Summary: 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 pullout 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.