ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES.

Greice Oliveira

A thesis submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Master of Science in the Department of Orthodontics in the School

of Dentistry.

Chapel Hill 2019

Approved by:

Tung Nguyen

Tate Jackson

Angela Broome

ii

© 2019 Greice Oliveira

ALL RIGHTS RESERVED

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ABSTRACT

Greice Oliveira: Accuracy and Precision of 3-Dimensional Printed Dental Models Produced by Different Additive Manufacturing Technologies (Under the direction of Tung Nguyen)

The primary aim of this investigation was to evaluate the accuracy of dental models manufactured

by different additive manufacturing processes: Stereolithography Apparatus (SLA), Liquid Crystal

Display with Unidirectional Peel (LCD-SLA/UDP), and Digital Light Processing (DLP). The secondary

objective was to test the accuracy of the models printed at different Z-plane resolutions. To accomplish

these goals, twelve 3-D digital surface models were selected, exported as surface model stereolithography

files, and printed using the additive manufacturing processes mentioned. Models were printed at 50 and

100µm resolution. The printed models were rescanned to create digital STL models and superimposed to

the original scan. No statistically significant difference between layer thicknesses was found for the SLA,

and for the DLP. The LCD-SLA/ UDP technology showed statistical significant differences between

different layer thicknesses. The SLA technology showed statistically significant difference from DLP and

LCD-SLA/ UDP. No significant difference was observed between DLP and LCD-SLA/ UDP.

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ACKNOWLEDGEMENTS

Thank you to my committee members, Dr. Nguyen, Dr. Jackson, and Dr. Broome, for your

guidance throughout this project. Thank you to my mentor, Dr. Nguyen, whose expertise was invaluable

in the formulating of the research topic and methodology. You provided me with the tools that I needed to

successfully complete my dissertation. Thank you to Ceib Phillips for her expertise in statistics. Thank

you to Nicki Parkstone and Cary Shepherd for their help and dedication with the laboratory work. Thank

you to the Southern Association of Orthodontists for the awarded research grant. A very special thank you

to my parents, Dejair and Cleide, for their constant support, and to my loving husband, Gustavo, and my

two daughters, Antonia and Cecilia for providing unending inspiration.

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TABLE OF CONTENTS

LIST OF TABLES …………………………………………………………………………….…………vii

LIST OF FIGURES …………………………………………………………….……………………….viii

LIST OF ABBREVIATIONS …………………………………………………………………………….ix

LIST OF SYMBOLS ………………………………………………………………………….…………..x

REVIEW OF THE LITERATURE …………………………………………………………….……….…1

Plaster Versus Digital Models ……………………………………………………………….….…1

3D Printing Technology ……………………………………………………………………..…….4

3D Manufacturing Technologies classification ……………………………………………..…….4

Additive Manufacturing Technologies …………………………………………………….….…..5

3D Printing Resolution ……………………………………………………………….………..…..6

Influence of Layer Thickness on 3D Printing …………………………………………..…………7

Accuracy of 3D Printed Dental Models ……………………………………………….….……….9

Clinical Applications of 3D Printed Dental Models ……………………………………………..10

References ………………………………………………………………………………………..12

ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES………………...15 Introduction ……………………………………………………………………………..………..15

Methods …………………………………………………………………………………..………17

Results ……………………………………………………………………………………...…….18

Discussion …………………………………………………………………………………..……19

Conclusion ………………………………………………………………………………….……22

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References……………………………………………………………………………………...…23

APPENDIX 1: TABLES AND FIGURES ………………………….……………………………………25

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LIST OF TABLES

Table 1: Difference in overall accuracy between different layer thicknesses for each 3D printer………..25

Table 2: Difference in overall accuracy between 3D printers by different layer thicknesses …………….26

Table 3. Photopolymer resins based on the manufacturers recommendations……………………………26

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LIST OF FIGURES

Figure 1: Study methodology flowchart ……………………………………………………………….....27

Figure 2: Chart comparing the 3D printing technologies and printers investigated in this study.……...…28

Figure 3: Close up view of models printed at 50 micron and 100 micron resolution ………………….....29

Figure 4: 3M™ High-Resolution Scanning Spray applied on printed models. Close up view of the spray coating…………………...…………………………………………………………………….……29

Figure 5: Superimposition of 3D printed models to their original counterpart …………………………..30

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LIST OF ABBREVIATIONS

3D Three Dimensional

AM Additive Manufacturing

RP Rapid Prototyping

CAD Computer-Aided Design

SLA Stereolithography Apparatus

DLP Digital Lighting Processing

FDM Fused Deposition Modeling

SLS Selective Laser Sintering

LCD-SLA/UDP Liquid Crystal Display with Unidirectional Peel

UV Ultraviolet

STL Stereolithography

ANOVA Analysis of Variance

SD Standard Deviation

x

LIST OF SYMBOLS

© Copyright symbol

® Registered Trademark

™ Trademark symbol

1

A REVIEW OF THE LITERATURE

Plaster versus digital models

Successful orthodontic treatment is dependent on comprehensive diagnosis and treatment

planning, and an accurate model analysis is critical for correct diagnosis. Three-dimensional (3D)

imaging has seen significant advances recently, allowing clinicians to develop the virtual orthodontic

patient. Advancements in 3D facial imaging, cone-beam computed tomography (CBCT), scanning

techniques, and dental model technology facilitate accurate reconstructions of teeth, soft tissue, and

bone.1

Plaster models are certainly the historic gold standard in dental diagnosis and treatment

procedures. Is has been shown that they present precise and reliable information about the patient’s dental

arches, teeth position and its dimensions. However, its use comes with a few important disadvantages.

Plaster models require physical storage space, they are prone to breakage and degradation over the years,

they lack in terms of efficiency when recovery is needed, and they represents a hurdle when sharing data

with other professionals is needed for multidisciplinary treatment.2-5

The benefits of using digital models in orthodontic practices are numerous. Primarily, digital

models are less labor intensive. Orthodontists can immediately send a digital scan of the teeth and

surrounding oral structures to an outside laboratory or share it with another dentist in cases which

multidisciplinary care is involved. Similarity, a digital diagnostic or treatment simulation can be

completed or models can be superimposed to evaluate treatment progress without risk of degradation or

loss of the model. There is no demand for physical storage as the patient data can be digitally archived

and only printed when needed.5-8

Digital study models were introduced commercially in 1999 by OrthoCad™ (Cadent, Carlstadt,

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NJ, USA), and in 2001 by E-model™ (GeoDigm, Chanhassen, MN, USA). New technologies were

introduced to the market later. 6 Currently, different technologies can be used to generate digital study

models, which may render discrepancies in accuracy between plaster and digital models.9

In 2003, Santoro et al. conducted a study to compare OrthoCad™ digital models to plaster

models. Two alginate impressions were taken from seventy-six subjects. One impression was

immediately poured to fabricate a standard plaster model, while the second impression was sent overnight

to OrthoCad™ to be digitized. Measurements such as tooth size, overjet, and overbite were taken from

the physical and digital models to the nearest 0.1mm. Physical models were measured with a Boley

gauge, and OrthoCad™ models were measured with OrthoCad™ software. The results regarding tooth

size and overbite showed significantly smaller measurements for digital models when compared to their

plaster counterpart. There was no statistically significant difference for overjet measurements. The

authors concluded that despite the differences reported, digital models produced by OrthoCad™ would be

considered an acceptable option for the purpose of routine diagnosis.4

In a similar study by Stevens et al. conducted in 2006, conventional plaster models were

compared to their digital models produced by E-Model™ software. Twenty-four subjects with eight

malocclusion types were enrolled in the study. Mean differences were recorded for Peer Assessment

Rating (PAR) and Bolton analysis. Measurements were recorded to the nearest 0.01mm. Conventional

models were measured with a digital caliper, while digital models were measured with the use of E-

Model™ software. Mean differences ranged from 0.04 - 0.83 for PAR index and from 0.04 - 0.38mm for

Bolton analysis. Although statistically significant differences were found between the 2 methods, the

authors concluded that no measurement associated with PAR index or Bolton analysis appeared to be

clinically significant, and that digital models are not a compromised choice for treatment planning and

clinical diagnosis.10

In 2011, Fleming et al. conducted a systematic review that included literature to assess validity of

the use of digital models. Various databases including MEDLINE, LILACS, BBO, ClinicalTrials.gov, the

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National Research Register and Pro-Quest Dissertation Abstracts and Thesis database were used to search

studies that would compare linear and angular measurements from conventional plaster and digital

models. Forty abstracts were initially selected. However, only 17 of them met the inclusion criteria. The

mean difference between measurements on digital versus plaster models was low. In all studies, the

differences were considered to be clinically insignificant. The authors concluded that digital models offer

a high degree of validity when compared to direct measurement on plaster models, and can be considered

clinically acceptable. However, the overall quality of the selected studies was variable, with generally

inadequate descriptions of the sample populations and rare reports of confidence intervals and standard

errors between different techniques.6

Another review by Luu et al., conducted in 2012, evaluated intrarater reliabilities in terms of

mean differences, intraclass correlation coefficient (ICC), and Pearson correlation coefficients (PCCs) of

measurements of digital models compared with plaster models. Agreement of measurements was

excellent. The authors were in agreement with Fleming et al., concluding that digital models are clinically

acceptable compared with plaster models in respect to intrarater reliability and validity of the selected

linear measurements.11

A more recent systematic review published in 2016 by Rossini et al. was conducted to assess

accuracy, validity, and reliability of measurements of digital versus plaster models. The databases used to

search for peer-reviewed articles included PubMed, PubMed Central, National Library of Medicine

Medline, Embase, Cochrane Central Register of Controlled Clinical trials, Web of Knowledge, Scopus,

Google Scholar, and LILACs. Thirty-five articles were selected. This review is an update from previous

reviews, with more articles with a lower risk of bias, and the addition of articles containing qualitative

ordinal measures (PAR, ABO Objective Grading System, and ICON). For most studies, no significant

differences in measurements were found with the exception of the Objective Grading System (ABO).12

With regard to this grade system, studies reported that digital models should be substituted by plaster

models for calculation of this index due to incorrect articulation and difficulties in landmark identification

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on digital models, and bias in the software used to calculate the ABO Objective Grading System.13-16

More recently, an improvement in this grading system was observed. Although the virtual setup model

resulted in greater deductions in overjet, occlusal contact, and total score compared with the plaster setup

model, the total reductions were not greater than 25, which represent the threshold for passing. This

finding provides evidence that supports clinical acceptance.13 In view of the many advantages such as

cost, time, and storage space required, digital models could be considered the new gold standard in

current practice.

3D printing technology

The evolution of 3D imaging and modelling in dentistry is progressing towards a more efficient

and cost-effective workflow using state-of-the-art technology. The practicability of this technique is

expanding in several dental fields such as prosthodontics, oromaxillofacial surgery and prosthesis, and

production of surgical guides or physical models in dental implant treatment. Three-dimensional printed

models can also be used to fabricate orthodontic appliances. Over the last years, the traditional

orthodontic workflow is shifting to a digital format. The 3D printing technology is becoming more

affordable, faster, and it is claimed to be able of producing dental models with a high resolution and

accuracy.

3D manufacturing technology classification

The 3D manufacturing process can be classified as subtractive or additive manufacturing.

Subtractive manufacturing is based on milling methodology while additive manufacturing is based on

layering methodology, usually joining materials layer upon layer. In subtractive manufacturing, a

prefabricated block of material is milled with the use of burs, diamonds, or diamond disks to manufacture

the desired shaped part.17 Although subtractive manufacturing can create complete shapes effectively, it is

not an economic alternative since material is wasted during the process. Around ninety percent of the

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prefabricated block is removed to fabricate a dental restoration, for instance.18 Other limitations can be

attributed to this method. Milling tools can only withstand short running cycles due to exposure to heavy

abrasion and wear. Also, the precision depends on the size of the smallest milling tool for each

material.19,20

The additive manufacturing methodology has the potential to overcome these limitations. In additive

manufacturing, the final physical part is fabricated by adding layer upon layer of material to form a 3D

object. The 3D computer-aided design (CAD) model is sliced into many layers and the geometric data

provided by the equipment software is used by the 3D printer to build each layer sequentially until the

part is completed. The use of this technique allows the fabrication of complex parts that include undercuts

and cavities, which is seen in dental anatomy. 21,22 Additionally, additive manufacturing is more cost-

effective since less material is used, and more efficient since more than 1 print can be done

simultaneously. Nowadays, there are a vast number of 3D printers with the ability to print various 3D

objects using different technologies. 23

Additive manufacturing technologies

A stereolithography apparatus (SLA) was one of the first additive manufacturing technologies to

be theorized by in the early 1980s, when Japanese researcher Dr. Hideo Kodama invented the modern

layered approach to stereolithography by using ultraviolet light to cure photosensitive polymers. 24,25 This

technique was then patented by Chuck Hull in 1984. 26 The process was defined as a "system for

generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed". The

SLA is a printing technique where an ultraviolet laser cures resin-based material in a desired shape. This

technique builds parts one layer at a time, in a vat of photopolymer resin. Each layer is traced-out and

light-cured by the laser on the surface of the liquid resin, at which point a build platform moves according

to the layer thickness desired, and another layer of resin is wiped over the surface. The process is then

repeated until complete. Supports are needed to attach overhanging parts to the build platform, prevent

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deflection due to gravity, and retain newly created sections during the peel process. The support is created

by the 3D printer software during the preparation of the 3D models and should be removed manually once

the printing process is concluded. Once printing is completed and the parts have reached their final shape,

the photopolymerization reaction is not yet driven to completion. At this point, mechanical and thermal

properties are not fully set. Approximately 80% of the total polymerization takes place while the part is

built in the 3D printer. Post curing is necessary to complete the remaining 20% by means of a

conventional UV curing unit. This enables parts to reach the highest possible strength and become more

stable. 27,28

Digital Light Processing (DLP) is a similar process to SLA. The major difference is the light

source. DLP uses an arc lamp with a liquid crystal display projector, which is applied to the entire surface

of the vat of photopolymer resin in a single pass, making the printing time significantly faster when

compared to SLA technology. The similarities also include the use of support to attach the printed parts

and the post-curing process to complete the solidification of the printed part.

Liquid Crystal Display-Stereolithography with Unidirectional Peel (LCD-SLA/UDP) technology

carries the same fundamentals from SLA. The LCD technology works similarly to DLP. However,

instead of using a light projector which beams light outward, a LCD screen beams light in straight lines.

Due to the nature of light reaching outward from a DLP projector, the light essentially creates a curved

lens or magnifying glass effect. As a result, the wider the cross section of the model, the lower the

resolution will be. In contrast, an LCD screen resolution is uniform across the entire area so it does not

present the same issue. What this allows is to print multiple models simultaneously, spread out around the

entire build platform, while maintaining the quality. The DLP technology would not be able to manage

this since objects towards the edge of the build platform would start to degrade in quality. As far as UDP

mode is concerned, it is an experimental technology that aims to produce large models extremely quickly.

Uni-directional peeling technology was created in an attempt to reduce the up-and-down peel to one-

directional peel action. However, due to its geometric limitations, it has seen limited marketability and

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usability in the dental field. 29

3D printing resolution

The 3D printer axis systems provide the object depth and design and are represented by the X, Y,

and Z planes. The XY plane is also known as the horizontal plane, while the Z plane is known as the

vertical plane. Normally, the X and Y-axes correspond to lateral movement while the Z axis corresponds

to vertical motion during the printing process. In other words, X and Y-axes refer to those that are parallel

to the 3D printer build platform. On the other hand, the Z-axis represents the direction in which the

printed part is built, which is perpendicular to the build platform. The Z resolution was the first major

numerical differentiation between early 3D printers. 30

The resolution that is measured microscopically in the XY dimension is described as the

minimum feature size. This is represented by the diameter of the laser beam and radical polymerization

kinetics in SLA technology and by the size of the pixel for technologies such as DLP and LCD-SLA, in

which a screen projector is used to solidify the liquid polymer. The resolution recommended for

orthodontic purposes should be equivalent to a minimum of 100 µm. Most resolution reports are related to

the Z dimension, represented by the layer thickness.

Influence of Layer Thickness on 3D printing

Generally, the layer thickness is based on the morphologic features and intended use of the object

to be printed. This feature is controlled by how much the 3D printer build platform elevates after a layer is

cured. Usually, if high accuracy is needed, the layer thickness is minimized. On the other hand, if cost and

time are more relevant, thicker layers are used. Improved resolution can be manifested by a smoother

surface finish and greater detail. Higher resolution in the z-direction corresponds to a reduced layer height

on a printed part. It is often assumed that an improved resolution should also equate to improved

accuracy. Although smaller layer height should result in a printed piece with more detail and improved

surface finish, it cannot be assumed that the printed piece has an improved accuracy compared to models

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presenting increased layer height or lower resolution. Also, high resolution comes with a tradeoff.

Thinner layers will require more repetitions, which in turn means more opportunities for artifacts and

errors. Layer thickness selection must provide a satisfactory balance between speed and resolution. 31

It was not until 2017 that Favero et al. published a study that investigated the effect of layer

thickness on the accuracy of 3D printed dental models. Thirty-six typodont models were scanned and then

fabricated by a SLA printer. Three layer thicknesses were evaluated: 25, 50, and 100 µm (N=12).

Forty-eight additional models were printed using 4 commercially available 3D printers: Juell 3D (12

models printed with 100 µm layer), Objet Eden260V Dental Advantage (12 models printed with 28 µm

layer), large-frame Vector 3SP (12 models printed with 100 µm layer height), and Perfactory Desktop

Vida (EnvisionTEC) (12 models printed with 100 µm layer height). All printed models were digitally

scanned and superimposed using a best-fit algorithm to assess accuracy. As the layer thickness decreased,

the deviation values increased, with the 25 µm layer models showing the greatest deviations, and the 100

µm layer models showing the least deviation. The authors attributed that to the increased potential for

errors and artifacts as the quantity of layers are added. An inconsistent X-Y resolution may have also

produced surface irregularities. Similarly, as the number of layers increased, it also increased the amount

of potential variation cause by the x-y resolution. Although statistically significant differences were found

in the average overall deviations, all values were considered clinically acceptable. Based on this study, it

can be stated that increased resolutions do not necessarily mean increased accuracy. Factors such as cost,

efficiency, surface quality, and accuracy should be considered when selecting layer thickness.30

More recently, a study was conducted by Loflin et al. to compare the accuracy of 3D printed

models and traditional plaster models using three layer heights: 25, 50, and 100 µm. Twelve sets of stone

models previously submitted as ABO cases were digitally scanned and then printed on a SLA Form2

printer using three layer heights. All stone models and 3D printed models were scored by 4 faculties and 7

residents from the Department of Orthodontics at the University of Texas Health Science Center at

Houston School of Dentistry. The ABO Cast-Radiograph Evaluation (CRE) grading system was used to

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score each model. Based on that, 8 criterias were evaluated: alignment/rotations, marginal ridges,

buccolingual inclination, overjet, occlusal contacts, occlusal relationships, interproximal contacts, and

root angulation.31

Models with a 100 µm layers showed the strongest correlation of total score to stone models,

while 25 µm models showed the weakest correlation of total score. The 100 µm layer models were highly

correlated to stone models in regards to buccolingual inclination, overjet, occlusal relationship, and

interproximal contacts, and presented a weak correlation in regards to alignment/rotations and marginal

ridges. All models showed very strong correlation with stone models for occlusal contacts.31

Accuracy of 3D printed dental models

In order to incorporate the use of 3D printed models in orthodontics, accuracy must be warranted.

Accuracy can be determined by the superimposition of the printed model to the original CAD model and

it is reported as a percentage of its original volume. Precision is the closeness of the superimposition of

repeatedly printed models. In other words, it can be referred to the reproducibility of the printer to

fabricate multiple models with the exact same dimensions. Precision is especially important when

fabricating clear aligners.

The accuracy of 3D printed models can range from 20 to 100 µm. This can be influenced by the

3d printer technology and its resolution. Dietrich et al reported a superior precision, but less accuracy for

SLA technology when compared with Polyjet. Although Kim et al showed better trueness values for DLP

and Polyjet, SLA had better accuracy when the occlusion was evaluated. However, it is important to

recognize the discrepancy observed in the results from those studies ranged from 20 to 50 µm, which may

not be significant for many clinical applications. Also, since then the laser resolution of the current SLA

3D printers have improved.33,34

Other factors can also influence the accuracy of a 3D printed part. The material selection can play

a role on that. The use a flexible resin, like those used for indirect bonding trays have less dimensional

stability than the standard rigid resins. If the resin does not flow properly, the cured layer will be uneven.

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The color can also affect it since the more transparent the resin is, the easier for the laser to pass through

and cure undesired areas, introducing artifacts.

Accuracy can also be altered by the print orientation. Horizontal prints are known to be more

accurate than vertical ones due to shrinkage of unsupported parts. To avoid distortion, current 3D

software will add the support to the printed part.

It is critical that the build platform rises perfectly parallel. Otherwise it will create distortion and

may skew the print. Different technologies are used to allow this parallel raise of the platform. Despite of

the mechanism used, 3D printers should be calibrated in a regular basis to provide reliable prints. The

intervals between calibrations should not exceed 3 to 4 months.

Clinical Applications of 3D printed dental models

The use of 3D printed models in orthodontics is not limited for diagnosis and treatment planning

but also include the fabrication of indirect bonding trays, metal appliance fabrication, retainers and clear

aligners. Two different methods can be used to fabricate indirect bonding trays. One method consists of

printing the dental model with the brackets. The indirect tray is then fabricated with the material of

choice, and the brackets are inserted into the tray. The materials used can be silicone-based, thermoplastic

or dual materials. The second option is to directly print the indirect bonding tray. The disadvantage of this

method is that the absence of a custom base may allow for excess of composite around the brackets. For

both methods, the patient’s dentition is digitized and the software provides the clinical outcome of the

orthodontic treatment and based on that position the brackets in a “straight-wire” setup, but also allowing

the clinician to change the position of the bracket as desired. The bracket position relative to each tooth is

then transferred to the initial alignment. Unfortunately, there are only a few companies that fabricates

these trays with transparent material to allow proper light curing while also providing adequate material

flexibility to prevent debonding of brackets during tray removal. Additionally, literature is still lacking

data on accuracy and precision of 3D printed indirect bonding trays.

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Metal appliances can be produced by Selective Laser Sintering (SLS) technology. This technology

can use several materials such as Stainless Steel, Cobalt Chromium, T64 Titanium, porcelain, plastics,

and glass, providing high strength and stiffness to the printed part. Orthodontic metal appliances such as

skeletal anchorage attachments, lingual holding arches, Rapid Palatal Expanders can be fabricated using

SLS. However, a multi-piece part will often involve a 2-step process in which bands, support arms, and

lingual pads are 3D printed and then welded to the commercially available expansion screw to complete

the RPE appliance.35

As a result of the more reasonable prices of current 3D printers and the availability of commercial

software that will align the teeth and produce sequential models following the teeth movement, it is

possible to fabricate “in-office” clear aligners. The scan of the patient’s dentition is imported into a

software such as OrthoAnalyzer (3Shape, Copenhagen, Denmark), Orchestrate 3D (Orchestrate

Orthodontic Technologies, Rialto, CA), or OnyxCeph (Image Instruments, Chemnitz, Germany). After

models are cleaned, oriented and based, the teeth are segmented so it can be positioned to achieve ideal

alignment. The number of trays fabricated is calculated by the software based on tooth movement and its

preset algorithm. Another option is to use online services such as AccuSmile, ArchForm or ARCAD. The

setup is completed by them and the STL files with the tooth movement is sent to the clinician for a cost.

Also, companies are developing resins that will permit to directly print clear aligners, making the process

more efficient and cost-effective.36

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digital study models. Biomed Eng Online 2013;12:1-11.

24. Hideo Kodama. A Scheme for Three-Dimensional Display by Automatic Fabrication of Three Dimensional Model. IEICE Transactions on Electronics (Japanese Edition) 1981; J64-C:237–41.

25. Hideo Kodama. Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer. Review of Scientific Instruments 1981; 52: 1770–73. 26. Hull CV. Apparatus for production of three-dimensional objects by stereolithography. U.S. Patent No. 4,575,330. March 11, 1986. 27. Dandekeri S, Sowmya MK, Bhandary S. Stereolithographic surgical template: A review. J Clin Diagnostic Res 2013; 7:2093-5. 28. www.formlabs.com 29. www.uniz.com 30. Favero CS, English JD, Cozad BE, Wirthlin JO, Short MM, Kasper FK. Effect of print layer height and printer type on the accuracy of 3-dimensional printed orthodontic models. Am J Orthod

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Dentofac Orthop 2017;152:557-565. 31. www.all3dp.com 32. Loflin, W. (2018) The Effect of Print Layer Thickness on the American Board of orthodontics

(ABO) Cast-Radiograph Evaluation (CRE) Grading of 3D-printed Models (masters dissertation) 33. Dietrich CA, Ender A, Baumgartner S, Mehl A. A validation study of reconstructed rapid prototyping models produced by two technologies. Angle Orthod 2017;87:782-787. 34. Kim SY, Shin YS, Jung HD, Hwang CJ, Baik HS, Cha JY. Precision and trueness of dental models manufactured with different 3-dimensional printing techniques. Am J Orthod Dentofac Orthop. 2018;153:144-153. 35. Graf S, Cornelis MA, Hauber Gameiro G, Cattaneo PM. Computer-aided design and manufacture of hyrax devices: Can we really go digital? Am J Orthod Dentofac Orthop. 2017;152:870-874. 36. EnvisionTec Press release, Feb 2018. https://envisiontec.com/orthodontic-materials-launched-at-lmt-lab-day-chicago

15

ACCURACY AND PRECISION OF 3-DIMENSIONAL PRINTED DENTAL MODELS PRODUCED BY DIFFERENT ADDITIVE MANUFACTURING TECHNOLOGIES.

Introduction

Three-dimensional (3D) printing technology has undergone significant advances in recent years,

rapidly changing dentistry. This technology is becoming more affordable, efficient, and capable to

provide high printing accuracy. In orthodontics, 3D printing is used to produce study models for diagnosis

as well as working models for appliance fabrication.1,2 Additive Manufacturing (AM), also referred to as

Rapid Prototyping (RP) is described as a variety of techniques used to rapidly fabricate a scale model of a

physical part or assembly using three-dimensional computer-aided design (CAD) data. . The printer reads

the CAD data and deposits consecutive layers of liquid, powder, or plastic filaments to build a model

from a series of cross sections that are joined together to establish the final shape.3

As defined by its name, three-dimensional printing involves 3 dimensions where “X” and “Y”

represent the horizontal plane. The “Z” dimension represents the vertical axis and it is related to the layer

height in which a part will be printed. As with any 3D printed structures, the final accuracy of dental

models produced by AM can be influenced by the 3D printing system of choice. Currently, many

different AM technologies are available. It is important for clinicians to be aware of each system’s

properties and characteristics so to appropriately utilize it for the different orthodontic needs.4

Some of the most common 3D printing techniques used in dentistry are Stereolithography (SLA),

Digital Lighting Processing (DLP), Fused Deposition Modeling (FDM), and Selective Laser Sintering

(SLS), and Liquid Crystal Display with Unidirectional Peel (LCD-SLA/UDP). In our study, we

investigated SLA, DLP, and LCD-SLA/UDP. The LCD-SLA/UDP is a more recent technology. The

principles of SLA, DLP, and LCD-SLA/UDP printing are essentially the same. These technologies use a

16

similar mode of operation in which a light source is used to promote photopolymerization of resin

monomers. The method used to apply this principle is what sets them apart. The SLA printing technique

uses a laser beam as the light source, the DLP technique uses a projector, and the LCD-SLA/UDP uses an

array of ultraviolet (UV) LCD.5,6

In the SLA technique, the laser must pass through every point of the cross-sectional area to be

printed. The DLP projector and the LCD screen permits that an entire layer of polymer be

photopolymerized at once, allowing to a faster printing cycle when compared to SLA. The light directed

from a DLP projector expands as it reaches the cross-section area to be printed. Due to the nature of the

projected cone of light, the periphery of a printed part may have a higher chance of presenting distorted

pixels. Unlike previous technologies, the LCD technology uses panels that have the ability to project the

light in a parallel mode, instead of expanding the light. It also selectively filters UV light, which allows

the entire layer to be printed faster than DLP.6

Currently, the two most popular printers used amongst orthodontists are the Form2 (Formlabs,

Somerville, MA, USA) and the MoonRay (SprintRay, Los Angeles, CA, USA). They employ SLA and

DLP technologies, respectively. The Slash Plus UDP uses LCD printing method, which is a more recent

technology and, for that reason, not as popular as Form2 and Moonray.5,6

As the adoption of 3D printing technology by orthodontics practices sees a significant increase, it

is important to investigate the accuracy of different systems, notably of 3D printed dental models since

they are used for a variety of clinical purposes. To this day, limited data exists on the accuracy of 3D

printed dental models and possible differences between various additive processes using different layer

thicknesses.

The aims of this study were to evaluate the accuracy of dental models manufactured by different

3D print technologies (SLA, DLP, and LCD-SLA/UDP), and to test the accuracy of the dental models

printed at Z-plane resolutions of 50 and 100 microns (µm). The null hypotheses were that there would be

no significant differences in accuracy between the printed models manufactured by different 3D printing

17

technologies, and that there would be no significant differences between printed models printed at

different Z-plane resolutions.

Methods

Ethical approval for this study was obtained from the Institutional Review Board (IRB) at the

University of North Carolina (Institutional Review Board number 18-3245; reference number 236961). A

flowchart illustrating the study methodology is shown in Figure 1. Twelve consecutive 3D digital models

(6 maxillary and 6 mandibular) from the UNC Orthodontic database were randomly selected. According

to power analysis done on similar precursor studies, a total of 12 dental models were required to

demonstrate significant results in regards to dimensional accuracy of different additive manufacturing

technologies.7,8 Models were selected based on the following inclusion criteria: 1) Complete intra-arch

permanent dentition from first molar to first molar; 2) Morphology of all involved teeth within normal

limits.

Digital scans were obtained via TRIOSⓇ scanner (3Shape, Copenhagen, Denmark) and exported as

surface model stereolithography (STL) files. Digital models were coded to remove any identifiable

information, and stored in an encrypted drive. The STL files were converted into physical models using 3

different AM techniques: 1) SLA (Form2), 2) DLP (MoonRay S100), and 3) LCD-SLA/ UDP (Slash Plus

UDP) (Figure 2). For every technology, dental models were printed at resolutions of 50 and 100 microns

in the Z plane (Figure 3). Photopolymer resins were selected based on the manufacturers

recommendations. All 3D printed models were printed as solid models with a horseshoe-shaped base and

in the horizontal position. The occlusal plane was parallel to the building platform, and the tooth cusps

were oriented away from it. At the completion of each printing cycle, each model was carefully removed

from the building platform and immersed in 2 separate tanks with 90% isopropyl alcohol, for 20 minutes,

to remove any uncured resin. Then, the model was removed from the second tank and allowed to air dry

at room temperature. Each model was post-cured for 15 minutes. All 72 replicas were stored in a dark

18

container to prevent exposure to sunlight.

The printed models were rescanned with the TRIOSⓇ scanner to create new digital STL models.

The scanner was calibrated before every use and all models were placed in the same orientation during

the scanning process. Each model was positioned on the back of its base and was oriented with the dental

arch pointing toward the scanner. A scan spray (3M™ High-Resolution Scanning Spray) (Figure 4) was

applied in each printed model prior to scanning. A minimal surface coating was sprayed to ensure optimal

scanning properties of the models. These new digital models produced STL files that were superimposed

to the original ones, which were obtained by the first scanning process. Prior to the superimposition, all

models, including the original ones, were cropped digitally to eliminate non-anatomic parts.

A Procrustes analysis, which overlaps 3D images, was done using an open source software (Slicer

3D CFM 4.0). 9 Reports were generated for each superimposition that listed any deviations between the

mesh surfaces of the original files and their corresponding 3D printed models. Figure 4 shows examples

of superimpositions of the digitized SLA, DLP, and LCD-SLA/UDP replicas with the corresponding STL

original files. Positive discrepancies are represented by a range from yellow to red, and show areas of the

print that are larger when compared to the original dataset. Negative discrepancies are represented by a

range from light to dark blue, and show areas of the print that are smaller when compared to the original

dataset. The green areas indicate few or no deviations between the superimposed models equivalent to

less than 0.1 millimeter when compared to the original scan. Five randomly selected models were re-

measured after 2 weeks to ensure the measurements could be accurately replicated. The same investigator

recorded all measurements on two separate occasions. Any differences between the two sets of

measurements greater than or equal to 0.5mm were identified and repeated until acceptable reliability was

achieved. Absolute and root mean square errors were calculated along the entire surface of the models to

determine accuracy. The T-test and ANOVA statistical analysis were performed.

19

Results

The results of the difference in the overall accuracy between 50 and 100 µm layer thicknesses for

each 3D printer are presented in Table 1. No statistical significant difference between 50 and 100 µm

layer thicknesses was found for the SLA technology (Form2) (p 0.0248), and for the DLP technology

(MoonRay S100) (p 0.426). The LCD-SLA/ UDP technology (Slash Plus UDP) showed statistical

significant differences between 50 and 100 µm layer thicknesses (p 0.019), presenting a lower mean error

value for the 50 µm layer (0.147 mm) when compared to the 100 µm layer (0.164 mm). Among the 3D

printers investigated, the lowest mean error value was found for models produced by the SLA technology

(Form2), with a 50 µm layer thickness (0.172 mm). The highest mean error value was observed on

models produced by the DLP technology (MoonRay S100), also with a 50 µm layer thickness (0.172

mm). The SLA technology (Form2) showed the lowest value for mean maximum error, followed by

MoonRay, and Slash Plus UDP.

Table 2 shows the results of overall accuracy between 3D printers by different layer thicknesses.

The SLA technology (Form2) showed statistically significant difference from DLP (MoonRay S100) and

LCD-SLA/ UDP (Slash Plus UDP) for both 50 µm (p 0.0001) and 100 µm (p 0.039) layer thicknesses. No

significant difference was observed between DLP (MoonRay) and LCD-SLA/ UDP (Slash Plus UDP).

Discussion

The objectives of the present study were to investigate the accuracy of dental models manufactured

by different 3D printing technologies, and to evaluate the accuracy of the 3D-printed dental models at

different layer thicknesses..

The accuracy of a printed structure is determined by factors such as the XY resolution, the Z

resolution or layer thickness, and the printing technology. A higher Z resolution means a reduced layer

thickness on a 3D printed model. In theory, a reduced layer thickness would be physically expressed by a

20

smoother surface finish and better surface detail of the model. For this reason, it could be assumed that a

50 µm layer thickness 3D print would exhibit a better resolution than a 100 µm layer thickness print.

Despite the fact that a reduced layer thickness is expected to provide more detail in a 3D printed model, it

does not necessarily mean that it will provide superior accuracy when compared to a model produced with

a larger layer thickness, as it is often assumed. Our data showed no statistical significance in accuracy for

different layer thicknesses for both SLA (Form2) and DLP (MoonRay S100) technologies. Although the

difference between 50 and 100 µm layer models produced by the LCD-SLA/ UDP (Slash Plus UDP)

technology was statistically significant. This difference corresponds to 0.017mm and is not considered

clinically relevant.

The superior overall accuracy found for the SLA (Form2) technology, when compared to DLP

(MoonRay S100) and LCD-SLA/UDP (Slash Plus UDP), may be attributed to the fact that this

technology uses a laser beam that cures every point of the entire printing layer, turning the process time-

consuming but also highly accurate. The DLP and LCD-SLA/UDP technologies are able to cure an entire

layer at once. The cone of light projected by the DLP can also interfere with the XY resolution since it

may create distorted pixels, which can affect the periphery of a printed part. The resulting irregularities

found on the boundaries of each layer will affect the accuracy of the 3D printed model. The XY

resolution, also known as “minimal feature size”, corresponds to 140 µm, 100 µm, and 75 µm for SLA,

DLP, and LCD-SLA UDP, respectively. This feature is set by the manufacturer, so standardization for

comparison purposes is not possible.10-12

An important characteristic of our methodology is that the original digital models were obtained

directly from the oral environment, and then compared to the scanned 3D printed models. This allowed us

to assess accuracy of 3D printed models while eliminating variables that could have been introduced

during acquisition by means of alginate impressions. On the other hand, the 3D printed models had to be

scanned to be digitally superimposed with the original models. However, the TRIOSⓇ scanner provides

high accuracy, and the errors introduced by the digitalization process should be considerably less

21

significant than those introduced by the printing techniques.13

Previous studies on accuracy of 3D printed models compared plaster to 3D printed models through

linear measurements using a digital caliper.14-17 To evaluate the discrepancies between the original scans

and the 3D printed models, we opted to use digital superimposition. Mathematical superimposition has

the ability to overcome human errors related to landmark identification.

A digital model produced by additive manufacturing can be influenced by factors that would

include a possible distortion produced with data conversion and manipulation during formatting to STL

file, and also model shrinkage during fabrication and post-curing process. If these errors were part of the

present study, all 3D printed models would have been influenced. Previous studies have determined a

range of error that would be considered clinically acceptable when evaluating accuracy of 3D printed

models. The data from these studies showed 0.20 to 0.50 mm to be an acceptable range for clinical

accuracy.14,18-22 Taking this range in consideration, the 3D printing technologies included in our study

produced 3D printed models that are suitable for diagnostic and treatment planning.

A previous study conducted by Favero et al. showed an increase in deviation values directly

proportional to an increase in Z resolution values. The 100 µm layer thickness group showed increased

accuracy when compared to the 50 µm and 25 µm layer thickness groups in terms of deviation from the

original digital model. The 25 µm layer models presented the greatest deviations, while the 100 µm layer

models showed the smallest deviation. The authors attributed that to the increased potential for errors and

artifacts as the amount of layers required to print a given part increases. Despite of the differences, all

values were considered clinically acceptable. The selection of layer thickness must take in consideration

not only accuracy, but also factors such as cost, surface quality, and efficiency. In our study, No

statistically significant differences were observed between the 50 µm and 100 µm layer thickness, except

for the LCD-SLA/UDP technology. The methodology used in both studies were similar, however our

study used a different scanner and scan protocol since we sprayed the models with a scan powder, and

also used a different software to superimpose the digital models onto their original counterparts.23

22

The intraoral scanner (TRIOSⓇ) used to scan the printed 3D models was the same used to obtain

the original intraoral scans. The TRIOSⓇ scanner is a powder-free system based on ultrafast optical

scanning technology and has documented high accuracy.24, 25 Although the original intraoral scan used

does not require a scan spray, difficulties were found during the scanning process of the 3D printed

models. The scan was not able to capture continuous images of the models, likely due to the highly

different optical properties of oral tissues and printing polymers. The scanners produced were grossly

inaccurate and irregular. In an attempt to eliminate the issue and obtain quality scans of the models, we

opted for applying a thin layer of the 3M™ High-Resolution Scanning Spray over all printed models.

This scanning spray has the ability to adjust the optical reflective properties of intraoral surfaces to allow

for proper capture of 3D digital images. In our study, we found this to be true for models surfaces as well.

Although the 3D printed models had to be coated by scanning spray powder, the coating thickness was

kept to a minimal. A previous study comparing the surface morphologies and coating thickness of 3

different spray scan systems was done using confocal laser scanning microscopy. The mean coating

thickness ranged from of 18.9 µm to 25.3 µm. The study concluded that all coating thickness values were

clinically acceptable.26

The color map illustrated in Figure 4, shows examples of superimposition of a 3D printed model to

their original model. Less pronounced errors were observed on the SLA (Form2) technology. The SLA

model was slightly wider by approximately 0.1mm labiolingually, which translates into an appliance that

is slightly bigger, but still fits properly. The DLP (Moonray S100) and LCD-SLA/UDP (Slash Plus UDP)

technologies showed shrinkage of the model. Additionally, while the bulk of the error for the SLA is

located on the labial surface, the DLP and LCD-SLA/UDP presented errors along all the cusp tips, which

is not desirable in a tight-fitting appliance. This is also reported by the maximum error values that were

greater for the LCD-SLA/UDP models. The post-curing process of the 3D printed models can lead to

additional shrinkage of the resin. This impact is more significant to the Z plane.27

23

Conclusion

Models produced with the Form2 were more accurate than the models produced with the MoonRay

and Slash Plus UDP. Changing the print resolution from 50 to 100 µm showed to affect the overall

accuracy only for models produced with the Slash Plus UDP.

24

REFERENCES

1. Christensen LR. Digital workflows in contemporary orthodontics. APOS Trends Orthod 2017; 7: 12-8. 2. Graf S, Cornelis M, Gameiro G, and Cattaneo P. Computer-aided design and manufacture of hyrax devices: Can we really go digital? Am J Orthod Dentofacial Orthop 2017; 152: 870-4. 3. Beguma Z, Chhedat P. Rapid prototyping—when virtual meets reality. Int J Comput Dent 2014; 17: 297-306. 4. Wu W, Ye W, Wu Z, Geng P, Wang Y, Zhao J. Influence of Layer Thickness, Raster Angle, Deformation Temperature and Recovery Temperature on the Shape-Memory Effect of 3D-Printed Polylactic Acid Samples. Materials 2017; 10: 970. 5. Nguyen T, Jackson T. Understanding the Fundamentals of 3D printing. In: Kim-Berman H, Franchi L, Ruellas A, eds. Effective, Efficient and Personalized Orthodontics: Patient-centered Approaches and Innovations. Craniofacial Growth Series, Center for Human Growth and Development, The University of Michigan, Ann Arbor, MI 2018; 55:281-293. 6. LCD vs DLP - Resin 3D Printing Technologies Compared. https://all3dp.com/2/lcd-vs-dlp-3d-printing-technologies-compared/ 7. Hazeveld A, Huddleston Slater, and Ren Y. Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques. Am J Orthod Dentofacial Orthop no. 2014; 104: 108-15. 8. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of three-dimensional dental resin models created by fused deposition modeling, stereolithography, and Polyjet prototype technologies: A comparative study. Angle Orthod. 2018; 3: 363-69. 9. 3D Slicer. 2017 [cited 2017 March 7, 2017]; Available from: slicer.org. 10. Form2 Tech Specs. https://formlabs.com/3d-printers/form-2/tech-specs/ 11. Technical Specifications. https://sprintray.com/moonray-desktop-3d-printer/technical-specifications/ 12. Slash plus UDP. https://www.uniz.com/us_en/page/3d-printers-slashplus-udp 13. Nedelcu RG, Persson AS. Scanning accuracy and precision in 4 intraoral scanners: an in vitro comparison based on 3-dimensional analysis. J Prosthet Dent 2014; 6:1461-71. 14. Hazeveld A, Huddleston Slater, and Ren Y. Accuracy and reproducibility of dental replica models reconstructed by different rapid prototyping techniques. Am J Orthod Dentofacial Orthop 2014; 145:108-15. 15. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of three-dimensional dental resin models created by fused deposition modeling, stereolithography, and Polyjet prototype technologies: A comparative study. Angle Orthod. 2018; 3: 363-69. 16. Kasparova M et al. Possibility of reconstruction of dental plaster cast from 3D digital study models. Biomed. Eng. Online 2013; 1:1.

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17. Murugesan K, Anandapandian PA, Sharma SK, and Kumar MV. Comparative evaluation of dimension and surface detail accuracy of models produced by three different rapid prototype techniques. J. Indian Prosthodont 2012; 1:16-20. 18. Hassan WN, Yusoff Y, Mardi NA. Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees ofcrowding. Am J Orthod Dentofacial Orthop 2017; 151:209-18. 19. Schirmer UR, Wiltshire WA. Manual and computer-aided space analysis: a comparative study. Am J Orthod Dentofacial Orthop 1997; 112:676-80. 20. Hirogaki Y, Sohmura T, Satoh H, Takahashi J, Takada K. Complete 3-D reconstruction of dental cast shape using perceptual grouping. IEEE Trans Med Imaging 2001; 20:1093-101. 21. Halazonetis DJ. Acquisition of3-dimensional shapes from images. Am J Orthod Dentofacial Orthop 2001; 119:556-60. 22. Bell A, Ayoub AF, Siebert P. Assessment of the accuracy of a three- dimensional imaging system for archiving dental study models. J Orthod 2003; 30:219-23. 23. Favero CS, English JD, Cozad BE, Wirthlin JO, Short MM, Kasper FK. Effect of print layer height and printer type on the accuracy of 3-dimensional printed orthodontic models. American Journal of Orthodontics and Dentofacial Orthopedics 2017; 4:557-565. 24. Hack GD, Patzelt SBM. Evaluation of the Accuracy of Six Intraoral Scanning Devices: An in-vitro Investigation. ADA Professional Product Review 2015;4:1-5. 25. Ender A, Zimmermann M, Attin T, Mehl A. In vivo precision of conventional and digital methods for obtaining quadrant dental impressions. Clin Oral Investig 2015; 7:1495-504. 26. Lehmann KM, Azar MS, Wentaschek S, Scheller H. The Effect of Optical Conditioning of Preparations with Scan. Acta Stomatol Corat 2011; 2:86-92. 27. Keating AP, Knox J, Bibb R, Zhurov AI. A comparison of plaster, digital and reconstructed study model accuracy. J. Orthod. 2008; 3:191-201.

26

APPENDIX 1: TABLES AND FIGURES

Table 1. Difference in overall accuracy between different layer thicknesses (Z resolution) for each 3D

printer. Level of significance set to ≤ 0.05.

Printer Z Resolution

(µm)

Mean

Maximum

Error (mm)

Mean Error

(mm)

SD P-Value

50 0.488 0.093 0.041

Form2 100 0.593 0.117 0.044

0.248

50 0.741 0.172 0.036

MoonRay 100 0.726 0.159 0.030

0.426

50 0.907 0.147 0.017

Slash Plus

UDP

100 1.001 0.164 0.019

0.037

27

Table 2. Difference in overall accuracy between 3D printers by different layer thicknesses (Z resolution).

Level of significance set to ≤ 0.05.

Resolution Form2 vs. MoonRay Form2 vs. Slash Plus

UDP

MoonRay vs. Slash Plus UDP

50 µm 0.0001 0.002 0.066

100 µm 0.039 0.007 0.679

Table 3. Photopolymer resins based on the manufacturers recommendations.

Printer Form2 MoonRay Slash Plus / UDP

Resin Formlabs Gray Resin (RS-F2-GPGR-04)

SprintRay Model Gray (SGR – 12419)

zOrtho Gray (ZSLM07GR001)

28

Figure 1: Study methodology flowchart.

Original scans (N=12)

Conversion to STL format

50 µm layer 100 µm layer 50 µm layer 100 µm layer 50 µm layer 100 µm layer

Models superimposition

29

Figure 2: Chart comparing the 3D printing technologies and printers investigated in this study.

Printer

Form2

MoonRay

Slash Plus UDP

Technology SLA DLP LCD-SLA/UDP

Layer thickness 25, 50, and 100 µm 20, 50, and 100 µm 10, 25, 50, 100, 150, 200, and 300 µm

Build Volume 5.7” x 5.7” x 6.9” 5” x 3.2” x 8”

7.5" × 4.7" × 7.9"

Footprint 13.5” x 13” x 20.5” 15” x 15” x 20” 14" × 16" × 21"

MSRP $ 3,499 $ 4,430 $ 3,349

Image sources: formlabs.com, sprintray.com, uniz.com

30

Figure 3: Close up view of models printed at 50 micron and 100 micron resolution.

Form2 MoonRay Slash Plus UDP

50 microns

100 microns

Figure 4: 3M™ High-Resolution Scanning Spray (A) applied on printed models (B). Close up view of

the spray coating.

31

Figure 5: Superimposition of 3D printed models to their original counterpart.

Form2 MoonRay Slash Plus UDP