Biomechanical Comparison of Lumbosacral Fixation Techniques in a Calf Spine Model

Study Design. In vitro biomechanical testing of the strength and stability of lumbosacral fixation constructs. Objectives. The purpose of this study was to quantify and compare the biomechanical properties of five different lumbosacral fixation constructs and determine the benefit of adding supplementary fixation to S1 screws. Summary of Background Data. Extension of long fusions to the sacrum remains a difficult clinical challenge. Only a limited number of biomechanical studies have evaluated the different fixation methods available, and none has included both nondestructive and load to failure testing of these fixation methods. Methods. Six fresh-frozen calf spines were prepared and tested for each construct. The five constructs tested included the following: S1 screws alone, S1 screws and S2 proximally directed screws, S1 screws and S2 distally directed screws, S1 screws and intrasacral rods, and S1 screws and iliac screws. Nondestructive, multidirectional flexibility analyses included four loading methods followed by a destructive flexural load to failure. Lumbosacral peak range of motion (millimeters or degrees) and ultimate failure load (Nm) of the five reconstruction techniques were statistically compared using a one-way analysis of variance combined with a Student-Newman-Keuls post hoc test. 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 (P > 0.05). In axial compression, only the addition of iliac screws significantly reduced S1 screw strain. In destructive testing under flexion loading, only iliac screws statistically increased the load at failure (P = 0.005). Conclusion. This study demonstrates the effectiveness of adding a second fixation point distal to the S1 screws in reducing S1 screw strain. Iliac fixation is more effective than secondary sacral fixation points but may not be necessary in all clinical situations. 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. Whether testing range of motion, 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. 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.