A telemetric knee is being developed with the intent of measuring and predicting patellofemoral and tibiofemoral forces in the knee joint following total knee arthroplasty (TKA). A patellar transducer has been developed that is implanted during a TKA in place of the posterior half of the patella, and measures the magnitude and location of the patellofemoral force. With appropriate kinematic modeling, tibiofemoral forces can be resolved from this data as well. The transducer was designed using Finite Element Analysis (FEA) to optimize the thickness and geometry of the transducer to maximize sensitivity yet still provide appropriate structural integrity.

Calibration of the transducer is accomplished by rotating the orthogonal coordinate system off-axis from the normal load such that calibration loading vectors are all at equal angles from the transducer plane. This alleviates stress-concentration artifacts that may be present when loading the transducer along its plane. The transducer is cyclically loaded with a known magnitude along all three axes. This data is then used to derive a calibration matrix that can be applied to any force vector. A wireless data transfer system is used in the current prototype, and an embedded wireless system is proposed. In situ testing in a cadaver leg segment illustrates the ability of the transducer to measure relative load, but brings to light concerns regarding repeatability and temperature sensitivity. Further in vitro testing evaluates probable causes of these problems and ways to address them. Design modifications based on the experimental analysis of this research have been recommended. These modifications should improve robustness, repeatability and sensitivity to such variables as temperature.