Biomechanical effect of the correction on the anchoring strength of de-orbiting S1 bicortical pedicle screw — An in-vitro investigation in normal and osteoporotic conditions

• The de-orbiting S1 bicortical pedicle screw is biomechanically unclear.
• The effect of the correction procedure on the de-orbiting S1 screw was investigated.
• For normal bone quality, correction could restore the comparable stability.
• For osteoporotic bone quality, correction could not prevent the loss of stability.
• Additional augmentation fixations are needed in osteoporotic condition.

BackgroundThe proper anchoring zone at the anterior sacral cortex is vital for the stability of S1 bicortical pedicle screw. During implantation, de-orbiting S1 bicortical pedicle screw is encountered occasionally and immediate correction is biomechanically required, especially in osteoporotic condition. Nevertheless, the biomechanical changes of the de-orbiting S1 bicortical pedicle screw after correction in different bone qualities remain unclear. The purpose of this study was thus to evaluate the biomechanical changes of the de-orbiting S1 bicortical pedicle screw after correction.Methods18 fresh-frozen human sacra were divided into the normal and osteoporotic groups. The control and de-orbiting S1 bicortical pedicle screws were established randomly on either side of the sacrum. Following fatigue loading test, the subsidence displacement and axial pull-out strength were measured and analyzed.FindingsIn normal group, the subsidence displacement of the de-orbiting screw was increased significantly by comparison with the control one (median 0.37 vs. 0.36 mm) (P = 0.009); but no significant difference of the maximum pull-out strength between the two screws (median 359 vs. 353 N) (P = 0.262) was observed. In osteoporotic group, when compared with the control screw, the subsidence of the de-orbiting screw increased significantly (median 1.14 vs. 0.74 mm) (P = 0.012) and the maximum pull-out strength decreased obviously (median 151 vs. 252 N) (P = 0.012).InterpretationFor normal bone quality, comparable stability could be achieved in the de-orbiting S1 bicortical pedicle screw after correction. Nevertheless, in osteoporotic condition, the correction procedure could not effectively prevent the loss of anchoring strength, and hence additional immediate augmentation fixation should be considered.