Background: First-generation annealed and second-generation sequentially annealed, highly crosslinked polyethylenes (HXLPEs) have documented reduced clinical wear rates in their first decade of clinical use compared with conventional gamma inert-sterilized polyethylene. However, for both types of annealed HXLPE formulations, little is known about their reasons for revision, their in vivo oxidative stability, and their resistance to mechanical degradation.
Questions/purposes: We asked whether retrieved sequentially annealed HLXPE acetabular liners exhibited: (1) similar reasons for revision; (2) lower oxidation; (3) improved resistance to wear and degradation of mechanical properties; and (4) improved resistance to macroscopic evidence of rim damage when compared with acetabular liners fabricated from single-dose annealed HXLPE.
Methods: One hundred eighty-five revised acetabular liners in two cohorts (annealed and sequentially annealed) were collected in a multicenter retrieval program between 2000 and 2013. We controlled for implantation time between the two cohorts by excluding annealed liners with a greater implantation time than the longest term sequentially annealed retrieval (5 years); the mean implantation time (± SD) for the annealed components was 2.2 ± 1.4 years, and for the sequentially annealed liners, it was 1.2 ± 1.2 years. Reasons for revision were assessed based on medical records, radiographs, and examinations of the retrieved components. Oxidation was measured at the bearing surface, the backside surface, the locking mechanism, and the rim using Fourier transform infrared spectroscopy (ASTM F2102). Penetration was measured directly using a micrometer (accuracy: 0.001 mm). Mechanical behavior (ultimate load) was measured at the superior and inferior bearing surfaces using the small punch test (ASTM F2183). We used nonparametric statistical testing to analyze for differences in oxidation, penetration rates, and ultimate load when adjusting for HXLPE formulation as a function of implantation time.
Results: The acetabular liners in both cohorts were revised most frequently for instability, loosening, and infection. Oxidation indices (OIs) of the sequentially annealed liners were lower than annealed liners at the bearing surface (mean OI difference = 0.3; p < 0.001), the backside surface (mean OI difference = 0.2; p < 0.001), and the rim (mean OI difference = 2.6; p < 0.001). No differences were detected in linear penetration rates between the cohorts (p = 0.10). Ultimate strength at the bearing surface of the HLXPE was not different between sequentially annealed and annealed cohorts (p = 0.72).
Conclusions: We observed evidence of in vivo oxidation in retrieved annealed and, to a lesser extent, retrieved sequentially annealed acetabular liners. However, we observed no association between the levels of oxidation and clinical performance of the liners.
Clinical relevance: The findings of this study document the oxidative and mechanical behavior of sequentially annealed HXLPE. The reduced oxidation levels in sequentially annealed liners support the hypothesis that annealing in sequential steps eliminates more free radicals. However, as a result of the short-term followup, analysis of longer-term retrievals is warranted.