Initial biomechanical responses during maxillary first molar intrusion across different root-sinus configurations and sinus floor cortical thicknesses: a three-dimensional finite element study

This study quantitatively characterized the initial biomechanical responses of five Kwak root-sinus configurations and different maxillary sinus floor cortical thicknesses under standardized axial loading. Five finite element models representing Kwak Types I-V were generated from the same baseline cone-beam computed tomography dataset to control for interindividual anatomical variation. Total axial loads of 0.49, 0.98, 1.47, and 1.96 N were divided equally between buccal and palatal crown loading areas and directed apically. Outcomes included axial root-apex displacement and von Mises stress within predefined maxillary sinus floor cortical bone and apical periodontal ligament regions of interest. Cortical thicknesses of 0.20, 0.35, and 0.50 mm were evaluated in the Type III model, with an additional cortical-thickness analysis across all five types at 1.96 N. Root-apex displacement increased with load magnitude but differed only slightly among configurations. Sinus floor cortical stress generally increased from separation to penetration configurations, with a larger increase between Types II and III. Mean apical periodontal ligament stress changed little among configurations, whereas maximum stress differed among models. Increasing cortical thickness caused only small reductions in root-apex displacement but reduced mean and 95th-percentile cortical stresses; maximum cortical stress did not show a consistent trend. Root-sinus configuration and sinus floor cortical thickness affected local cortical stress more clearly than axial displacement. These findings provide baseline biomechanical data for interpreting Kwak-classified anatomy in the posterior maxilla.