FRICTIONAL RESISTANCE OF NEW AND EXPLANTED ARTIFICIAL HIP JOINTS

The frictional resistance of 54 explanted Charnley joints and 32 new j oints (various types) has been measured in a hip function simulator in dry and lubricated environments. The friction factors (torque/(normal load X radius)) for new prostheses were remarkably similar no matter what material or head size was used provided the acetabular component was made of ultra high molecular weight polyethylene (UHMWPE). Charnle y joints, implanted for up to 17 years, were removed from the patients at revision operations and showed that most had a friction factor sim ilar to that of new joints. However, 30% had a friction factor greater than 0.16 when dry and 39% showed friction factors in excess of 0.07 when lubricated (cf 0.1 and 0.04 respectively when new). All the joint s operated under a mixed lubrication regimen. When the acetabular comp onent was made from UHMWPE, the friction factors were hardly affected by the material of the femoral component. 30% of explanted joints had friction almost twice as great as new joints, but loosening of the pro stheses also occurred in some joints whose friction remained low.