Subject Specific Modeling for Lumbar Spine Surgery Complications Analysis: A Preliminary Case Report

Lumbar spine surgery mechanical complications include implant failure and early adjacent disc degeneration. Finite element models (FEM) allow better understanding of underlying mechanisms, providing they are patient specific, specifically for geometry and loads. The aim of this study is to explore the ability of a FEM to explain mechanical complications of lumbar spine surgery. A preliminary study on a selected case was conducted. A lumbar spine nonlinear FEM was used. The model with a mean geometry was extensively evaluated based on in vitro experiments. A 72-years-old female patient was considered. She had a proximal disc failure requiring revision surgery after a three-level rod-screws fixation. The L3-L5 FEM was used in three configurations: 1/ reference mean geometry with mean lever arm (REF); 2/ Subject specific geometry (GEOM) was built from medical images 3/ because of the patient postural trouble, a worst-case scenario considered an increased lever arm (GEOM + LOAD). Stress in the adjacent disc fibers (L2-L3) and in the implant were computed. Compared to the REF configuration, changing the conditions yielded an increase in disc fibers loads, from 2.8 N to 4.2 N (GEOM) and 6.4 N (GEOM + LOAD). Maximum Von Mises stress increased from 87 MPa to 115 MPa (GEOM) and 173 MPa (GEOM + LOAD). This preliminary study only considers a single patient. However, it highlights the importance of taking into account both subject specific geometry and loads when considering clinical use of models to investigate surgery complications. Work is in progress on a cohort of patients.