Mechanical Properties of Dental Occlusal Splint Materials After Accelerated Aging in Artificial Saliva: An In Vitro Study

Background: Occlusal splints are exposed to prolonged intraoral conditions, including saliva, temperature fluctuations, and mechanical loading, which may alter their mechanical performance over time. The increasing use of SLA 3D-printed photopolymer resins for splint fabrication has introduced new materials whose aging-related mechanical stability remains insufficiently characterized. Objectives: This in vitro study evaluated the effects of accelerated aging in artificial saliva on the Shore D hardness and flexural properties, including flexural modulus, flexural strength, and strain at failure, of four polymeric materials used for occlusal splint fabrication: a thermoformed PETG material (Duran, C), a milled PMMA (Bilkim Polywax, F), and two SLA-printed photopolymer resins (HARZ Labs Dental Clear, H; NextDent Ortho Rigid, N). Materials and Methods: Eighty rectangular specimens (n = 20/material) were fabricated following ASTM D790 and subjected to accelerated aging in Fusayama artificial saliva at 60 °C for 24, 48, and 72 h. Shore D hardness was assessed first as a non-destructive measurement, followed by three-point bending tests yielding flexural modulus, flexural strength, and strain at failure. Data were analyzed using one-way ANOVA and post hoc multiple comparisons at a significance level of α = 0.05. Results: Aging effects were material-dependent. Duran (C) showed no statistically significant changes in Shore D hardness (p = 0.651) and only non-progressive variations in flexural behavior. Bilkim Polywax (F) exhibited a significant hardness reduction at 48 h (p = 0.004) and selective changes in flexural strength at 72 h. HARZ Labs Dental Clear (H) showed the most severe and progressive deterioration, with significant reductions in all evaluated parameters, including hardness decreases of up to 5.8% and flexural modulus reductions exceeding 49%. NextDent Ortho Rigid (N) demonstrated an initial hardness decrease followed by stabilization, with a significant reduction in flexural modulus at 72 h. Conclusions: Under the accelerated hydrothermal aging conditions employed in this study, the two investigated SLA-printed photopolymer resins (HARZ Labs Dental Clear and NextDent Ortho Rigid) exhibited greater mechanical deterioration than the tested thermoformed PETG and milled PMMA materials. These findings are limited to the investigated materials, standardized rectangular specimens, and the specific accelerated aging protocol used in this study. Among the tested materials, thermoformed PETG demonstrated the greatest mechanical stability. Material-specific evaluation of aging resistance remains advisable before routine clinical application, particularly for digitally fabricated occlusal splints intended for prolonged intraoral service.