Victoria Good

Water and bovine serum lubrication compared in simulator PTFE/CoCr wear model

Controversy surrounds wear data from laboratory hip simulator studies, whether derived from water-based or serum-based studies or whether a major design parameter such as the size of the femoral head has an effect on the volume of wear particulate released. To investigate these relationships, we studied cup wear in water- and serum-based lubricants using as our standard the polytetrafluoroethylene (PTFE) data derived by Charnley. To model Charnleys clinical experience, PTFE acetabular cups were used in sets of three each of four sizes of CoCr femoral heads: 22.25-, 28-, 32-, and 42-mm diameters. Six criteria were used to evaluate the performance of the lubricants against clinical accuracy and scientific methods. The PTFE wear data from the serum-based tests was consistently linear with the duration of the test, exhibited a precision within +/-3% about the average for each set of three cups, and copious amounts of wear debris were clearly seen circulating and settling to the bottom of the wear chambers. The wear data clearly demonstrated Charnleys thesis that volume of wear increases with regard to the size of the femoral head in a linear manner. This increase was considered satisfactory at 9%/ mm. However, in terms of clinical accuracy, the simulator wear rates averaged 3 to 4 times greater than the comparable clinical data for wear magnitude. Thus, the serum-based tests satisfied three of the six criteria used. The PTFE wear data from the water-based tests was generally nonlinear, continually increasing with test duration. These wear trends were examined in three discrete phases to estimate the changing wear rates. By the end of the tests, the wear rates had increased from 1.3 to 3.9 times, with the 42-mm heads showing the greatest change. The resulting precision was never better than +/-26% and deteriorated to +/-70%. In terms of clinical accuracy, the water-based wear rates varied from 2 to 7 times less than the Charnley PTFE wear magnitudes, averaging 4 times less. The water-based data did not satisfactorily model the relationship between increased wear with increased head size. Minimal PTFE wear debris was observed, and what did emerge after thousands of wear cycles appeared as streamers up to 30 mm long and up to 5 mm wide. When these detached, they floated up to the surface where they could be separated into smaller particulates. A similar phenomenon was noted for polyethylene wear tests conducted with water lubrication. Thus the water-based tests satisfied none of the six validation criteria evaluated. These data raise serious doubts as to the validity of testing implant and material combinations in water as a predictor of clinical performance. Bovine serum was not totally satisfactory, but the wear data did model some of the important clinical characteristics of hip joint behavior.

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

Charnleys laboratory wear studies of non-gamma sterilized polytetrafluoroethylene (PTFE) and polyethylene (PE) found that the PTFE to PE wearrate ratio of 250:1 was much higher than the in-vivo wear ratio of 20:1. Tests of PTFE and PE in our laboratory showed a wear ratio of 150:1, using bovine serum as the lubricant and 190:1 with water as the lubricant. Our hypothesis was that the wear-rates of PTFE and PE cup materials were related to the concentration of protein in the serum. We studied the wear behavior of PTFE and PE cups in varied protein concentrations, using 4 femoral head sizes to validate the clinical range reported by Charnley. The PTFE wear-rates increased with increasing protein concentration and conversely, PE wear-rates decreased with increasing protein concentration. This inverse relationship made it possible to bring the wear ratio closer to the desired clinical wear ratio. We found that the clinically relevant PTFE/PE wear ratio corresponded to 3-10 mg/mL of protein in bovine serum.

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

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 Charnleys 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 Charnleys 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 ceramics 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.

Needs of Bioceramics to Longevity of Total Joint Arthroplasty

Wear on alumina / UHMWPE-THP decreased by 25-30% of that on met al / UHMWPE in hip simulator test and clinical results. Wear on THP of alum ina / alumina was near zero in hip simulator test. In knee simulator test, UHMWPE wear against lumina decreased to 1/10 of that against metal. Clinically we have no revision case due to PE we ar problems for 23 years. In retrieved cases, UHMWPE surface against alumina was very s mooth. On UHMWPE surface against metal, many fibrils and scratches were found. In IBBC loosening in acetabulum occurred in 2.5% in 268 joints in only early cases at 16 to 14 years after THA. Only one joint was revised. At revision THA with massive bone defect, HA granules were filled. Socket migrations in two joints and partial spaces in two joints occurred in total 40 joints at 17 to 5 years. Bioceramics was found to be indispensable in enduring total joint arthroplasty . Introduction In order to keep the longevity of total joint arthroplasty, extrem ely low wear bearing materials and maintaining adequate fixability to the bone forever are desired. In our experimental and long term clinical experiences, it has been found that bioceramics, including bioinnert and bioactive ceramics, have been playing a maj or p rt in enduring total joint arthroplasty. The production of particulate wear debris from implant materials a nd subsequent osteolysis has been recognized as the major cause of long term failure in tot al hip replacement. The basic strategy to address the problem of osteolysis should be to reduce the number of polyethylene particles generated by improving the materials at the articulating count erfaces. The use of a ceramic femoral head has been advocated especially in young active patients because it produces less polyethylene wear compared with a conventional metal femoral head. However, an attempt to eliminate the use of polyethylene has been made through the use of metal-on-metal and ceramic-on-ceramic articulations. In 1970, to increase the wear resistance of polyethylene, wear te sts w re performed on RCH 1000 [ultrahigh molecular weight polyethylene (UHMWPE), molecular we ight, 106] irradiated at several levels of high-dose gamma radiation emitted by 60Co. The wear ra te was smallest at 100 Mrad. Sockets cross-linked by gamma radiation at 100 Mrad were used clinically from 1971 to 1978. We also experimentally confirmed that UHMWPE (molecular weig ht, 6 x 106) showed less wear in an alumina-on-UHMWPE combination than in the metal-on-UHMWPE combina tion. In 1977, we began to use 28 mm alumina balls. In our clinical experience, it was found that the thicker the poly ethylene socket, the lower the wear rate. To use a thicker UHMWPE sockets, the femoral head size was decreased with time: 26 mm alumina femoral heads were used from 1989 to 1994, 22 mm alumina femoral h eads were used from 1994 to 1995, and 22 mm zirconia femoral heads were used from 1995 to 1996. S ince 1996, Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 735-754 doi:10.4028/www.scientific.net/KEM.240-242.735

Low-Wear Effect of High-Dose Gamma-Irradiated Crosslinked Polyethylene in Total Hip Prostheses

The wear rate of crosslinked sockets irradiated in air with gamma-irradiation of 100 Mrad, which were used in total hip arthroplasty clinically between 1971 and 1978, was 18% of that of non-crosslinked sockets. As the sockets were irradiated in air, the surface was oxidized. The weight-bearing portion of retrieved crosslinked sockets presented a clearly outlined pattern, irregularly lined with smooth ripples of 0.1 um. Delamination, flaking, burnishing, and scratch were not observed. In our wear tests, using cylinder-on-flat, sphero-flat reciprocating, and hip simulator test, crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) irradiated with 50 to 1000 Mrad showed extremely low wear, even when the UHMWPE was irradiated in the air and the surface was oxidized. In our hip simulator test with 12 channels using 30% bovine serum as lubricant, crosslinked sockets irradiated with 50 to 150 Mrad, in which the oxidized surface was eliminated, showed no wear. After radiation, the mechanical properties of elongation and tensile strength of polyethylene decreased. The hardness and the ultimate tough strength increased. The chemical properties of gel contents, oxidation (C = O), and double bond (C = C) increased.