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.

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

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

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

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

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

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

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

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

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.

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.

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.

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

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

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