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Effect of layer thickness setting on the adaptation of stereolithography apparatus–fabricated metal frameworks for removable partial dentures: An in vitro study
Corresponding author: Dr Ji-Hwan Kim, Department of Dental Laboratory Science and Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, REPUBLIC OF KOREA
A staircase effect is noted in the fabrication of metal frameworks for removable partial dentures (RPDs) when using stereolithography apparatus (SLA). It affects the adaptation of the definitive metal framework depending on the layer thickness setting. However, studies on the effect of the layer thickness setting on the adaptation of metal frameworks are lacking.
Purpose
The purpose of this in vitro study was to determine the optimal layer thickness through comparative analysis of the adaptation of SLA-fabricated metal frameworks with different layer thickness settings.
Material and methods
A total of 15 metal RPD frameworks were SLA-fabricated by using 3 different layer thickness settings (16 μm, 50 μm, and 100 μm). The adaptation of the frameworks was measured by using the silicone replica technique, sectioned at the canine, first molar, and second molar regions by using a guide. The thickness of the light-body silicone was measured with a digital microscope at 3 points in each of the 3 areas. The measurements of the adaptation were statistically analyzed using the nonparametric Kruskal-Wallis test and post hoc Mann-Whitney U test with Bonferroni correction.
Results
The gaps measured in each area showed statistically significant differences in all 3 groups (P<.05). In the anterior, middle, and posterior areas, the 16-μm metal framework group showed the narrowest gaps (207 ±46 μm, 195 ±49 μm, and 188 ±40 μm, respectively). The 3 groups showed statistically significant differences in total gaps in the RPD frameworks relative to the layer thickness settings (P<.05); the total gap was lowest (197 ±42 μm) for the 16-μm group.
Conclusions
For SLA, 50 μm is the recommended layer thickness considering the effect of layer thickness settings on the adaptation of the RPD framework and the fabrication time.
Clinical Implications
In the fabrication of the removable partial denture framework using stereolithography apparatus, differences in layer thickness settings affect the adaptation and fabrication time. Of the layer thickness settings tested, a layer thickness of 50 μm is recommended.
The metal removable partial denture (RPD) framework has traditionally been fabricated with the lost wax casting technique,
a time-consuming and labor-intensive technique; moreover, its accuracy varies with the operator’s skills and experience. Computer-aided design and computer-aided manufacturing (CAD-CAM) systems have been adopted to partially address these challenges.
Virtual evaluation of the accuracy of fit and trueness in maxillary poly (etheretherketone) removable partial denture frameworks fabricated by direct and indirect CAD/CAM techniques.
Recently, additive manufacturing CAD-CAM systems have become popular in dentistry because the use of materials is efficient and complex shapes can be reproduced effectively.
Additive manufacturing, widely known as 3D printing, is a method of using 3D model data to fabricate the definitive product by adding the materials in layers. In accordance with the International Organization for Standardization/American Society for Testing and Materials 52900:2015 standards, additive manufacturing is subclassified into the following 7 techniques: binder jetting; directed energy deposition; material extrusion; material jetting; powder bed fusion; sheet lamination; and vat photopolymerization.
The fabrication of an RPD framework by using SLM involves the selective irradiation and melting of metallic powder placed on a bed with a high-power laser beam. A cross section is derived from the CAD-based shape of the framework. Another layer of powder is then arranged on top of the cross section and selectively irradiated and melted using the high-power laser to combine the layers. This process is repeated until the desired metal framework is obtained.
The fabrication of an RPD framework by using SLA involves a casting process where the liquid photoreactive resin is partially irradiated and polymerized with an ultraviolet laser beam to create a layer-by-layer castable resin pattern from the CAD-based shape of the framework. After the conventional investing of the castable resin pattern, wax elimination, and casting, the metal framework is obtained.
In spite of recent attempts to replace the manual fabrication of RPD frameworks with SLM and analyzing the effects, the expensive materials and equipment required for SLM and the need for substantial improvements hinder its clinical application.
In fabricating a castable resin pattern by using SLA, it is possible to designate or set the layer thickness, which substantially affects the accuracy, build time, and adaptation and surface roughness of the definitive prosthesis.
To meet the clinical requirement for higher accuracy, the layer thickness setting may be minimized. In contrast, the layer thickness setting may be increased to shorten the build time. Thus, the layer thickness of the castable resin pattern must be set appropriately when fabricating an RPD framework with SLA.
The adaptation of RPD frameworks fabricated by conventional methods has been reported to be in a range of 90 to 680 μm.
The authors are unaware, however, of a study that comparatively analyzed the layer thickness settings when fabricating RPD frameworks. Therefore, the purpose of the present study was to determine the optimal layer thickness by comparatively analyzing the adaptation of RPD frameworks fabricated by using SLA with different layer thickness settings. The null hypothesis was that no difference in adaptation would be found among the RPD frameworks fabricated by using SLA with different layer thickness settings.
Material and methods
The workflow of this study is shown in Figure 1. To fabricate the RPD frameworks, a maxillary edentulous model (EDE1001; Nissin) was selected as the master model. An epoxy model was fabricated by using replica silicone (Deguform; DeguDent GmbH) and dental epoxy resin (POLYUROCK; Sterngold Dental).
The replicated epoxy model was scanned with a dental model scanner (D900 Scanner; 3Shape A/S) and saved as a standard tessellation language (STL) file. The STL file was uploaded to the CAD software program (3Shape Dental Designer; 3Shape A/S), and a skilled expert (S.G.Y.) developed the clinically appropriate CAD design for the RPD frameworks.
The CAD design STL file was transmitted to a 3D printer program (ZENITH Z512; Dentis), and appropriate positioning and support locations were set. The slicing data files with 3 different layer thickness settings (16 μm, 50 μm, and 100 μm) were saved. These 3 settings were determined from previous studies and the manufacturer’s recommendations, the 50-μm and 100-μm settings from previous studies,
and the 16-μm setting added from the manufacturer’s literature that included 16-μm, as well as 50-μm and 100-μm thickness settings. The saved slicing data file was transmitted to the SLA printer (ZENITH U; Dentis), and castable resin liquid (ZMD-1000B Castable; Dentis) was injected into the printer’s resin vat to fabricate castable resin patterns for each layer thickness setting.
A power analysis was conducted to estimate the required sample size. Assuming 3 test groups, an effect size of 1.43 and Type I and Type II error probabilities of 0.05 and 0.95. As a result, 4 per group were required, making 12 specimens in total, and the actual power calculated was 0.97. Therefore, a sample size of 5 per group was used (N=15).
The 15 printed castable resin patterns were conventionally invested with the investment material (Optivest; DeguDent GmbH), eliminated, and cast with a cobalt-chromium alloy (Biosil F; DeguDent GmbH). After casting, the investment material was eliminated, the framework airborne-particle abraded (Cobra; Renfert GmbH), the sprues removed, and the prostheses finished.
To form identical record bases and occlusion rims, one of the 15 fabricated metal frameworks was placed on the model, and baseplate wax (Atria Modeling Wax; Atria Inc) was used to fabricate the mesh part in the form of the record base. At the alveolar ridge area, the occlusion rim was created horizontal to the base plane of the model, and silicone putty (Platinum 95; Zhermack) was used to fabricate a silicone mold, which was used to duplicate the other 14 metal frameworks (Fig. 2).
Figure 2Fabrication of castable resin patterns and specimens. A, Castable resin patterns of metal framework fabricated according to 3 different layer thickness settings (n=5). B, Specimens fabricated with the same record bases and occlusal rims (n=5).
Light-body silicone (Aquasil Ultra XLV; Dentsply Sirona) was evenly injected into the intaglio surface of the RPD framework and fitted on the epoxy model. A static load of 19.6 N was applied for 9 minutes, and the excess silicone was removed. After the RPD framework had been separated from the model, wax (Boxing Wax; Daedong Industry) was used for boxing the land area of the model, and fluid silicone (Dublisil 30; Dreve Dentamid GmbH) was injected. Once the fluid silicone had polymerized, the model was removed for reboxing, and fluid silicone was injected and allowed to polymerize before completion of the silicone block.
A guide was fabricated to sequentially and consistently cut the completed 15 silicone blocks in the canine, first molar, and second molar areas.
The adaptation of the cut silicone blocks was measured by using a digital microscope (KH-7700; Hirox) at ×160 magnification. The 3 cut areas were subdivided into the anterior, middle, and posterior parts, and the gaps of the light-body silicone blocks were measured at 3 points each in the anterior, middle, and posterior parts (9 points in total) (Fig. 3).
Figure 3Measurement of adaptation using silicone replica technique. A, Measurement position of silicone blocks cut into anterior, middle, and posterior areas using guide. B, Gap value of light-body silicone measured at measurement position with digital microscope.
Before the measurements, the operator (S.G.Y.) calibrated the digital microscope to ×160 magnification (KH-7700; Hirox Co, Ltd) by making repeated trials. Calibration in between the measurements was performed with the digital microscope’s autocalibration selection function. While zooming, calibration values are selected automatically, which reliably prevented the incorrect selection of calibration values during image measuring and recording.
The adaptation measurements were analyzed by a statistical software program (IBM SPSS Statistics, v24.0; IBM Corp). The Kolmogorov-Smirnov and Shapiro-Wilk tests were used to test the normality of the measured adaptation of the RPD frameworks relative to their layer thickness settings. Because the normality assumption was not satisfied (P<.05), the nonparametric Kruskal-Wallis test was performed with the Type 1 error threshold (α=.05). The Bonferroni method was used to correct for multiple testing, and a post hoc analysis was performed by using the Mann-Whitney U test.
Results
The adaptation of the fabricated RPD frameworks was analyzed with respect to the SLA layer thickness settings. Table 1 shows the gaps of the RPD frameworks in the anterior, middle, and posterior areas. The 16-MF and 100-MF groups showed the narrowest and widest gaps in the anterior, middle, and posterior areas (207 ±46 μm and 293 ±21 μm; 195 ±49 μm and 340 ±37 μm; and 188 ±40 μm and 292 ±59 μm). All gaps in the anterior, middle, and posterior areas showed statistically significant differences among the groups (P<.05). The post hoc analysis found significant differences in the anterior (P=.022), middle (P=.002), and posterior (P=.027) areas between the 16-MF and 100-MF groups only.
Table 1Mean ±SD of gap of metal framework for removable denture fabricated with different layer thickness settings in anterior, middle, and posterior regions (μm)
Table 2 shows the 16-MF group with the narrowest total gap (197 ±42 μm), whereas the 100-MF group had the widest total gap (309 ±32 μm). The differences in the total gap among the groups were statistically significant (P<.05). The post hoc analysis found a significant difference between the 16-MF and 100-MF groups only (P=.003).
Table 2Mean ±SD of total gap of metal framework for removable denture fabricated with different layer thickness settings (μm)
a,bValues followed by different letters indicate statistical significance based on Mann-Whitney U test with Bonferroni correction (significant P<.017).
The null hypothesis that the adaptation of the RPD frameworks fabricated with different SLA layer thickness settings was rejected as differences were found (P<.05). In addition, as seen in Tables 1 and 2, all adaptation measurements for the RPD frameworks fabricated with different layer thickness settings were statistically significantly different (P<.05). However, the adaptation of all groups was within a clinically acceptable range of the adaptation suggested by previous in vitro studies.
A clinically optimal layer thickness was determined by analyzing the layer thickness settings that may affect the adaptation of the definitive prosthesis when fabricating with SLA castable resin RPD framework patterns.
which is preferred to seating and sectioning the frameworks on their models and measuring the gap, a process that may displace the measurement points, thereby producing measurement errors.
The silicone replica technique has been widely used to assess the adaptation of removable dentures because it prevents damage to the model and prosthesis and facilitates convenient measurement of the adaptation. However, the thin silicone film can be torn during the removal from the model,
the room temperature was kept constant, and a guide was used to produce uniform cuts in each of the 15 silicone blocks.
Among the 3 groups, the 16-MF group had the lowest total adaptation and lowest adaptation in each area for the removable dentures, followed by the 50-MF and 100-MF groups (Tables 1 and 2). The lowest adaptation of the 16-MF group among the 3 groups may be attributable to the lowest layer thickness setting. The staircase effect inherent in additive manufacturing affects the accuracy and surface roughness of the final products.
Figure 4Digital microscope images of staircase effect on intaglio surface of metal frameworks fabricated with different layer thicknesses. A, Staircase effect on intaglio surface of 16-MF group. B, Staircase effect on intaglio surface of 50-MF group. C, Staircase effect on intaglio surface of 100-MF group. 16-MF, 16 μm-metal framework; 50-MF, 50 μm-metal framework; 100-MF, 100 μm-metal framework.
approximately 9.5 hours to fabricate the castable resin pattern in the 16-MF group, 3.5 hours in the 50-MF, and 2 hours in the 16-MF group. Therefore, decreasing the layer thickness is desirable with respect to adaptation, but not for the fabrication time.
The present study suggested that the 50-MF group was the group with a clinically optimal layer thickness setting because it did not show a significant difference in the adaptation compared with the 16-MF group, whose adaptation was the best among the 3 groups and was within the clinically acceptable adaptation range; its deposition speed (build time) exceeded that of the 16-MF group by approximately 6 hours and showed only a 1.5-hour difference compared with the 100-MF group.
Limitations of the present study included that differences between different manufacturer specifications for their SLA equipment may result in differences in the quality of the final printed product. In addition, errors arising in investing, pattern eliminating, and casting of the resin patterns may have affected adaptation. Therefore, a wider range of SLA devices should be included in future studies. Furthermore, adaptation should be analyzed with respect to the pattern elimination and casting temperature settings to identify the optimal temperature for pattern elimination and casting in the fabrication of RPD frameworks with castable resin patterns.
Conclusions
Based on the findings of this in vitro study, the following conclusions were drawn:
1.
The layer thickness setting affects the adaptation of the RPD framework when using SLA.
2.
In fabricating the RPD frameworks with SLA, considering the adaptation and build time, a layer thickness of 50 μm is recommended.
Acknowledgments
The authors thank Dentis for providing the material used to conduct this experiment.
Virtual evaluation of the accuracy of fit and trueness in maxillary poly (etheretherketone) removable partial denture frameworks fabricated by direct and indirect CAD/CAM techniques.
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