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

O'Brien, Joseph R. MD, MPH*; Yu, Warren MD*; Kaufman, Brian E. MD*; Bucklen, Brandon PhD; Salloum, Kanaan BS; Khalil, Saif PhD; Gudipally, Manasa MS

doi: 10.1097/BRS.0b013e31829e17ff

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

S2 alar-iliac screws applied with bicortical purchase in the ilium (quad-cortical screws) may allow for shorter screw lengths with equivalent biomechanics. Cadaveric spines with differing lumbopelvic fixation are compared with a construct ending in quad-cortical screws. Results show quad-cortical screws are as effective in limiting range of motion after long-segment fixation.

*George Washington University MFA, Washington, DC; and

Globus Medical, Inc., Valley Forge Business Center, Audubon, PA.

Address correspondence and reprint requests to Joseph R. O'Brien, MD MPH, The George Washington Medical Faculty Associates, 2150 Pennsylvania Ave, NW, 7th Floor, Washington, DC 20037; E-mail:

Acknowledgment date: March 7, 2013. First revision date: April 15, 2013. Second revision date: May 7, 2013. Third revision date: May 22, 2013. Acceptance date: May 26, 2013.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Globus Medical, Inc. funds were received to support this work.

Relevant financial activities outside the submitted work: consultancy, stock/stock options, royalties and patents.

Authors KS, BB, SK, and MG are employees of Globus Medical Inc.

© 2013 by Lippincott Williams & Wilkins