Tribology of replacement hip joints

Abstract

The clinical wear rates of current designs of artificial hip joints which pair either stainless steel or CoCrMo alloy femoral heads with ultra high molecular weight polyethylene (UHMWPE) sockets are found to vary from 50-140 mm 3 /year depending on the radius of the head of the hip joint. A 28 mm diameter CoCrMo femoral head will produce about 62-75 mm 3 /year of UHMWPE wear particles. The rate of wear is important because the failure of artificial joints by loosening in the bone, has been shown to occur when the total volume of wear debris reaches about 600 mm 3 . Thus the lower the wear rate, the longer a joint will last. These clinical wear rates compare with modern joint simulator results of 60 mm 3 /10 6 cycles (over 0 to 2 million cycles) to 48.2 mm 3 /10 6 cycles (over 0-5 million cycles). Ceramic heads (zirconia) have a very smooth finish and had UHMWPE wear rates of only 80% of those of CoCrMo alloy which were rougher. Studies of friction in these artificial joints have led to the conclusion that all of these designs operate under mixed lubrication conditions. Thus they wear. In an attempt to reduce this wear, some designs have moved towards generating a full fluid film lubrication mechanism.