A TELEMETRY SYSTEM FOR MEASURING STRESS IN A HIP JOINT PROSTHESIS

Abstract

It has been clinically observed that a significant fraction of hip joint implants fail due to mechanical loosening, particularly in younger, more active patients. Design enhancements are hampered by the lack of in vivo data of the actual forces produced on the prosthesis by various recipient activities. This paper describes a telemetry system small enough to fit in the spherical ball joint of a hip joint prosthesis. The system is capable of transmitting sufficient in vivo data to allow reconstruction of major forces through the prosthesis. The design allows for total hermetic enclosure of the electronic parts within the prosthesis which is implanted within the human body. Figure 1 shows 2 engineering model prosthesis assemblies. Input power coupling is provided through a cuff temporarily placed over the area of the device. Telemetry readout from the transmitter antenna, also totally enclosed, allows periodic out-patient checkup and monitoring. In this manner, since no batteries are used, in vivo monitoring of load forces on the prosthesis can be accomplished periodically over a several year period without surgical revision. The data obtained will be used to design stronger implants which will have very low failure rates even when subjected to activities of younger recipients.