Tibiofemoral forces determine polyethylene wear and affect the longevity of total knee prostheses. Previously, investigators relied on theoretic data from mathematical models to predict mechanical forces in the knee. Predictions of tibiofemoral forces are highly variable because of the complex interplay of the muscles involved in activities. Ideally, knee forces should be directly measured. An electronic total knee prosthesis (e-Knee) was developed to directly measure tibiofemoral compressive and tensile forces in vivo. After 13 years of research and development, the e-Knee was implanted into a patient in 2004. Tibiofemoral force data were collected intraoperatively and throughout the postoperative period during activities of daily living and during exercise. Direct measurement of knee forces can lead to a better understanding of the stresses seen following total knee arthroplasty. Information generated by the e-Knee will aid in the improvement of implant design and patient care.