Wear of large ceramic bearings

Abstract

Large diameter ceramic bearings are of increasing interest due to the enlarged range of motion, enhanced stability of the artificial hip joint [1], and reduced risk of dislocations. Larger diameter hard on hard bearings may as well change the wear characteristics due to larger wear areas or different lubrication behaviour from changed diameter tolerances as known from Me-Me large bearings. But not only hard-on-hard bearings are of interest. With new low wear highly cross-linked polyethylene, wear behaviour of large ball heads against XPE liners is in focus. Three different wear studies were conducted: 1. Ce-Ce: Alumina matrix bearings 40 mm and 44 mm with different diameter tolerances were tested according to DIN EN 14242. Roundness of the ball head and insert as well as clearance of the bearing partners have been varied. 2. Ce-XPE: Mean volume change rates have been compared between a 36 mm Biolox® forte ceramic ball head and CoCr ball heads. Both heads articulated against a XPE liner. 3. Ce-Ce: 36 mm diameter bearings in microseparation mode with two different ceramic materials were tested, one a pure alumina, the other an alumina matrix composite. The different wear studies show: 1. Large ceramic bearings have a very low wear rate. The influence of the clearance on the wear rate is negligible. 2. Using a ceramic ball head against a highly crosslinked polyethylene liner reduces the wear rate by 40% compared to metal ball heads. 3. Even in microseparation mode the wear volume is very low compared to other bearing materials. The wear volume is similar to previously performed microseparation wear studies of 28 mm bearings. The wear volume depends on the used combination of the two different bearing materials. The alumina matrix composite (Biolox® delta) has 6 fold less wear when compared with the alumina couple. The superior wear characteristics of large ceramic bearings have been proven in all tribological test setups. The use of ceramics in a hip replacement will significantly reduce the risk of particle induced osteolysis leading to an increased longevity in the human body.