Biomechanical Study of the Novel Dual-Core/Double-Thread Pedicle Screw Systems– Effects of Perforation, Expansion and Cement Augmentation

This study aims to develop the novel Dual-Core/Double-Thread pedicle screw system with three different design concepts (solid, perforation, and expansion) to assess their mechanical performance in spine instrumentation. Pedicle screws have been widely used in lumbar spine instrumentation. However, loosening and breakage of the pedicle screws are not uncommon. Previous reports have demonstrated that numerous factors, such as screw designs (thread, thread pitch, shaft diameter), screw types (perforation or expansion), hole preparation, insertion torque and bone quality are related to screw anchoring strength. However, until now, few reports are available to systematically assess the biomechanical performance (insertion torque, pullout, bending and fatigue strength) of dual-core/double-thread screws equipped with perforated or expansive designs, under different insertion techniques (cement augmentation, pilot hole preparation) and bone quality In current study, three novel screw systems with dual-core/double-thread design are proposed: solid, perforation, and expansion. The dual-core/double-thread screws are designed by adding a second (fine and shallow) thread adjacent to the first standard coarse thread but only in the proximal shaft. We hypothesize that the anchoring strength of pedicle screws can be improved if the additional fine thread is inserted within the cortical bone (at the proximal shaft), whereas cancellous bone is purchased by standard coarse threads. Based on the solid dual-core/double-thread screws, the perforated and expansive dual-core/double-thread screws are equipped with perforation and four separated anterior fins, respectively, to further enhance screw anchoring power. We propose to perform this study within three years. Both static (pullout and bending tests) and dynamic (fatigue) tests will be conducted to evaluate the mechanical performance of these novel screw systems. With the completion of these works, the influence of three novel screw designs (solid dual-core/double-thread, perforated dual-core/double-thread, expansive dual-core/double-thread), cement augmentation, pilot hole preparation, insertion torque and bone quality on screw fixation strength will be examined. The contents of this two-year study are summarized as follows: A. The first-year study: Design and manufacture of novel pedicle screws Three types of pedicle screws (solid, perforation, and expansion) with dual-core/double-thread designs will be developed. The potential advantage of various screws will be compared with the conventional bone screws. B. The second-year study: Static experiment (screw pullout and screw bending) Uniform synthetic bones (test block) with different densities (7.5 lbm/ft3, 15 lbm/ft3 and 20 lbm/ft3) will be used as substitute for the human bone. Each novel screw system will be inserted into the test material with or without cement augmentation. The insertional torque will be measured as the screw is advanced into the material. By using MTS hydraulic test machine, static experiments including screw pullout and screw bending will be conducted according to the related ASTM testing standards (F543-02, F2193-02). B. The third-year study: Dynamic cyclic experiment (fatigue test for spinal implants assembly) Following assemble of spinal units (screw, rod and nut), dynamic tests for spinal construct will be conducted according to ASTM 1717-04 testing standards, to compare the integral stability in various screw designs.

Project ID:PB10507-2959
External Project ID:MOST105-2221-E182-005