SHORT-SEGMENT PEDICLE INSTRUMENTATION - BIOMECHANICAL ANALYSIS OF SUPPLEMENTAL HOOK FIXATION

Study Design. This biomechanical study of fractures in cadaver vertebr ae used specially designed pedicle screws to determine screw strains d uring loading of two different fixation constructs. Objectives. The au thors determined the relative benefit of adding offset sublaminar hook s to standard pedicle screw constructs to reduce screw bending moments and prevent fixation failure and sagittal collapse. Summary of Backgr ound Data. Clinical studies have demonstrated a high incidence of earl y screw failure in short-segment pedicle instrumentation constructs us ed to treat unstable burst fractures. Strategies to prevent early cons truct failure include longer constructs, anterior strut graft reconstr uction, and use of offset sublaminar hooks at the ends of standard sho rt-segment pedicle instrumentation constructs. Methods. Human cadaver spines with an L1 burst fracture were instrumented with a standard sho rt-segment pedicle instrumentation construct using specially instrumen ted pedicle screws. Mechanical testing was carried out in flexion, ext ension, side bending, and torsion, and stiffness and screw bending mom ents were recorded. Offset hooks were applied initially, then removed and testing repeated. Stiffness data were compared to intact and postf racture results, and between augmented and standard constructs. Result s. Addition of offset laminar hooks, supralaminar at T11 and infralami nar at L2, to standard short-segment pedicle instrumentation construct s increased stiffness in flexion by 268%, in extension by 223%, in sid e bending by 161%, and in torsion by 155% (all were significant except torsion). Sublaminar hooks also reduced pedicle screw bending moments to roughly 50% of standard in both flexion and extension (P<0.05). Co nclusions, Supplemental offset hooks significantly increase construct stiffness without sacrificing principles of short-segment pedicle inst rumentation, and absorb some part of the construct strain, thereby red ucing pedicle screw bending moments and the likelihood of postyield de formation and clinical failure.