Biomechanical testing of a new design for Schanz pedicle screws

Standard 5-mm AO Schanz pedicle screws were biomechanically compared with a new design, featuring 6-mm threads with a 5-mm core diameter continuous with the shaft. One each of the two screw designs was surgically inserted into the matching pedicles of 32 cadaveric vertebrae. The pull-out strengths of the screws were then determined by recording the peak force values during extraction under servohydraulic displacement control. The screws were also tested in three-point bending, varying the inner load point with respect to the shaft thread junction, within a clinically anticipated range. The mean pull-out strength for the 6-mm screw was 597 N, which was significantly greater than the mean strength of 405 N for the 5 mm screw (p = 0.002). The 6-mm screw was also stronger in three-point bending, and failed at the point of inner load application, with no evidence of a stress-raising effect at the shaft/thread junction. In contrast, the 5-mm screw withstood lower loads, and failed at the shaft/thread junction, regardless of the point of loading. Pedicle screw breakage and pull-out are the recognized modes of failure of spinal implants, which are dependent on pedicle screw fixation. The results suggest distinct biomechanical advantages for the 6-mm screw, which should be used whenever clinically feasible.