Fusion angle affects intervertebral adjacent spinal segment joint forces-Model-based analysis of patient specific alignment.

This study addresses the hypothesis that adjacent segment intervertebral joint loads are sensitive to the degree of lordosis that is surgically imposed during vertebral fusion. Adjacent segment degeneration is often observed after lumbar fusion, but a causative mechanism is not yet clearly evident. Altered kinematics of the adjacent segments and potentially nonphysiological mechanical joint loads have been implicated in this process. However, little is known of how altered alignment and kinematics influence loading of the adjacent intervertebral joints under consideration of active muscle forces. This study investigated these effects by simulating L4/5 fusions using kinematics-driven musculoskeletal models of one generic and eight sagittal alignment-specific models. Models featured different spinopelvic configurations but were normalized by body height, masses, and muscle properties. Fusion of the L4/5 segment was implemented in an in situ (22°), hyperlordotic (32°), and hypolordotic (8°) fashion and kinematic input parameters were changed accordingly based on findings of an in vitro investigation. Bending motion from upright standing to 45° forward flexion and back was simulated for all models in intact and fused conditions. Joint loads at adjacent levels and moment arms of spinal muscles experienced changes after all types of fusion. Hypolordotic configuration led to an increase of adjacent segment (L3/4) shear forces of 29% on average, whereas hyperlordotic fusion reduced shear by 39%. Overall, L4/5 in situ fusion resulted in intervertebral joint forces closest to intact loading conditions. An artificial decrease in lumbar lordosis (minus 14° on average) caused by an L4/5 fusion lead to adverse loading conditions, particularly at the cranial adjacent levels, and altered muscle moment arms, in particular for muscles in the vicinity of the fusion.