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

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???m CMOS process. The tested resolution is 60.1???V_pp (1.35?g) with ±100?mV_pp input. The chip can operate under 1.2 to 3.6?V voltage supply for single battery application with 116?C160???A 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.