The influence of passive spinal structures on spinal geometric compensation during flexion-extension: A finite element analysis.

Abstract

The objective of this work was to develop a finite element model of the thoracolumbar spine to assess the effects of passive structures, rib cage, intervertebral disc (IVD), iliolumbar ligament (ILL), and facet cartilage-capsular ligament (FC-FCL), on segmental range of motion (RoM) and thoracolumbar curvature. The model included the vertebrae, rib cage, IVDs, and pelvis, with ligaments and the FC modeled as tension-only and compression-only spring elements, respectively. The model was subjected to 60° of flexion and 55° of extension. A simulated follower load of 1175 N was applied, increasing by 2.4% at each segmental level. Changes in lumbar intervertebral rotations (IVR), lumbar and thoracic RoM and lumbar lordotic (LLA) and thoracic kyphotic angles (TKA) were analyzed for cases involving removal of the ILL and rib cage and removing the L4-L5 and L5-S1 FC-FCL and increasing the elastic moduli of the L4-L5 and L5-S1 IVD, independently. Removing the L5-S1 FC-FCL increased segmental motion (21.7°) in extension with compensatory reductions at L4-L5 (5.5°). Increases in lumbar lordosis (7°) were proportional to increases in thoracic kyphosis (7°). Similar yet smaller effects were observed when removing the rib cage and ILL, with the inverse observed following increasing IVD stiffness. Removal of the rib cage and ligaments or changes in the stiffness of the IVD influence segmental mobility and drives compensatory adjustments in adjacent segments to maintain congruency. Understanding how these tissues affect spinal alignment may inform surgical strategies aimed at preserving or restoring tissue function to maintain spinal stability.