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Affiliate Professor, Graduate Prosthodontics, Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, Wash; Faculty and Director of Research and Digital Dentistry, Kois Center, Seattle, Wash; Affiliate Professor, Graduate Prosthodontics, Department of Prosthodontics, School of Dental Medicine, Tufts University, Boston, Mass
Postgraduate Advanced in Implant-Prosthodontics, Department of Conservative Dentistry and Prosthodontics, School of Dentistry, Complutense University of Madrid, Madrid, Spain
Founder and Director, Kois Center, Seattle, Wash; Affiliate Professor, Graduate Prosthodontics, Department of Restorative Dentistry, University of Washington, Seattle, Wash; Private Practice, Seattle, Wash
Associate Professor, Department of Conservative Dentistry and Prosthodontics, Director of postgraduate program of Advanced in Implant-Prosthodontics, School of Dentistry, Complutense University of Madrid, Madrid, Spain
Different factors can affect the manufacturing accuracy of additively manufactured dental devices; however, the influence of print orientation and wet-dry storage time on their intaglio accuracy remains uncertain.
Purpose
The purpose of this in vitro study was to assess the effect of print orientation (0, 45, 70, and 90 degrees) and wet-dry storage time (0, 30, 60, and 90 days) on the intaglio accuracy of additively manufactured occlusal devices.
Material and methods
An occlusal device design was obtained in a standard tessellation language (STL) file format (control file) which was used to fabricate all the specimens by using a stereolithography printer (Form 3+) and a biocompatible resin material (Dental LT Clear Resin, V2). Four groups were created based on the print orientation used to manufacture the specimens: 0, 45, 70, and 90 degrees. Each group was divided into 4 subgroups depending on the time elapsed between manufacturing and accuracy evaluation: 0, 30, 60, and 90 days. For the subgroup 0, a desktop scanner (T710) was used to digitize all the specimens. The 30-day subgroup specimens were stored for 30 days with the following daily storage protocol: 16 hours inside a dry lightproof container, followed by 8 hours in artificial saliva (1700-0305 Artificial Saliva) inside the same lightproof container. The specimens were then digitized by following the same procedures used for subgroup 0. For the subgroups 60 and 90, the identical procedures described for subgroup 30 were completed but after 60 and 90 days of storage, respectively. The reference STL file was used to measure the intaglio discrepancy with the experimental scans obtained among the different subgroups by using the root mean square error calculation. Two-way ANOVA and post hoc Tukey pairwise comparison tests were used to analyze the data (α=.05).
Results
Print orientation (P<.001) and usage time (P<.001) were significant predictors of the trueness value obtained. Additionally, the 0-degree print orientation at day 0 group demonstrated the best trueness value among all the groups tested (P<.05). No significant trueness discrepancies were found among the 45-, 70-, and 90-degree print orientation, or among the 30, 60, and 90 days of storage. A significant precision difference was found in the variance between print orientation groups across usage time subgroups.
Conclusions
The print orientation and wet-dry storage times tested influenced the trueness and precision of the intaglio surfaces of the occlusal devices manufactured with the 3D printer and material selected.
Clinical Implications
When using the stereolithography printer and biocompatible resin tested for fabricating occlusal devices, a 0-degree print orientation maximizes manufacturing accuracy. When stored between 30 days and 90 days after their manufacture, no changes in the intaglio accuracy of the dental devices should be expected.
Among the different additive manufacturing (AM) categories,
vat polymerization, including stereolithography (SLA) and digital light processing (DLP) technologies, provides options for fabricating dental devices and restorations including occlusal devices.
A review on chemical composition, mechanical properties, and manufacturing work flow of additively manufactured current polymers for interim dental restorations.
The manufacturing accuracy and mechanical properties of AM dental devices have been reported to be affected by the AM technology, printer, and material used,
3D printing parameters, supporting structures, slicing, and post-processing procedures of vat-polymerization additive manufacturing technologies: a narrative review.
Effects of build orientation on adaptation of casting patterns for three-unit partial fixed dental prostheses fabricated by using digital light projection.
Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Effects of environmental conditions, aging, and build orientations on the mechanical properties of ASTM type I specimens manufactured via stereolithography.
3D printing parameters, supporting structures, slicing, and post-processing procedures of vat-polymerization additive manufacturing technologies: a narrative review.
Influence of postpolymerization methods and artificial aging procedures on the fracture resistance and flexural strength of a vat-polymerized interim dental material.
The effect of other factors on the manufacturing accuracy and mechanical properties of AM dental devices, including the composition of the printed biocompatible material, optimal manufacturing protocol based on the AM technology, printer and material used, mechanical properties, and clinical performance of vat-polymerized AM occlusal devices, is still unclear.
A review on chemical composition, mechanical properties, and manufacturing work flow of additively manufactured current polymers for interim dental restorations.
3D printing parameters, supporting structures, slicing, and post-processing procedures of vat-polymerization additive manufacturing technologies: a narrative review.
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Effects of build orientation on adaptation of casting patterns for three-unit partial fixed dental prostheses fabricated by using digital light projection.
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Effects of build orientation on adaptation of casting patterns for three-unit partial fixed dental prostheses fabricated by using digital light projection.
Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
Effects of environmental conditions, aging, and build orientations on the mechanical properties of ASTM type I specimens manufactured via stereolithography.
Influence of postpolymerization methods and artificial aging procedures on the fracture resistance and flexural strength of a vat-polymerized interim dental material.
that evaluated the influence of print orientation of the clinical fit of SLA AM occlusal devices, which reported no differences between the occlusal devices manufactured with different print orientations. Therefore, the optimal print orientation for maximizing the manufacturing accuracy of AM devices is unclear. Additionally, if the manufacturing conditions (AM technology, printer, and material) and the fabricating protocol change, the outcome would be expected to vary.
Storage time has been reported as a variable that may reduce the dimensional stability of AM occlusal devices and surgical implant guides.
However, the available data are scarce. Additionally, variations in resin composition among the biocompatible resins used to fabricate occlusal devices may produce different results.
The aim of this in vitro study was to assess the effect of different print orientations (0, 45, 70, and 90 degrees) and wet-dry storage time (0, 30, 60, and 90 days) on the manufacturing accuracy (trueness and precision) of the intaglio surface of SLA vat-polymerized occlusal devices. The null hypotheses were that no difference would be found in the trueness and precision values of the intaglio surfaces of the occlusal devices manufactured with different print orientations or after the different wet-dry storage times tested.
Material and methods
An occlusal device design was obtained in a standard tessellation language (STL) file format (reference STL file) (Fig. 1). The STL file was used to manufacture all the study specimens by using an SLA printer (Form 3+; FormLabs) and a biocompatible resin material for fabricating occlusal devices (Dental LT Clear Resin, V2; FormLabs). The printer had been previously calibrated according to the manufacturer’s protocol. All the specimens were fabricated using nitrile gloves by a prosthodontist with 6 years of previous experience handling 3-dimensional (3D) polymer printers.
Four different groups were created based on the print orientation used to fabricate the specimens: 0- (group 0 or control group), 45- (group 45), 70- (group 70), and 90-degree (group 90) print orientation (Fig. 2). The manufacturer of the material and printer selected recommended 0-degree print orientation; therefore, group 0 was considered the control. All the specimens were manufactured using the same printing parameters with automatic support generation, except for the print orientation, that varied depending on the experimental group. All the specimens were produced from the same new resin bottle, and the identical postprocessing procedures were completed for all the groups.
3D printing parameters, supporting structures, slicing, and post-processing procedures of vat-polymerization additive manufacturing technologies: a narrative review.
After printing, the specimens were removed from the build platform with a removal tool provided by the manufacturer. Then, the specimens were fully submerged in a bath (Form Wash; FormLabs) with 99% isopropyl alcohol (IPA) (Isopropyl alcohol 99%; Cumberland Swan) for 15 minutes and subsequently submerged in a second bath with clean 99% IPA for 5 minutes. The specimens were placed on a paper towel, dried in ambient air for 30 minutes, and then placed in the ultraviolet (UV)-polymerization machine (Form Cure; FormLabs) for 60 minutes at 60 °C according to the manufacturer’s recommendations. The support material was removed from all specimens by using a removal tool provided by the manufacturer. No further polishing or postprocessing procedures were completed. The specimens were stored in a lightproof container for no more than 24 hours until the initial accuracy measurements were made.
Figure 2Representative print orientation tested. A, Group 0. B, Group 45. C, Group 70. D, Group 90.
Each group was divided into 4 subgroups depending on the time elapsed between the manufacturing of the occlusal device and the accuracy evaluation: 0 (subgroup 0), 30 (subgroup 30), 60 (subgroup 60), and 90 days (subgroup 90). For subgroup 0, no later than 24 hours after manufacturing, a laboratory scanner (T710; Medit) was used to digitize all the occlusal devices. The laboratory scanner was calibrated before starting data acquisition and after every 10 scans by following the manufacturer’s calibration protocol. The manufacturer of the laboratory scanner reports a 4-μm scanning accuracy according to the International Organization for Standardization (ISO) 12 836/2015 standard.
International Organization for Standardization. ISO 12836:2015. Dentistry — Digitizing devices for CAD/CAM systems for indirect dental restorations — Test methods for assessing accuracy. Accessed January 2, 2020. https://www.iso.org/standard/68414.html.
For subgroup 30, the specimens were stored with a daily storage protocol designed to replicate the clinical use and storage time of an occlusal device. Every day, the occlusal devices were stored in a dry lightproof container for 16 hours, followed by 8 hours submerged in artificial saliva (1700-0305 Artificial Saliva; Pickering Laboratories) inside a lightproof container. After 30-days of this storage protocol, the specimens were placed on a paper towel and dried in ambient air for 10 minutes. Subsequently, the specimens were digitized by following the same procedures as in subgroup 0. For subgroups 60 and 90, the same storage and digitizing procedures described for subgroup 30 were completed, but the specimens were digitized after 60 and 90 days of storage, respectively.
The reference STL file was used to measure the discrepancy with the experimental scans obtained for the different subgroups tested. The discrepancy was assessed only on the intaglio surface of the specimens, as the other surfaces could have been altered by the removal of the support material. The STL files were imported into a reverse engineering software program (Geomagic Control X; 3D Systems). The reference STL file and the experimental file were defined and aligned by using the best fit technique.
The root mean square (RMS) error calculation was computed in the same area using the following formula: , where X1,i are the reference data, X2,i are the scan data, and n indicates the total number of measurement points measured in each analysis (Fig. 3). The discrepancy calculations for each group were used to analyze the data. Trueness was defined as the average RMS error discrepancies between the reference file and experimental scans, while precision was described as the RMS error variations per group or standard deviation (SD).
International Organization for Standardization. ISO 5725-1:1994 Accuracy (trueness and precision) of measurement methods and results - Part 1: General principles and definitions.
The Shapiro-Wilk and Kolmogorov-Smirnov tests indicated that the data had a normal distribution (P>.05). Two-way ANOVA and post hoc Tukey pairwise comparison tests (α=.05) were used to analyze the data. A statistical program (SPSS Statistics for Windows, v27; IBM Corp) was used to perform the statistical analysis.
Results
The mean ±SD trueness and precision values of all subgroups tested are presented in Table 1. Regarding trueness, 2-way ANOVA showed that print orientation (df=3, MS=0.081825, F=19.93, contribution=14.94%, P<.001) and wet-dry storage time (df=3, MS=0.041779, F=10.17, contribution=7.63%, P<.001) were significant predictors of the trueness (RMS) value obtained (Fig. 4A). With respect to the group factor, the Tukey pairwise comparison revealed significant trueness value discrepancies among the different print orientations tested. The group 0 (0-degree print orientation) demonstrated the best trueness value (lowest mean RMS error=0.199 mm) among all the groups tested (P<.05). The 45 (trueness mean value of 0.261 mm), 70 (trueness mean value of 0.265 mm), and 90 (trueness mean value of 0.265 mm) groups were not significantly different from each other (P>.05) (Fig. 4B). With respect to the subgroup factor, the Tukey pairwise comparison showed significant trueness values discrepancies among the different wet-dry storage times tested (P<.05). Subgroup 0 (0 days) demonstrated the best trueness value (lowest mean RMS error= 0.217 mm) among all the subgroups tested (P<.05). The 30 (trueness mean value of 0.246 mm), 60 (trueness mean value of 0.256 mm), and 90 (trueness mean value of 0.271 mm) subgroups were not significantly different from each other (P>.05) (Fig. 4C).
Table 1Trueness and precision values measured among subgroups tested
Regarding precision, a significant difference was found in the variance between print orientation groups across the wet-dry storage time subgroups using the Levene test for equality of variance (nonoverlapping confidence intervals showed significant difference) (P<.001) (Fig. 4D).
Discussion
Based on the results obtained in this in vitro study, the print orientation and wet-dry storage times tested influenced the manufacturing accuracy of the intaglio surfaces of the occlusal devices manufactured with the SLA 3D printer and material selected. The 0-degree printed specimens evaluated within 24 hours after manufacturing showed the best trueness and precision values among the different print orientations and wet-dry storage times tested. Additionally, the higher the print orientation degree and the longer the wet-dry storage time, the lower the manufacturing trueness and precision values measured. However, the accuracy discrepancies were not statistically significant between the 45-, 70-, and 90-degree print orientation and were also not statistically significant between the 30-, 60-, and 90-day wet-dry storage time.
While digital techniques have been reported for fabricating single- and dual-material additively manufactured occlusal devices,
the optimal printing protocol and clinical manufacturing accuracy tolerance for these devices is unclear. In the present study, the manufacturing accuracy discrepancies measured among the different print orientations tested ranged from 0.156 ±0.066 mm to 0.223 ±0.039 mm at day 0 and from 0.243 ±0.099 mm to 0.291 ±0.043 mm at day 90. These manufacturing accuracy values are consistent with previous reported values.
How these manufacturing accuracy discrepancies may affect the clinical fit of these occlusal devices is unclear. Further studies are needed to establish the optimal printing protocol and clinically acceptable manufacturing accuracy for fabricating AM occlusal devices.
Print orientation has been reported to influence the manufacturing accuracy,
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Effects of build orientation on adaptation of casting patterns for three-unit partial fixed dental prostheses fabricated by using digital light projection.
Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
Effects of environmental conditions, aging, and build orientations on the mechanical properties of ASTM type I specimens manufactured via stereolithography.
Influence of postpolymerization methods and artificial aging procedures on the fracture resistance and flexural strength of a vat-polymerized interim dental material.
of AM dental devices. While a print orientation may maximize the manufacturing accuracy of a dental device, the same print orientation may not maximize other properties of the same printed object. If the AM technology, printer, resin, printing parameters or postprocessing procedures vary, the outcome of the printed device also changes. Therefore, generalizations of these studies should be done with care. One clinical study assessed the influence of print orientation on the manufacturing accuracy of 3D-printed occlusal devices at 0, 30, and 90 degrees in a patient.
The authors reported no fit differences among the different occlusal devices fabricated with these 3 different print orientations; however, the sample size was small (n=2); only 1 patient was considered; and the intraoral scanner used to digitize the patient, the design considerations and printing parameters used to produce the devices tested, and the criteria to clinically assess fit were not provided. Therefore, comparisons with the results of previous studies are difficult.
Storage condition and time have been identified as factors that can affect the dimensional stability of AM dental devices
another study described dimensional changes on printed surgical implant guides caused by dry-condition storage time. Reliable study comparisons should only involve additively manufactured devices with the same clinical application.
assessed the influence of the storage conditions (dry under natural light conditions, dry in a dark container, and submerged in water) on the dimensional stability of the intaglio surfaces of the occlusal devices after 1-, 7-, and 27-day storage. The specimens were fabricated by using an SLA printer (Form 2; FormLabs) and a biocompatible resin (Dental Clear LT; FormLabs).
The dimensional stability evaluation was completed by measuring the RMS error between the digitized specimen after 0 days and the scanned specimen after 1 or 7 or 27 days; therefore, the virtual design of the occlusal device was not used as a reference file. The results revealed that the storage conditions did not affect the dimensional stability of the occlusal devices within 27 days.
In the present study, the virtual occlusal device design was considered the reference file and used to measure manufacturing accuracy at different evaluation times. Additionally, a combination of dry and artificial saliva submerged conditions was developed to replicate the dry storage and wet intraoral usage conditions of occlusal devices within 90 days. The differences in research methodology between the 2 studies prevented simple comparison of the respective results.
has revealed that AM occlusal devices are more prone to water sorption and, therefore, have a higher susceptibly to aging compared with pressed and milled devices. Dry-, wet- (water or artificial saliva) or the combination of dry- and wet-storage conditions in laboratory settings may not reflect the same changes over time that occur with the clinical use of AM occlusal devices. In the present study, a novel laboratory storage protocol was used to simulate the 24-hour conditions of an occlusal device worn at night; the device was stored in a dry environment for 16 hours and then submerged in artificial saliva for 8 hours. Clinical studies are needed to further evaluate the clinical long-term dimensional stability of AM occlusal devices.
Characteristics related to the occlusal device design can also impact the accuracy of the AM device. The different design factors include path of insertion, presence of undercuts, extension of the design, offset, or minimum material thickness.
Digitally programmed (CAD) offset values for prototyped occlusal splints (CAM): assessment of appliance-fitting using surface-based superimposition and deviation analysis.
Artificial intelligence (AI) models are being developed for optimizing the manufacturing procedure, which may assist in establishing the optimal printing protocol based on the manufacturing trinomial and clinical application of the device being printed.
Limitations of the present study included the limited vat-polymerization technologies, printers, and materials tested, as well as the limited print orientations assessed. Additionally, a novel storage protocol was used. Further studies are needed to evaluate the chemical composition and biocompatibility characteristics of conventional, milled, and AM occlusal devices. Additional laboratory and clinical studies are recommended to further evaluate the printing variables that affect the manufacturing accuracy of AM occlusal devices, including print orientation. Additionally, the optimal printing protocols should be further assessed based on the AM technology, printer, material, and clinical application of the printed dental device.
Conclusions
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
Print orientation and storage time influenced the trueness and precision of the intaglio surfaces of the occlusal devices manufactured with the 3D printer and material selected.
2.
The 0-degree print orientation obtained the best manufacturing accuracy values, which coincides with the manufacturer’s recommendations for the printer and materials used.
3.
The accuracy values decreased after 30 days of storage time, but no statistically significant differences were found among specimens stored for 30, 60, or 90 days.
References
Revilla-León M.
Özcan M.
Additive manufacturing technologies used for processing polymers: current status and potential application in prosthetic dentistry.
A review on chemical composition, mechanical properties, and manufacturing work flow of additively manufactured current polymers for interim dental restorations.
3D printing parameters, supporting structures, slicing, and post-processing procedures of vat-polymerization additive manufacturing technologies: a narrative review.
Effects of build orientation on adaptation of casting patterns for three-unit partial fixed dental prostheses fabricated by using digital light projection.
Fracture load of 3D-printed fixed dental prostheses compared with milled and conventionally fabricated ones: the impact of resin material, build direction, post-curing, and artificial aging-an in vitro study.
Surface roughness and shear bond strength to composite resin of additively manufactured interim restorative material with different printing orientations.
Effects of environmental conditions, aging, and build orientations on the mechanical properties of ASTM type I specimens manufactured via stereolithography.
Influence of postpolymerization methods and artificial aging procedures on the fracture resistance and flexural strength of a vat-polymerized interim dental material.
International Organization for Standardization. ISO 12836:2015. Dentistry — Digitizing devices for CAD/CAM systems for indirect dental restorations — Test methods for assessing accuracy. Accessed January 2, 2020. https://www.iso.org/standard/68414.html.
Digitally programmed (CAD) offset values for prototyped occlusal splints (CAM): assessment of appliance-fitting using surface-based superimposition and deviation analysis.
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