Biomechanical evaluation of contemporary posterior spinal internal fixation configurations in an unstable burst-fracture calf spine model: special references of hook configurations and pedicle screws.

STUDY DESIGN: This study attempts to determine the most biomechanically rigid posterior spinal instrumentation configuration in a burst-fracture calf spine model.

OBJECTIVES: To compare the biomechanical stability of contemporary posterior spinal instrumentation in various hook and screw configurations in an unstable calf spine model.

SUMMARY OF BACKGROUND DATA: Burst-fractures are relatively common injuries seen in the setting of spinal trauma. The use of posterior-only configurations in the treatment of this deformity has become a much more popular approach because of the relative ease of applying the instrumentation.

METHODS: Fresh frozen in vitro study using 10 calf spines involving the T11-S1 vertebral segments. Pure moment forces including flexion, extension, axial rotation, and lateral bending were applied to the top of the spinal column at T11. Testing was first performed on all intact specimens. A corpectomy was then performed at L2. Testing was then repeated on each of the ten specimens after internal fixation with different posterior spinal configurations using ISOLA instrumentation (DePuy AcroMed Inc., Raynham, MA).

RESULTS: With regards to flexion-extension and lateral bending, all configurations except for distraction hook-rod construct provided stability greater than the intact spine. The distraction hook-rod configuration failed to control extension (P > 0.05) above the intact specimen. All pedicle screw constructs were more rigid than the hook-rod constructs in axial rotation at the level of injury (P < 0.001).

CONCLUSIONS: The motion segment at the corpectomy site is adequately stabilized by contemporary spinal internal fixation configurations tested except for the distraction-hook stabilization. Axial rotation is generally poorly controlled by posterior-only internal fixation. Pedicle screw instrumentation was the most rigid compared with other forms of stabilization in stabilizing a burst-corpectomy defect. Based on this study, pedicle screw configurations are preferred over hook-rod strategies in the posterior stabilization of a burst-corpectomy anterior defect. Among hook-rod configurations, the distraction hook-rod strategy provided the least stability.