INTRODUCTION: Even though superior facet joint violation could have implications for enhancing the facet degeneration and adjacent segment degeneration after lumbar fusion, the biomechanical influence still remain uncertain. Therefore, the purpose of this study is to investigate the association between the position of inserted pedicle screw and corresponding facet contact force or intra‐discal pressure and the effect of removal of screws, using finite element model.
METHODS: In the validated intact lumbar FE model (L2‐5), four scenarios of L4‐5 posterolateral fusion were simulated, which consisted of the model with the violated both L3/4 facet joints by pedicle screws (FV), the model with L3/4 facet joint preservation (FP), and models with removal of pedicle screws in former two models (rFV, rFP). Under extension and torsion moment, the intradiscal pressure and facet contact force at L3/4 segment were analyzed in each scenario, and compared with intact model.
RESULTS: The FV model led to a 14.3% increase of intradiscal pressure and 100% increase of facet contact force at L3/4 segment under extension, while the change of intradiscal pressure and facet contact force amounted to a 5.2 % and 7.7% increase under extension in the FP model. In each rFV and rFP model, the amount of an increase of intradiscal pressure reduced to 6.5% and 0% increase, and the amount of an increase of facet contact force reduced to 34.7% and 3.9% increase under extension. Under torsion, the change of intradiscal pressure and facet contact force had similar trends to extension moment.
DISCUSSION: This biomechanical study elicited that pedicle screws which violated corresponding facet joints could lead to the increase of stress at superior adjacent segments, compared to in‐violated pedicle screws. Even after bony fusion was accomplished, removal of violated pedicle screws had a beneficial effect on decrease of overstress at adjacent disc and facet joint.