Low-power circuits for the wireless ligament balance measurement system in TKA

Abstract

A low power system for the ligament balance measuring in Total Knee Arthroplasty is presented in this paper. The system consists two parts: a front-end Ligament Balance Measuring System (LBMS) which is inserted into the knee joint during the operation, and the display part. LBMS is comprised of a sensors array including eight precise force sensors, signal conditioning circuits that support up to 15 force sensors, a sub-threshold microprocessor, power circuits and a 433 MHz RF transceiver for data transmission. The force corresponding to its distribution is transmitted wirelessly and displayed in 3-D in real time with an accuracy of 0.049 N. The signal conditioning circuits, the sub-threshold 8 bit microprocessor and the application specific integrated circuits chip have been designed and fabricated in 0.18 μm CMOS process. The tested resolution is 60.1 μVpp (1.35 g) with ±100 mVpp input. The chip can operate under 1.2 to 3.6 V voltage supply for single battery application with 116---160 μA power consumption. The testing results of the microprocessor show that the leakage power is 46 nW and the dynamic power is 385 nW @ 165 kHz with operating voltage of 350 mV. The simulation results show that the power circuits can provide the supply voltage ranging from 0.3 V to 0.6 V for the sub-threshold microprocessor. Experimental results verified the system. Some clinical experiments will be carried out in the future.