Biomechanical finite element methods study of a novel internal fixation system for lumbar spondylolysis

This study aimed to evaluate the biomechanical efficacy of a novel internal fixation system for the treatment of lumbar spondylolysis (LS). In addition, the changes in the mechanical performance of the proposed fixation construct during progressive compression were systematically investigated.

A healthy 25-year-old male volunteer was recruited for lumbar spine CT data acquisition to construct and validate a nonlinear finite element model of the L4-S1 spinal segment (A); Based on this, models were established for L5 spondylolysis (B); L5 spondylolysis model with traditional internal fixation (C); L5 spondylolysis model with the pressurization process of the novel LS repair device fixation (D→E→F). For these six models, we constrained the lower surface of the S1 vertebral body while applying an axial compression force of 500 N and a moment load of 7.5 N m on the upper surface of the L4 vertebral body to simulate six motions of the lumbar spine. The performance of each finite element model was evaluated by comparing the range of motion (ROM), maximum displacement, and maximum pressure experienced by the lumbar spine under different motion conditions.

Compared with Model C, Models D, E, and F exhibited a reduced ROM and maximum displacement under left axial rotation and right axial rotation conditions. Notably, compared with Model C, the novel internal fixation models consistently demonstrated a decreasing trend in the maximum stress on the intervertebral discs (IVD) and an increasing trend in the maximum stress on the articular cartilage and maximum stress and displacement of the bone graft. Moreover, The progressive pressurization (D→E→F) of the novel internal fixation model further enhanced stress transfer between the isthmic defect ends and the bone graft.

Compared with the conventional fixation model (Model C), the novel internal fixation models (Models D, E, F) provided superior spinal stability, more effectively restored physiological stress levels in the facet cartilage, and generated greater mechanical stimulation within the bone graft region. These findings suggest that the proposed fixation system may provide a more favorable biomechanical environment for pars defect healing and graft fusion.