Development of predictive algorithms for the wear resistance of denture teeth materials.

OBJECTIVES: To investigate the wear resistance of conventional, CAD-milled and 3D-printed denture teeth in vitro with simulated aging. To use the collected data to train single time series sample model LSTM and provide proof of concept. METHODS: Six denture teeth materials (Three Conventional; Double-cross linked PMMA (G1), Nanohybrid composite (G2), PMMA with microfillers (G3), CAD-milled (G4), two 3D-printed teeth (G5, G6) (Total n = 60) underwent simulation for 24 and 48 months of linear reciprocating wear using a universal testing machine (UFW200, NeoPlus) under 49 N load, 1 Hz and linear stroke of 2 mm in an artificial saliva medium. Single samples were parsed through Long Short-Term Memory (LSTM) neural network model using Python. To determine minimal simulation times, multiple data splits for training were trialled (10/20/30/40%). Scanning electron microscopy (SEM) was performed for material surface evaluation. RESULTS: 3D printed tooth material (G5) had the lowest wear resistance (59 ± 35.71 µm) whereas conventional PMMA with microfillers (G3) shown the highest wear rate (303 ± 0.06 µm) after 48 months of simulation. The LSTM model successfully predicted up to 48 months wear using 30% of the collected data. Compared with the actual data, the model had a root-mean-square error range between 6.23 and 88.56 µm, mean-absolute-percentage-error 12.43-23.02% and mean-absolute-error 7.47-70.71 µm. SEM images revealed additional plastic deformations and chipping of materials, that may have introduced data artifacts. CONCLUSIONS: 3D printed denture teeth materials showed the lowest wear out of all studied for 48 months simulation. LSTM model was successfully developed to predict wear of various denture teeth. The developed LSTM model has the potential to reduce simulation duration and specimen number for wear testing of various dental materials, while potentially improving the accuracy and reliability of wear testing predictions. This work paves the way for generalized multi-sample models enhanced with empirical information.

Effect of various printing parameters on the accuracy (trueness and precision) of 3D-printed partial denture framework.

OBJECTIVES: To measure and compare the accuracy of 3D-printed materials used for RPD production to improve workflow and eliminate errors in manufacturing. METHODS: A partially edentulous maxilla (Kennedy Class III, modification 1) was prepared and designed with proximal plates, rest seats and clasps in one first premolar, one canine and two second molars. A total of 540 3D printed RPD frameworks were 3D printed with three different types of resin (DentaCAST (Asiga, Australia), SuperCAST (Asiga, Australia) and NextDent (3D Systems, Netherlands)). To evaluate the trueness of the printing materials, they were printed with three types of layer thickness: 50 µm, 75 µm and 100 µm, using two types of build angles: 0° and 45° and three types of plate locations: side, middle and corner. After production, all specimens were scanned and superimposed with a control sample that was digitally designed. Using the initial alignment and best-fit alignment method, the root mean square error (RMSE) was calculated. To capture region specific discrepancy, 10 points of XYZ internal discrepancy within RPDs were measured and Euclidean error was calculated. Data was statistically analysed using Shapiro-Wilk and Kruskal-Wallis tests, one-way ANOVA and T-test (SPSS Version 29) and MATLAB (R2022b). RESULTS: Optimal results were found using 45°, middle of the build plate and layer thicknesses of 100 µm (115 ± 19 µm, DentaCAST), 75 µm (143 ± 14 µm, NextDent), 50 µm (98 ± 35 µm, SuperCAST), which were clinically acceptable. Results were statistically significant when comparing layer thickness in each testing group (p < 0.001). Layer thickness was a primary parameter in the determination of print accuracy among all materials (p < 0.001). Higher discrepancies and failures were observed in 0° prints. No statistically significant difference was found in material usage between build angles or layer thickness (p > 0.005). CONCLUSIONS: All three 3D printing materials exhibited clinically acceptable RMSE results with a build angle of 45° with a printing layer thickness of 50 µm for SuperCAST, 75 µm NextDent and 100 µm for DentaCAST. The highest discrepancies were mostly found in posterior clasps, while the lowest discrepancy was found in palatal straps. Despite unoptimized spacing of prints, frameworks configured to print in the middle of the build plate result in the least printing failures.

Evaluation of wear behaviour of various occlusal splint materials and manufacturing processes.

OBJECTIVES: To investigate the volumetric and vertical loss of occlusal splints manufactured by conventional (heat-cure), subtractive (CAD/CAM) and additive (3D-printing) methods. METHODS: Six occlusal splint materials were investigated (n = 126), using three manufacturing methods: heat-cured, CAD/CAM and 3D-printed built-in three different printing angles (0°,45°and 90°). Block-on-ring wear testing was performed with extracted human molars as the antagonist. All samples were tested with an applied force of 49N at 1 Hz and 60RPM in artificial saliva at 37 °C for six and 12 months. Scanning electron microscopy images were analysed to evaluate the wear on the tooth enamel and in the splint material. Volumetric and vertical wear loss were statistically analysed. RESULTS: The lowest volumetric and vertical loss was observed in CAD-CAM materials (6.44 ± 1.77 mm3 and 48.3 ± 7.14 µm) with no statistical significance to the heat-cured material (17.22 ± 9.23 mm3 and 148 ± 121.1 µm) after 12 months (p < 0.172). The mean volumetric loss of 3D printed materials ranged from 0.25 ± 0.15 mm3 to 0.29 ± 0.1 4mm3 with no statistical difference, whereas, the differences in vertical loss from 131.63 ± 44.1 µm to 493 ± 79.19 µm were statistically significant (p < 0.001). The highest vertical loss was observed in the KeySplint Soft 3D printed at 90° (385.35 ± 82.37 µm), whereas FreePrint Splint 2.0 with a build angle of 0° had the highest volumetric loss (204.59 ± 25.67 mm3). CONCLUSION: CAD-CAM material had the highest wear resistance followed by heat-cured material.KeySplint Soft and FreePrint Splint 2.0 3D printed materials would be preferred for patients that do not have severe bruxing episodes. No significant wear of human enamel after six and 12 months was observed under SEM for any tested materials.

Wear Behavior of Occlusal Splint Materials Manufactured By Various Methods: A Systematic Review.

PURPOSE: To systematically review studies on various materials and methods used for wear testing of occlusal devices and their antagonists in vitro and in vivo. METHODS: An electronic search in OVID, Web of Science, PubMed and Scopus was conducted using the following terms (MeSH words) with any synonyms and closed terms: "Splint*" OR "occlusal splint*" OR "night guard" OR "occlusal device" OR "occlusal devices" OR "deprogrammer" OR "bite splint" OR "bite plane" OR "orthotic appliance*" OR "orthotic devices" AND "wear" OR "two-body wear" OR "three-body wear" OR "tooth wear" OR "wear measurement*" OR "wear behaviour" OR "wear behavior" OR "abrasion" AND "Polymethyl Methacrylate" OR "PMMA" OR "acrylic resin*" OR "dental material*" OR "dental enamel" OR "CAD" OR "CAM" OR "PEEK" OR "material* testing". Database search was limited to English-language publications and published between 2001 and 1st of September 2021. A further hand search was done to ensure all materials were captured. RESULTS: After the removal of duplicates, 115 studies were identified, and 11 were chosen for review. Studies showed that the lowest volumetric loss was observed in PEEK occlusal device materials, whereas heat-cure, CAD-milled, and 3D printed occlusal device materials had no significant difference in wear. Vacuum-formed materials showed the highest wear among all groups. Testing parameters were found to be inconsistent across all studies. CONCLUSION: There is a need for standardization of in vitro and in vivo wear measurement and testing protocols as this study revealed a wide variety of testing protocols which potentially could influence the outcome. Polishing procedures are required for the material. Limited studies are available on 3D printed occlusal device materials and would therefore require further investigation, especially on printing build angles and settings. Further clinical studies would be advantageous to provide guidance on the selection of the best occlusal device material that would last the longest without remake.

The effect of light curing intensity on bulk-fill composite resins: heat generation and chemomechanical properties.

OBJECTIVES: The aim of this study was to assess the effect of light curing intensity and wavelength spectrum on heat generation and chemomechanical properties of bulk-fill composites. METHODS: Four bulk-fill restorative materials (Filtek bulk-fill, Tetric PowerFill bulk-fill, Beautifil Bulk restorative and Admira Fusion X-tra were used in this study. A total of 100 cylindrical specimens of each composite (n = 25/group) were prepared, then cured using monowave light curing unit (LCU) with a single light intensity of 1470 mW/cm2, and polywave LCU with three different light intensities (1200,2100, 3050mW/cm2). The temperature change during polymerisation was measured by five K-type thermocouples placed in each 1 mm layer from top to bottom. Hardness and degree of conversion of composites at each level were evaluated. Results were statistically analysed. RESULTS: The use of polywave LCU resulted in statistically higher peak temperatures ranging between 31.4-63.5 °C compared to the temperature generated by monowave LCU ranging between 29.5-60 °C (p < .05). Curing using polywave LCU with the highest light intensity of 3050 mW/cm2 caused the highest peak temperature irrespective of the composite types. There was no significant difference in hardness with different light curing intensities and curing times, regardless of the bulk-fill resin materials (p > .05). A positive correlation was also found between the hardness and the DoC of the four bulk-fill composites. CONCLUSION: The change in temperature during polymerisation of bulk-fill composites were found to be proportional to the increase in light curing intensity. Mechanical properties of the bulk-fill composites were dependent on the composition and the type of photoinitiators.

Fibre-reinforced and repaired PMMA denture base resin: Effect of placement on the flexural strength and load-bearing capacity.

OBJECTIVES: To measure the effect of placement of glass fibre mesh on the flexural strength and load bearing capacity of repaired polymethylmethacrylate (PMMA) denture base resin. MATERIALS AND METHODS: A total of 150 heat-polymerised acrylic resin specimens were fabricated with dimensions of 5 × 30 × 50 mm for flexural strength testing. Specimens were divided into 5 groups according to repair width and placement of the fibre mesh. Three groups (n = 90) had a repair width of 20 mm (including the control group), and two groups (n = 60) had a narrower repair width of 16 mm. Fibre mesh was either embedded at the neutral (bottom of the repair area) or tension (top of the repair area) zone of the specimen when subjected to flexural strength testing. Half of the specimens from each group were subjected to artificial ageing by thermocycling (5 °C and 55 °C, 30s dwell time) for 10,000 cycles to stimulate 12 months in vivo. All the specimens were stored in distilled water at 37 °C for 24 h prior to testing. The flexural strength of the specimen was obtained by three-point bend testing, and data were statistically analysed using ANOVA and post-hoc analysis (SPSS; significance level p < 0.05). Probability of failure was calculated using Weibull analysis. Scanning electron microscopy analysis was used to identify the mode of failure. RESULTS: Specimens repaired with the 20 mm fibre mesh placed in the tension zone showed the highest mean flexural strength (101.33 ± 12.66 MPa) with statistical significance (p = 0.05) to the other groups except for the specimens repaired with the 16 mm fibre mesh placed in the tension zone (p = 0.072). The highest Weibull modulus was found in the thermal cycling group of the specimens with 20 mm repair width repaired with the fibre mesh embedded at the neutral zone (10.01). The lowest Weibull modulus was found in the non-thermal cycling group of the control group (3.15). CONCLUSION: Placing fibre mesh in the tension zone of a PMMA specimen significantly improved the flexural strength of the repair. Placing the fibre mesh in the neutral zone or the fibre mesh width was short of the lower support rollers resulted in no significant increase in flexural strength compared to the non-reinforced control group. Ageing via thermal cycling resulted in a decrease in flexural strength across all sample groups. This study highlights the importance of recognising the valid repair region and to have the mesh embedded in certain dimensions, otherwise it will have no significant contribution towards the repair and increase of flexural strength of the denture.

Comparison of hardness and polishability of various occlusal splint materials.

OBJECTIVES: To measure polishability of occlusal splint materials manufactured by various methods. METHODS: Seven occlusal splint materials manufactured by four different methods - Heat cured (Vertex Rapid simplified Clear), CAD-milled (Ceramill a-splint), Vacuum-formed (Proform splint) and 3D-printed (Freeprint Ortho, KeySplint Soft, DentaClear and FreePrint Splint 2.0) were tested for gloss, roughness, and surface hardness and elastic modulus. For all groups, the tests were repeated with the materials polished with three different polishing burs, pumice and high shine. All polishing procedures were standardised by applying the force of 1 N for 1 min at the set speed. 3D printed materials were further tested with additional specimens manufactured at different printing angles of 0°, 45° and 90°. Data was statistically analysed using ANOVA (SPSS Version 26) and MatLab (R2020a). Polished surfaces of each specimen were analysed under scanning electron microscope. RESULTS: Vacuum-formed materials showed the highest polishability (80.61 ± 0.98 GU) with no statistical significance to heat-cured or CAD-milled (p = 1.00). Pumice and high shine polish significantly improved the gloss for all groups. The mean gloss and surface roughness for all 3D-printed materials ranged from 75.24 ± 25.05 GU to 0.18 ± 0.21 GU and 2.73 ± 3.18 µm to 0.06 ± 0.01 µm, which was significantly lower (p < 0.001) than heat-cured, CAD-milled and vacuum-formed materials. The highest hardness (0.40 ± 0.009 GPa), elastic modulus (6.06 ± 1.49 GPa) and gloss were found when materials were 3D-printed at 45°, with the lowest surface roughness. CONCLUSION: Statistically significant differences in polishability were found among the different occlusal splint materials. The polishability and surface hardness of 3D-printed occlusal splint materials was influenced by the print angle. The 0° 3D-printed occlusal splint materials produced the highest gloss and the lowest surface roughness pre-polished, indicating that no polishing is required. While the 3D-printed occlusal splint materials at 45oand 90° required polishing with burs, pumice and high shine to reduce the surface roughness, there were layering structures created during printing.