Enhanced wear performance of highly crosslinked UHMWPE for artificial joints

Abstract

It is well known that osteolysis induced by polyethylene wear debris is the main cause of long-term failure of hip and knee prostheses. We developed a treatment of medical-grade ultra-high molecular-weight polyethylene (UHMWPE) in order to improve its tribologic properties and reduce its wear. Medical-grade UHMWPE was irradiated with a 200 kGy dose of radiation, thermally stabilized at a temperature close to the melting point, and then sterilized with ethylene oxide. The irradiation treatment was performed to crosslink the UHMWPE. The thermal stabilization treatment, contributing to the reaction between the free radicals generated by the irradiation process, was chosen to enhance crosslinking and to prevent oxidation and the shortening of chains. The non-invasive sterilization process with ethylene oxide was chosen to prevent the re-formation of free radicals. The wear performance of this material was compared to UHMWPE, untreated or treated with different sterilization techniques, using gamma and beta irradiation. Insoluble crosslinked constituents were measured with an extraction method. Wear was evaluated using a flat-on-ring wear test machine. While small differences were found among the different sterilization processes, 200 kGy-irradiated UHMWPE followed by thermal treatment and sterilization with ethylene oxide had the least wear and the greatest amount of crosslinking.

Mechanical properties of ultra-high molecular weight polyethylene irradiated with gamma rays

“…It is therefore concluded that, post-irradiation heat treatment at temperatures lower than the melting temperature is favored over post-irradiation remelting, which should reduce crystallinity of the material. 2,5,6 Conclusions Crystalline structure and mechanical properties including creep resistance were investigated for the UHMWPEs that were gamma-irradiated and heat-treated at a temperature lower than the melting temperature. ESR analysis ascertains that most of residual radicals are stabilized by the annealing.…”

“…Upon gamma irradiation, following the formation of free radicals, chain scission as well as crosslinking occurs. [3][4][5][6][7][8][9][10] During shelf storage or in vivo, residual radicals react with oxygen diffusing from the surface, which generates peroxy radicals. The peroxy radicals continue to react over the period of years to lead to oxidative degradation.…”

“…Hence, irradiation or post-irradiation heat treatment under an oxygenfree atmosphere is required. 2,5,6 Since the late 1990s a group of new prostheses with enhanced wear and oxidation resistance have been introduced. 2,5,6 They utilize gamma irradiation and post-irradiation remelting at temperatures above the melting temperature to stabilize the residual radicals remaining mainly in the crystalline region.…”

“…2,5,6 Since the late 1990s a group of new prostheses with enhanced wear and oxidation resistance have been introduced. 2,5,6 They utilize gamma irradiation and post-irradiation remelting at temperatures above the melting temperature to stabilize the residual radicals remaining mainly in the crystalline region. However, heat treatment at such a high temperature may result in the decrease in crystallinity and creep resistance, which is also important for the longevity of prosthesis material, and consequently reduce the positive effect of irradiation-induced crosslinking.…”

Wear evaluation of cobalt–chromium alloy for use in a metal‐on‐metal hip prosthesis

“…Research in other laboratories [43][44][45][46][47] has documented the improvements in wear properties and the changes in physical properties of polymers that have been cross linked as a part of the material processing or sterilization process. In addition, research on the same equipment used for the current study with the same protocols 48 -50 has shown that the cross-linked polyethylenes may exhibit wear properties that yield gravimetric weight losses very similar to those for the metal-on-metal samples.…”