TELEMETRY SYSTEM FOR MONITORING ANTERIOR CRUCIATE LIGAMENT GRAFT FORCES IN-VIVO

Quantifying changes in the tension of an anterior cruciate ligament (A CL) graft in vivo during rehabilitative exercises is useful for develo ping the optimum rehabilitation for patients who have had reconstructi ve surgery. The purpose of the work reported is to design, build and t est a telemetry system that can measure the in vivo ACL graft tension post-operatively. A commercially available fixation device is modified to sense the graft tension, house electronic components, transmit an output signal and pass the power generating signal. A transcutaneous i nductive link is used to power the implanted telemetry electronics. Th e current difference technique is used to measure changes in two resis tance strain gauges that monitor shear strain developed on the femoral fixation device by the ACL graft. This current regulates a frequency- modulated output signal that is transmitted using a new technique. Har nessing the ionic and volume conduction properties of the body fluids, the new technique involves injecting current subcutaneously into the tissue and then sensing the potential developed on the skin by surface electrodes. The waveform shape, amount of charge injected, charge den sity and current density are regulated to avoid tissue damage, pain an d unwanted muscular stimulation. A signal conditioning board detects a nd converts the output to an analogue voltage far collection by a comp uter data-acquisition system. A performance evaluation demonstrates th at the telemetry system either meets or exceeds all of the criteria ne cessary for the application.