INTRODUCTION: Sacral screws in long posterior fusion constructs have been implicated in pseudarthrosis related to screw strains and motion. Pedicle screw strains have been quantified previously in longitudinal orientation but these studies have not differentiated the magnitude of these based on the direction of force applied to the spine, nor have they captured resultant strain vectors in all screw bending moments. The objective of this study was to compare the resulting bending moments on the S1 screws during simulated physiologic loading in sagittal and lateral bending and spinal torsion.
METHODS: Six L2‐pelvis specimens were dissected and potted. Four instrumentation states were tested: 1)posterior fixation with pedicle screws from L2‐S1 (PS), 2)iliac screws +PS, 3)AxiaLIF +PS and 4)ALIF +PS. Pure moments were applied at ±7.5 Nm in each anatomic plane and torsion with no compressive preload. The S1 screws were instrumented to directly measure biplanar screw bending with strain gages in two independent half bridge configurations, individually calibrated in Nm. Vector addition of the strain channels was used to calculate the resultant magnitudes of the bending moments during peak applied moments.
RESULTS: The S1 screw bending moment magnitudes were consistently greatest in torsion for all constructs, followed by flexion and extension, and least in lateral bending. AxiaLIF and iliac screws significantly reduced the strains in all motion directions and were statistically equivalent. ALIF was less effective to reduce the S1 bending moments. (cont'd)
DISCUSSION: Long posterior fixation is oriented parallel to the long axis of the spine, which may lead to the incorrect assumption that the most strain is applied in this direction. Typical spine loads are significantly more complex during daily activities. These data suggest that the most strain on the S1 screws occurs during torsion and that iliac bolts and AxiaLIF are similar in reducing S1 bending moments.