MAX UV (Asiga, Alexandria, Australia), which may influence the internal
and marginal fit of these restorations due to errors on the intaglio surface. In addition to that, errors
presented on the external surface may have an effect on the adjacent teeth, soft tissue, contact
points, and occlusion.
36
Although the Cara Print 4.0 (Kulzer, Hanau, Germany) has a different light
source that is usually recommended by the resin manufacturers, Asiga MAX UV (Asiga, Alexandria,
Australia) on other hand performed significantly higher, in terms of trueness and numbers of crown
printed, however, it was slower. It is also important to mention that the evaluated printed crowns in
our study were with the buccal surface included in the analysis that has the supporting structures,
which in case if it was not included, it might provide different results. Also, it was evident that
crowns that failed to be printed were all located far from the center, which confirms some of the
speculations with regards to the effect of the light intensity, however, we did not check the trueness
of crowns that were often close to the center. More importantly, the dimeters of the supporting
28
structures were not standardized between the two printers, if that was taken into account, different
results would be expected.
One of the limitations of this study is that it was conducted by using a 3D metrology analysis
software program (Geomagic Qualify 12, 3D Systems, Rock Hill, SC, USA). This software provided
a table of results based on a 3D comparison, which includes different values and deviations. The
available software package only reported the trueness analysis based on the mean absolute deviation,
which relies on averaging the positive average deviation and the negative average deviation.
However, other studies have reported their accuracy, including trueness, based on root mean square
estimate values (RMSE).
24,26,31,36
Geomagic 12 doesn’t provide such values, unlike higher versions,
such as Geomagic Studio 2014 (3D Systems). Also, samples were not coated with special powder
(Helling 3D, Laser Design, Minneapolis, MN, USA) before scanning, as the provided powder
resulted in scanning errors when it was used for few of our samples in a pilot study. This was
observed that all samples showed positive deviation (red color) in the areas of the occlusal grooves.
Possibly, scattering of the scanning light or inability of the scanner to reach the depth of the grooves
properly might have had an influence on the accuracy in these areas.
29
5. Conclusion
Within the limitation of this study, it can be concluded that the two evaluated DLP printing
machines can print restorations with high trueness that is within the clinically acceptable range for
external and intaglio surfaces. However, the amount of accuracy in terms of trueness is device
dependent as one of the printing systems performed significantly better than the other system.
30
6. Conflicts of Interest
The authors do not report any conflict of interest.
31
7. Funding
This study was funded by the Advanced Operative and Adhesive Dentistry Program in the
Division of Restorative Sciences at the Herman Ostrow School of Dentistry of USC.
32
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Abstract (if available)
Abstract
Objective: The purpose of this in vitro study is to evaluate the trueness of three-dimensionally (3D) printed provisional crowns using two high accuracy Digital Light Processing (DLP) printers. ❧ Methods: A total of 20 provisional crowns were printed using UV-polymerized resin in two different DLP printers, Cara Print 4.0 (Kulzer, Hanau, Germany) and Asiga MAX UV (Asiga, Alexandria, Australia). These crowns were based on a reference crown obtained from a Standard Tessellation Language (STL) file of a crown for a mandibular molar. Once the printing was completed, the post-processing protocol followed the manufacturing instructions. Specimens were placed in an alcohol bath using 99% isopropyl for two consecutive five-minute cycles, followed by trimming of the supporting structures. Finally, printed crowns were soaked again for one minute in a 99% isopropyl alcohol bath. The additional light-cure step was not performed, which was the purpose of this study to check for trueness of the printers without interference of additional light polymerization. The external and intaglio surfaces of all 20 samples were scanned using a desktop scanner (3Shape D710) to generate new STL files. To perform the trueness analysis, a surface matching software (Geomagic 12) was used for superimposing the reference STL file with the new STL files of the printed crowns and was used to perform a 3D comparison. Statistical analysis was performed with two-way ANOVA with Dunnett’s T3 post-hoc for groupwise comparison at α=0.05. ❧ Results: There was a statistically significant difference in trueness between the two printers (p<0.05). However, there was no significant difference between intaglio and external surfaces, and there was no interaction between the two variables, printers and surfaces, and they were found to be not significant (p>0.05). ❧ Conclusion: Within the limitations of this study, it can be concluded that trueness of 3D printed provisional crowns using DLP printers is device dependent. While they print the external and intaglio surfaces of the restorations equally well within the clinically acceptable range, one performed significantly better than the other. ❧ Clinical Significance: 3-dimensional (3D) printing technology is emerging in the dental field, offering varieties of open opportunities to perform fast and reliable products. 3D printed provisional crowns will help both the patient and clinician to decrease number of visits and deliver durable provisional restoration in one visit without the need for multiple visits.
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Asset Metadata
Creator
Alshanbari, Mohammed
(author)
Core Title
Trueness evaluation of three-dimensionally (3D) printed provisional crowns by two digital light processing (DLP) printers
School
School of Dentistry
Degree
Master of Science
Degree Program
Biomaterials and Digital Dentistry
Publication Date
09/21/2020
Defense Date
06/29/2020
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
3D printed provisional restoration,DLP technology,OAI-PMH Harvest,surface matching,trueness
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Phark, Jin-Ho (
committee chair
), Duarte, Sillas Jr. (
committee member
), Son, Jenny (
committee member
)
Creator Email
alshanba@usc.edu,dr.mohd07@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c89-375747
Unique identifier
UC11665829
Identifier
etd-Alshanbari-9006.pdf (filename),usctheses-c89-375747 (legacy record id)
Legacy Identifier
etd-Alshanbari-9006.pdf
Dmrecord
375747
Document Type
Thesis
Rights
Alshanbari, Mohammed
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
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...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Tags
3D printed provisional restoration
DLP technology
surface matching
trueness