The fallacy of evaluating biomaterial wear-rates with water as lubricant: A hip simulator study of alumina-PTFE and 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.

Wear of ultra-high molecular weight polyethylene and polytetrafluoroethylene in a hip simulator: A dose-response study of protein concentration

“…Waterlubricated tests of both materials showed wearrates much lower than the clinical rates (Saikko 1993, Clarke et al 1995, Good et al 1996, Phipatanakul et al 1998). All such in vitro studies maintained the high PTFE/PE wear ratio, as first described by Charnley. However, Charnley's PTFE revision cups showed that volumetric wear increased linearly with head-diameter (Charnley 1976).…”

“…The ball-size effect for both PTFE and PE has been duplicated in the laboratory with serum lubrication (Good et al 1996, Clarke et al 1997. However, in water-lubricated tests the wear-rates were 4 times lower than clinical and the ball-size effect was lost (Good et al 1996, Phipatanakul et al 1998.…”

Tribological properties and structure of ultra-high molecular weight polyethylene after gamma irradiation

“…It is generally accepted that wear under these conditions is not realistic and the wear rates obtained are not directly applicable to artificial hip joints. The use of distilled water did not prevent the transfer film, formed on the ceramic counterface, while the use of more realistic protein containing bovine serum would prevent the formation of such a transfer film [29,30]. Wear under a more realistic multi-directional motion would be increased significantly, compared with the uniaxial motion considered [31,32].…”

The fundamentals of biotribology and its application to spine arthroplasty

“…40 – 42 The large joint literature indicates that use of nonphysiological lubricants, such as deionized water and saline, can lead to wear, which is highly unrepresentative of in vivo results. 40 , 43 – 45 Physiological based lubricants with protein concentrations similar to that found in human synovial fluid, coupled with appropriate load and motion inputs, have been shown to allow for close predictions of in vivo wear performance for total hip and knee replacements.…”