Biomechanical Evaluation of S2 Alar-Iliac Screws Effect of Length and Quad-Cortical Purchase as Compared With Iliac Fixation

Study Design. A biomechanical study conducted on cadaveric specimens. Objective. (1) To compare the biomechanical strength of the S2 alar-iliac (S2AI) screw to traditional iliac fixation and (2) to examine the effect of length and trajectory on the S2AI screw. Summary of Background Data. A recent technique to attain spinal fixation distal to S1 pedicle screws is the S2AI screw using either an open or a percutaneous approach with an altered S2 alar screw trajectory to obtain purchase in the ilium. A novel modification of the S2AI screw is placement with bicortical purchase in the ilium (quad-cortical screw). This may allow for a shorter-length screw with equivalent biomechanics. Methods. Seven human cadaveric spines (L2-Pelvis) were fixed at L2 proximally and the pubis distally. Pedicle screws were placed from L3-S1 with S2AI screw lengths of 65-mm, 80-mm, or 90-mm iliac screws. S2AI screws were tested with and without quad-cortical purchase. Each specimen was tested on the 6 degrees of freedom spine simulator. A load control protocol with an unconstrained pure moment of 10 Nm was used in fl exionextension, lateral bending, and axial rotation for a total of 3 load/ unload cycles. The range of motion was normalized to the intact cadaveric spine (100%). Results. All the instrumented constructs significantly reduced range of motion compared with the intact spine. The L3-S1 construct was statistically significantly less stable than all instrumented constructs in fl exion-extension. There was statistically no significant difference between the S2AI screws of all lengths and the iliac screw constructs with offset connectors. Conclusion. S2AI screws are biomechanically as stable as the test constructs using iliac screws in all loading modes. Sixty-fivemillimeter S2AI screws were biomechanically equivalent to 90-mm iliac screws and 80-mm S2AI screws. Quad-cortical purchase did not statistically significantly improve the biomechanical strength of S2AI screws.