Combined Rigid-Deformable Modeling of Lumbar Spine Mechanics

Finite element (FE) models of the spine have been used to assess natural and pathological spine mechanics and evaluate performance of various fusion and posterior stabilization devices [1–3]; however, analysis times may be prohibitive for clinical and design phase assessments. Muscle-actuated, rigid body models have also been developed and used to estimate spinal loading conditions during simulated activities [4]. Although rigid body dynamics platforms typically require less computational time, they are unable to evaluate internal stresses and strains in deformable structures. This study proposes to develop a combined rigid – deformable surrogate spine model where the behavior of the intervertebral disc, facet cartilage and ligaments are replicated by simulated mechanical constraint at desired levels. The explicit FE platform is able to accommodate the spectrum of representations, including fully deformable, fully rigid body, implanted, or any combination. Accordingly, the objective of the current study was to assess the ability of a combined rigid-deformable spine model to accurately reproduce the behavior of the fully deformable representation in the natural state with improved computational efficiency. Specifically, this study compared results for a lumbar (L1-L5) spine under follower load and moment conditions for representations ranging from fully deformable to fully rigid. The combined rigid-deformable model includes the deformable disc, facet cartilage contact, ligament representations at L4-L5, while the other levels are modeled using a simplified mechanical constraint.