A biomechanical study on the direct repair of spondylolysis by different techniques of fixation.

To compare the immediate stability of different fixation techniques for the direct repair of spondylolytic defects in the pars interarticularis.Eight fresh-frozen and thawed calf cadaveric lumbar spines were used for mechanical testing. A modified Scott, screw-rod-hook, screw-rod construct fixation and Buck techniques were applied to calf lumbar spines in which bilateral spondylolytic defects had been created in the L(4) vertebra. The multidirectional flexibility of each specimen was measured under flexion/extension, right/left lateral bending, and right/left axial rotation.After creating a pars interarticularis defects at L(4), the range of movement (ROM) at both the L(3)-L(4) and L(4)-L(5) levels were increased under all loading conditions. Each fixation technique significantly increased stability and almost restored intervertebral rotation mobility to normal levels. Under flexion-extension, the screw-rod-hook and screw-rod construct techniques of fixation provided more rotational stability than did the other two techniques (P < 0.05). The screw-rod-hook, screw-rod construct and Buck techniques also provided more flexion/extension stability than the modified Scott technique.The current study has shown that the ROM at the involved and upper adjacent level of spondylolysis is greater than in the intact spine. All four fixation techniques accorded with spinal biomechanical principles and restored intervertebral rotation displacements under flexion, rotation and bending loads to the intact condition. The screw-rod-hook and screw-rod construct fixation techniques provided more stability than did the modified Scott and Buck techniques, and are therefore good prospects for direct repair.