Higher Adjacent Segment Shear Forces after Simulated Lumbar Fusions with Reduced Lordosis

Introduction: Sagittal spinopelvic alignment has been shown to affect loads at the intervertebral joints and has been linked to spine health. Furthermore, the link between degenerative changes and loading of the IVD seems evident with pathophysiological loading widely thought to detrimentally affect the intervertebral disc. If changes in lumbosacral anatomy and kinematics are induced by lumbar fusion, a shift in loads may occur too rapidly for the disc to adequately adapt. Instead it may become prone to microstructural damage, potentially triggering a vicious degenerative cycle eventually leading to a degenerated disc at the adjacent segment. Previous biomechanical studies have experimentally investigated and characterized the effects of fusion. Significant differences in adjacent segment motion have been reported when segmental lordosis at the fusion level is not maintained [1]. On this basis we hypothesized that fusion angle also affects joint reaction loads. Employing a musculoskeletal model of the spine and using data from experimental studies, post-operative loads were quantified and are presented in relation to joint loads in a simulated configuration prior to fusion. Methods: A publicly available musculoskeletal model of the lumbar spine and torso [2] for OpenSim [3] was adopted, substantially refined, and benchmarked against experimental data. This model was consistently used throughout the study. Intervertebral joints were equipped with stiffness and body specific coordinate systems were changed, allowing a straightforward modification of spinopelvic sagittal profile. Corrections to establish symmetry with respect to the sagittal plane were carried out and adjustments to mass properties of trunk segments reflected population mean values. Eight distinctive spinopelvic configurations were selected from clinical subjects of a previous study, covering the range of observed alignments in patients. Subsequently, a model was fitted onto each sagittal spinopelvic anatomy while maintaining total body height, mass assignments and sagittal plane symmetry. Results therefore capture the influence of sagittal profile yet are independent of other parameters and are therefore directly comparable. Simulation of a sinusoidal forward/backward bending motion from upright standing to 45° forward flexion and back in 5s was performed for each of the alignment-specific models in its pre-and post-op state. Different fusion configurations were modeled analogously to a previous experimental study [1], with segmental lordosis at the fusion level being maintained (in-situ), increased (hyperlordotic) and decreased (hypolordotic). The same study provided information on vertebrae kinematics, being used to control the simulation. Mean predicted post-fusion joint loads were then contrasted with corresponding results prior to fusion. Furthermore, the …