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Adjacent segment degeneration (ASD) is an abnormal process that develops at spinal segments adjacent to the fused segment caused by biomechanical changes after spinal fusion. To avoid the adverse effects of spinal fusion on the adjacent segments, various flexible stabilization systems have been developed, of which the Graf system is one of the most widely used. We assessed the biomechanical influence of the Graf system and spinal fusion. The L3-L6 vertebrae were taken from porcine lumbar spines. A spinal motion tester displaced the end of the L3 vertebra to simulate continuous flexion/extension. Three cycles of flexion/extension were applied and the angular motion and intradiscal pressure were recorded during the third cycle for each test. The angular deformity at the flexible stabilized segment was suppressed until 4° of deformation; the deformity gradually increased after 4° and then finally equaled that of the intact spine. The maximum intradiscal pressure increased significantly in each segment fused and in the adjacent spinal segments using the Graf system. In conclusion, the Graf system reduces the motion of adjacent segments and may reduce the risk of ASD. Nevertheless, the relationship between ASD and the increase in the intradiscal pressure remains controversial.

An in Vitro Biomechanical Evaluation of the Mobility of Adjacent Segments after Spinal Instrumentation