**2.3 Metal-on-metal bearings in hip arthroplasty**

Metal-on-metal hip prostheses made of cobalt-chromium-molybdenum alloys represent an alternative to metal-on-polyethylene bearings because of their substantially lower wear rates (Kurtz, 2004). MoM bearings were proposed in an effort to eliminate wear-induced osteolysis. However, the size, the shape, the number and the chemical characteristics of ion particles are different from the polyethylene particles (Sieber et al., 1999). Because of the difference in particle size, metal-on-metal bearings have been estimated to produce about 100 times more wear debris particles than CoCr/UHMWPE bearings (Doorn et al., 1999, Firkins et al., 2001). As a result of increased biological activity of the smaller metal particles different problems from those of MoPE bearings emerged. Adverse reactions to metal debris have been reported to be a cause of pain in metal-on-metal hip arthroplasty (Wynn-Jones et al., 2011) and potential carcinogenesis raised concerns (Keegan et al., 2007). Two important features determine the survivorship of each type of metal implant: metallurgy and implant design. They are interrelated and determine the biological response to an implant and the survivorship of THA. The properties of the bearing surface are dependent on the manufacturing process. This will result in different wear pattern and metal ion release (Catelas et al., 2003).

Large femoral heads used with MoM bearing have certain advantages: allow accelerated rehabilitation, good range of motion, lesser possibility for impingement, greater intrinsic stability. However, MoM bearing is very sensitive to improper surgical technique. Suboptimal or improper positioning of the components of the arthroplasty leads to impingement, edge loading, reduced clearance that subsequently results in elevated production of metal wear debris.
