Association of vertical facial morphology with occlusal force changes after masseter botulinum toxin ınjection in bruxism: a prospective T-scan study

Botulinum toxin type A (BTX-A) is increasingly used in bruxism management, yet its effects across different vertical skeletal patterns remain unclear. This study investigated whether vertical facial morphology is associated with differences in the early occlusal force response to standardized masseter BTX-A injection using the T-Scan system. Thirty-eight patients with probable bruxism were initially recruited and stratified according to lateral cephalometric measurements. Four patients were lost to follow-up, resulting in a final sample of 34 participants, including a low-angle group (GoGn/SN < 26°; n = 17) and a normal-angle group (GoGn/SN 26–38°; n = 17). Each patient received bilateral masseter injections of 25 units of BTX-A (Botox®, Allergan). T-Scan Novus™ recordings were obtained at baseline (T0) and day 15 (T1). Outcome measures included right–left occlusal force imbalance (%) , anterior and posterior force percentages, and occlusion time. Statistical analyses included the Wilcoxon signed-rank test, the Mann–Whitney U test, linear mixed-effects models, ANCOVA with robust standard errors, and Fisher's exact test. Following injection, a statistically significant within-group reduction in right–left occlusal force imbalance was observed in the low-angle group (13.45 ± 10.00% to 8.87 ± 6.94%; Wilcoxon p = 0.019). However, the between-group comparison of change scores ( p = 0.965) and the time × group interaction ( p = 0.920) were not statistically significant. A significant interaction between baseline right–left occlusal force imbalance and group assignment was observed in ANCOVA ( p = 0.008). Between-group differences in anterior and posterior force percentages were no longer statistically significant at day 15 ( p = 0.157 and p = 0.160, respectively). Occlusion time remained unchanged in both groups. A statistically significant within-group reduction in right–left occlusal force imbalance was observed in the low-angle group at day 15 following masseter BTX-A injection. However, the between-group comparison of change scores ( p = 0.965) and the time × group interaction ( p = 0.920) were not statistically significant. Therefore, this within-group observation should not be interpreted as evidence of a differential treatment effect between the low-angle and normal-angle groups. Confirmation in larger comparative studies with longer follow-up is warranted. ClinicalTrials.gov NCT07673471, registered 23 June 2026. Retrospectively registered.