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CrN‐coated metal‐on‐metal hip bearings have the poten6al to reduce wear and cobalt ion release
*1Royle, M; 1Lu, F; 2Fox, A; 2Housden, J; 3Leyland, A; 4Ellison, P; 4Collins, S; 5Lali, F V; 5Hart, A J; 1Shelton, J C 1Queen Mary University of London, UK; 2Tecvac, UK; 3University of Sheffield, UK; 4Corin, UK; 5Imperial College, UK *Corresponding author: MaQ Royle, [email protected]. School of Engineering and Materials Science
INTRODUCTION With increased use of large diameter metal‐on‐metal (MoM) hip bearings there are concerns with metal par\cles, corrosion products and increased metal ion levels on the incidence of adverse biological reac\ons [1]. More recently there have been reports of pseudotumours [2, 3], metallosis [4] and asep\c lymphocy\c vasculi\c‐associated lesions (ALVAL) [5]. MoM bearings contain 27 ‐ 30% Cr and 57 ‐ 68% Co. In vitro wear par\cle cytotoxicity inves\ga\ons have studied the effects of bulk CoCr par\cles [6]. However, it has been suggested that Co induces a greater cytotoxic reac\on compared to Cr, both in par\culate and ionic form [7]. Chromium nitride (CrN) coa\ngs have been used in a variety of engineering applica\ons including extrusion dies [8], forming and machine tools [9], and aerospace [10], and have demonstrated excellent sliding wear resistance [11]. Furthermore, nanometer‐sized CrN par\cles have shown improved biocompa\bility compared to CoCr par\cles [12].
AIMS To compare the wear of conven\onal MoM bearings with a hard CrN‐coated surface, and to inves\gate the poten\al of the coa\ng to reduce Co release.
MATERIALS AND METHODS Test components Ten Ø48 mm heads and cups were manufactured from hot isosta\cally pressed and solu\on annealed CoCrMo alloy to ASTM F75 (Corin, UK). Five bearings were tested uncoated as MoM. Five bearings were coated with CrN using electron beam physical vapour deposi\on (EBPVD) (Tecvac, UK), to a mean thickness of 8µm. The mean diametrical clearance was 157 and 165 µm for the MoM and CrN‐coated, respec\vely. Wear tes\ng was performed on an 8‐sta\on orbital hip joint simulator (MTS Systems, USA). • Load profile: 2.45 kN peak load at 1 Hz frequency • Lubricant: Newborn calf serum diluted to 25% (protein: 17g/L) with 0.1% w/v sodium azide added
• Fixtures: PTFE‐coated to prevent ion contamina\on • Cup inclina\on angle: 35° (MoM n=3; CrN‐coated n=2)
60° (MoM n=2; CrN‐coated n=3) Wear and ion release Wear was determined gravimetrically at 0.17, 0.33, 0.67, 1.00 and 2.00 million cycles (mc) using the densi\es 0.0083 and 0.0059 g/mm3 for MoM and the CrN coa\ng, respec\vely. Total Co and Cr content of the lubricant (ions plus par\cles) was measured using graphite furnace atomic absorp\on spectrometry (GFAAS) over the first 24 hrs (86,400 cycles), at 0.17 and 0.33 mc, and then every 0.33 mc up to 2.00 mc.
DISCUSSION Ion measurement is a complimentary technique to the standard gravimetric method, which may avoid the piqalls associated with taper‐fixture wear and protein deposi\on etc. During phases of mass gain, established from the gravimetric measurements, Cr release measured using GFAAS con\nued from the CrN‐coated bearings indica\ng ultra‐low wear behaviour. The reduced variance of gravimetric and ion results indicated that the CrN coa\ng may produce a more homogeneous bearing surface, with more reproducible wear proper\es compared to MoM.
REFERENCES [1] Visuri et al. (2005). Proc IMechE Part H: Eng Med, 220. [2] Pandit et al. (2008). JBJS Br, 90. [3] Campbell et al. (2010). CORR, 468. [4] Campbell et al. (2006). 52nd Mee\ng of ORS, Chicago, USA. [5] Willert et al. (2005). JBJS Am, 87. [6] Brown et al. (2006). Proc IMechE Part H: Eng Med, 220. [7] Kwon et al. (2009). J Biomed Mater Res B, 4. [8] Björk et al. (2001) Surf Coat Tech, 146‐147. [9] Fuentes et al. (2005). J Mater Process Tech, 167. [10] Swadzda et al. (1996). Surf Coat Tech, 78. [11] Bienk et al. (1995). Surf Coat Tech, 76‐77. [12] Williams et al. (2003). Proc IMechE Part H: Eng Med, 217. [13] Leslie et al. (2009). J Biomed Mater Res B, 90. [14] Vassiliou et al. (2006). Proc IMechE Part H: Eng Med, 220.
Figure 1. Mean gravimetric wear volume • Rela\ve standard devia\on (%RSD) shown at end of each wear phase; it generally decreased as cycle number increased, tending to be smaller for CrN‐coated
• Mass gain for low angle CrN‐coated (35°) indicated ultra‐low volumetric wear as reported previously [13]
Figure 2. Three different wear phases iden6fied • 35° MoM wear rates consistent with literature [13, 14] • 60° cup inclina\on had minimal effect on wear • CrN‐coated bedded‐in sooner than MoM; the ultra‐low wear became difficult to measure precisely using the standard gravimetric technique
RESULTS
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