Abstract
Introduction
Sequentially annealed highly crosslinked polyethylenes (HXLPEs) were introduced in total knee replacement (TKR) starting in 2005 to reduce wear and particle-induced osteolysis. Few studies have reported on the clinical performance of HXLPE knees. In this study, we hypothesized that due to the reduced free radicals, sequentially annealed HXLPE would have lower oxidation levels than gamma inert-sterilized controls.
Methods
145 tibial components were retrieved at consecutive revision surgeries at 7 different surgical centers. 74 components were identified as sequentially annealed HXLPE (X3, Stryker) while the remainder (n = 71) were conventional gamma inert sterilized polyethylene. The sterilization method was confirmed by tracing the lot numbers by the manufacturer. The conventional inserts were implanted for 1.7 years (Range: 0.0–9.3 years), while the X3 components were implanted 1.1 years (Range: 0.0–4.5 years). Surface damage was assessed using the Hood method. Oxidation analysis was performed in accordance with ASTM 2102 following submersion in boiling heptane for 6 hours to remove absorbed lipids. 30 of the conventional and 29 of the HXLPE inserts were available for oxidation analysis.
Results
The predominant reasons for revision were loosening, instability, and infection for both material cohorts. None of the highly crosslinked tibial inserts were revised for osteolysis. Pitting, scratching, and burnishing were the predominant damage mechanisms within both material groups. Delamination was not present within the highly crosslinked inserts. Oxidation indices were similar between the two cohorts at the bearing surface, AP face, and the post (p>0.30). Oxidation was lower at the backside surface in the HXLPE group when compared to the gamma inert group (p=0.04). In the HXLPE group, the backside surface had lower oxidation indices than the bearing surface (p = 0.02), post (p < 0.02), and AP Face (p = 0.001). In the HXLPE cohort, only the bearing surface was positively correlated with implantation time (Rho = 0.51; p = 0.01); whereas, the oxidation indices in the gamma inert cohort were not correlated with implantation time.
Discussion
This study evaluated the surface damage mechanisms and oxidative stability of 2nd generation HXLPE in total knee replacement. Sequentially annealed HXLPE inserts exhibited comparable surface damage mechanisms and oxidation indices as compared with conventional inserts. Additional, longer-term HXLPE retrievals are necessary to ascertain the long term in vivo stability of these materials in total knee replacement.
