Most of us have heard from our knees after a tennis game, but how about a knee that can deliver real-time data during various activities? Jerry Ward, a retired aerospace engineer, is walking around on one today. This total knee prosthesis, implanted at the Scripps Clinic Division of Orthopaedic Surgery, is instrumented with force transducers, a microtransmitter, and an antenna that transmits to an external receiver. Some of the components were made by MicroStrain Inc. (www.microstrain.com), a developer of wireless microsensors. The prosthesis will allow real-time measurements on the distribution of forces to the implant and load-carrying capabilities of the bone as Ward walks, climbs stairs, and exercises. The top of the implant is similar to a standard model, with a typical femoral component and polyethylene articular surface. The lower component, made of titanium, is customized to allow measurement of loads across the knee. Four metal posts separate the top and bottom plates; strain sensors under the posts monitor changes in the metal as Ward moves. A removable coil around the knee activates the system, then sends data to a computer for conversion into loads in pounds at each of the four posts.