Abstract
Introduction
Over 40-years the dominant form of implant fixation has been bone cement (PMMA). However the presence of circulating PMMA debris represents a 3rd-body wear mechanism for metal-on-polyethylene (MPE). Wear studies using PMMA slurries represent tests of clinical relevance (Table 1). Cup designs now use many varieties of highly-crosslinked polyethylene (HXPE) of improved wear resistance. However there appears to be no adverse wear studies of vitamin-E blended cups.1–4 The addition of vitamin E as an anti-oxidant is the currently preferred method to preserve mechanical properties and ageing resistance of HXPE. Therefore the present study examined the response of vitamin-E blended liners to PMMA abrasion combined with CoCr and ceramic heads. The hip simulator wear study was run in two phases to compare wear with, (i) clean lubricants and (ii) PMMA slurries.
Methods
The vitamin-e blended polyethylene liners (HXe+) were provided by DJO Surgical (Austin, TX) with 40mm CoCr and ceramic femoral heads (Biolox-delta). Polyethylene liners were run in standard “Inverted” test. (Table 1) All cups were run in ‘clean’ serum lubricant for 6-million load cycles (6Mc)5 and in a debris slurry (PMMA: 5mg/ml concentration) for 2Mc.4 A commercial bone cement powder was used as “abrasive” (Biomet, Warsaw, IN). PMMA slurries were added at test intervals 6, 6.5, 7 and 7.5Mc.4 Wear was assessed gravimetrically and characterized by linear regression. Bearing roughness was analyzed by interferometry and SEM.
Results
The acetabular cups showed low wear-rates to 6Mc duration with both ceramic and CoCr heads (Fig. 1). The debris slurries created much higher wear-rates, whereas control liners continued as before (Table 2).
Discussion
This is the 1st study of vitamin-E blended polyethylene under 3rd-body wear conditions. With clean lubricant conditions, CPE combination wore approximately 50% less than MPE combination. Under abrasive lubricant conditions, CPE and MPE combinations wore at approximately same rate but more than an order of magnitude greater than the clean test phase. This was typical of such PMMA abrasion tests (Table 1). However there are no guidelines as to optimal choice of particulate type, morphology, dosage, frequency of injection or duration of test intervals (Table 1). The production of particulates in vivo is an unpredictable phenomenon and consequently laboratory simulation is fraught with uncertainties.6 On completion of the abrasive challenge (6–8Mc duration), our study will continue under clean conditions to 10Mc to define the important recovery phase of the vitamin-E blended polyethylene.3
