Evaluation of the biomechanical stability of lumbar dynamicly fixed by socket track pedicle screws

Abstract

Objective To investigate the stability of lumbar single-level dynamicly fixed by socket track pedicle screws after decompression. Methods Socket track pedicle screws are made of titanium-alloy, swing like universal screw within the scope of 18° in the sagittal plane and 8° in the horizontal plane. Motion constraints were due to the hole which the screw goes through. Six pig spines (L1-5) were tested by applying a pure moment of 4.018 Nm in 3 directions of loading. The range of motion (ROM) of the following groups underwent different surgical procedures was measured: (1) intact; (2) dynamic fixation 1 (decompression by total laminectomy and instrumentation with socket track pedicle screws); (3) dynamic fixation 2 (decompression by total laminectomy and bilateral facetectomy while instrumentation with socket track pedicle screws); (4) instability resulted from decompression (decompression by total laminectomy plus bilateral facetectomy without any rod); (5) hybrid fixation (decompression by total laminectomy and bilateral facetectomy while instrumentation with 2 socket track pedicle screws connected with 2 unilateral pedicle screws ). Results Compared with the intact specimens (in flexion, extension, lateral bending and rotation, corresponding angular displacements were 1.84°±0.75°, 1.55°±0.34°, 2.2°±1.07°, 2.06°±0.76° respectively), the ROM of segment L3, 4 of each group were significantly restricted except for axial rotation. In flexion, extension and lateral bending, the ROM was reduced to 0.71°±0.39°, 0.46°±0.18°, 0.85°±0.66° in dynamic fixation 1 group; 0.79°±0.43°, 0.71°±0.20°, 1.17°±0.48° in dynamic fixation 2 group and 0.63°±0.50°, 0.37°±0.19°, 0.54°±0.60° in hybrid fixation group. The rotation ROM of dynamic fixation 1 group and hybrid fixation group were decreased to 1.50°±0.64° and 1.22°±0.36°, whereas the ROM of dynamic fixation 2 had a non-significant reduction to 1.85°±0.67°. The extension and rotation ROM of dynamic fixation 2 increased significantly comparing to dynamic fixation 1 group; the extension, lateral bending and axial rotation ROM of hybrid fixation group were reduced comparing to the state of dynamic fixation 2 group. Conclusion The dynamic instrumentation of single-level lumbar spine with socket track pedicle screws could offer stability in all directions. The flexion, extension, lateral bending and rotation ROM of spine with dynamic instrumentation after total laminectomy and bilateral facetectomy could be reduced. Key words: Lumbar vertebrae; Bone screws; Biomechanics