Multiphysics finite element analysis of vertebral stability in simulated osteolytic lumbar defects treated with 125I brachytherapy and vertebroplasty.

OBJECTIVE: This study evaluates the impact of radiation dose, thermal effects, and mechanical factors on vertebral stability and safety in a finite element model of simulated osteolytic defects and compared 125I seed brachytherapy alone with 125I brachytherapy combined with percutaneous vertebroplasty (PVP). METHODS: A 3D FE model of the L3-L5 lumbar spine was reconstructed from CT data of a healthy adult subject. Idealized osteolytic defects were introduced into the anterior-middle L4 region at three volume grades: T ≤ 30%, 30% < T < 50%, and T ≥ 50%. Three scenarios were simulated for each grade: (i) osteolytic defect model, (ii) 125I seed implantation alone, and (iii) 125I-PVP combination. Ninety-day cumulative dose distributions (TG-43U1) were used to model dose-dependent degradation of material properties. PMMA polymerization was modeled using Arrhenius kinetics, and the temperature field was coupled into the structural analysis. Under physiological loading (500N + 7.5 N m), maximum displacement, maximum von Mises stress, peak temperature, and thermally damaged tissue volume fraction were calculated. RESULTS: For T ≤ 30%, 125I implantation alone caused minimal changes in displacement and stress. Adding PMMA increased flexion displacement to 7.12 mm and maximum flexion von Mises stress to 36.29 MPa. For T > 30%, 125I implantation alone did not substantially improve mechanical performance. Compared with 125I implantation alone, 125I seed brachytherapy combined with PVP reduced flexion displacement from 8.20 to 4.36 mm and maximum flexion von Mises stress from 32.41 to 28.92 MPa in the 30% < T < 50% group. In the T ≥ 50% group, the combined treatment reduced flexion displacement from 8.81 to 4.76 mm and maximum flexion von Mises stress from 53.53 to 29.97 MPa. PMMA polymerization generated peak interface temperatures of 74 °C-84 °C and thermally damaged tissue fractions of 8%-25%, while spinal canal temperatures remained below 43 °C. CONCLUSION: For T ≤ 30%, 125I seed implantation maintained vertebral stability, whereas adding PVP increased local stiffness without clear mechanical benefits. For T > 30%, particularly T ≥ 50%, 125I seed brachytherapy combined with PVP improved stress distribution and displacement control compared with 125I alone. These findings represent model-based trends and should not be interpreted as failure-load predictions or clinical thresholds.