S1 SCREW BENDING MOMENT WITH POSTERIOR SPINAL INSTRUMENTATION ACROSS THE LUMBOSACRAL JUNCTION AFTER UNILATERAL ILIAC CREST HARVEST

Gary M. Alegre, MD,
Munish C. Gupta, MD,
Brian K. Bay, PhD,
Tait S. Smith, MS,
Justin E. Laubach, BS
* · (a – DePuy, b – DePuy)
University of California, Davis, California, USA

PURPOSE:
To determine which long construct across the lumbosacral junction produced the least bending moment on the S1 screw when only one ilium is available for fixation.

METHODS:
Four L2–sacrum constructs were tested on 6 sawbone models 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 anterior support at L5–S1. 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:
The moment on the left S1 screw decreased by 70% with the addition of an iliac bolt (p=0.000). The addition of a right S2 screw did not significantly decrease the moment on the left S1 screw when it was already supported with a left iliac bolt (p=0.434). Anterior support decreased the moment on the left S1 screw by 33% in flexion/extension (p=0.01). The moment on the left S1 screw with an iliac bolt was 56% less than the moment on the S1 screw with anterior support alone (p=0.003). The flexion/extension moment on the right S1 screw decreased by 26% with the addition of a left iliac bolt (p=0.000). The iliac bolt and right S2 screw decreased the right S1 screw flexion/extension moment by 73% (p=0.000). The moment on the right S1 screw decreased by 80% when using bilateral S2 screws Anterior support decreased the right S1 screw flexion/extension bending moment by 33% (p=0.002). There was no significant difference in the bending moment on the right S1 screw between adding a left iliac bolt or anterior support (p=0.35).

DISCUSSION:
This study is unique in the use of internal strain gauges at the S1 pedicle screw to compare constructs across the lumbosacral junction. This technique allows one to measure the moment directly in the pedicle screw at the location where these screws are known to fail. 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 screw over the S2 screw in decreasing the moment on the S1 screw in flexion and extension. Adding anterior support to L2–S1 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, when distal fixation past S1 is already present, adding anterior support does not significantly decrease the moment on S1 in flexion and extension

* If noted, the author indicates something of value received. The codes are identified as: a research or institutional support, b–miscellaneous funding, c–royalties, d–stock options, e–consultant.