**2.4 Ceramics in hip arthroplasty**

298 Recent Advances in Arthroplasty

exclusively included use of alumina (Al2O3) (Boutin & Blanquaert, 1981). In the late 1980s, alumina was replaced with zirconia (ZrO2) due to its superior strength and toughness as compared with alumina (Willmann, 1998). Drawback of ceramic materials used for bearing couples is their inherently lower strength and toughness under tension and bending. Fracture of the ceramic bearing component is the main mode of failure that

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

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

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

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

resistance, and abrasion resistance (Muratoglu & Kurtz, 2002, Kurtz, 2004).

subluxation and dislocation, and less frequent impingement.

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

occurs even with modern ceramic composites.

**2.2 Ultra-high molecular weight polyethylene** 

(Muratoglu & Kurtz, 2002).

After the drawback of implant fracture in the 1970's and early 1980's, the very low wear rate of ceramic materials renewed interest in developing new designs of CoC bearings for clinical use in the 1990's. Ceramics are brittle, polycrystalline hard bodies, characterized with high hardness and friction endurance. Particles produced by CoC articulations are considered biologically inert and could reduce the rate of osteolysis observed with conventional PE bearings. Despite their brittleness, ceramic materials have several tribological properties, including hardness, which contribute to wear and scratch resistance (Kurtz, 2004). There are three types of ceramics that are of interest in THA, including alumina, zirconia, and alumina matrix composites (Kurtz, 2004). The strength of the ceramics depends on the size of the alumina or zirconia powder grains and the distribution of internal defects, as well as on its composition (i.e., percentage of alumina versus zirconia). Advances in technology with diminishing of grain size have resulted in improved strength. However, the survival of the CoC hip arthroplasty is highly dependent on surgical implantation technique. This articulation is less forgiving than conventional UHMWPE bearing to improper positioning of the components with a subsequent risk of fracture. Although fracture risk is low, it continues to be an issue of debate among orthopedic surgeons. Another potential problem is chipping of the liner that occurs with impingement or during insertion with improper placement.

Biomechanical studies show excellent wear resistance of ceramic bearings. However, clinical studies do not show significant advantage of ceramics compared to polyethylene. In two randomized studies, an alumina-on-alumina bearing was compared with cobalt-chrome-onpolyethylene bearing (Bierbaum et al., 2002, Capello et al., 2005). There was no significant difference in clinical outcome between CoC and MoPE bearings.
