Do in vivo kinematic studies provide insight into adjacent segment degeneration? A qualitative systematic literature review

While much evidence suggests that adjacent segment degeneration is merely a manifestation of the natural degenerative process unrelated to any spine fusion, a significant body of literature supports the notion that it is a process due in part to the altered biomechanics adjacent to fused spine segments. The purpose of this study was to review and critically analyze the published literature that investigated the in vivo kinematics of the adjacent segments and entire lumbar spine in patients receiving spinal fusion or motion-preserving devices. A systematic review of the PubMed database was conducted, initially identifying 697 studies of which 39 addressed the in vivo kinematics of the segments adjacent to spinal implants or non-instrumented fusion of the lumbar spine. Twenty-nine articles studied fusion, of which three reported a decrease in range of motion of the caudal adjacent segment post-fusion. Examining the rostral adjacent segment, twelve studies observed no change, nine studies found a significant increase, and three studies reported a significant decrease in sagittal plane range of motion. Of the six studies that analyzed motion for the entire lumbar spine as a unit, five studies showed a significant decrease and one study reported no change in global lumbar spine motion. Kinematics of the segment rostral to a total disc replacement was investigated in six studies: four found no change and the results for the other two showed dependence on treatment level. Fifteen studies of non-fusion posterior implants analyzed the motion of the adjacent segment with two studies noting an increase in motion at the rostral level. There appears to be no overall kinematic changes at the rostral or caudal levels adjacent to a fusion, but some patients (similar to 20-30 %) develop excessive kinematic changes (i.e., instability) at the rostral adjacent level. The overall lumbar ROM after fusion appears to decrease after a spinal fusion.