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 biomechanical stabilization of a rigid versus a dynamic posterior fixation device. OBJECTIVES To compare flexibility of a dynamic pedicle screw fixation device 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 contain the potential to enhance load sharing and optimize fusion potential while maintaining stability similar to that of rigid systems. METHODS Load-displacement tests were performed on intact and stabilized calf spines for the dynamic and rigid devices. Stability across a destabilized 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 inserted, and the construct then was re-tested. Axial pull-out tests were performed to assess the likely effects of pedicle screw replacement on the load-displacement data. RESULTS Results indicated a 65% reduction in motion in flexion-extension and a 90% reduction in lateral bending across the destabilized level for both devices, compared with intact spine values. Reduction in axial rotation motion was much 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 magnitudes in flexion, extension, and lateral bending. In axial rotation, the devices only could restore stability to levels similar to those in an intact spine. The dynamic device offers a design that may enhance load sharing without sacrificing construct stability.