Bruxism Simulation in Aligner Therapy: Effects on Restored Posterior Teeth.

Background/Objectives: Parafunctional habits such as bruxism generate high occlusal forces that can significantly compromise the performance of dental restorations during orthodontic treatment. This ex vivo study aimed to evaluate the surface wear of Class II composite restorations and the integrity of clear aligners (CAs) under simulated parafunctional loading. Methods: Thirty-four posterior teeth restored with composite materials were subjected to either normal masticatory forces or high-intensity cyclic forces mimicking bruxism while being fitted with orthodontic aligners. The collected experimental data were analyzed using R (version 4.3) under the Jamovi project (version 2.5.3). Differences between groups were assessed using paired samples t-tests and Wilcoxon tests for paired samples, with robust t-tests applied when data normality could not be confirmed. Statistical significance was set at α = 0.05. Results: Parafunctional loading led to significantly greater surface degradation of restorations and increased aligner wear. Compared with functional forces, RMS errors were substantially higher under parafunctional forces (33.5 vs. 21.5 units; p < 0.001), indicating reduced positional accuracy. Aligner thickness decreased more under parafunctional conditions (0.0304 mm) than under normal function (0.0122 mm), with all comparisons showing high statistical significance and large effect sizes. Conclusions: Parafunctional forces were found to significantly increase surface wear in Class 2 resin composite restorations during clear aligner therapy. Simulated bruxism also compromised aligner integrity, indicating the need for more durable materials and tailored treatment strategies for patients with bruxing habits. These findings highlight the importance of selecting durable restorative and aligner materials for bruxer patients to ensure long-term treatment success.