Total hip joint prosthesis for in vivo measurement of forces and moments

A new instrumented hip joint prosthesis was developed which allows the in vivo measurement of the complete contact loads in the joint, i.e. 3 force and 3 moment components. A clinically proven standard implant was modified. Inside the hollow neck, 6 semiconductor strain gauges are applied to measure the deformation of the neck. Also integrated are a small coil for the inductive power supply and a 9-channel telemetry transmitter. The neck cavity is closed by a titanium plate and hermetically sealed by electron beam welding. The sensor signals are pulse interval modulated (PIM) with a sampling rate of about 120 Hz. The pulses are transmitted at radio frequencies via a small antenna loop inside the ceramic head, which is connected to the electronic circuit by a two-pin feedthrough. Inductive power supply, calculation of the loads from the measured deformations and real time load display are carried out by the external equipment. The maximum error of the load components is 2% including crosstalk.