Analysis of the thickness of 3-dimensional-printed orthodontic aligners

Introduction

This study aimed to investigate the effect of digitally designed aligner thickness on the thickness of the corresponding 3-dimensional (3D)-printed aligner.

Methods

Digitally designed aligners of 3 different thicknesses (0.500 mm, 0.750 mm, and 1.000 mm) were 3D printed in 2 different resins—Dental LT (n = 10 per group) and Grey V4 (n = 10 per group)—using a stereolithography format 3D printer. The Dental LT aligners were coated with a contrast spray and scanned with an optical scanner. The Grey V4 aligners were scanned before and after the application of the spray. Aligner scans were superimposed onto the corresponding digital design file. Average wall thickness across the aligner for each specimen was measured with metrology software.

Results

Superimpositions showed that 3D-printed aligners were thicker overall than the corresponding design file. The Dental LT aligners had the largest thickness deviation, whereas the Grey V4 without spray had the smallest. For the 0.500-mm, 0.750-mm, and 1.000-mm groups, Dental LT average thickness deviation from the input file was 0.254 ± 0.061 mm, 0.267 ± 0.052 mm, and 0.274 ± 0.034 mm, respectively, and average thickness differences between the Grey V4 with and without spray was 0.076 ± 0.016 mm, 0.070 ± 0.036 mm, and 0.080 ± 0.017 mm, respectively. These results indicate that the excess thickness in the Dental LT groups could not be attributed to spray alone.

Conclusions

Fabrication of clear aligners directly by 3D printing with the workflow applied resulted in an increased thickness that may deleteriously affect the clinical utility of the aligners.

Highlights

  • Three-dimensional printing enabled direct fabrication of aligners of different thicknesses.

  • Clear aligners fabricated by 3D printing presented wall thicknesses greater than designed.

  • Thickness deviation may affect the clinical utility of 3D-printed clear aligners.

The increasing popularity of clear aligner therapy is due, in part, to both adolescents and adults seeking an esthetic alternative to traditional brackets and wires. The preparation of treatment plans involving clear aligner therapy requires careful consideration of the diagnosis while selecting appropriate features of aligners, which can include the mechanical properties of the aligner material, the thickness of the aligner, the amount of activation, and the interaction of the aligner with auxiliary attachments. The thickness of an aligner, for example, influences the magnitude of force it delivers—with thicker appliances producing significantly greater force than thinner appliances.

The thickness of the appliance must be carefully considered so as to realize a mechanical environment appropriate to generate optimal tooth movement over time. A survey of the literature and information available from various manufacturers indicates that aligner thicknesses generally range from 0.400 mm to 1.500 mm. The thickness of clear aligners fabricated via thermoforming depends on the thickness of the available polymer films used in the process, and the thickness cannot be controlled across the arch. However, 3-dimensional (3D) printing technologies present the capability to support direct fabrication of clear aligners with customized thicknesses and spatial control of aligner thickness across the arch. Accordingly, the fabrication of aligners directly via 3D printing presents the potential to enable spatial control of aligner properties, such as thickness, that can, in turn, influence tooth movement.

As the number of resins on the U.S. market cleared for 3D printing of appliances for long-term intraoral use increases, the direct fabrication of clear orthodontic aligners by means of 3D printing appears to be on the horizon. , Direct fabrication of clear aligners by 3D printing would mark a paradigmatic shift in clear aligner production and enable new frontiers in aligner therapy. However, the clinical utility of clear aligners fabricated directly by 3D printing will depend on realization of appliances with suitable dimensional accuracy, among a variety of considerations.

Jindal et al found the dimensional accuracy and the compressive mechanical properties of 3D-printed clear aligners cured after printing to be superior in comparison with thermoformed aligners. The study compared tooth height measurements in assessing dimensional accuracy and reported absolute differences from the digital file ranging from 0.02 mm to 0.86 mm for the 3D-printed clear aligners and from 0.00 mm to 0.88 mm for the thermoformed clear aligners. McCarty et al applied 3D surface analysis techniques to investigate the effect of print angle and duration of postprint curing on the dimensional accuracy of 3D-printed clear aligners. The overall deviations for all investigated groups fell under 0.25 mm with respect to the reference digital file, but 3D surface comparison maps suggested overbuilding of the aligners in certain key areas, such as the intaglio surface, which could potentially affect the seating and utility of the aligners.

These previous studies investigated the dimensional accuracy of 3D-printed clear aligners in terms of tooth height or overall surface deviations. , However, the fidelity with which 3D printing technologies can realize designed aligner thickness is unknown, and it could affect the clinical utility of directly printed aligners. Accordingly, the objective of this study was to investigate the effect of the digitally designed aligner thickness on the thickness of the corresponding 3D-printed aligner.

Material and methods

For this study, aligners with uniform thicknesses of 0.500 mm, 0.750 mm, and 1.000 mm were digitally designed on the basis of a surface scan of the anatomy of a typodont maxillary arch form (Nissin, Kyoto, Japan) and exported as 3 separate stereolithography (STL) files. The arch form had a full complement of teeth except for third molars. The designed aligner files were imported into PreForm software (version 2.19.3; Formlabs, Inc, Somerville, Mass) and prepared for printing. The occlusal plane of each aligner was oriented at a 45° angle with respect to the build platform—with the gingival margin and the anterior dentition oriented away from the platform—as previously reported. Supports with a touchpoint size of 0.60 mm and a density of 1.00 were generated automatically by the software in contact with the cameo surface of each aligner and were edited as needed to facilitate printing without disturbing the intaglio surface. A clear dental resin (Dental LT Clear V1; Formlabs) was used in 1 group, and a grey model resin (Grey V4; Formlabs) was used in the other group. The Dental LT resin was selected because it is a clear Class IIa biocompatible resin marketed for direct fabrication of long-term orthodontic appliances and has been previously investigated in the fabrication of clear aligners. , The Grey V4 resin enabled the fabrication of opaque 3D-printed aligners of the same design that could be registered with optical scanning in the absence of a contrast spray. Ten aligners of each thickness for each material group were printed using a print layer height of 0.100 mm, because it was the only option available for the Dental LT resin and was also available for the Grey V4 resin. After printing, the specimens underwent postprocessing in accordance with the manufacturer’s instructions. Specifically, parts were separated from the build platform, then washed in 2 successive isopropyl alcohol (>99%) baths under sonication for 2 minutes and 3 minutes, respectively, to remove the uncured resin. The aligners then were air-dried before postprint curing for 20 minutes at 80°C for Dental LT and 30 minutes at 60°C for the Grey V4 in a curing chamber (Form Cure; Formlabs). After postprocessing, each aligner was separated manually from its supports using flush cut nippers. Before digitization, all Dental LT aligners were sprayed by a single operator (A.E.) with a contrast spray (Spotcheck SKD-S2 Aerosol Developer; Magnaflux, Ltd, Glenview, Ill) to create an opaque coating of the intaglio and cameo surfaces to facilitate digital scanning. Then all Dental LT prints were scanned with a digital scanner (iTero Element Flex; Align Technology, Inc, San Jose, Calif). For the Grey V4 group, each aligner was scanned with a desktop scanner (E3; 3Shape, Copenhagen, Denmark) before and after application of the spray, because the Grey V4 resin could not be captured with the iTero Flex scanner.

After scanning, STL files of each aligner were imported into Geomagic Control metrology software (version 2015.1.1; 3D Systems, Rock Hill, SC) along with the corresponding design file for evaluation of 3D surface deviation and aligner thickness. For the Dental LT group, the digital input file was set as the “reference,” and the scanned STL file of the sprayed clear aligner was set as the “test” for 3D surface deviation analysis. Aligner thickness was calculated using the “Evaluate Wall Thickness” tool in the software. To quantify the potential contribution of spray to the measured thickness of Dental LT aligners, we repeated the study with the Grey V4 material, which the scanner could register without a contrast spray. The Grey aligners were scanned before and again after the application of the spray. Each nonsprayed Grey aligner scan served as the reference, and the corresponding sprayed Grey aligner scan served as the test in the superimpositions. Wall thickness was calculated as with the Dental LT aligners. The 3D comparison tools calculated the surface deviations across the parts and generated color maps illustrating 3D surface deviation.

Statistical analysis

Tests of normality were performed using R statistical software to assess deviations from normality. A generalized linear mixed model and post-hoc Tukey contrasts, when appropriate, were applied using R statistical software for all analyses with a level of significance of P <0.05.

Results

The average wall thicknesses for the 3 aligner thickness groups (0.500 mm, 0.750 mm, and 1.000 mm) for the Dental LT and Grey V4 resins are presented in Tables I and II , respectively. Intaglio views of the STL files used for printing each aligner thickness as well as representative photographs of aligners printed in Dental LT and Grey V4 resins are shown in Figure 1 . Cameo and intaglio views of aligners printed in Dental LT of each thickness investigated are shown in the Supplementary Figure . Wall thickness was determined using the Geomagic wall thickness measurement tool. A printing error was observed for 1 sample of the Dental LT group (a region of the aligner failed to manifest), and a scanning error was encountered for 1 sample of the Grey V4 group (the scanner failed to render a digital model). Accordingly, those 2 samples were excluded from the analysis. Average wall thickness for the Dental LT resin aligners for the respective thicknesses of 0.500 mm (n = 9), 0.750 mm (n = 10), and 1.000 mm (n = 10) exceeded the designed input part thickness by 0.254 ± 0.061 mm, 0.267 ± 0.052 mm, and 0.274 ± 0.034 mm, respectively. Average wall thickness for the Grey V4 resin aligners for the respective thicknesses of 0.500 mm (n = 10), 0.750 mm (n = 9), and 1.000 mm (n = 10) exceeded the designed input part thickness by 0.050 ± 0.012 mm, 0.044 ± 0.011 mm, and 0.051 ± 0.010 mm, respectively, without spray; whereas the difference in part thickness between the post- and presprayed Grey V4 aligners registered 0.076 ± 0.016 mm, 0.070 ± 0.036 mm, and 0.080 ± 0.017 mm, respectively.

Table I
Effect of designed aligner wall thickness on the thickness of the 3D-printed aligner (Dental LT; sprayed)
0.500 mm (n = 9) 0.750 mm (n = 10) 1.000 mm (n = 10) GLMM Tukey post-hoc
Mean ± SD Mean ± SD Mean ± SD P 0.500 mm vs 0.750-mm 0.500 mm vs 1.000 mm 0.750 mm vs 1.000 mm
Average wall thickness 0.754 ± 0.061 1.017 ± 0.052 1.274 ± 0.034 <2.2 × 10 −6 <2 × 10 −6 <2 × 10 −6 <2 × 10 −6
Difference from designed wall thickness 0.254 ± 0.061 0.267 ± 0.052 0.274 ± 0.034 0.6666
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Jan 9, 2021 | Posted by in Orthodontics | Comments Off on Analysis of the thickness of 3-dimensional-printed orthodontic aligners
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