Changes in the design philosophy for femoral hip implants are resulting in new cementless designs that attempt to increase bone strain levels by increasing stem flexibility. Composite materials are often selected for these new implants because of the inherent ability to custom design the mechanical response of the material. Traditional test methods currently used for the evaluation of relatively stiff, homogeneous metallic implants may not be appropriate for the evaluation of these new prosthesis designs.
This paper presents a new test fixture design that uses a two-beam structure to simulate the support provided in-vivo by the femoral bone. A prototype fixture was designed, built, and tested with composite hip prostheses to demonstrate the viability of the concept. Net head deflections and prosthesis strain levels were measured and compared to the response of the same implants when press fit and cemented in composite femurs.
Data from these tests suggest that such a fixture design is capable of providing the required support and endurance for high cycle prosthesis fatigue testing. The net support was greater than that of a press fit implant but less than that of a cemented implant in a composite bone. Simulation of the most extreme stress state for the implant is most likely represented by the press fit case and therefor some modifications of the fixture design will be required to improve the support model.