Stability analysis of an enhanced load sharing posterior fixation device and its equivalent conventional device in a calf spine model

Study Design. An in vitro test of calf spine lumbar segments to compare bio mechanical stabilization of a rigid versus a dynamic posterior fixation dev ice. Objectives. To compare flexibility of a dynamic pedicle screw fixation devi ce with an equivalent rigid device. Summary of Background Data. Dynamic pedicle screw device studies are not as prevalent in the literature as studies of rigid devices. These devices con tain the potential to enhance load sharing and optimize fusion potential wh ile maintaining stability similar to that of rigid systems. Methods. Load-displacement tests were performed on intact and stabilized ca lf spines for the dynamic and rigid devices, Stability across a destabilize d L3-L4 segment was restored by insertion of either a 6 mm x 40 mm dynamic or rigid pedicle screw fixation device across the L2-L4 segment. The screws then were removed, 7 mm x 45 mm pedicle screws of the opposite type were i nserted, and the construct then was re-tested. Axial pull-out tests were pe rformed to assess the likely effects of pedicle screw replacement on the lo ad-displacement data. Results. Results indicated a 65% reduction in motion in flexion-extension a nd a 90% reduction in lateral bending across the destabilized level for bot h devices, compared with intact spine values. Reduction in axial rotation m otion was muck smaller than in other modes. Axial pull-out tests showed no weakening of the bone-screw interface. Conclusions. Both devices provided significant stability of similar magnitu des in flexion, extension, and lateral bending. In axial rotation, the devi ces only could restore stability to levels similar to those in an intact sp ine. The dynamic device offers a design that may enhance load sharing witho ut sacrificing construct stability.