The effect of spinal destabilization and instrumentation on lumbar intradiscal pressure: an in vitro biomechanical analysis.

STUDY DESIGN: In vitro biomechanical testing was performed in human cadaveric lumbar spines, using pressure needle transducers to analyze the effects of spinal destabilization and instrumentation on lumbar intradiscal pressures. OBJECTIVES: To quantify changes in lumbar intradiscal pressures at three adjacent disc levels under conditions of spinal reconstruction, and to evaluate the possibility of pressure-induced disc pathology secondary to spinal instrumentation. SUMMARY OF BACKGROUND DATA: Lumbar intradiscal pressures under in vivo and in vitro conditions and the use and development of spinal instrumentation have been investigated comprehensively. However, the effects of spinal destabilization and instrumentation on lumbar intradiscal pressure have not been delineated clearly. METHODS: In 11 human cadaveric lumbosacral specimens, specially designed pressure needle transducers quantified intradiscal pressure changes at three adjacent disc levels (L2-L3, proximal; L3-L4, operative; and L4-L5, distal) under four conditions of spinal stability: intact, destabilized, laminar hook and pedicle screw reconstructions. Biomechanical testing was performed under axial compression (0-600 N), anterior flexion (+12.5 degrees) and extension (-12.5 degrees), after which the level of degeneration and disc area (cm2) were quantified. RESULTS: In response to destabilization and instrumentation, proximal disc pressures increased as much as 45%, and operative pressure levels decreased 41-55% (P < 0.05), depending on the instrumentation technique. Linear regression and correlation analyses comparing intradiscal pressure to the grade of disc degeneration were not significant (r = 0.24). CONCLUSIONS: Changes in segmental intradiscal pressure levels occur in response to spinal destabilization and instrumentation (P < 0.05). Intradiscal cyclic pressure differentials drive the metabolic production and exchange of disc substances. Conditions of high or low disc pressure secondary to spinal instrumentation may serve as the impetus for altered metabolic exchange and predispose operative and adjacent levels to disc pathology.

The role of spinal instrumentation in augmenting lumbar posterolateral fusion.

STUDY DESIGN: Using a sheep model, clinically practical posterolateral intertransverse process fusion was successfully achieved and biomechanically tested to determine the load-sharing environment provided by spinal instrumentation and posterolateral fusion mass following solid arthrodesis. OBJECTIVES: To quantify the in vivo load-sharing capacity of spinal instrumentation on augmenting the posterolateral intertransverse fusion. The hypothesis was that transpedicular screw fixation maintains the biomechanical contribution to the posterolateral fusion stability even after successful arthrodesis because of its providing anterior and middle column support. SUMMARY OF BACKGROUND DATA: Although many previous studies have documented the biological and biomechanical advantages of posterolateral fusion, it is known that posterolateral fusion without spinal instrumentation allowed significant remaining motion at the fused segment even after the solid arthrodesis. Whether spinal instrumentation, especially transpedicular screw fixation, augments in vivo posterolateral fusion stability after solid arthrodesis has not been previously investigated. METHODS: Radiographic, macroscopic, and biomechanical analyses of a posterolateral intertransverse process fusion model were performed on 18 sheep at 4 months postoperatively. The load-sharing contribution of the spinal instrumentation was calculated based on the stability with or without spinal instrumentation tested in five loading modalities. Histomorphometry of the vertebral body spanned by spinal instrumentation provided the information regarding the biological effect of the load-sharing capacity of spinal instrumentation on bone remodeling. RESULTS: All sheep who received posterolateral intertransverse process fusion demonstrated successful solid arthrodesis and high biomechanical quality of the posterolateral fusion mass when compared to previous posterolateral fusion models. The significant difference in stiffness between fixation and subsequent fixation removal was observed in flexion, despite maintaining high lateral bending stiffness equivalent to the fixation (with instrumentation) level. This significant load-sharing contribution of spinal instrumentation detected in flexion corresponded to 27% when compared to the fixation level. The qualitative and quantitative bone histology showed 64% of the volumetric density of bone in the fixation group when compared to that of the sham group as well as narrow trabeculae and reduced connection of trabeculae. CONCLUSIONS: The continuance in support offered by transpedicular screw fixation was assured in vivo after the solid posterolateral intertransverse process fusion. This was clearly demonstrated under eccentric loads in a sagittal plane, suggesting that transpedicular screw fixation was able to provide anterior and middle column support and resist eccentric loads.

The effects of spinal fixation and destabilization on the biomechanical and histologic properties of spinal ligaments. An in vivo study.

STUDY DESIGN: An animal study was conducted to assess whether different surgical procedures of spinal fixation and destabilization would influence the biomechanics and histology of lumbar spinal ligaments. OBJECTIVES: To investigate the effects of spinal fixation and destabilization as well as surgical intervention itself on the biomechanical and histologic properties of lumbar spinal ligaments. SUMMARY OF BACKGROUND DATA: Although several investigators have reported normal biomechanical properties of different spinal ligaments, there have been no studies in which changes in spinal ligament properties, secondary to the altered biomechanical environment provided by such surgical procedures as spinal fixation and destabilization, have been investigated. METHODS: Thirty-six mature sheep were divided into four groups: Group I: nonsurgical control: Group II: sham operation consisting of bilateral posterolateral exposure at L4-L5; Group III: spinal fixation using transpedicular screws and plates and bilateral posterolateral bone graft at L4-L5; and Group IV: spinal destabilization consisting of bilateral facetectomy and anterior discectomy at L4-L5. Four months after surgery, the biomechanical analysis included destructive tensile testing of four different bone-ligament-bone complexes at the operative and proximal adjacent levels: anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, and supraspinous and interspinous ligaments combined. Histomorphometric analyses of the vertebral body and spinal ligaments were performed histomorphometrically. RESULTS: Biomechanical analysis results demonstrated remarkable changes in the structural and mechanical ligament properties at the operative level. The fixation group's ligaments showed consistent decreases in the ultimate load and elastic modulus compared with those parameters in the control group (P < 0.05). Histologically, the fixation group's ligamentum flavum showed marked vacuolation in the ligament substance, whereas the interspinous ligament exhibited significant insertion changes compared with little change in substance. In all eight sheep in Group IV, unintentional bilateral facet fusions were obtained; and in all eight animals in Group III with pedicle instrumentation and posterolateral fusion, solid arthrodesis was exhibited. This allowed a distinction to be made between the stress-shielding effect of spinal instrumentation and arthrodesis (Group III) versus spinal fusion alone (Group IV) on both spinal ligament and vertebral body. Group II (sham) had a significant decrease in supraspinous and interspinous ligaments, but nonsignificant decreases in the stress-shielding effect of 10-12% in other ligaments. CONCLUSIONS: Posterior spinal instrumentation and fusion led to decreased biomechanical properties of the ligamentum flavum, posterior longitudinal ligament, and interspinous and supraspinous ligaments. The stress-shielding effects were ligament dependent and were most pronounced on the posterior side. The altered biomechanical environment produced by spinal fixation, surgical intervention itself, or nonphysiologic mobilization can affect the ligamentous properties in vivo, possibly serving as the impetus for low back pain.

Video-assisted thoracoscopic surgery versus open thoracotomy for anterior thoracic spinal fusion. A comparative radiographic, biomechanical, and histologic analysis in a sheep model.

STUDY DESIGN: In this in vivo investigation, a sheep model was used to compare the efficacy of a video-assisted thoracoscopic approach and a traditional thoracotomy in promoting a successful interbody spinal arthrodesis. OBJECTIVES: To compare the incidence of successful anterior spinal arthrodesis among three stabilization techniques-iliac crest, Bagby and Kuslich device, and Z-plate--performed using a video-assisted thoracoscopic approach and conventional open thoracotomy approaches. SUMMARY OF BACKGROUND DATA: A clinical outcome study on open versus endoscopic spinal fusion is not yet available. Moreover, no basic scientific investigations have been conducted to determine whether the success of an endoscopic arthrodesis is comparable to that of a conventional open procedure. METHODS: Fourteen Western Crossbred sheep underwent three identical destabilization procedures at T5-T6, T7-T8, and T9-T10, in which the anterior and middle osteoligamentous columns of the spine were resected, followed by three randomized reconstruction procedures using iliac autograft alone, and Z-plate stabilization with iliac autograft. In seven sheep, the entire destabilization-reconstruction procedure was performed using a video-assisted thoracoscopic surgical approach. In the remaining seven, the procedure was performed by conventional open thoracotomy. RESULTS: Histomorphometric and biomechanical evaluation demonstrated that the video-assisted thoracoscopic approach and open thoracotomy arthrodesis had comparable bone formation and biomechanical properties (P > 0.05). However, the Z-plate fusions, as a group, demonstrated increased flexion-extension stiffness properties and trabecular bone formation compared with the autograft and Bagby and Kuslich device fusions (P < 0.05). CONCLUSIONS: Thoracic interbody spinal fusions performed by thoracoscopy have demonstrated histologic, biomechanical, and radiographic equivalence to those performed by a thoracotomy approach. However, in the endoscopy group, intraoperative complications causing longer operative times, higher estimated blood loss, and increased animal morbidity indicated a substantial learning curve associated with the adoption of this surgical technique.