Biomechanical Comparison of 2 Different Pedicle Screw Systems During the Surgical Correction of Adult Spinal Deformities.

STUDY DESIGN A biomechanical spine model was used to evaluate the impact of screw design on screw-vertebra interface loading during simulated surgical corrections of adult scoliosis. OBJECTIVES To evaluate differences in screw-vertebra interface forces during adult scoliosis correction between favored angle (FA) screws with extension tabs and standard polyaxial screws while varying deformity severity and curve rigidity. SUMMARY OF BACKGROUND DATA Pedicle screws enable surgeons to safely and effectively realign spinal deformities. The risk of perioperative screw pullout increases when presented with adult deformities that have less flexible spines and lower bone mineral density. An FA screw with reduction tabs is believed to enable surgical techniques permitting load distribution on multiple screws, thereby reducing screw pullout potential. METHODS The researchers constructed 3 finite element spine models from adult scoliosis patients. Mechanical properties of intervertebral discs were modeled to reflect less flexible adult spines and their stiffness was varied to evaluate impact on screw-vertebra forces. Models simulated scoliosis surgery according to clinical data using FA or polyaxial screws. Forces measured at the screw-vertebra interface were monitored and compared for each patient with FA and then polyaxial screws. RESULTS Simulations using FA screws reduced screw-vertebra interface forces significantly compared with polyaxial screws. Favored angle screws caused 18%, 14%, and 16% reductions in peak forces and 29%, 35%, and 22% reductions in average forces compared with polyaxial screws for patients 1, 2, and 3, respectively. Favored angle screws also provided consistent relative reduction in average forces by 28% when varying properties of intervertebral discs among 8, 10, and 12 MPa. CONCLUSIONS Using a virtual finite element platform, FA screws reduced screw-vertebra interface forces encountered during simulated correction of less flexible adult scoliosis compared with standard polyaxial screws. These results show a potential benefit of using this modified screw design to reduce screw-vertebra forces and potential intraoperative pullout failures.