The fallacy of evaluating biomaterial wear-rates with water as lubricant: a hip simulator study of alumina-PTFE nd CoCr-PTFE combinations.

Abstract

Controversy surrounds wear data from hip-simulator studies, whether from the choice of lubricants or other parameters such as the particular biomaterial combinations used, and whether any such interactions could bias the resulting wear predictions. To investigate these phenomena, we studied the wear performance of CoCr and alumina femoral heads, in water and serum-based lubricants, using as our standard the polytetrafluoroethylene wear data derived clinically by Charnley. To model Charnley's clinical experience, PTFE acetabular cups were used in sets of three each with each size of femoral head for 22.25, 28, and 42-mm diameters in a nine-channel hip simulator. From the serum-based tests, the CoCr-PTFE wear data were consistently linear with duration of test, exhibited very large wear rates of 3,000-8,400 mm3/10(6), cycles had a precision within +/- 4% for each set of three cups, and copious amounts of small particulate were clearly seen circulating. The wear data clearly demonstrated Charnley's thesis that volume of wear increased with regard to size of femoral head. From the water-based tests, the CoCr-PTFE wear data were nonlinear with duration of test, had much reduced wear rates compared to the serum tests, lost the clinical relationship with ball size, and precision deteriorated to +/- 27% for each set. The wear debris appeared as 1-2 cm long ribbons which floated to the surface. For the alumina-PTFE combination in serum, the wear data appeared identical in performance to the CoCr-PTFE data in serum. Thus, the PTFE wear rates were not sensitive to the choice of femoral-head material. The most surprising outcome in this study was the zero-wear performance of the ceramic-PTFE combination in water. This contrasted remarkably with the large wear rates established for the same combinations run in serum. The zero-wear performance of the ceramic-PTFE combination in water was unexpected, but a similar phenomenon was noted in published simulator tests of ceramic-UHMWPE run in water. It now seems likely that such data may reflect the capricious behavior of water lubrication rather than any other variables under evaluation. The water-based experiments clearly favored the ceramic's superior tribological performance and placed metal bearings at a decided disadvantage. Therefore, for an in vitro simulation of materials wear-ranking of clinical relevance, it may be advisable to use a serum-based lubricant.