Wear reduction effect on ultra-high-molecular-weight polyethylene by application of hard coatings and ion implantation on cobalt chromium alloy, as measured in a knee wear simulation machine

The most important factor affecting performance and longevity of hip joint and knee implants is the wear rate of the ultra-high-molecular-weight polye thylene (UHMWPE) component. UHMWPE wear debris has been linked to complicat ions including tissue inflammation, bone loss (osteolysis) and implant loos ening. Reduction of debris has been addressed by investigating new polyethy lene formulations, manufacturing and finishing processes, including surface treatments and coatings both on plastic and metallic components. There hav e been many studies on the effect of surface treatments and hard coatings o n cobalt chromium and titanium alloys for prosthetic applications. However, most of them have used laboratory tribo-testers without much correlation t o articulating movements in human joints. In this work, hard coatings (TiN and DLC) and surface treatments (nitrogen ion implantation) have been inves tigated as potential candidates to reduce wear of UHMWPE, when applied on t he counter face cobalt chromium alloy. Also the effect of applying nitrogen ion implantation on the UHMWPE surface itself has been investigated. To ev aluate wear performance a special knee wear simulator has been used with a combined rolling-sliding movement that corresponds to the most unfavourable situation in the knee. As testing materials, conventional coated and uncoa ted Co-Cr as well as Al2O3 femoral heads were used against UHMWPE plates to obtain comparative data. Testing was carried out at 50 MPa using distilled water at 37 +/- 2 degreesC as a lubricant. Wear measurements on UHMWPE wer e made following ASTM F732 standard by weight loss measurements and also by laser profilometry. The results, LIP to 5 million wear cycles that represe nt approximately 3 years of implant life, clearly have demonstrated the ben eficial effects of diamond-like carbon (DLC) and ion implantation (both on Co-Cr and UHMWPE) in reducing wear of UHMWPE. Similar values were also obta ined for Al2O3 ceramic material. A wear reduction up to approximately five times was obtained by the former materials, in comparison with the uncoated control specimen. However, the ion plated TiN coating increased significan tly wear on the UHMWPE. Transference of UHMWPE to the ball was very evident with the uncoated control and TiN coated Co-Cr, as observed by SEM, but wi th DLC and the ceramic material this occurred to a much lesser extent. Some micro-delamination at the worn surface of the UHMWPE was observed when wea ring against uncoated Co-Cr, but was not clear in the rest options. Ion imp lantation and DLC can thus be two good candidate treatments to reduce wear of UHMWPE in cobalt chromium knee and hip joint implants in substitution of a more expensive ceramic material. This DLC has passed all biocompatible t ests in accordance with FDA regulations and ISO 10993 standards for implant able devices. An example of I uniformly DLC coated femoral head and knee im plant is shown. (C) 2001 Elsevier Science B.V. All rights reserved.