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.