Heat Generated by Knee Prostheses

Temperature sensors were placed in 50 knees in 25 patients who had one or both joints replaced. Temperature recordings were made before walking, after walking, and after cycling. The heat generated in healthy, arthritic, and replaced knees was measured. The knee replacements were done using eight different prostheses. A rotating hinge knee prosthesis generated a temperature increase of 7°C in 20 minutes and 9°C in 40 minutes. An unconstrained ceramic femoral pros- thesis articulating with a polyethylene tibial prosthesis generated a temperature increase of 4°C compared with a healthy resting knee. The other designs using a cobalt- chrome alloy and high-density polyethylene had temperature increases of 5°-7°C with exercise. Frictional heat generated in a prosthetic knee is not immediately dissipated and may result in wear, creep, and other degenerative processes in the high-density polyethylene. Extended periods of elevated temperature in joints may inhibit cell growth and perhaps contribute to adverse performance via bone resorption or component loosening. Prosthetic knees generate more heat with activity than healthy or arthritic knees. More-con- strained knee prostheses generate more heat than less- constrained prostheses. A knee with a ceramic femoral component generates less heat than a knee with the same design using a cobalt-chromium alloy.