Finite element simulation for the prediction of mechanical failure in the lumbar spine surgery

Abstract

Finite Element spine modeling can be useful for computer assisted surgery planning providing help in assessing the risk of mechanical failure. The aim of this study is to provide a patient specific finite element model of the lumbar spine where surgical gesture was considered and to evaluate its relevance as a predictive tool for lumbar spine surgery. A three-dimensional FEM of the L1-Sacrum lumbar spine with geometrical personalization was used. Mechanical properties were based on published data. Then, main characteristics of degenerative pathologies and surgical gestures were considered, and posterior implants were modeled. The sacrum was fixed and Flexion compression loads were applied on the L1 vertebra. Then, results of simulations were post-processed to obtain stresses in the discs and along implants. Clinical cases were collected to evaluate the models relevance. 66 cases instrumented with rigid screw rod systems were considered. 13 of the collected cases were “failed cases” in which patients had screw breakage or screw loosening. Blind simulations were performed for all these patients and the results were qualitatively compared with the clinical outcome. Numerical results highlighted the specific behavior of 12 models for which loads on screws and rods were markedly higher than those of all others. The analysis of the clinical outcome emphasized that 9 of these 13 models corresponded to patients with a mechanical failure. For the first time a patient specific model was proposed for lumbar spine surgery planning. By increasing the number of modeled clinical cases, it could be possible to improve this model and to provide an help for surgery planning.