UCLA

Orthopedic implants are too stiff relative to bone. This results in stress shielding of the bone tissue, which leads to bone resorption and implant loosening. The development of metallic alloys that maintain mechanical strength with a lower Young’s modulus has been unsuccessful. Mechanical designs that mitigate stress shielding do so by introducing bearing surfaces, which create wear debris that is harmful to the body. Other novel solutions lack long-term clinical trials to validate their efficacy. This manuscript proposes the implementation of compliant mechanism design for orthopedic implants. Compliant mechanisms rely on the elastic property of flexure elements to achieve motion. Two applications are investigated: orthopedic screws and tibial stems in total knee replacements. The compliant screw design possesses a self-anchoring property to improve its fixation ability. It is analyzed and optimized with finite element software. The results of the benchtop tests indicate the compliant screw better protects the surrounding material and can displace further and endure more load per mass before failure. It has also demonstrated improved anti-vibrational properties. The compliant tibial stem design accommodates the internal-external rotation of the knee. It is simulated for the entire load cycles of multiple activities of daily living using finite element software and referencing instrumented knee implant data. Benchtop tests show close agreement with the finite element analysis predictions, suggesting the design can accommodate the rotation without failing from cyclic loads.