Early outcomes of sequentially cross-linked thin polyethylene liners with large diameter femoral heads in total hip arthroplasty.

UNLABELLED: Two important issues affecting the outcome of total hip arthroplasty have been dislocation and wear, despite excellent clinical results. Larger femoral heads have had success in decreasing dislocation rates; however, there are concerns regarding the subsequent use of thinner polyethylene liners, and their effects on wear rates. Historically, high stresses on thin polyethylene bearings have caused concerns, including rim cracking and catastrophic implant failure with polyethylene thicknesses less than 5 millimeters. Recently, sequentially cross-linked and annealed polyethylene has been shown to reduce the wear rate significantly, compared to conventional ultra-high-molecular-weight polyethylene (UHMWPE) in vitro. The purpose of this study was to analyze the clinical and radiographic outcomes in a cohort of patients treated with sequentially cross-linked and annealed polyethylene bearings with a nominal thickness of 3.8 millimeters. Outcomes were compared to a similar cohort of patients who were treated with total hip arthroplasty during the same time period and utilizing the same polyethylene thickness of 5.8 millimeters or greater.
MATERIALS AND METHODS: We identified 50 patients (53 hips) who had a minimum 2-year clinical and radiographic follow-up after a standard total hip arthroplasty, performed with a thin, sequentially cross-linked and annealed polyethylene bearing surface. There were 15 males and 35 females, with a a mean age of 60 years (range, 16 to 93 years) and a mean body mass index of 28.6 kg/m² (range, 17.2 to 47.5 kg/m²). Overall survivorship was compared to a cohort of 50 patients (53 hips) treated with total hip arthroplasty during the same time period, utilizing the same polyethylene of 5.8 millimeters or greater thickness. Radiographic analysis of polyethylene wear was performed on a subset of 26 hips, using a previously validated two-dimensional computer-aided technique. Volumetric wear was calculated and subsequent annual volumetric and linear wear rates were derived for each patient in the study cohort. Additionally, radiographic analysis was performed to assess for any progressive radio-lucencies or malalignment.
RESULTS: The overall survivorship of the study cohort was 100%, compared to a 96% survivorship in the comparison group (two failures due to infection). The mean Harris hip scores in the thin polyethylene cohort improved from 43 points (range, 10 to 67 points) pre-operatively to 91 points (range, 69 to 100 points) postoperatively. Upon radiographic review, no malalignment, radiolucencies, or polyethylene fracture was noted in the study cohort. The mean volumetric wear rate was 0.4122 mm³/year (range, 0.2311 to 0.7310 mm³/year), and the mean linear wear rate was 0.0004 mm/year (range, 0.0002 to 0.0007 mm/year) for the thin polyethylene group. The mean volumetric wear was 0.8839 mm³ (range, 0.4621 to 1.5839 mm³) for this cohort. Excellent clinical and radiographic outcomes were found for patients treated with thin, sequentially cross-linked and annealed polyethylene bearings utilized in total hip arthroplasty, with a nominal thickness of 3.8 millimeters. We have not seen any failures with thin polyethylene liners that have undergone this manufacturing process, which is in contradistinction to results of previously reported thin polyethylene liners. Wear rates were lower than other bearing surfaces at similar periods.