SCREW OMISSION AND THE STABILITY OF POSTERIOR PEDICLE SCREW CONSTRUCTS FOR SHORT-SEGMENT STABILIZATION

To determine the net contribution of a spinal construct to stability, and whether extending the construct to another level in situations in which a defective pedicle cannot have a screw inserted, mie performed biomechanical tests in which we evaluated three-, four-, and five-leve l synthetic spinal constructs in which the location and number of pedi cle screws were varied above and below a vertebrectomy defect. We subj ected all constructs to axial, compression, lateral bending, flexion, extension, and torsional forces with the use of an Instron biaxial mac hine. Left-right symmetrical constructs were more stable than asymmetr ical ones. Three-level constructs were statistically stiffer than the longer ones In compression, left bending, and flexion. Torsional stabi lity, however, was greater in the longer constructs. Five-level constr ucts with both end screws in place had greater torsional stiffness tha n when they were missing a screw, In vertebrectomy defects, if four sc rews cannot be placed across it, then the engagement of two screws is indicated, The stability provided by a single screw at a spinal level is minimal, Additional screws augment the purchase of the construct in the bone; however, they do not afford further protection to the defec t.