The Influence of Thermal and Mechanical Aging on the Flexural Properties of Conventional and 3D-Printed Materials Used in Occlusal Splints Manufacturing

Occlusal splints are a type of intraoral appliance that are widely used for the management of temporomandibular disorders and bruxism, yet limited evidence exists regarding the comparative effects of combined aging on conventional and digitally manufactured materials. This in vitro study evaluated the influence of thermal and mechanical aging on the flexural properties of three materials commonly used for the manufacturing of occlusal devices: self-curing poly(methyl methacrylate) (PMMA, control), light-cured urethane dimethacrylate (UDMA)-based resin, and stereolithography (SLA)-printed photopolymer. Seventy-two standardized specimens (<i>n</i> = 24 per material; 64 × 10 × 3.3 mm) were fabricated, then randomly allocated to three groups (<i>n</i> = 8): control, thermocycling (10,000 cycles, 5 °C/55 °C), and combined thermocycling with mechanical loading (1000 cycles). Flexural strength and modulus were determined by three-point bending tests and analyzed using a two-way analysis of variance (ANOVA) with Tukey's post hoc test (α = 0.05). Thermocycling significantly reduced flexural strength in PMMA (65.19 ± 6.68 to 57.94 ± 7.15 MPa) and SLA (67.67 ± 1.54 to 59.37 ± 8.80 MPa) groups (<i>p</i> < 0.05), while UDMA group (45.489 ± 3.905 to 43.123 ± 4.367 MPa) demonstrated no significant changes (<i>p</i> ≥ 0.05). UDMA exhibited substantially and significantly lower flexural properties compared to PMMA and SLA across all conditions (<i>p</i> < 0.0001). Thermal aging slightly compromises the mechanical properties of PMMA and SLA-printed materials, whereas UDMA-based resins exhibit good aging resistance but considerably lower initial values. While UDMA-based resin showed superior aging resistance, its lower baseline mechanical properties may limit its application in high-stress clinical scenarios compared to PMMA and SLA-printed materials. Material selection should consider both initial properties and long-term environmental changes.