Biomechanical Comparison of Lumbosacral Fixation Techniques in Calf Spine Model

Information provided by

· (a, b - Depuy Acromed) *University of Miami Spine Institute. Miami, FL, USA; **Orthopaedic Biomechanics Laboratory, Union Memorial Hospital. Baltimore, MD, USA

PURPOSE: The purpose of this study was to compare the biomechanical properties of five different lumbosacral fixation constructs, and determine the benefit of adding supplementary fixation to S1 screws.

MATERIALS AND METHODS: 6 fresh frozen calf spines were prepared and tested for each construct. The five constructs tested were: S1 screw alone, S1 screw and S2 proximally directed screw, S1 screw and S2 distally directed screw, S1 screw and intrasacral rod, S1 screw and iliac screw. Biomechanical analysis was performed utilizing a servohydraulic MTS 858 bionix testing device configured with the OptoTrak 3020-motion analysis system and data acquisition software.

RESULTS: S1 screw strain tested in flexion/extension was significantly reduced by the addition of any second point of distal fixation. There was no significant difference between any of the different sacral fixation constructs. Iliac screws reduced the S1 screw strain more effectively than a second fixation point in the sacrum (p ‹ .05). In destructive testing under flexion loading, only the iliac screws statistically increased the load to failure (p=.005).

DISCUSSION/CONCLUSIONS: 1) This study demonstrates the effectiveness of adding a second fixation point distal to the S1 screw in reducing S1 screw strain. Reduction in screw strain may correlate with reduction in screw loosening in clinical use. The configuration of the second fixation point in the sacrum does not seem to be important in reducing strain. Iliac fixation is more effective than secondary sacral fixation points, but may not be necessary in all clinical situations. 2) Only iliac fixation effectively increased the load to failure under catastrophic loading conditions. Supplementary sacral fixation failed to significantly protect against catastrophic failure. These findings support the clinical observation that iliac fixation is least likely to fail in high risk, long fusions. 3) Whether testing range of motion, or screw strain, or load to failure, no benefit could be demonstrated for intrasacral rod placement when compared with other supplementary sacral fixation techniques. Intrasacral rod placement was equal to a second sacral screw in reducing S1 screw strain during flexion/extension loading. It was not as effective as iliac fixation in reducing screw strain or preventing catastrophic failure. 4) When choosing fixation methods in long fusions to the sacrum, this study supports the use of iliac fixation as the method least likely to loosen or pull out. A second point of sacral fixation also offers biomechanical advantages when compared with S1 fixation alone, and may be an appropriate choice in less 'high risk' fusions to the sacrum.