A comparison of wear of 36 mm and 28 mm metal-on-highly cross-linked polyethylene articulations in primary total hip replacements

Abstract

Total hip replacement is one of the most frequently performed and successful surgical procedures. Its most common modes of failure identified in joint registries are dislocation in the short term and aseptic loosening associated with wear and osteolysis in the long term. Therefore, the ideal articulation would have both a low incidence of dislocation and low wear. Metal-on-highly cross-linked polyethylene (HXLPE) articulations of 36 mm diameter have been demonstrated in a randomised controlled trial to have a significantly lower incidence of dislocation at one year postoperatively compared to 28 mm articulations. Historically, large articulations (femoral head size ≥32 mm) have been associated with increased wear rates of conventional polyethylene compared to smaller articulations. Advances in polyethylene manufacture with cross-linking for clinical use in total hip replacements has significantly reduced early wear rates compared to conventional polyethylene. This has prompted reconsideration of the ideal femoral head size to enhance the longevity of articulations. This study aims to compare the wear of 36 mm and 28 mm metal-on-highly crosslinked polyethylene total hip replacements through a post hoc analysis of radiographs of patients enrolled in the randomised controlled trial referred to above. Comparison of wear rates between cohorts was undertaken by use of computer-assisted analysis (PolyWare™) of patient radiograph sets. Radiograph sets for 326 patients, 164 with 28 mm and 162 with 36 mm articulations, were analysed. 36 mm metal-on-HXLPE articulations were found to have a statistically significant higher magnitude of bedding-in and creep at three but not twelve months when compared to the 28 mm cohort. The mean annual two-dimensional wear rate from 1 year until final radiograph was 0.00mm/yr for both cohorts. There were no differences between 36 mm to 28 mm cohorts in mean annual volumetric wear rates or significant differences in the proportion of patients in each cohort with two-dimensional wear rates ≥ 0.1 mm/yr or volumetric wear rates ≥ 80 mm³/yr. These wear rates have previously been associated with osteolysis when using metal-on-conventional polyethylene articulations. While the use of large articulations had been reported to be associated with comparatively greater wear rates of articulations incorporating conventional PE, this appears not to apply to large articulations incorporating HXLPE. The low wear rates measured combined with the findings of the RCT of a significantly reduced incidence of dislocation at one year of 36mm compared to 28mm articulations, support the use of 36 mm metal-on-highly cross-linked polyethylene articulations. Longer term follow-up is required to assess whether low wear rates are maintained for both 36mm cohorts and whether wear of HXLPE is associated with the development of periprosthetic osteolysis.