The effect of sagittal alignment on adjacent joint mobility after lumbar instrumentation––a biomechanical study of lumbar vertebrae in a porcine model
Introduction
Spinal fusion with instrumentation has been widely accepted for the treatment of lumbar spinal disorders such as spondylolisthesis and traumatic spine injury. Clinical reviews show that spinal fusion with spinal instrumentation will accelerate the degeneration of the adjacent motion segment (Aota et al., 1995; Chow et al., 1996). A 45% rate of segmental instability above the lumbar fusion after a median follow-up of 33 years has been reported (Lehmann et al., 1987). There have been several demographic reports and biomechanical studies regarding adjacent instability after lumbar fusion. Biomechanical studies have proved that instrumentation would increase stress at the neighboring disc (Ha et al., 1993). Except facet absence, facet asymmetry, W type of facet joints and increased laminar joint angle, the predisposing factors for developing adjacent segment instability are not clear.
The importance of the maintenance of physiologic spinal alignment in spinal fusion to minimize the acceleration of degenerative changes at the levels adjacent to the fused segments and to achieve good long-term clinical results has been widely accepted (Lorenz et al., 1983; Oda et al., 1999; Schlegel et al., 1996). To maintain normal posture and an optimum center of gravity after surgery, the parafusion motion segments may be required to compensate for the changes in spinal alignment and the lost spinal motion at the fused segments. Therefore, theoretically postoperative lumbar malalignment may accelerate adjacent segment deterioration by over-loading the motion segment, particularly in a non-physiologic fusion. However, the mechanical effects of sagittal malalignment on the adjacent motion segments have been investigated in only a few biomechanical studies. Therefore, we designed a biomechanical study using fresh porcine spines to investigate the effect of sagittal alignment on the adjacent unfused segment after instrumented lumbar fusion.
Section snippets
Methods
Eight fresh porcine lumbar spines, aged six months old, were used in this study. The surrounding soft tissue and muscle were dissected off the lumbar spine and sacrum, with care being taken to preserve the bone, capsule and spinal ligament. The specimens were mounted with crossed transfixing pins at the twelfth thoracic body superiorly and the sixth lumbar body embedded in a low-melting alloy jig inferiorly. This mounting method left six mobile intervertebral discs and five vertebrae from the
Results
The typical diagram of vertebral displacement versus applied moment on the superior adjacent segment under flexion motion is shown in Fig. 3. The curve pattern demonstrates that the vertebral displacement of the adjacent segment decreased significantly with the increasing flexion moment in the early period. However, the decreasing rate (slope) decreased gradually with increasing flexion moment. The maximal vertebral displacements under conditions of 7200 N mm were collected for analysis. The
Discussion
During the past two decades, transpedicle spinal instrumentation has been widely accepted for the treatment of spinal instability. Pedicle screw fixation guarantees rigid fixation and increases the fusion rate (Niu et al., 1996). Since the advent of spinal instrumentation, some surgeons have reported a shorter time to develop adjacent instability. The risk factors for developing lumbar postfusion adjacent joint instability have not been clearly defined. Older patients have higher baseline
Conclusion
In the current study, the vertebral displacements at the superior adjacent segment (L1–L2) were significantly larger than those of the inferior adjacent segment in all testing conditions. This may cause the greatest amount of abnormal stress on the superior adjacent segment and leads to the acceleration of degeneration in the superior adjacent segment. Besides, the kyphotic lumbar spine caused larger adjacent joint mobility as compared to the lordotic lumbar spine under the conditions of
Acknowledgements
This work was funded by the National Science Council of Taiwan, grant number NSC 90-2314-B-182-047.
No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
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