Biomechanical comparison of spondylolysis fixation techniques

Study Design. A load-controlled biomechanical analysis of flexion, extension, and torsional stiffness in instrumented calf spines. Objectives. To compare biomechanically the performance of various fixation techniques for the repair of spondylolytic defects in the pars interarticularis. Summary of Background Data. Several techniques have been developed to stabilize a spondylolytic defect in the lumbar spine. There are, however, no comprehensive biomechanical studies in which these techniques are compared. Methods. Nine fresh-frozen and thawed calf cadaveric lumbar L2-L6 spines were used for mechanical testing. Scott's technique, Buck's technique (screw fixation in the lamina across the defects), modified Scott's technique (wire loops around cortical screws placed into both pedicles and tightened under the spinous process), and screw-rod-hook fixation were applied on the calf lumbar spines in which bilateral spondylolytic defects were created in the L4 vertebra. Motion across the defects for each direction of loading in flexion, extension, and rotation was measured using extensometers. The intervertebral rotations and the strain at the site of the spondylolytic defect were computed from the acquired load-displacement data. Results. Each fixation technique significantly increased stiffness and returned the intervertebral rotational stiffness to nearly intact levels. Displacement across the defect under flexion loading was significantly suppressed by each instrumentation technique, but the least motion (P < 0.05) was allowed with the screw-rod-hook fixation or Buck's technique. Conclusions. All four fixation techniques restored the intervertebral rotational displacements under flexion and torsional loading to the intact condition. The screw-rod-hook fixation allowed the least amount of motion across the defect during flexion.