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

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).

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.

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.

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.