Among the multiplicity of thread shapes, the buttress thread design remains the historic paradigm for the shape of current orthopedic screws. The popularity of buttress threads in current orthopedic screw designs is reflected by the advantage of handling high axial thrust in one direction, which leads to increased shear strength and improved unidirectional pullout resistance compared to other conventional thread shapes. However, orthopedic screws are typically not challenged by axial loading forces from physiological motion in- vivo. Thus, standard buttress screws remain at a significant risk of failure when exposed to multidirectional loading forces. To address the physiological multiaxial loading environment, newer generation locking plates have been able to reduce the risk of implant failure, particularly in osteoporotic bone. Locked plating relies on the benefit of a fixed-angle construct that does not rely on friction and compression between implant and bone. However, locking head screws have been shown to have their own set of shortcomings, including the stiffness of plate-screw constructs, asymmetric callous formation and increased cost, hence the need for continued research towards more effective and cost-conscientious solutions. SMV Scientific has designed a new bone- screw-fastener, entitled SMV Bone Interlocking Thread Geometry (BITG) that distributes forces from the implant onto the bone and subsequently resists loads in all directions. The new fastener consists of a ?female thread? bone cutting technology designed to maximize bone volume, preserve bone architecture, and create a circumferential interlocking interface between the implant and bone. BITG resists multidirectional forces and bending moments to limit toggling and minimize radial forces; therefore, improving resistance to failure and decreasing risk of creating stress risers and iatrogenic fractures; BITG allows for higher finishing torques compared to buttress screws and resists screw stripping; and the BITG cutting mechanism curls the bone chips away from the cutting edges to create a debris free, solid bone-implant interface in order to present iatrogenic bone destruction during screw insertion. However, this novel thread geometry has not been optimized for cortical and trabecular bone orthopaedic applications in normal and osteoporotic bone stock, while subjected to different loading conditions. We hypothesize that the optimized BITG screws will provide improved multidirectional load resistance, when compared to buttress screws, for cortical and trabecular screws under different loading conditions and bone types. Therefore, we propose to conduct a parametric finite element (FE) analysis based study to optimize the BITG screws for orthopaedic use.
Specific Aim 1 : Validate our existing bone thread interface FE model with a cadaveric study using clinically relevant loading conditions.
Specific Aim 2 : Use the validated FE model to conduct a parametric FE analysis to optimize thread geometry for cortical and trabecular screws.
Currently, all commercially available orthopaedic screws use a buttressed thread design, which applies an outward force on the bone and is designed to resist load in only one direction. However, the real life loading on orthopedic implants is multiaxial and can, therefore, result in loosening or ?toggling?, causing the screw to erode through the bone and enlargement of the hole within which the screw resides, thereby leading to failure of fixation. The novel thread design presented in this proposal instead provides a radial inward force from the screw on the bone, and subsequently resists loads in all directions. The innovation presented here consists of a screw with bone cutting technology to increase bone volume, preserve bone architecture, and create a uniquely threaded bone interface.
