ABSTRACT During the past decade, the use of bone screws in spinal stabilization has dramatically increased. Failure of implanted screws to provide adequate stabilization can necessitate additional surgical procedures. Modifications of factors previously shown to be associated with increased screw pullout strength have shown to be insignificant when applied in osteoporotic bone. The objective of this study was to investigate the feasibility of a new bone anchor design in providing superior biomechanical performance when compared to metal screws. We conducted a finite element study simulating implant pullout testing of a metal bone screw and a polymer bone anchor. The results indicated that the polymer bone anchor, while having inferior material properties, has superior biomechanical behavior. The pullout strength was increased by 40% with the new design, while stiffness was increased by more than four fold. We conclude from this study that bone anchors made out of polymers may be suitable for medical applications, however, their design needs to deviate from the traditional screw shape for adequate fixation. With material properties matching bone, polymers may prove to be more successful in long-term clinical applications, especially in osteoporotic bone. INTRODUCTION Surgical management of fractures has historically been accomplished by fixation of the fragments with metallic implants. Despite substantial improvements in metallurgy, design, and the understanding of the biomechanical forces acting on the implant system, the screw-bone interface has remained a major site of complications leading to failure of treatment [1]. Biomedical polymers with properties matching bone tissue may be a better alternative. The overall objective of this study was to investigate the feasibility of a new bone anchor design in providing superior biomechanical performance in osteoporotic bone when compared to metal screws. In this first phase we conducted a finite element study simulating implant pullout testing of a metal bone screw and a new concept design using a polymer bone anchor. MATERIALS AND METHODS Three-dimensional finite element models of a trabecular bone core with a cortical shell, a metal bone screw, and a new bone anchor were developed. Finite element models were of standard single-threaded TSRH screws (Medtronics Sofamor Danek, Memphis, TN, U.S.A) with properties of titanium. The polymer bone anchor was designed with an orthogonal beam network mimicking trabecular bone with channels allowing even distribution of an injectable material between the implant and the adjacent bone tissue. Material properties of the anchor were based on published data for the biomaterial. Trabecular bone was modeled as transversely isotropic osteoporotic bone. The outer diameter of the bone core was more than three times the diameter of the implants. The pullout test was simulated with a max displacement of 2.25 mm. Stiffness and strength were calculated from the load-deformation curves. RESULTS Metal bone screws can be considered as the gold standard to stabilize spinal functional units. Therefore, we compared the biomechanical behavior in the other construct with the behavior of the bone screw. Our results indicate that the initial pullout resistance of the bone anchor is about four fold higher than that of the bone screw and the pullout-strength is about 40% higher in the bone anchor. CONCLUSION The bone-screw interface is a critical component for spinal stabilization. Placement of a significantly stiffer implant into bone disperses the forces non-uniformly, and regions of increased stress result within the screw and the bone. Weakened mechanical properties of synthetic polymers require a paradigm shift in the design of the screw. The much larger bone-implant interface of the new design lead to a drastically increased pullout strength (>40%) in osteoporotic bone when compared to the metal screw. The properties of the bone anchor

1. NOVEL BONE ANCHOR CONCEPT FOR OSTEOPOROTIC BONE TISSUE Wettergreen MA, Sun K, Liebschner MAK Dept. of Bioengineering Rice University

12. Part 2: Generation of Bone Anchor Using Rapid Prototyping

14. Add dimensions Mimicking trabecular architecture 1mm 1.25 mm .675mm

19. PatternMaster Part with support Completed anchor Dissolve support Inject mold Mold

20. Completed PPF Bone Anchor

25. Questions?