Age- and force-dependent responses of the midpalatal suture to nonsurgical rapid palatal expansion in adult rats

PURPOSE: To investigate how age-related changes in midpalatal suture morphology influence skeletal response to varying force magnitudes in nonsurgical rapid palatal expansion (RPE) using a rat model. METHODS: Nine rats were allocated to an age-based analysis (2, 4, and 8 months; n = 3 per group) to evaluate physiologic age-related development in midpalatal suture morphology. Thirty-five mature rats (8 months) were further assigned to light force (50 g), heavy force (200 g), or control groups. Forces were applied via custom-fabricated expansion devices for 3, 7, and 14 days. Micro-computed tomography (micro-CT), histological analysis, and immunohistochemistry were performed. Histological analysis included qualitative and quantitative assessments. Micro-CT was used for evaluating suture separation, bone volume fraction (BV/TV), and bone mineral density (BMD). Immunohistochemistry assessed the expression of bone remodeling markers. RESULTS: Increased age correlated with reduced midpalatal suture separation and bone formation. Heavy forces resulted in significantly greater suture separation at early time points but also induced more bone resorption, as indicated by lower BV/TV and BMD values. Immunohistochemistry confirmed increased RANKL expression in heavy force groups, indicating enhanced bone remodeling activity. CONCLUSION: Midpalatal suture response is significantly influenced by both age and the magnitude of applied force. While heavy force promotes faster suture separation, it leads to increased bone resorption and reduced bone quality. In contrast, light force results in a more gradual separation but supports earlier osteoclastic activity, greater vascularization, and better preservation of bone structure. These findings underscore the importance of age-appropriate force selection in nonsurgical RPE protocols.