Evaluation of adjacent segments mobility after spinal instrumentation : an in vitro biomechanical study

I In nt tr ro od du uc ct ti io on n: : Spinal fusion with instrumentation is an essential component in the management of degenerative condition in the spinal motion segment. It is performed to improve initial stability with simplified external fixation for early ambulation1)2). The solid fixation, however, often induces an adjacent segment degeneration (ASD) above or below the fusion 3). ASD can be described as any abnormal process of disc degen-eration, listhesis, instability and etc that develops in the motion segment adjacent to a fused segment. Biomechanical changes of the segment due to the fusion, increase intradiscal pressure, increase facet loading, or increase mobility are considered to contribute ASD 4). To avoid the adverse effects of spinal fusion, various flexible stabilization systems have been developed to stabilize the unstable segment. Flexible stabilization systems were based on the concept to allow spinal motion but to restrict excessive motion. The Graf system was one of the most practiced methods of flexible stabilization. To be stabilized, the bands are pretensioned on application to place the motion segments into lordosis and to lock the facet joints. Kanayama et al reported that radiographic evidence of adjacent disc deterioration was observed more frequently in patients with posterolater-al fusion group than those with Graf system and concluded that the Graf system decreased risk of ASD compared with PF with instrumentation5). The objective of this study was to assess the biomechanical influence that occurred at the adjacent segment by flexible stabilization with Graf system and instrumented spinal fusion. M Ma at te er ri ia al ls s a an nd d M Me et th ho od ds s: : The in vitro experiments were performed with fresh frozen porcine lumbar spines. The lumbar spines were harvested from fifteen mature porcines. Each specimen was dissected into an L3-L6 spinal column consisting of four adjacent vertebral bodies, including posterior elements, and intervertebral discs. The spines were tested intact first, then after bilateral medial facetectomy (MF), augmented by Graf system (Graf), and instrumented spinal fusion, respectively. Medial Facetectomy and the instrumentations were performed at L4. The spinal columns were tested using a spinal motion tester which was designed to apply displacement to the cranial vertebra to simulate sequential flexion-extension movement of the spine. During testing, axial com-pressive load of 70N was applied to the column. The actuator generated at speed of 5.0mm/sec with displacement of …