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Dimensional accuracy of extrusion- and photopolymerization-based 3D printers: In vitro study comparing printed casts
RESEARCH AND EDUCATION
Dimensional accuracy of extrusion- and photopolymerizationbased 3D printers: In vitro study comparing printed casts Norbert Nestler, DDS,a Christian Wesemann, DDS, Dr med dent,b Benedikt C. Spies, DDS, Dr med dent, PhD,c Florian Beuer, DDS, Dr med dent, MME, PhD,d and Axel Bumann, DDS, Dr med dent, PhDe In conjunction with the evolving digitization in modern dentistry, there is an increasing interest in 3D printing. Three-dimensional printing, also termed additive manufacturing (AM), can be integrated into a direct or indirect digital workflow. A digital work file, typically standard tessellation language (STL), can be either acquired directly by using an intraoral scanner (IOS) or a cone beam computed tomography (CBCT) system or generated indirectly by using a desktop scanner, scanning conventional casts or impressions. For further editing, the STL file can be transferred to a computer-aided design (CAD) software program and subsequently transmitted to a printer for producing custom structures directly.1,2
ABSTRACT Statement of problem. Reliable studies comparing the accuracy of complete-arch casts from 3D printers are scarce. Purpose. The purpose of this in vitro study was to investigate the accuracy of casts printed by using various extrusion- and photopolymerization-based printers. Material and methods. A master file was sent to 5 printer manufacturers and distributors to print 37 identical casts. This file consisted of a standardized data set of a maxillary cast in standard tessellation language (STL) format comprising 5 reference points for the measurement of 3 distances that served as reference for all measurements: intermolar width (IMW), intercanine width (ICW), and dental arch length (AL). The digital measurement of the master file obtained by using a surveying software program (Convince Premium 2012) was used as the control. Two extrusion-based (M2 and Ultimaker 2+) and 3 photopolymerization-based printers (Form 2, Asiga MAX UV, and myrev140) were compared. The casts were measured by using a multisensory coordinate measuring machine (O-Inspect 422). The values were then compared with those of the master file. The Mann-Whitney U test and Levene tests were used to determine significant differences in the trueness and precision (accuracy) of the measured distances. Results. The deviations from the master file at all 3 distances for the included printers ranged between 12 mm and 240 mm (trueness), with an interquartile range (IQR) between 17 mm and 388 mm (precision). Asiga MAX UV displayed the highest accuracy, considering all the distances, and Ultimaker 2+ demonstrated comparable accuracy for shorter distances (IMW and ICW). Although myrev140 operated with high precision, it displayed high deviations from the master file. Similarly, although Form 2 exhibited high IQR, it did not deviate significantly from the master file in the longest range (AL). M2 performed consistently. Conclusions. Both extrusion-based and photopolymerization-based printers were accurate. In general, inexpensive printers were no less accurate than more expensive ones. (J Prosthet Dent 2020;-:---)
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. a Doctoral student, Campus Benjamin Franklin, Center for Dental and Craniofacial Sciences (CC3), Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. b Graduate student, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. c Privatdozent, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. d Professor, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. e Professor, Mesantis 3D Dental Radiologicum, Clinic of Orthodontics, Berlin, Germany.
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MATERIAL AND METHODS
Clinical Implications Based on the present findings, both extrusion-based (fused deposition modeling) and photopolymerization-based (stereolithography, digital light processing) printers can be used to produce diagnostic casts. The cost of the evaluated systems did not correlate with accuracy.
The advantages of an AM-based digital workflow include the capability of a data set to be stored or shared conveniently and reproduced on demand.3 The production cost, time, and laboratory equipment required can be reduced from those for automatic manufacturing.4 Compared with subtractive manufacturing (such as milling), AM avoids abundant waste material and enables the production of more complex and accurate structures.5-7 Among AM technologies, photopolymerization, including stereolithography (SLA) and digital light processing (DLP), has been used in dentistry. High resolution for detailed reproduction, smooth surfaces, and reduced fabrication time are the most relevant advantages for the dental field.4,8 In this method, a light source is projected through a movable mirror onto a basin filled with liquid resin (photopolymer). After polymerization, the basin is lowered layer by layer so that each layer can be polymerized separately.4 Extrusionbased fused deposition modeling (FDM) is another established AM process. A heated plastic filament (thermoplastic) is deposited on a movable platform and then polymerized.9 Although this method is less expensive, rendition detail is reduced.10 In addition to the equipment and material costs, printing accuracy is an essential parameter for clinical applicability. The outcomes of the few studies conducted till now for assessing the dimensional accuracies of the different AM technologies11-13 have been inconsistent. Most of these studies used hand-held digital calipers,3,14,15 which provide low reproducibility16 or superimposition.17-19 In addition, the required digitization of the reference model is a source of error, depending on the type of digitization. Moreover, the statistical capabilities of the present studies are limited because of the low number of included casts (N=1 to 12).7,11,13 Therefore, the purpose of this in vitro study was to compare the dimensional accuracy (trueness and precision) of AM casts produced by 3 AM technologies (FDM, SLA, and DLP) with that of an adequate number of examined casts (n=37) and a reproducible measurement methodology. The null hypothesis was that no significant differences in trueness and precision would be found between the AM technologies considered. THE JOURNAL OF PROSTHETIC DENTISTRY
A specially developed resin cast served as the reference for all measurements.20-22 This neutrally interlocked maxillary cast displayed 5 measuring cubes in the area of the first maxillary molars, the canines, and between the central incisors. Drill holes were located in the center of these cubes. The highest point of the centerline of each hole was defined as the most reproducible measuring point. The measuring points were used to calculate the following distances for comparison: intermolar width (IMW), intercanine width (ICW), and arch length (AL) (Fig. 1). The master cast was scanned by using an IOS (TRIOS Color POD; 3Shape), and the corresponding STL file was generated. A palate-free base in horseshoe design was formed for the master file to simulate a clinical workflow. The master file was subsequently analyzed and measured by using a specialized surveying software program (Convince Premium 2012; 3Shape). The vertical walls of the drill holes were marked manually and identified by the software program as the vertical basic shape so that the ideal centerlines of the cylinders could be calculated. The highest point of the centerline was defined as the intersection of the centerline and the upper surface of the drilling cylinder. Thus, the individual measuring points could be connected and the distances IMW, ICW, and AL determined (Fig. 2). The measurement of the master file was repeated 15 times, and the average ±standard deviation digital master values (gold standard) were defined: IMW=50.003 mm ±0 mm, ICW=32.855 mm ±2 mm, and AL=77.695 mm ±5 mm. Thereafter, the master file was sent to the participating AM manufacturers and distributors with the requirement to print 37 identical casts as accurately as possible. Material selection and the printer-specific parameter setting were determined by the companies. The casts were produced by using 2 extrusion-based (FDM) and 3 photopolymerizationbased (SLA and DLP) printers (Table 1). Because of transportation damage, only 34 of the 37 casts fabricated by one of the printers (myrev140; Sisma) could be measured. Makergear delivered 38 casts, all of which were used for measurements. For the remaining printers, 37 casts were measured. The AM casts were measured in collaboration with the Fraunhofer Institute for Production Systems and Design Technology, Berlin. The measurement was carried out by using a multisensory coordinate measuring machine (CMM, O-Inspect 422; Zeiss) and software program (Calypso 5.4.20; Zeiss). For volumetric length measurements, the CMM operated with a maximum permissible deviation of 1.9 mm+L/250 in accordance with the international series of standards for CMM systems (ISO 10360).23 Each cast was calibrated and measured at constant environmental parameters (20 C, 50% humidity, and 1013 hPa ambient pressure).
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Figure 1. Master cast with reference measuring points yielding distances: 5=intermolar width, 6=intercanine width, and 1+2+3+4=dental arch length.
Measurements were carried out by contact with a 0.3mm-diameter, dimensionally stable ruby ball by scanning the drill holes over 169 to 200 measuring points to determine the defined measuring point for the distance measurement between the 5 reference cubes (Fig. 3). According to ISO 5725,24 the combination of trueness and precision results in accuracy. Trueness was defined as the proximity of a printer’s average to the digital master’s average, and precision was defined as the scatter of a printer’s readings and is correlated with the standard deviation (SD) or interquartile range (IQR). The null hypothesis assumed that the trueness and precision of the casts produced by the selected printers with regard to the calculated distances (IMW, ICW, and AL) would not display statistically significant differences compared with the digital master file (STL). The data were examined for normal distribution and likely outliers and were observed to be partially asymmetric. Therefore, the Mann-Whitney U test, a nonparametric test for independent samples, was applied to determine the methods that differed significantly (P<.05) from the values of the digital master file. In this testing method, the median as a measure of central tendency and the quartile ranges as measures of dispersion were used for pairwise comparisons. To verify the precision, the Levene tests were used in pairs so that significant differences (P<.05) among variances could be determined. The statistical analysis was performed by using a statistical software program (IBM SPSS Statistics, v22; IBM Corp). RESULTS Descriptive statistics are reported in Table 2. The ranges of the data sets for the printers investigated are shown in boxplots (Figs. 4-6). The vertical line (black) represents the median of the master values. The green boxplots and Nestler et al
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Figure 2. Digital master file imported to survey software (Convince Premium 2012; 3Shape).
blue boxplots depict extrusion-based and photopolymerization-based printers. For IMW, the printers myrev140 (P<.001), M2 (P<.001), and Form 2 (P=.046) differed significantly from the values of the master cast. However, Asiga MAX UV (P=.092) and Ultimaker 2+ (P=.443) exhibited no significant differences in comparison with the reference file. For ICW, all the printers deviated significantly from the master cast’s values. For AL, all the printers presented significant deviations from the master cast, except for Form 2 (P=.095). In terms of precision, no homogeneity of variances could be identified among the printers for any of the measured distances, indicating significant differences in all groups. DISCUSSION The purpose of the present study was to analyze the dimensional accuracy of AM complete-arch casts produced by a selection of extrusion- and polymerizationbased printers. According to the results, the null hypothesis that no significant differences would be found among the AM casts produced by the examined printers in trueness and precision was partially rejected. The accuracy measurement of anatomic casts is challenging because of their circular shapes and because the reproducibility of a suitable reference point cannot always be guaranteed.16,25 Therefore, superimposition, an approach generally used for accuracy analysis, was adopted.7,25 The reference file (master) and a scan of the cast to be analyzed were digitally superimposed by using a best-fit algorithm, and the dimensional surface deviations displayed in color or numerically.18 This approach is particularly suitable for explorative analyses of the entire surface and for evaluating surface resolution. However, superimposition is suitable only to a limited extent for assessing dimensional deviations because of the absence of a reference value. Furthermore, the THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 1. Printers evaluated and their properties from manufacturer’s specifications Category
Technology
Retail Price (V)
Resolution (mm)
Printer
Manufacturer
Build Volume (mm)
Material
M2
Makergear
Desktop
FDM
200×250×200
Acrylonitrile-Butadiene-Styrene copolymer (ABS)
1500
<50e250 Nozzle: 350
Ultimaker 2+
Ultimaker
Desktop
FDM
223×223×205
Polylactide (PLA)
2150
x, y: 12.5 z: 5
Form 2
Formlabs
Desktop
SLA (Laser)
145×145×175
Dental SG
3800
Layer thickness: 25e200 Laser spot: 140
Asiga MAX UV
Asiga
Desktop/Industrial
DLP (LED)
119×67×77
myrev140
Sisma
Industrial
SLA (Laser)
3×140×140×150
Imprimo LC Model
14 000
x, y: 62 z: 1
Optiprint
58 500
Laser spot: <20 z: 1
DLP, digital light processing; FDM, fused deposition modeling; LED, light-emitting diode; SLA, stereolithography.
necessary scanning process of the cast to be investigated has additional sources of error.26 Therefore, investigations focusing on dimensional accuracy generally use digital calipers, which involves additional uncertainties.3 To address the shortcomings of the reproducibility of anatomic reference points and the limited comparability of superimposition, artificial reference points were used in the present study. Other studies have also established artificial reference points located on study casts, reporting high measurement repeatability.12,27,28 A flat surface with a cylindrical hole was selected as the reference geometry. The aim was to design a reference body that was independent of different printing parameters to the extent feasible. The flat surface of the reference body should compensate for the influence of the different layer heights of the investigated printers in the vertical dimension. A circular shape was selected for the horizontal dimension. This was aimed at reducing the influence of different edge sharpness and x and y resolutions of the printers. However, the reference geometries are a possible limitation of the study because local deviations and errors in the printing of the reference bodies could influence the accuracy of the entire casts. Moreover, the measurement of the reference points used in the present study contained inaccuracies. The measurement was carried out with a multisensory CMM that operates with a maximum deviation of 1.9 mm+L/250. This implies a maximum deviation of 2.2 mm for the longest measured individual section (AL). In addition, there were maximum deviations of 2 mm between the largest and smallest bores of the reference points, with a maximum radius inaccuracy of 0.8 mm. The digital measurement of the master file resulted in a maximum deviation of 16 mm for AL. To enable better comparability, the algebraic signs of the individual deviations from the reference file were considered in the present investigation to determine accuracy. This may have caused the false-positive and false-negative measurements to balance each other out and achieve high trueness, notwithstanding the wide fluctuations in the results and consequent low precision. For the 3 investigated distances IMW, ICW, and AL, the THE JOURNAL OF PROSTHETIC DENTISTRY
Figure 3. Measurement of cast in multisensory coordinate measuring machine (O-Inspect 422; Zeiss).
deviations from the master file of all printers were between 12 mm and 240 mm (trueness). The IQR was between 17 mm and 388 mm (precision). The largest deviations were measured for AL, representing the longest distance included in the present experiment, whereas the shortest distance ICW exhibited a statistically significant decrease for all the printers (probably because of minor shrinkage effects), and the larger distance IMW revealed no significant deviation for certain printers. A likely explanation is the higher dispersion of the results for IMW, which made the deviations less likely to be statistically significant. In general, the myrev140 printer was highly precise, although it exhibited the highest median deviations from the master. The Ultimaker 2+ exhibited a high IQR for AL, Form 2 for all the distances. However, the latter did not deviate significantly from the master for AL. The Asiga MAX UV exhibited the highest accuracy with consistently low IQR and MD for all the distances. The Ultimaker 2+ achieved similarly high accuracy for both IMW and ICW. Nestler et al
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Table 2. Descriptive statistics Master and Printers N
Master
M2; Makergear
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma
15
38
37
37
37
34
Intermolar width -
-55 mm
12 mm
-80 mm
-16 mm
-175 mm
Min
50.003 mm
49.855 mm
49.915 mm
49.824 mm
49.927 mm
49.768 mm
Max
50.003 mm
50.047 mm
50.084 mm
50.127 mm
50.052 mm
49.868 mm
Median
50.003 mm
49.948 mm
50.015 mm
49.923 mm
49.987 mm
49.828 mm
IQR
0 mm
55 mm
56 mm
134 mm
47 mm
36 mm
SD
0 mm
39 mm
43 mm
94 mm
32 mm
28 mm
P GS
-
<.001
.443
.046
.092
<.001
PL
-
<.001
<.001
<.001
<.001
<.001
Median deviation from master
Intercanine width -
-73 mm
-25 mm
-77 mm
-23 mm
-113 mm
Min
32.849 mm
32.726 mm
32.802 mm
32.648 mm
32.802 mm
32.684 mm
Max
32.855 mm
32.829 mm
32.873 mm
32.957 mm
32.872 mm
32.768 mm
Median
32.855 mm
32.782 mm
32.830 mm
32.778 mm
32.832 mm
32.742 mm
IQR
3 mm
40 mm
19 mm
107 mm
18 mm
17 mm
SD
2 mm
27 mm
18 mm
85 mm
15 mm
20 mm
P GS
-
<.001
<.001
.010
<.001
<.001
PL
-
<.001
.001
<.001
<.001
.001
Median deviation from master
Arch length -
-90 mm
-163 mm
-136 mm
-52 mm
-240 mm
Min
77.683 mm
77.442 mm
77.374 mm
77.306 mm
77.521 mm
77.305 mm
Max
77.699 mm
77.739 mm
77.896 mm
77.883 mm
77.780 mm
77.532 mm
Median
77.695 mm
77.605 mm
77.532 mm
77.559 mm
77.643 mm
77.455 mm
IQR
2 mm
98 mm
247 mm
388 mm
50 mm
34 mm
SD
5 mm
71 mm
159 mm
199 mm
47 mm
56 mm
P GS
-
.001
.022
.095
<.001
<.001
PL
-
<.001
<.001
<.001
.001
.002
Median deviation from master
IQR, interquartile range; Max, maximum value; Min, minimum value; n, number of valid cases; P GS, significance toward digital master values produced by Mann-Whitney U tests (trueness); P L, P values produced by Levene tests for homogeneity of variances (precision); SD, standard deviation. Bold values indicate statistical significance (P<.05).
M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 49.7
49.8
49.9
50
50.1
50.2
50.3
Intermolar Width (mm) Figure 4. Boxplot of deviations for intermolar width.
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M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 32.7
32.6
32.8
32.9
33
33.1
Intercanine Width (mm) Figure 5. Boxplot of deviations for intercanine width.
M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 77.2
77.3
77.4
77.5
77.6
77.7
77.8
77.9
78
78.1
78.2
Arch Length (mm) Figure 6. Boxplot of deviations for arch length.
Meanwhile, the M2 exhibited average performance for most of the parameters studied. Dental casts need to provide both high surface resolution and high dimensional accuracy. The printer specifications mainly affect the resolution, whereas the AM geometry, material-related shrinkage, and warpage effects can further influence the dimensional accuracy. Although photopolymerization-based printers are better than extrusion-based printers in resolution,13 no differentiation in terms of dimensional accuracy was observed in this investigation. Different factors can influence the resolution THE JOURNAL OF PROSTHETIC DENTISTRY
and dimensional accuracy of AM casts: The resolution in the x and y dimensions is determined primarily by laser spot size (SLA), pixel size (DLP), and nozzle size (FDM) and in the z dimension by layer height. Marginal layer heights, resulting in a more detailed resolution, are essential for the clinical application of prosthetic components.29 However, a decreased layer height does not necessarily increase the dimensional accuracy of AM casts.19 The dimensional accuracy is affected mainly by shrinkage and warpage. For photopolymerization-based systems, a frequently mentioned reason for shrinkage Nestler et al
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and warpage is the incomplete polymerization of monomeric resin by the light source during the AM process, necessitating postpolymerization.8,11 Shrinkage and warpage can be observed in extrusion-based AM casts as well.25 Factors favoring this are an inadequately heated print bed, insufficient adhesion of the first layer, or removal of the casts too early when still warm. Furthermore, Camardella et al4 concluded that AM casts with a horseshoe-shaped base produced by using an SLA printer underwent a statistically significant reduction in the transversal dimension. In this study, all the SLA printers investigated exhibited dimensional inaccuracies for all the calculated distances. Accordingly, the absence of a transversal cast support of the horseshoe-shaped base may have increased shrinkage. The extent to which measurement deviations of complete-arch casts are clinically relevant is unclear. In general, deviations of over 500 mm are considered as clinically unacceptable.3,11,14 However, certain studies have reported that only deviations below 200 to 300 mm are clinically acceptable,12,18,30 corresponding to the tolerance of manual measurements.16,19,25 Consequently, all the examined printers may be used to produce clinically acceptable diagnostic casts in terms of precision and trueness, except for the myrev140 on AL (median deviation from master=-240 mm). For definitive casts, the deviations and surface resolution also appear acceptable for orthodontic purposes. However, these must be considered critically for prosthetic applications. In comparison with deviations of conventional cast fabrication methods by using high-precision impression materials, determined to be 17.4 mm to 36.7 mm,26 111.2 mm,30 and 134.7 mm,31 only certain printers match the accuracy of conventional methods. However, the inaccuracies inherent with conventional prosthesis fabrication have to be considered. With the digital workflow, the product is produced directly, and diagnostic casts are analyzed, avoiding deviations during the transition from digital to analog. Errors in the intermediate dental laboratory steps can be avoided, unlike with the conventional workflow. In addition to cast printing, photopolymerization-based printing can be used for directly fabricating surgical guides, occlusal devices, and removable complete dentures. To the authors' knowledge, medical class IIa filaments are not yet available for extrusion-based printers. Therefore, they should not be used to produce applications that are to remain in the patient’s mouth for an extended period. However, they are suitable for producing surgical templates.32 The selection of the AM system is user specific: In addition to the acquisition costs and printing accuracy, factors such as the intended application, maintenance costs, and planned production output (build volume and AM speed) should be considered.19 This study compared the accuracy of AM casts to an available STL Nestler et al
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file. The selection of the digitization system (IOS or CBCT) for generating the 3D data set is also of clinical relevance. This is because the resulting accuracy deviations have additional impact on the overall accuracy of the digital workflow. In methodically comparable studies, the mean deviations of different devices ranged from 0.03 mm to 397.46 mm while using IOS20 and from 1.41 mm to -228.62 mm while using CBCT systems.21 In addition to selecting a suitable digitizing system, the printed output can also be positively influenced by adapting the cast file before printing.29 However, a prerequisite for this is a continuously precise printing result and knowledge of the printer-specific deviation from the master file. Although the clinical efficacy and effectiveness of AM casts for preoperative planning of guided surgery have been investigated,33 studies on the treatment outcome of AM dental appliances with various AM techniques and digitization methods are sparse and need to be performed.12 CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. The extrusion-based and photopolymerizationbased printers were accurate and have limited suitability for clinical application. 2. The investigated printers did not generally match the accuracy of high-precision impression materials. 3. The cost of the AM system did not correlate with its accuracy. REFERENCES 1. Kaye R, Goldstein T, Zeltsman D, Grande DA, Smith LP. Three dimensional printing: A review on the utility within medicine and otolaryngology. Int J Pediatr Otorhinolaryngol 2016;89:145-8. 2. Al-Imam H, Gram M, Benetti AR, Gotfredsen K. Accuracy of stereolithography additive casts used in a digital workflow. J Prosthet Dent 2018;119:580-5. 3. Wan Hassan WN, Yusoff Y, Mardi NA. Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees of crowding. Am J Orthod Dentofacial Orthop 2017;151:209-18. 4. Camardella LT, de Vasconcellos Vilella O, Breuning H. Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases. Am J Orthod Dentofacial Orthop 2017;151:1178-87. 5. Barazanchi A, Li KC, Al-Amleh B, Lyons K, Waddell JN. Additive technology: Update on current materials and applications in dentistry. J Prosthodont 2017;26:156-63. 6. Osman RB, Alharbi N, Wismeijer D. Build angle: does it influence the accuracy of 3d-printed dental restorations using digital light-processing technology? Int J Prosthodont 2017;30:182-8. 7. Jeong YG, Lee WS, Lee KB. Accuracy evaluation of dental models manufactured by CAD/CAM milling method and 3D printing method. J Adv Prosthodont 2018;10:245-51. 8. Liaw CY, Guvendiren M. Current and emerging applications of 3D printing in medicine. Biofabrication 2017;9:024102. 9. Chen H, Yang X, Chen L, Wang Y, Sun Y. Application of FDM threedimensional printing technology in the digital manufacture of custom edentulous mandible trays. Sci Rep 2016;6:19207. 10. Groth C, Kravitz ND, Jones PE, Graham JW, Redmond WR. Three-dimensional printing technology. J Clin Orthod 2014;48:475-85. 11. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of threedimensional dental resin models created by fused deposition modeling,
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25. Lee KY, Cho JW, Chang NY, Chae JM, Kang KH, Kim SC, et al. Accuracy of three-dimensional printing for manufacturing replica teeth. Korean J Orthod 2015;45:217-25. 26. Ender A, Attin T, Mehl A. In vivo precision of conventional and digital methods of obtaining complete-arch dental impressions. J Prosthet Dent 2016;115:313-20. 27. Salmi M, Paloheimo KS, Tuomi J, Wolff J, Mäkitie A. Accuracy of medical models made by additive manufacturing (rapid manufacturing). J Craniomaxillofac Surg 2013;41:603-9. 28. Vogel AB, Kilic F, Schmidt F, Rübel S, Lapatki BG. Dimensional accuracy of jaw scans performed on alginate impressions or stone models: A practiceoriented study. J Orofac Orthop 2015;76:351-65. 29. Ishida Y, Miyasaka T. Dimensional accuracy of dental casting patterns created by 3D printers. Dent Mater J 2016;35:250-6. 30. Jin SJ, Kim DY, Kim JH, Kim WC. Accuracy of dental replica models using photopolymer materials in additive manufacturing: In vitro threedimensional evaluation. J Prosthodont 2019;28:e557-62. 31. Abduo J. Accuracy of casts produced from conventional and digital workflows: A qualitative and quantitative analyses. J Adv Prosthodont 2019;11: 138-46. 32. Sun Y, Ding Q, Tang L, Zhang L, Xie Q. Accuracy of a chairside fused deposition modeling 3D-printed single-tooth surgical template for implant placement: An in vitro comparison with a light cured template. J Craniomaxillofac Surg 2019;47:1216-21. 33. Diment LE, Thompson MS, Bergmann JHM. Clinical efficacy and effectiveness of 3D printing: a systematic review. BMJ Open 2017;7:e016891. Corresponding author: Dr Norbert Nestler CharitéeUniversitätsmedizin Berlin Corporate Member of Freie Universität zu Berlin Humboldt-Universität zu Berlin, and Berlin Institute of Health Campus Benjamin Franklin Center for Dental and Craniofacial Sciences (CC3) Aßmannshauser Str. 6, 14197 Berlin GERMANY Email: [email protected] Acknowledgments The authors thank the participating printer manufacturers and distributors for providing the study models, Fraunhofer Institute for Production Systems and Design Technology Berlin for the measurement of these models, and MESANTIS 3D Dental Radiologicum for bearing the costs of the measurement. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.11.011
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Dimensional accuracy of extrusion- and photopolymerizationbased 3D printers: In vitro study comparing printed casts Norbert Nestler, DDS,a Christian Wesemann, DDS, Dr med dent,b Benedikt C. Spies, DDS, Dr med dent, PhD,c Florian Beuer, DDS, Dr med dent, MME, PhD,d and Axel Bumann, DDS, Dr med dent, PhDe In conjunction with the evolving digitization in modern dentistry, there is an increasing interest in 3D printing. Three-dimensional printing, also termed additive manufacturing (AM), can be integrated into a direct or indirect digital workflow. A digital work file, typically standard tessellation language (STL), can be either acquired directly by using an intraoral scanner (IOS) or a cone beam computed tomography (CBCT) system or generated indirectly by using a desktop scanner, scanning conventional casts or impressions. For further editing, the STL file can be transferred to a computer-aided design (CAD) software program and subsequently transmitted to a printer for producing custom structures directly.1,2
ABSTRACT Statement of problem. Reliable studies comparing the accuracy of complete-arch casts from 3D printers are scarce. Purpose. The purpose of this in vitro study was to investigate the accuracy of casts printed by using various extrusion- and photopolymerization-based printers. Material and methods. A master file was sent to 5 printer manufacturers and distributors to print 37 identical casts. This file consisted of a standardized data set of a maxillary cast in standard tessellation language (STL) format comprising 5 reference points for the measurement of 3 distances that served as reference for all measurements: intermolar width (IMW), intercanine width (ICW), and dental arch length (AL). The digital measurement of the master file obtained by using a surveying software program (Convince Premium 2012) was used as the control. Two extrusion-based (M2 and Ultimaker 2+) and 3 photopolymerization-based printers (Form 2, Asiga MAX UV, and myrev140) were compared. The casts were measured by using a multisensory coordinate measuring machine (O-Inspect 422). The values were then compared with those of the master file. The Mann-Whitney U test and Levene tests were used to determine significant differences in the trueness and precision (accuracy) of the measured distances. Results. The deviations from the master file at all 3 distances for the included printers ranged between 12 mm and 240 mm (trueness), with an interquartile range (IQR) between 17 mm and 388 mm (precision). Asiga MAX UV displayed the highest accuracy, considering all the distances, and Ultimaker 2+ demonstrated comparable accuracy for shorter distances (IMW and ICW). Although myrev140 operated with high precision, it displayed high deviations from the master file. Similarly, although Form 2 exhibited high IQR, it did not deviate significantly from the master file in the longest range (AL). M2 performed consistently. Conclusions. Both extrusion-based and photopolymerization-based printers were accurate. In general, inexpensive printers were no less accurate than more expensive ones. (J Prosthet Dent 2020;-:---)
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. a Doctoral student, Campus Benjamin Franklin, Center for Dental and Craniofacial Sciences (CC3), Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. b Graduate student, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. c Privatdozent, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. d Professor, Department of Prosthodontics, Geriatric Dentistry and Craniomandibular Disorders, Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. e Professor, Mesantis 3D Dental Radiologicum, Clinic of Orthodontics, Berlin, Germany.
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MATERIAL AND METHODS
Clinical Implications Based on the present findings, both extrusion-based (fused deposition modeling) and photopolymerization-based (stereolithography, digital light processing) printers can be used to produce diagnostic casts. The cost of the evaluated systems did not correlate with accuracy.
The advantages of an AM-based digital workflow include the capability of a data set to be stored or shared conveniently and reproduced on demand.3 The production cost, time, and laboratory equipment required can be reduced from those for automatic manufacturing.4 Compared with subtractive manufacturing (such as milling), AM avoids abundant waste material and enables the production of more complex and accurate structures.5-7 Among AM technologies, photopolymerization, including stereolithography (SLA) and digital light processing (DLP), has been used in dentistry. High resolution for detailed reproduction, smooth surfaces, and reduced fabrication time are the most relevant advantages for the dental field.4,8 In this method, a light source is projected through a movable mirror onto a basin filled with liquid resin (photopolymer). After polymerization, the basin is lowered layer by layer so that each layer can be polymerized separately.4 Extrusionbased fused deposition modeling (FDM) is another established AM process. A heated plastic filament (thermoplastic) is deposited on a movable platform and then polymerized.9 Although this method is less expensive, rendition detail is reduced.10 In addition to the equipment and material costs, printing accuracy is an essential parameter for clinical applicability. The outcomes of the few studies conducted till now for assessing the dimensional accuracies of the different AM technologies11-13 have been inconsistent. Most of these studies used hand-held digital calipers,3,14,15 which provide low reproducibility16 or superimposition.17-19 In addition, the required digitization of the reference model is a source of error, depending on the type of digitization. Moreover, the statistical capabilities of the present studies are limited because of the low number of included casts (N=1 to 12).7,11,13 Therefore, the purpose of this in vitro study was to compare the dimensional accuracy (trueness and precision) of AM casts produced by 3 AM technologies (FDM, SLA, and DLP) with that of an adequate number of examined casts (n=37) and a reproducible measurement methodology. The null hypothesis was that no significant differences in trueness and precision would be found between the AM technologies considered. THE JOURNAL OF PROSTHETIC DENTISTRY
A specially developed resin cast served as the reference for all measurements.20-22 This neutrally interlocked maxillary cast displayed 5 measuring cubes in the area of the first maxillary molars, the canines, and between the central incisors. Drill holes were located in the center of these cubes. The highest point of the centerline of each hole was defined as the most reproducible measuring point. The measuring points were used to calculate the following distances for comparison: intermolar width (IMW), intercanine width (ICW), and arch length (AL) (Fig. 1). The master cast was scanned by using an IOS (TRIOS Color POD; 3Shape), and the corresponding STL file was generated. A palate-free base in horseshoe design was formed for the master file to simulate a clinical workflow. The master file was subsequently analyzed and measured by using a specialized surveying software program (Convince Premium 2012; 3Shape). The vertical walls of the drill holes were marked manually and identified by the software program as the vertical basic shape so that the ideal centerlines of the cylinders could be calculated. The highest point of the centerline was defined as the intersection of the centerline and the upper surface of the drilling cylinder. Thus, the individual measuring points could be connected and the distances IMW, ICW, and AL determined (Fig. 2). The measurement of the master file was repeated 15 times, and the average ±standard deviation digital master values (gold standard) were defined: IMW=50.003 mm ±0 mm, ICW=32.855 mm ±2 mm, and AL=77.695 mm ±5 mm. Thereafter, the master file was sent to the participating AM manufacturers and distributors with the requirement to print 37 identical casts as accurately as possible. Material selection and the printer-specific parameter setting were determined by the companies. The casts were produced by using 2 extrusion-based (FDM) and 3 photopolymerizationbased (SLA and DLP) printers (Table 1). Because of transportation damage, only 34 of the 37 casts fabricated by one of the printers (myrev140; Sisma) could be measured. Makergear delivered 38 casts, all of which were used for measurements. For the remaining printers, 37 casts were measured. The AM casts were measured in collaboration with the Fraunhofer Institute for Production Systems and Design Technology, Berlin. The measurement was carried out by using a multisensory coordinate measuring machine (CMM, O-Inspect 422; Zeiss) and software program (Calypso 5.4.20; Zeiss). For volumetric length measurements, the CMM operated with a maximum permissible deviation of 1.9 mm+L/250 in accordance with the international series of standards for CMM systems (ISO 10360).23 Each cast was calibrated and measured at constant environmental parameters (20 C, 50% humidity, and 1013 hPa ambient pressure).
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Figure 1. Master cast with reference measuring points yielding distances: 5=intermolar width, 6=intercanine width, and 1+2+3+4=dental arch length.
Measurements were carried out by contact with a 0.3mm-diameter, dimensionally stable ruby ball by scanning the drill holes over 169 to 200 measuring points to determine the defined measuring point for the distance measurement between the 5 reference cubes (Fig. 3). According to ISO 5725,24 the combination of trueness and precision results in accuracy. Trueness was defined as the proximity of a printer’s average to the digital master’s average, and precision was defined as the scatter of a printer’s readings and is correlated with the standard deviation (SD) or interquartile range (IQR). The null hypothesis assumed that the trueness and precision of the casts produced by the selected printers with regard to the calculated distances (IMW, ICW, and AL) would not display statistically significant differences compared with the digital master file (STL). The data were examined for normal distribution and likely outliers and were observed to be partially asymmetric. Therefore, the Mann-Whitney U test, a nonparametric test for independent samples, was applied to determine the methods that differed significantly (P<.05) from the values of the digital master file. In this testing method, the median as a measure of central tendency and the quartile ranges as measures of dispersion were used for pairwise comparisons. To verify the precision, the Levene tests were used in pairs so that significant differences (P<.05) among variances could be determined. The statistical analysis was performed by using a statistical software program (IBM SPSS Statistics, v22; IBM Corp). RESULTS Descriptive statistics are reported in Table 2. The ranges of the data sets for the printers investigated are shown in boxplots (Figs. 4-6). The vertical line (black) represents the median of the master values. The green boxplots and Nestler et al
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Figure 2. Digital master file imported to survey software (Convince Premium 2012; 3Shape).
blue boxplots depict extrusion-based and photopolymerization-based printers. For IMW, the printers myrev140 (P<.001), M2 (P<.001), and Form 2 (P=.046) differed significantly from the values of the master cast. However, Asiga MAX UV (P=.092) and Ultimaker 2+ (P=.443) exhibited no significant differences in comparison with the reference file. For ICW, all the printers deviated significantly from the master cast’s values. For AL, all the printers presented significant deviations from the master cast, except for Form 2 (P=.095). In terms of precision, no homogeneity of variances could be identified among the printers for any of the measured distances, indicating significant differences in all groups. DISCUSSION The purpose of the present study was to analyze the dimensional accuracy of AM complete-arch casts produced by a selection of extrusion- and polymerizationbased printers. According to the results, the null hypothesis that no significant differences would be found among the AM casts produced by the examined printers in trueness and precision was partially rejected. The accuracy measurement of anatomic casts is challenging because of their circular shapes and because the reproducibility of a suitable reference point cannot always be guaranteed.16,25 Therefore, superimposition, an approach generally used for accuracy analysis, was adopted.7,25 The reference file (master) and a scan of the cast to be analyzed were digitally superimposed by using a best-fit algorithm, and the dimensional surface deviations displayed in color or numerically.18 This approach is particularly suitable for explorative analyses of the entire surface and for evaluating surface resolution. However, superimposition is suitable only to a limited extent for assessing dimensional deviations because of the absence of a reference value. Furthermore, the THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 1. Printers evaluated and their properties from manufacturer’s specifications Category
Technology
Retail Price (V)
Resolution (mm)
Printer
Manufacturer
Build Volume (mm)
Material
M2
Makergear
Desktop
FDM
200×250×200
Acrylonitrile-Butadiene-Styrene copolymer (ABS)
1500
<50e250 Nozzle: 350
Ultimaker 2+
Ultimaker
Desktop
FDM
223×223×205
Polylactide (PLA)
2150
x, y: 12.5 z: 5
Form 2
Formlabs
Desktop
SLA (Laser)
145×145×175
Dental SG
3800
Layer thickness: 25e200 Laser spot: 140
Asiga MAX UV
Asiga
Desktop/Industrial
DLP (LED)
119×67×77
myrev140
Sisma
Industrial
SLA (Laser)
3×140×140×150
Imprimo LC Model
14 000
x, y: 62 z: 1
Optiprint
58 500
Laser spot: <20 z: 1
DLP, digital light processing; FDM, fused deposition modeling; LED, light-emitting diode; SLA, stereolithography.
necessary scanning process of the cast to be investigated has additional sources of error.26 Therefore, investigations focusing on dimensional accuracy generally use digital calipers, which involves additional uncertainties.3 To address the shortcomings of the reproducibility of anatomic reference points and the limited comparability of superimposition, artificial reference points were used in the present study. Other studies have also established artificial reference points located on study casts, reporting high measurement repeatability.12,27,28 A flat surface with a cylindrical hole was selected as the reference geometry. The aim was to design a reference body that was independent of different printing parameters to the extent feasible. The flat surface of the reference body should compensate for the influence of the different layer heights of the investigated printers in the vertical dimension. A circular shape was selected for the horizontal dimension. This was aimed at reducing the influence of different edge sharpness and x and y resolutions of the printers. However, the reference geometries are a possible limitation of the study because local deviations and errors in the printing of the reference bodies could influence the accuracy of the entire casts. Moreover, the measurement of the reference points used in the present study contained inaccuracies. The measurement was carried out with a multisensory CMM that operates with a maximum deviation of 1.9 mm+L/250. This implies a maximum deviation of 2.2 mm for the longest measured individual section (AL). In addition, there were maximum deviations of 2 mm between the largest and smallest bores of the reference points, with a maximum radius inaccuracy of 0.8 mm. The digital measurement of the master file resulted in a maximum deviation of 16 mm for AL. To enable better comparability, the algebraic signs of the individual deviations from the reference file were considered in the present investigation to determine accuracy. This may have caused the false-positive and false-negative measurements to balance each other out and achieve high trueness, notwithstanding the wide fluctuations in the results and consequent low precision. For the 3 investigated distances IMW, ICW, and AL, the THE JOURNAL OF PROSTHETIC DENTISTRY
Figure 3. Measurement of cast in multisensory coordinate measuring machine (O-Inspect 422; Zeiss).
deviations from the master file of all printers were between 12 mm and 240 mm (trueness). The IQR was between 17 mm and 388 mm (precision). The largest deviations were measured for AL, representing the longest distance included in the present experiment, whereas the shortest distance ICW exhibited a statistically significant decrease for all the printers (probably because of minor shrinkage effects), and the larger distance IMW revealed no significant deviation for certain printers. A likely explanation is the higher dispersion of the results for IMW, which made the deviations less likely to be statistically significant. In general, the myrev140 printer was highly precise, although it exhibited the highest median deviations from the master. The Ultimaker 2+ exhibited a high IQR for AL, Form 2 for all the distances. However, the latter did not deviate significantly from the master for AL. The Asiga MAX UV exhibited the highest accuracy with consistently low IQR and MD for all the distances. The Ultimaker 2+ achieved similarly high accuracy for both IMW and ICW. Nestler et al
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Table 2. Descriptive statistics Master and Printers N
Master
M2; Makergear
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma
15
38
37
37
37
34
Intermolar width -
-55 mm
12 mm
-80 mm
-16 mm
-175 mm
Min
50.003 mm
49.855 mm
49.915 mm
49.824 mm
49.927 mm
49.768 mm
Max
50.003 mm
50.047 mm
50.084 mm
50.127 mm
50.052 mm
49.868 mm
Median
50.003 mm
49.948 mm
50.015 mm
49.923 mm
49.987 mm
49.828 mm
IQR
0 mm
55 mm
56 mm
134 mm
47 mm
36 mm
SD
0 mm
39 mm
43 mm
94 mm
32 mm
28 mm
P GS
-
<.001
.443
.046
.092
<.001
PL
-
<.001
<.001
<.001
<.001
<.001
Median deviation from master
Intercanine width -
-73 mm
-25 mm
-77 mm
-23 mm
-113 mm
Min
32.849 mm
32.726 mm
32.802 mm
32.648 mm
32.802 mm
32.684 mm
Max
32.855 mm
32.829 mm
32.873 mm
32.957 mm
32.872 mm
32.768 mm
Median
32.855 mm
32.782 mm
32.830 mm
32.778 mm
32.832 mm
32.742 mm
IQR
3 mm
40 mm
19 mm
107 mm
18 mm
17 mm
SD
2 mm
27 mm
18 mm
85 mm
15 mm
20 mm
P GS
-
<.001
<.001
.010
<.001
<.001
PL
-
<.001
.001
<.001
<.001
.001
Median deviation from master
Arch length -
-90 mm
-163 mm
-136 mm
-52 mm
-240 mm
Min
77.683 mm
77.442 mm
77.374 mm
77.306 mm
77.521 mm
77.305 mm
Max
77.699 mm
77.739 mm
77.896 mm
77.883 mm
77.780 mm
77.532 mm
Median
77.695 mm
77.605 mm
77.532 mm
77.559 mm
77.643 mm
77.455 mm
IQR
2 mm
98 mm
247 mm
388 mm
50 mm
34 mm
SD
5 mm
71 mm
159 mm
199 mm
47 mm
56 mm
P GS
-
.001
.022
.095
<.001
<.001
PL
-
<.001
<.001
<.001
.001
.002
Median deviation from master
IQR, interquartile range; Max, maximum value; Min, minimum value; n, number of valid cases; P GS, significance toward digital master values produced by Mann-Whitney U tests (trueness); P L, P values produced by Levene tests for homogeneity of variances (precision); SD, standard deviation. Bold values indicate statistical significance (P<.05).
M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 49.7
49.8
49.9
50
50.1
50.2
50.3
Intermolar Width (mm) Figure 4. Boxplot of deviations for intermolar width.
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M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 32.7
32.6
32.8
32.9
33
33.1
Intercanine Width (mm) Figure 5. Boxplot of deviations for intercanine width.
M2; Makergear
Printers
Ultimaker 2+; Ultimaker
Form 2; Formlabs
Asiga MAX UV; Asiga
myrev140; Sisma 77.2
77.3
77.4
77.5
77.6
77.7
77.8
77.9
78
78.1
78.2
Arch Length (mm) Figure 6. Boxplot of deviations for arch length.
Meanwhile, the M2 exhibited average performance for most of the parameters studied. Dental casts need to provide both high surface resolution and high dimensional accuracy. The printer specifications mainly affect the resolution, whereas the AM geometry, material-related shrinkage, and warpage effects can further influence the dimensional accuracy. Although photopolymerization-based printers are better than extrusion-based printers in resolution,13 no differentiation in terms of dimensional accuracy was observed in this investigation. Different factors can influence the resolution THE JOURNAL OF PROSTHETIC DENTISTRY
and dimensional accuracy of AM casts: The resolution in the x and y dimensions is determined primarily by laser spot size (SLA), pixel size (DLP), and nozzle size (FDM) and in the z dimension by layer height. Marginal layer heights, resulting in a more detailed resolution, are essential for the clinical application of prosthetic components.29 However, a decreased layer height does not necessarily increase the dimensional accuracy of AM casts.19 The dimensional accuracy is affected mainly by shrinkage and warpage. For photopolymerization-based systems, a frequently mentioned reason for shrinkage Nestler et al
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and warpage is the incomplete polymerization of monomeric resin by the light source during the AM process, necessitating postpolymerization.8,11 Shrinkage and warpage can be observed in extrusion-based AM casts as well.25 Factors favoring this are an inadequately heated print bed, insufficient adhesion of the first layer, or removal of the casts too early when still warm. Furthermore, Camardella et al4 concluded that AM casts with a horseshoe-shaped base produced by using an SLA printer underwent a statistically significant reduction in the transversal dimension. In this study, all the SLA printers investigated exhibited dimensional inaccuracies for all the calculated distances. Accordingly, the absence of a transversal cast support of the horseshoe-shaped base may have increased shrinkage. The extent to which measurement deviations of complete-arch casts are clinically relevant is unclear. In general, deviations of over 500 mm are considered as clinically unacceptable.3,11,14 However, certain studies have reported that only deviations below 200 to 300 mm are clinically acceptable,12,18,30 corresponding to the tolerance of manual measurements.16,19,25 Consequently, all the examined printers may be used to produce clinically acceptable diagnostic casts in terms of precision and trueness, except for the myrev140 on AL (median deviation from master=-240 mm). For definitive casts, the deviations and surface resolution also appear acceptable for orthodontic purposes. However, these must be considered critically for prosthetic applications. In comparison with deviations of conventional cast fabrication methods by using high-precision impression materials, determined to be 17.4 mm to 36.7 mm,26 111.2 mm,30 and 134.7 mm,31 only certain printers match the accuracy of conventional methods. However, the inaccuracies inherent with conventional prosthesis fabrication have to be considered. With the digital workflow, the product is produced directly, and diagnostic casts are analyzed, avoiding deviations during the transition from digital to analog. Errors in the intermediate dental laboratory steps can be avoided, unlike with the conventional workflow. In addition to cast printing, photopolymerization-based printing can be used for directly fabricating surgical guides, occlusal devices, and removable complete dentures. To the authors' knowledge, medical class IIa filaments are not yet available for extrusion-based printers. Therefore, they should not be used to produce applications that are to remain in the patient’s mouth for an extended period. However, they are suitable for producing surgical templates.32 The selection of the AM system is user specific: In addition to the acquisition costs and printing accuracy, factors such as the intended application, maintenance costs, and planned production output (build volume and AM speed) should be considered.19 This study compared the accuracy of AM casts to an available STL Nestler et al
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file. The selection of the digitization system (IOS or CBCT) for generating the 3D data set is also of clinical relevance. This is because the resulting accuracy deviations have additional impact on the overall accuracy of the digital workflow. In methodically comparable studies, the mean deviations of different devices ranged from 0.03 mm to 397.46 mm while using IOS20 and from 1.41 mm to -228.62 mm while using CBCT systems.21 In addition to selecting a suitable digitizing system, the printed output can also be positively influenced by adapting the cast file before printing.29 However, a prerequisite for this is a continuously precise printing result and knowledge of the printer-specific deviation from the master file. Although the clinical efficacy and effectiveness of AM casts for preoperative planning of guided surgery have been investigated,33 studies on the treatment outcome of AM dental appliances with various AM techniques and digitization methods are sparse and need to be performed.12 CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. The extrusion-based and photopolymerizationbased printers were accurate and have limited suitability for clinical application. 2. The investigated printers did not generally match the accuracy of high-precision impression materials. 3. The cost of the AM system did not correlate with its accuracy. REFERENCES 1. Kaye R, Goldstein T, Zeltsman D, Grande DA, Smith LP. Three dimensional printing: A review on the utility within medicine and otolaryngology. Int J Pediatr Otorhinolaryngol 2016;89:145-8. 2. Al-Imam H, Gram M, Benetti AR, Gotfredsen K. Accuracy of stereolithography additive casts used in a digital workflow. J Prosthet Dent 2018;119:580-5. 3. Wan Hassan WN, Yusoff Y, Mardi NA. Comparison of reconstructed rapid prototyping models produced by 3-dimensional printing and conventional stone models with different degrees of crowding. Am J Orthod Dentofacial Orthop 2017;151:209-18. 4. Camardella LT, de Vasconcellos Vilella O, Breuning H. Accuracy of printed dental models made with 2 prototype technologies and different designs of model bases. Am J Orthod Dentofacial Orthop 2017;151:1178-87. 5. Barazanchi A, Li KC, Al-Amleh B, Lyons K, Waddell JN. Additive technology: Update on current materials and applications in dentistry. J Prosthodont 2017;26:156-63. 6. Osman RB, Alharbi N, Wismeijer D. Build angle: does it influence the accuracy of 3d-printed dental restorations using digital light-processing technology? Int J Prosthodont 2017;30:182-8. 7. Jeong YG, Lee WS, Lee KB. Accuracy evaluation of dental models manufactured by CAD/CAM milling method and 3D printing method. J Adv Prosthodont 2018;10:245-51. 8. Liaw CY, Guvendiren M. Current and emerging applications of 3D printing in medicine. Biofabrication 2017;9:024102. 9. Chen H, Yang X, Chen L, Wang Y, Sun Y. Application of FDM threedimensional printing technology in the digital manufacture of custom edentulous mandible trays. Sci Rep 2016;6:19207. 10. Groth C, Kravitz ND, Jones PE, Graham JW, Redmond WR. Three-dimensional printing technology. J Clin Orthod 2014;48:475-85. 11. Rebong RE, Stewart KT, Utreja A, Ghoneima AA. Accuracy of threedimensional dental resin models created by fused deposition modeling,
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25. Lee KY, Cho JW, Chang NY, Chae JM, Kang KH, Kim SC, et al. Accuracy of three-dimensional printing for manufacturing replica teeth. Korean J Orthod 2015;45:217-25. 26. Ender A, Attin T, Mehl A. In vivo precision of conventional and digital methods of obtaining complete-arch dental impressions. J Prosthet Dent 2016;115:313-20. 27. Salmi M, Paloheimo KS, Tuomi J, Wolff J, Mäkitie A. Accuracy of medical models made by additive manufacturing (rapid manufacturing). J Craniomaxillofac Surg 2013;41:603-9. 28. Vogel AB, Kilic F, Schmidt F, Rübel S, Lapatki BG. Dimensional accuracy of jaw scans performed on alginate impressions or stone models: A practiceoriented study. J Orofac Orthop 2015;76:351-65. 29. Ishida Y, Miyasaka T. Dimensional accuracy of dental casting patterns created by 3D printers. Dent Mater J 2016;35:250-6. 30. Jin SJ, Kim DY, Kim JH, Kim WC. Accuracy of dental replica models using photopolymer materials in additive manufacturing: In vitro threedimensional evaluation. J Prosthodont 2019;28:e557-62. 31. Abduo J. Accuracy of casts produced from conventional and digital workflows: A qualitative and quantitative analyses. J Adv Prosthodont 2019;11: 138-46. 32. Sun Y, Ding Q, Tang L, Zhang L, Xie Q. Accuracy of a chairside fused deposition modeling 3D-printed single-tooth surgical template for implant placement: An in vitro comparison with a light cured template. J Craniomaxillofac Surg 2019;47:1216-21. 33. Diment LE, Thompson MS, Bergmann JHM. Clinical efficacy and effectiveness of 3D printing: a systematic review. BMJ Open 2017;7:e016891. Corresponding author: Dr Norbert Nestler CharitéeUniversitätsmedizin Berlin Corporate Member of Freie Universität zu Berlin Humboldt-Universität zu Berlin, and Berlin Institute of Health Campus Benjamin Franklin Center for Dental and Craniofacial Sciences (CC3) Aßmannshauser Str. 6, 14197 Berlin GERMANY Email: [email protected] Acknowledgments The authors thank the participating printer manufacturers and distributors for providing the study models, Fraunhofer Institute for Production Systems and Design Technology Berlin for the measurement of these models, and MESANTIS 3D Dental Radiologicum for bearing the costs of the measurement. Copyright © 2019 by the Editorial Council for The Journal of Prosthetic Dentistry. https://doi.org/10.1016/j.prosdent.2019.11.011
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