Pelvic fixation: Biomechanics and complications

When extending a fusion construct to the sacrum in spinal deformity, there has long been difficulty in obtaining bony fusion at the L5-S1 level. This is due to a number of factors, including the altered morphology of the sacrum as compared to the lumbar vertebrae, adequate bone stock, and large cantilever forces at the lumbosacral junction, among others. This problem can be partially rectified by extending the construct to the pelvis, but even this does not result in a 100% fusion rate at the lumbosacral junction. The techniques currently used to fuse the spine to the pelvis primarily consist of iliac screws and S2 alar-iliac screws, but these techniques are not without complications. Here we review the biomechanics of pelvic fixation, techniques of these two screw types, and the complications associated with each.