Battery-less wireless instrumented tibial tray.

Abstract

Previous research has found that over 400,000 total knee replacement procedures (TKR) are annually performed in the United States. Therefore, it is important that the loads on TKR implants be fully understood to improve the reliability of the implants. This paper presents the development of a battery-less wireless instrumented tibial tray for TKR implants. Previous instrumented tibial trays were powered by inductive coupling which required the patient to wear an externally-powered coil. Whereas, the proposed instrumented tibial tray is powered internally by an integrated piezoelectric energy harvesting system. This paper also presents the development of capacitive force sensors and an ultra low-power method to measure the capacitive force sensors. Two capacitive force sensor designs were considered and neither design could meet all of the performance requirements for the intended application. Despite this finding, several sensors were produced to demonstrate the concept behind capacitive force sensing using piezoelectric energy harvesting. With a 316 lb applied force, the energy harvesting system could fully charge the storage capacitors in 11 steps and could harvest an average of 1051 μJ per step. To power the force measurement system for ten seconds and to transmit the data, the piezoelectric energy harvesting system must be charged before the force measurement process is initiated by a minimum of 11 steps with a force of 316 lbs and a minimum of two steps must be taken during the force measurement process. During the force measurement process, each force sensor was sampled at a frequency of 10 Hz for 10 seconds; thereafter, all of the data was transmitted to the RF base station. The resulting capacitive force sensors showed good results when a set of cyclic loads were applied; however, the sensors demonstrated issues with repeatability when the applied force was increased and then reduced to the original value. The force sensors require improvements, but once this is completed, the system shows promise to be an effective measurement device for TKR implants.