Abstract
Introduction
Revision of total hip replacements (THRS) is predominantly due to aseptic loosening, pain and infection [1]. The current method used to address the risk of infection is to administer antibiotics and to include antibacterial agents into bone cement (if used) and on implant coatings [2–4]. Currently, silver (Ag) coatings have only been applied to titanium hip stems [3]. Cobalt chromium alloy (CoCr) is a widely used orthopaedic alloy which is commonly used as a bearing surface; revisions of joints using this material often describe adverse reactions to the particulate wear debris [1]. This study considers an Ag containing CrN based coating on a CoCr substrate with the aim to reduce cobalt (Co) release and promote antibacterial silver release.
Methods
Silver Chromium Nitride (CrNAg) coatings were developed and applied onto the bearing surfaces of 48 mm diameter metal-on-metal THRs. Three coatings were evaluated: high Ag at the surface (CrNAg+), low Ag at surface (CrNAg-) and uniform Ag (CrNAg=). All bearings were tested under ISO 14242-3 conditions for 0.17 million cycles (mc) representing approximately 2 months use in vivo. Wear was determined gravimetrically; Ag and Co levels in the lubricant were measured using graphite furnace atomic absorption spectroscopy. Testing of the CrNAg= bearings were continued to 2mc under standard conditions; CrNAg- bearings to 5mc incorporating lateralisation, which created separation at swing phase and rim contact at heel strike. Wear volume and Ag/Co release were monitored at 0.33, 0.67, 1mc and every mc thereafter.
Results
All bearings produced low levels of wear and released silver into the lubricating fluid. An increase in silver concentration at the surface of the bearings was found to increase both the silver released and wear, Figure 1. Negligible cobalt was released.
Testing of the CrNAg= coating to 2mc showed the wear rate to decrease after 0.17mc, Figure 2. Ag release continued up to 2mc but at a decreased rate.
The CrNAg- coating tested under lateralisation conditions to 5mc showed no coating failure and negligible cobalt release. Wear and silver release showed similar patterns and reached a steady state after 1mc, Figure 3.
Discussion
The AgCrN coatings on bearing surfaces of a hip joint are capable of releasing Ag at concentrations within the ‘No Observable Adverse Effect Limit’ [5]. These coatings also prevented Co release while maintaining a low wear rate. All coatings remained intact and did not delaminate, even under adverse conditions. These coatings have been tested in a metal-on-metal hip bearing surface, the most controversial and challenging condition for a coating, wearing against itself. CoCr is used as a bearing surface against polyethylene in hips and knees, in stems and tapers, as tibial trays in knee replacement and as shells for acetabular cups. This coating may be applied to a wide range of applications, removing some of the challenges over the use of CoCr while reducing infections.
