Characteristics of immediate and fatigue strength of a dual-threaded pedicle screw in cadaveric spines.

Abstract

Novel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation. Tests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws. In vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks. Seventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested. Measures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure. For each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity. In bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156). Although dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.