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 flexion-extension, 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 flexion-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-five-millimeter 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.

Level Of Evidence: N/A.