Third-body abrasive wear challenge of 32 mm conventional and 44 mm highly crosslinked polyethylene liners in a hip simulator model

Abstract

Hip simulator studies have shown that wear in the polyethylene liners used for total hip replacements increased with the larger-diameter femoral balls and could also be exacerbated by third-body abrasion. However, they also indicated that the more highly cross-linked polyethylene (HXPE) bearings were more wear resistant than conventional polyethylene (CXPE) bearings. Unfortunately the HXPE bearings appeared to be particularly sensitive to adverse wear conditions. One simulator study in particular indicated that poly(methyl methacrylate) (PMMA) debris increased wear sixfold by means of two-body abrasive interactions rather than the supposed third-body abrasion or roughening effects of the Co—Cr surfaces. There has been no confirmation of such novel theories. Therefore the goal of this study was to investigate the sensitivity of large-diameter HXPE bearings to the third-body PMMA wear challenge in a hip simulator model. An orbital hip simulator was used in standard test mode with a physiological load profile. The 32 mm control liners were machined from moulded GUR1050 and gamma irradiated to 35 kGy under nitrogen (CXPE). The 44 mm liners were also from moulded blanks, gamma irradiated to 75 kGy, machined to shape, given a proprietary heat treatment, and sterilized by gas plasma (HXPE). As in the published simulator model, the study was conducted in three phases. In phase 1, all cups were run in standard (‘clean’) lubricant for 1.5×10⁶ cycles duration. In phase 2, three CXPE cups and six HXPE cups were run for 2×10⁶ cycles with a slurry of PMMA particles added to the lubricant. In phase 3, the implants were again run in ‘clean’ lubricant for 2×10⁶ cycles duration. In addition, three HXPE cups were run as wear controls for 5.5×10⁶ cycles duration in clean lubricant. In phase-1, the HXPE liners demonstrated twelvefold reduced wear compared with the CXPE controls. The 32 mm and 44 mm Co—Cr balls were judged of comparable roughnesses. However, the surface finish of HXPE liners was superior to that of CXPE liners. In phase-2 abrasion, wear rates increased sixfold and eightyfold for CXPE and HXPE bearings respectively. These data confirmed that HXPE bearings were particularly sensitive to ‘severe’ test modes. The Co—Cr balls revealed numerous surface patches representing transferred PMMA with average transient roughness increased to 25 nm and 212 nm for the 32 mm and 44 mm balls respectively. These PMMA patches produced an aggressive two-body abrasion wear of the polyethylene. After cleaning, the ball roughness returned to near normal. Therefore the Co—Cr roughness was not an issue in this severe test mode. In phase 3, the wear decreased to near the index values of phase 1, while liner roughness dropped by more than 90 per cent. The control CXPE liners now demonstrated twice the wear of the HXPE, as would be predicted comparing the diameter and cross-linking algorithms. No previous study has correlated polyethylene roughness profiles to wear performance. In phase 2, PMMA abrasion created significant damage to the polyethylene surfaces. The average roughness S_a of CXPE liners increased to 3.6 μm, a twenty-fourfold increase with some scratches up to 40 μm deep. The HXPE roughness also increased but only to 1.5 μm, a ninefold increase. The scratch indices S_z and S_p for HXPE surfaces were also 50 per cent less severe than on CXPE surfaces. However, within 2×10⁶ cycles duration of phase 3, all liners had recovered to virtually their original surface finish in phase 1. In all test phases, the surface finish of the HXPE liners remained superior to control liners. These experimental data confirmed many of the results from the previous simulator study with the PMMA abrasion models. Thus the 44 mm liners appeared an excellent clinical alternative to the smaller ball designs used in total hip replacements.