Biomechanical characteristics of osteotomy correction for kyphosis secondary to ankylosing spondylitis: a finite element study

BACKGROUND: This study comparatively analyzed the biomechanical performance of different vertebral level osteotomies in patients with severe kyphosis secondary to ankylosing spondylitis (ASK) using finite element analysis. METHODS: The hilus pulmonis-hip axis (HP-HA) method was employed to determine osteotomy angles. Posterior vertebral column resection (PVCR) was simulated at four distinct vertebral levels (T12, L1, L2, and L3), followed by pedicle screw-rod fixation combined with interbody fusion cages. Total displacement, von Mises stress distribution in the pedicle screw-rod construct, titanium cage, and cage-vertebral interface were evaluated and compared across models. RESULTS: Four finite element models (A-D, corresponding to T12, L1, L2, and L3 osteotomies) were successfully established and validated. Total displacement decreased progressively from model A to model D, measuring 6.88 mm, 5.44 mm, 3.90 mm, and 2.26 mm, respectively. Peak von Mises stresses in the pedicle screw-rod systems were 115.58 MPa, 139.53 MPa, 191.35 MPa, and 184.14 MPa, respectively. Model D (L3 osteotomy) demonstrated the smallest displacement and well-dispersed stress distribution, while model A (T12 osteotomy) exhibited the largest displacement despite having the lowest screw-rod stress. All stresses remained below the yield strength of the internal fixation devices. CONCLUSION: This finite element study demonstrates that L3 osteotomy yields superior stability compared to T12 osteotomy, though the conclusion is based on a single-case model and requires further validation through multicase studies. These findings underscore the importance of considering segment-specific biomechanical factors in surgical planning for ASK patients.