Introduction: The purpose of this study was to evaluate the micromotion of a femoral prosthesis relative to the femur in a revision hip replacement model.

Methods: A series of Ovine hip hemiarthroplasties were mechanically tested to detect micromotion of the femoral prosthesis relative to the femur 12 weeks following implantation. A mechanical testing device utilising muscle simulation of the major groups around the femur was designed. A 3D targeting system was developed using non-contact LASER transducers on the implant referenced to a second target on the overlying femur. Movement of this second target was measured with three LVDT’s (linear variable differential transformers).

Results: The system error was quantified in each femur to a resolution of the order of 15 microns. The mean micromotion, in 3D at two points assuming rigid body mechanics, was less than 50 microns for clinically stable implants. One stem was determined to be clinically loose and had a corresponding mean micromotion of 150 microns.

Conclusion: The method enabled measurement of 3Dmicromotion of a femoral prosthesis within the femur, during a laboratory approximation of normal physiological load cycles. The micromotion values corresponded to clinical outcomes, in a manner consistent with other reports in the literature. This system can be modified to allow targeting of different implants within a variety of bone types.