In Vivo Hip Joint Forces Recorded on a Strain Gauged ‘English’ Prosthesis Using an Implanted Transmitter

Abstract

This paper describes the results obtained from implanting a strain gauged version of an ‘English’ hip joint replacement together with a totally implantable FM radio transmitter. The implant is based upon a new concept in the design of femoral hip components having a diminished head offset to reduce head load and improved stem shape permitting alignment of the neck along the theoretical axis of peak load transmitted during the gait cycle. The implant was inserted using the ‘English’ trochanteric approach (English, 1975) which further reduces the load on a prosthetic hip joint with the use of a spacer out from the redundant femoral head to rearrange the trochanteric muscle lever arms. The resulting axial load is detected by four strain gauges mounted on a ‘piston in cylinder’ arrangement contained within the thickened neck of the prosthesis. The single channel FM transmitter relays the gauge output to a signal processing unit to give a direct output of activity for recording on a UV recorder. Recordings were taken during implantation, recovery, walking (at three days) physiotherapy, stair climbing and walking over a period of forty days.