Biomechanical Comparison of Iliac Screws Versus Femoral Ring Allograft on Sacral Screw Strain

Purpose: This in-vitro biomechanical study was undertaken to evaluate sacral screw strain as the length of a scoliosis construct is increased in the lumbar spine-historically there is a high failure rate of posterior long constructs ending with S1 pedicle screws. Our main objective was to determine the biomechanical advantage of anterior load sharing with femoral ring allograft or supplementing the S1 pedicle screws with screws in the iliac crest.

Methods: Seven human cadavaric lumbosacral spines including the pelvis were biomechanically evaluated following six transpedicular screw reconstruction conditions - 1) Intact spine, 2) L5-S1 pedicle fixation, 3) L4-S1, 4) L3-S1, 5) L2-S1 and 6) L1-S1 - with and without bilateral iliac screws. As a final series, a femoral ring allograft was added at L5-S1 and the testing repeated without the use of iliac screws. Six-degree-of-freedom stability testing included pure unconstrained moments (±8Nm) in axial rotation, flexion / extension and lateral bending, with quantification of sacral screw strain (ue) and lumbosacral (L5-S1) range of motion (ROM). RESULTS: Anterior Flexion: With increasing levels of pedicle screw fixation alone, S1 screw strain significantly increased when scoliosis constructs extended up to the L2 and L1 levels (p<0 .05). The addition of iliac screws were not effective in shorter constructs at L5 or L4 reducing S1 screw strain, but demonstrated significance compared to pedicle alone when the extended cephalad L3 (p<0.05). use interbody femoral allograft L5-S1 significantly reduced strain fixation reached L2, was as sacral overall (Figure 1). Extension: Similar findings noted extension with increasing long scoliosis extending L1 Moreover, above In contrast, ring reconstructions less strain.

Discussion: This study has clinical relevance in treating patients with adult lumbar scoliosis, isthmic spondylolisthesis with supra-adjacent disc degeneration and multilevel degerneative disc disease requiring fusion to the sacrum. For long scoliosis fusions above L3, the sacral screws should be protected with supplementation of either iliac screws or anterior column support at L5-S1. Of the two, iliac screws seem to be more valuable. For isthmic spondylolisthesis and degenerative conditions requiring instrumented fusions extending to only L4 or L5, it is not biomechanically justified to require either iliac screw fixation (instrumenting across the sacroiliac joint) and it is not required to add anterior femoral ring allograft provided the S1 screws obtain bicortical purchase.