Impact of Screw Diameter on Pedicle Screw Fatigue Strength-A Biomechanical Evaluation

OBJECTIVE: Loosening of pedicle screws is a frequently observed complication in spinal surgery. Because additional stabilization procedures such as cement augmentation or lengthening of the instrumentation involve relevant risks, optimal stability of the primarily implanted pedicle screw is of essential importance. The aim of the present study was to investigate the effect of increasing the screw diameter on pedicle screw stability.

METHODS: A total of 10 human cadaveric vertebral bodies (L4) were included in the present study. The bone mineral density was evaluated using quantitative computed tomography and the pedicle diameter using computed tomography. The vertebrae underwent instrumentation using 6.0-mm × 45-mm pedicle screws on 1 side and screws with the largest possible diameter (8-10-mm × 45-mm) on the other side. Fatigue testing was performed by applying a cyclic loading (craniocaudal sinusoidal 0.5 Hz) with increasing peak force (100 N + 0.1 N/cycle) until screw head displacement of 5.4 mm was reached.

RESULTS: The mean fatigue load was 334 N for the 6-mm diameter screws and was increased significantly to 454 N (+36%) for the largest possible diameter screws (P < 0.001). With an increase in the fatigue load by 52%, this effect was even more pronounced in vertebrae with reduced bone density (bone mineral density <120 mg/cm3; n = 7; P < 0.001). The stiffness of the construct was significantly greater in the largest diameter screw group compared with the standard screw group during the entire testing period (start, P < 0.001; middle, P < 0.001; end, P = 0.009).

CONCLUSIONS: Increasing the pedicle screw diameter from a standard 6-mm screw to the largest possible diameter (8-10 mm) led to a significantly greater fatigue load.