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Abstract

Abstract.

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Abstract

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  1. Abstract Intramedullary nails (IN) present one method of repairing long bone fractures. Upon insertion into the intramedullary space the nail is fastened with 2 proximal and 2 distal screws. Holes must be drilled into the bone that align with these 4 holes. Currently a jig assembly is used to guide surgeons drilling these holes. Sometimes this assembly fails to align the drill correctly resulting in insecure nails. The assembly appears to fail at the extension-nail interface. A re-designed nail-extension unit has been developed to address this issue. Initial results suggest that a solid unit with stress-raisers reduces the movement that occurred about the old connection.

  2. Client Information • R. Tass Dueland, DVM • Ray Vanderby, Professor • William L. Murphy, Professor and Advisor

  3. Background & Motivation • IN are ~97% effective in repairing clean long bone fractures • IN allows necessary movement to stimulate osteoblasts while providing rigid support • IN nails are not effective when not secured properly • Screws fail to fasten ~4% of the time

  4. Current Products • Intramedullary Nail • Produced by Innovative Animal Products • Magnetic Targeting Device • Locates field produced by magnets in the nail Fig. 1: IN and jig assembly Fig. 2: Magnetic targeting device

  5. Problem Statement • Decrease drill guide misalignments with the intramedullary nail holes to reduce failure rates.

  6. Client Design Requirements • Maintain integrity of the nail • Implement into current procedures • Compose of biocompatible materials • Improve fastening success rate

  7. Final Design Concept • Eliminate extension-nail interface • Stress-raisers allow control of yield point via stress factor, Kt • Solid unit should be stronger then any threaded connection Push Rod IN Fig. 3: Push Rod that applies tension to nail at stress raiser.

  8. t h P P di r σyA Kt Final Design Continued • Prototype stress raiser: t=0.5mm h=0.9mm r~0.5mm • Results in a Kt~1.6 • Breaking Point: σy=485MPa (Schmid, 1999) • P = ~ 3.5 KN • or ~800 lbs Fig. 4: Variables that determine Kt Stress concentration factors, Kt for a tube in tension with a fillet, (ESDU 1981).

  9. Final Design Fracture • Fracture was not obtained with current design • Obtained with minimal outside force application • Increasing h will yield higher Kt, reduce A, and allow for a larger push rod Fig. 5: The push rod can be seen protruding from the break point.

  10. Testing Methods • Goal: Quantify force necessary to cause screw misalignment • Procedure: • A force gauge was used to apply a force at the distal end of the IN. The value was recorded upon achieving misalignment. Fig. 6: Picture depicting testing methods. The jig is clamped to the table. A force gauge is used deviate the nail.

  11. Force Results • Prototype required ~1.5X more force to deviate nail to misalignment Fig. 7: Average force applied at a constant position to deviate the nail to misaligment.

  12. Moment Results • Larger moment required to achieve deviation in prototype • Assuming this moment remains constant, a relationship between the force and where it is applied can be found. Fig. 8: Average force applied at a constant position to deviate the nail to misaligment.

  13. Conclusion • Vast improvement in resistance to bending • Greater force required to deviate nail • Stress raiser cleanly fractures • Prototype requires further modifications

  14. Future Work • Increase diameter of center hole • Increases diameter of plunger • Decreases fracture force • New material for plunger rod • Increase elastic and shear modulus • More testing

  15. Acknowledgements • R. Tass Dueland, DVM • Ray Vanderby, Professor • William L. Murphy, Professor • Carrie Lowrey, Surgical Assistant • UW-Madison Veterinary School • Paul Manley, DVM • Daniel Ruys, Professional Welder • Machine Shop Personnel • Jay Warrick, BME Graduate Student

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