Frictional heating of total hip implants. Part 1: measurements in patients

Hip implants heat up due to friction during long lasting, high loading activities like walking. Thermal damage in the surrounding soft and hard tissues and deteriorated lubrication of synovial fluid could contribute to implant loosening. The goal of this study was to determine the implant temperatures in vivo under varying conditions. Temperatures and contact forces in the joints were measured in seven joints of five patients using instrumented prostheses with alumina ceramic heads and telemetry data transmission. The peak temperature in implants with polyethylene cups rose up to 43.1 degrees C after an hour of walking but varied considerably individually. Even higher temperatures at the joints are probable for patients with higher body weight or while jogging. The peak temperature was lower with a ceramic cup, showing the influence of friction in the joint. During cycling the peak temperatures were lower than during walking, proving the effect of force magnitudes on the produced heat. However, no positive correlation was found between force magnitude and maximum temperature during walking. Other individual parameters than just the joint force influence the implant temperatures. Based on the obtained data and the available literature about thermal damage of biological tissues a detrimental effect of friction induced heat on the stability of hip implants cannot be excluded. Because the potential risk for an individual patient cannot be foreseen, the use and improvement of low friction implant materials is important.