**2.2 Ultra-high molecular weight polyethylene**

Since 1962, ultra-high molecular weight polyethylene has been successfully used in hip replacement for four decades. UHMWPE is a polymer with outstanding chemical and mechanical characteristics. The chemical composition of polyethylene is simple, consisting only of carbon and hydrogen. However, at molecular level the polymer is a complex crystalline structure and at super-molecular level it is in the form of powder (also known as resin) than must be consolidated by melting or pressure to form a solid body. By further processing complexity could be added to the chemical structure of the polymer. In joint replacement, of special importance are its chemical inertness, low friction, lubricity, impact resistance, and abrasion resistance (Muratoglu & Kurtz, 2002, Kurtz, 2004).

After recognition of the problem of wear of the artificial bearing and the ensuing adverse tissue reaction to wear debris the issue is a focus of ongoing research. Alternative bearings were proposed with the goal of elongating longevity of the hip arthroplasty. Polyethylene is a crystalline polymer and its mechanical properties are dependent on its molecular weight and crystalinity. In its solid state, UHMWPE is a two-phase material with crystalline domains embedded within an amorphous phase. The complexities in the microstructure of UHMWPE give rise to a range in mechanical behavior depending upon the processing, thermal and radiation exposure, storage, and prior mechanical history of the polymer (Muratoglu & Kurtz, 2002).

In the 1990s, radiation crosslinking combined with thermal treatment has emerged as a technology to improve the wear and oxidation resistance of UHMWPE acetabular components (McKellop et al., 1999, Muratoglu et al., 2001). The new bearing showed reduced wear rate in numerous in vitro and clinical studies (McKellop et al., 1999, Muratoglu et al., 2001, Campbell et al., 2010, Kurtz et al., 2010). The wear rate of the alternative bearing of highly cross-linked polyethylene and CoCr was significantly lower compared to conventional articulation (UHMWPE/CoCr) (McKellop et al., 1999). The reduced wear rates and enhanced strength allow wider clinical application of the highly cross-linked polyethylene. There is an opportunity to enlarge the diameter of the femoral head used in total hip arthroplasty. This will allow increased range of motion of the joint, increased activities in daily living, greater stability of the joint and reduced incidence of subluxation and dislocation, and less frequent impingement.
