Biomechanical analysis of adjacent segments after spine fusion surgery using a novel geometrically-parametric patient-specific finite element model

Abstract

Abstract OBJECTIVE: This study aimed to perform mechanical analysis of adjacent segments after spine fusion surgery using a novel geometrically-parametric patient-specific finite element model and to elucidate the mechanism of adjacent segments degeneration（ASD）thus providing theoretical evidence for early prevention. METHODS: Fourteen parameters based on patient specific spinal geometry were extracted from the patient's preoperative computed tomography(CT), and the relative positions of each spinal segment were determined using image match method. A preoperative patient-specific model of spine was established by the above method. The postoperative model of L45 posterior lumbar interbody fusion(PLIF) surgery was constructed using the same method except for removing the lamina and the intervebral disc, and inserting a cage, 4 pedicle screws, and 2 connecting rods. Range of motion(ROM) and stress changes were compared between each anatomic structure of the preoperative and postoperative models. RESULTS: The overall ROM of the lumbar spine decreased after fusion, while the ROM, stress at facet joint, and stress at the intervertebral disc of adjacent segments all increased. The stress distribution in the annulus fibrosus, nucleus pulposus, and facet joints also showed that not only the maximum stress in these tissues were elevated, but the areas of the moderate-to-high stress region were also expanded. During torsion, the stress at facet joint and annulus fibrosus of the proximal adjacent segment (L34) increased to a larger extent compared with the distal adjacent segment (L5S1). Conclusion: While fusion surgery causes an overall restriction of motion at the lumbar spine, it also causes more loads sharing by the adjacent segments in order to compensate for the fused segment, thus increasing the risk of ASD. The proximal adjacent segment is more prone to degeneration than the distal adjacent segment after spinal fusion due to the significant increase in stress changes.