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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.

Quantitative measurement of spinal rotational mobility is critical in assessing spinal instability. We have developed a system for intraoperative measurement of spinal mobility. The system consists of a motor-driven mechanical apparatus with a computerized controller, load cell, optical displacement transducer, and spinous process holders that allow handling of the spinal process without damaging the ligaments. In this study, we examined the ability of this measurement system to detect mobility and corresponding clinical instability of a destabilized model spine comprised of porcine lumbar segments. From the load-displacement curve, we determined three motion parameters: stiffness, neutral zone (NZ), and absorption energy (AE). To demonstrate the potential of the measurement system for clinical application, we applied five cycles of axial rotation to the segment ten times. The measurements were highly reproducible. To verify the utility of the measurement system, we tested destabilized functional spinal units in vitro. In this setting, stiffness and AE decreased and NZ increased with progressive destabilization of the model spine. Significant relationships were observed between the destabilized model and the biomechanical data.