The quantitative assessment of contribution of risk factors to overstress at adjacent segments after lumbar fusion: removal of posterior ligaments, pedicle screws

Adjacent segment deterioration (ASD) is a serious sequela of spinal fusion surgery that has recently become much more widespread due to an increasing number of spinal fusions performed [1]. Moreover, previous studies [2] of potential risk factors for ASD have found that instrumentation, posterior lumbar interbody fusion, injury to the facet joint of the adjacent segment, fusion length, age, and sagittal alignment, are all risk factors for ASD. However, the actual significance of these risk factors remains controversial, possibly due to differing patient populations and methodologies [2]. Even though fusion probably affects motion and causes stress to adjacent segments, the following questions remain: to what extent does each risk factor impact ASD? What is the most decisive factor among the many risk factors for ASD? And, are there interactions between risk factors for ASD? In order to search for the answers to these questions, we planned to investigate the stress and the range of motion (ROM) at adjacent segments after fusion, using finite element (FE) model of lumbar spine. Two important risk factors were chosen such as instrumentation (pedicle screws) or the ablation of continuity of proximal ligament complex (PLC) among many risk factors. Therefore, the purpose of this study is to investigate changes of the range of motion (ROM) at adjacent segments after lumbar fusion according to whether or not pedicle screws is removed and whether or not the continuity of proximal ligament complex (PLC) is preserved.