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

Abstract

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 Charnley's 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 Charnley's 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.