Abstract
Is is believed that 3rd-body wear of polyethylene, be it from particles of bone, bone-cement (PMMA), or metal, is an unavoidable risk in total hip arthroplasty (THA). Simulator studies have demonstrated that wear in conventional polyethylene (CXPE) and highly crosslinked polyethylene (HXPE) cups increased 6 and 20-fold respectively when challenged by circulating 3rd-body PMMA particulates. There was no corresponding change in head roughness, i.e. the PMMA did not roughen CoCr surfaces. Many contemporary cup designs now use the vitamin-E process combined with higher crosslinking dosage (VEPE). However, little if anything is known about the VEPE debris. Therefore in this study we analyzed the morphology of VEPE particles from cups that had been run in, a) standard simulator test mode and b) adverse PMMA debris-challenge mode. The aim of this study was to determine how a clinically relevant challenge, such as addition of PMMA particles affected the wear debris. This had not been attempted previously due to contamination polyethylene by PMMA debris. The hypotheses were that, a) during the ‘clean’ test, VEPE would yield smaller debris of standard globular shape compared to controls (XPE) and b) in adverse PMMA challenge mode, VEPE debris size would increase and become more flake-like.
The XPE and vitamin-E blended cups (VEPE) cups were gamma-irradiated at 7.5 Mrad and 15 Mrad, respectively. Cups were run Inverted and mated with ceramic femoral heads of diameter 44 mm (Biolox-delta, Ceramtec). The three test phases included; ‘clean’ for 6 million cycles (6 Mc), abrasive slurry 6–8 Mc (concentration 10g/L), and ‘clean’ 8–10 Mc. The debris was isolated using standard procedure for ‘clean’ tests and a modified procedure for the abrasive slurries. Particles were imaged using SEM and the micrographs analyzed (Image J). Approximately 600 particles were analyzed from each sample (4.5 Mc and 8 Mc) and morphology defined via aspect ratio (AR), equivalent circular diameter (ECD), and circular shape factor (CSF).
The clean test revealed slight differences in shape factors for XPE and VEPE (AR, CSF within 30%: p <0.0001) but none with regard to size (p > 0.9999). The median ECD for both XPE and VEPE was approximately 0.55 μm. The abrasive test revealed a statistical difference (p < 0.0001) in shape compared to the clean test, but varied less than 25%. The greater change in debris morphology between the abrasive test and clean test was size, which increased 3.6 fold for VEPE particles (ECD = 2.0 μm) and 4.3 fold for XPE particles (ECD = 2.3 μm).
It was determined that addition of vitamin E to the PE did not change the size, but did change the shape of PE debris particles up to 30%. This study was the first to isolate debris particles during an abrasive slurry test and determine morphology under such conditions. Debris particles formed in abrasive conditions were found to be 4-fold larger in diameter, suggesting a larger volume of shreds in comparison to the mostly submicron population observed under standard testing conditions.
Figure 1: Boxplot of equivalent circular diameter values.
Figure 2: Boxplot of aspect ratio values.
Figure 3: Boxplot of circular shape factor values.
