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Novel Bone Anchor Concept for Osteoporotic Bone Tissue, 02/2003

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

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Novel Bone Anchor Concept for Osteoporotic Bone Tissue, 02/2003

  1. 1. NOVEL BONE ANCHOR CONCEPT FOR OSTEOPOROTIC BONE TISSUE Wettergreen MA, Sun K, Liebschner MAK Dept. of Bioengineering Rice University
  2. 2. Overview <ul><li>Motivation </li></ul><ul><li>Previous bone screw design </li></ul><ul><li>New bone anchor design </li></ul><ul><li>Evaluation and creation of new concept </li></ul>
  3. 3. Background <ul><li>Biomechanical failure of spine </li></ul><ul><ul><li>Osteoporosis </li></ul></ul><ul><ul><li>Loss of BMD </li></ul></ul><ul><ul><li>Trauma </li></ul></ul><ul><li>Fixation devices to restrain or restore function </li></ul><ul><li>Compatibility with bone </li></ul>
  4. 4. Previous Work on Bone Screw Pullout Strength Increase <ul><li>Double Threaded Screws </li></ul><ul><ul><li>No significant increase in pullout resistance </li></ul></ul><ul><ul><li>Mummaneni et al. (2002) </li></ul></ul><ul><li>Hollow Modular Anchorage System </li></ul><ul><ul><li>No significant difference in pullout strength </li></ul></ul><ul><ul><ul><li>Schramm et al. (2002) </li></ul></ul></ul><ul><li>Allogenic bone screws, bioabsorbable screws </li></ul><ul><ul><li>60% reduced pullout strength </li></ul></ul><ul><ul><ul><li>Rano et al. (2002) </li></ul></ul></ul><ul><li>Numerical Simulations on composite screws </li></ul><ul><ul><li>Better long-term results </li></ul></ul><ul><ul><ul><li>Gefen A. (2002) </li></ul></ul></ul>
  5. 5. Basic Concept of Screw Working Principle <ul><li>Designed for connecting solids </li></ul><ul><li>Pullout strength </li></ul><ul><li>dependent on </li></ul><ul><ul><li>Screw length </li></ul></ul><ul><ul><li>Screw diameter </li></ul></ul><ul><ul><li>Thread height </li></ul></ul><ul><ul><li>Number of threads </li></ul></ul><ul><ul><li>Surface area in contact </li></ul></ul>
  6. 6. Motivation <ul><li>Current screw designs not osteointegrative </li></ul><ul><li>Develop suitable replacement for bone screws </li></ul><ul><li>New design must include following characteristics </li></ul><ul><ul><li>High pullout strength </li></ul></ul><ul><ul><li>Osteointegration through bone anchorage sites </li></ul></ul><ul><ul><li>Biocompatibility preventing rejection </li></ul></ul><ul><ul><li>Strength matching to bone </li></ul></ul>
  7. 7. Working Hypothesis <ul><li>By matching porosity and architecture of bone in the design of a bone fixation screw, improved tissue integration will result </li></ul>
  8. 8. Materials and Methods Part 1: Finite Element Analysis <ul><li>Standard bone screw </li></ul><ul><ul><li>Single right-hand threaded screw </li></ul></ul><ul><ul><li>Material: Titanium; E = 207GPa; K = 5GPa </li></ul></ul><ul><ul><li>Screw Diameter = 6 mm </li></ul></ul><ul><ul><li>Screw Length = 27.5 mm </li></ul></ul><ul><li>Bone complex </li></ul><ul><ul><li>Trabecular Bone, E = 200 MPa </li></ul></ul><ul><ul><li>Core length = 35 mm </li></ul></ul><ul><ul><li>Vertebral Shell, E = 1 GPa </li></ul></ul>
  9. 9. Materials and Methods Part 1: Finite Element Analysis <ul><li>Bone Anchor Plug </li></ul><ul><ul><li>Material: PMMA; E=2.5 GPa; K=113 MPa </li></ul></ul><ul><ul><li>Plug Diameter = 6 mm </li></ul></ul><ul><ul><li>Plug Length = 22.5 mm </li></ul></ul><ul><li>Properties of Augmented Area </li></ul><ul><ul><li>2 mm layer surrounding the plug </li></ul></ul><ul><ul><li>Bone-PMMA composite E=800 MPa </li></ul></ul><ul><ul><li> Ult of composite = 30 MPa </li></ul></ul>
  10. 10. Results Part 1: Finite Element Analysis <ul><li>Screw Results </li></ul><ul><li>Stiffness 890 N/mm </li></ul><ul><li>Ultimate Load 716 N </li></ul><ul><li>Plug Results </li></ul><ul><li>Stiffness Plug 4037 N/mm </li></ul><ul><li>Ultimate Load 1007 N </li></ul>Non-linear finite element analysis with a maximum displacement of 2.25 mm
  11. 11. Conclusions Part 1: Finite Element Analysis <ul><li>Bone anchor pullout resistance is four fold higher than that of the bone screw </li></ul><ul><li>Pullout-strength is about 40% higher in the bone anchor </li></ul>
  12. 12. Part 2: Generation of Bone Anchor Using Rapid Prototyping
  13. 13. Methods Part 2: Rapid Prototyping <ul><li>Generation of porous architecture with orthogonal beams using IronCAD </li></ul><ul><li>Bone anchor design scaled from trabecular architecture </li></ul><ul><ul><li>Beam spacing = 1.25 mm </li></ul></ul><ul><ul><li>Beam diameter = .675 mm </li></ul></ul><ul><li>Design modified to include syringe port at delivery end </li></ul><ul><li>Hollow inner channel allows delivery of biomaterial, bone cement, therapeutic fluid </li></ul>
  14. 14. Add dimensions Mimicking trabecular architecture 1mm 1.25 mm .675mm
  15. 15. <ul><li>Fused Deposition Modeling (FDM) </li></ul><ul><ul><li>Two thermoplastic wax materials: build and support </li></ul></ul><ul><ul><li>Process deposits in layers to generate model </li></ul></ul><ul><li>Removal of material with differential dissolving </li></ul><ul><ul><li>Build dissolves with organic solvents </li></ul></ul><ul><ul><li>Support dissolves with kerosene based liquid </li></ul></ul><ul><li>Model and mold built with PatternMaster </li></ul><ul><li>(SolidScape Inc., Merrimac, NH) </li></ul>Methods Part 2: Rapid Prototyping
  16. 17. Methods Part 2: Manufacturing Process <ul><li>Positive of model generated with IronCAD </li></ul><ul><li>Model built with PatternMaster </li></ul><ul><li>Support material dissolved </li></ul>Completed Plug Plug without support Plug with support CAD
  17. 18. <ul><li>Negative of CAD file generated </li></ul><ul><li>Model built with PatternMaster </li></ul><ul><li>Support dissolved with BioAc </li></ul><ul><li>Mold injected with poly(propylene fumarate) </li></ul><ul><li>Mold dissolved with DMSO </li></ul>Methods Part 2: Manufacturing Process
  18. 19. PatternMaster Part with support Completed anchor Dissolve support Inject mold Mold
  19. 20. Completed PPF Bone Anchor
  20. 21. Results Part 2: Rapid Prototyping <ul><li>Successful creation of bone anchor using both positive and negative build processes </li></ul><ul><ul><li>Positive = Build bone anchor with RP </li></ul></ul><ul><ul><li>Negative = Build bone anchor mold with RP </li></ul></ul><ul><li>Injection molding successful </li></ul><ul><ul><li>Bone plug injected with poly(propylene) fumarate </li></ul></ul><ul><ul><li>Could not use PMMA due to differential solvents problem </li></ul></ul>
  21. 22. Conclusions <ul><li>Shown effectiveness of new design using Finite Element Analysis </li></ul><ul><li>Shown feasibility of generating complex interconnected architecture using rapid prototyping and molding techniques </li></ul><ul><li>FEA of new design indicates paradigm shift required </li></ul><ul><li>Demonstrated success of injection molding with poly(propylene fumarate) </li></ul><ul><ul><li>Injection process not optimized, air holes exist </li></ul></ul><ul><li>Resolution of bone can be achieved with injection molding </li></ul>
  22. 23. Future Work <ul><li>Revise model to incorporate architecture and fill volume </li></ul><ul><li>Perform pullout tests with vertebral bodies </li></ul><ul><li>Build and test anchor with several different biomaterials </li></ul>
  23. 24. Acknowledgements <ul><li>Dr. Antonios Mikos, Rice University </li></ul><ul><li>Mark Timmer, Rice University </li></ul><ul><li>Laboratory of Computational and Experimental Biomechanics </li></ul><ul><li>Texas ATP Grant </li></ul>
  24. 25. Questions?

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