This paper describes two useful metrics to estimate the ligament imbalance for post Total Knee Arthroplasty (TKA) scenario: the center of pressure and the net moments (varus-valgus and anterior posterior). Both metrics have been evaluated using high level models and experimental measurement. Self-powered analog and digital architectures for the center of pressure are elaborated here and complies with the Medical Implant Communication Service (MICS) standard. It is shown that the analog architecture is advantageous in terms of surface area and overall power consumption.