Design and performance of spinal fixation pedicle screw system.

Pedicle screw-rod bilateral constructions are extensively used in spinal fixation. In this study, the common cause for failure of bilateral constructions has been determined to be the high stress concentration at the rod-setscrew interface. In order to overcome this problem, a design modification has been made by using a supplementary part (shoe) between rod and setscrew. Performance comparison of the conventional design and modified design has been done by conducting static tests. Design modification has resulted in 11%, 27%, 42% and 31% improvements in axial gripping capacity, torsional gripping capacity, flexion/extension resistance and subassembly compression strength, respectively. The most outstanding achievement has been obtained in the fatigue life, which was extended by almost three times.