S1 screw bending moment with posterior spinal instrumentation across the lumbosacral junction after unilateral iliac crest harvest

Study Design. A biomechanical study comparing fixation across the lumbosacral junction. Objectives. To determine which long posterior construct across the lumbosacral junction produces the least bending moment on the S1 screw when only one ilium is available for fixation. Summary of Background Data. Recent in vitro studies have demonstrated the benefit of anterior support and fixation into the ilium when instrumenting a long posterior construct across the lumbosacral junction. Methods. Four L2-sacrum constructs were tested on six synthetic models of the lumbar spine and pelvis simulating that the right ilium had been harvested. Construct 1: L2-S1 bilateral screws. Construct 2: L2-S1 + left iliac bolt. Construct 3: L2-S1 + left iliac bolt + right S2 screw. Construct 4: L2-S1 + bilateral S2 screws. The four constructs were then retested with an anterior L5-S1 strut. A flexion-extension moment was applied across each construct, and the moment at the left and right S1 pedicle screw was measured with internal strain gauges. Results. Iliac bolt fixation was found to significantly decrease the flexion- extension moment on the ipsilateral S1 screw by 70% and the contralateral screw by 26%. An anterior L5-S1 strut significantly decreased the S1 screw flexion- extension moment by 33%. Anterior support at L5-S1 provided no statistical decrease in the flexion-extension moment when bilateral posterior fixation beyond S1 was present with either a unilateral iliac bolt and contralateral S2 screw, or bilateral S2 screws. Conclusions. There is a significant decrease in the flexion-extension moment on the S1 screw when extending long posterior constructs to either the ilium or S2 sacral screw. There is no biomechanical advantage of the iliac bolt over the S2 screw in decreasing the moment on the S1 screw in flexion and extension. Adding anterior support to long posterior constructs significantly decreases the moment on the S1 screw. Adding distal posterior fixation to either the ilium or S2 decreases the moment on S1 screws more than adding anterior support. Further, adding anterior support when bilateral distal fixation past S1 is already present does not significantly decrease the moment on the S1 screws in flexion and extension.