Characterization of wear particles generated from CoCrMo alloy under sliding wear conditions

• Published in: Wear 2011-07, Vol.271 (9), p.1658-1666
• Main Authors: Pourzal, R., Catelas, I., Theissmann, R., Kaddick, C., Fischer, A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Biological and medical sciences; Chemical composition; Chromium; Cobalt; CoCrMo; Cylinders; Diseases of the osteoarticular system. Orthopedic treatment; Exact sciences and technology; Friction, wear, lubrication; Hip joint; Machine components; Mechanical engineering. Machine design; Medical sciences; Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects); Simulation; TEM; Tribometers; Wear; Wear particles
• ISSN: 0043-1648; 1873-2577
• DOI: 10.1016/j.wear.2010.12.045

Biological effects of wear products (particles and metal ions) generated by metal-on-metal (MoM) hip replacements made of CoCrMo alloy remain a major cause of concern. Periprosthetic osteolysis, potential hypersensitivity response and pseudotumour formation are possible reactions that can lead to early revisions. To accurately analyse the biological response to wear particles from MoM implants, the exact nature of these particles needs to be characterized. Most previous studies used energy-dispersive X-ray spectroscopy (EDS) analysis for characterization. The present study used energy filtered transmission electron microscopy (TEM) and electron diffraction pattern analysis to allow for a more precise determination of the chemical composition and to gain knowledge of the crystalline structure of the wear particles. Particles were retrieved from two different test rigs: a reciprocating sliding wear tribometer (CoCrMo cylinder vs. bar) and a hip simulator according to ISO 14242-1 (CoCrMo head vs. CoCrMo cup). All tests were conducted in new born calf serum (30 g/l protein content). Particles were retrieved from the test medium using a previously published enzymatic digestion protocol. Particles isolated from tribometer samples had a size of 100–500 nm. Diffraction pattern analysis clearly revealed the lattice structure of strain induced hcp ɛ-martensite. Hip simulator samples revealed numerous particles of 15–30 nm and 30–80 nm size. Most of the larger particles appeared to be only partially oxidized and exhibited cobalt locally. The smallest particles were Cr 2O 3 with no trace of cobalt. It optically appeared that these Cr 2O 3 particles were flaking off the surface of larger particles that depicted a very high intensity of oxygen, as well as chromium, and only background noise of cobalt. The particle size difference between the two test rigs is likely related to the conditions of the two tribosystems, in particular the difference in the sample geometry and in the type of sliding (reciprocating vs. multidirectional). Results suggest that there may be a critical particle size at which chromium oxidation and cobalt ionization are accelerated. Since earlier studies have shown that wear particles are covered by organic residue which may act as a passive layer inhibiting further oxidation, it would suggest that this organic layer may be removed during the particle isolation process, resulting in a change of the particle chemical composition due to their pyrophor