**3.2.2 Crystallinity**

178 Recent Advances in Arthroplasty

noted above, the average wear rate of the polyethylene against cobalt-chrome alloy is typically reported to be in the range of 0.1 to 0.2 mm/yr. However, it should be noted that this average includes those implants that have accelerated wear rates due to excessive thirdbody damage to the bearing surfaces, radiation-induced oxidative degradation of the polyethylene, or other causes. Thus, the inherent wear rate of a polyethylene cup with a cobalt-chrome alloy ball under clean conditions is probably somewhat below the clinical average wear rate, possibly as low as 0.05 mm/yr. Ion implanting and other surface hardening techniques also have been applied to cobalt-chromium alloy. Laboratory tests of hardened cobalt-chromium alloy have been reported to both markedly reduce wear and to increase wear of the opposing polyethylene, and clinical results are not yet sufficient to resolve this contradiction. Whereas it seems likely that surface hardening of cobalt-chrome may improve its resistance to moderate amounts of third-body abrasion, the uncertainty of

Alumina and zirconia femoral balls have been used widely as bearing surfaces against polyethylene cups, and most clinical studies have shown substantially lower polyethylene wear rates than with metal balls, with the wear ratios ranging from 0.75 to as low as 0.25 with alumina balls. A comparable advantage has been reported with zirconia against polyethylene.(Urban 2001) Unacceptably high rates of polyethylene wear, lysis, and loosening with an early type of zirconia ball were reported in a study. Similarly, although the majority of the laboratory tests have indicated lower wear of polyethylene with alumina or zirconia than with metal, in one hip simulator study slightly greater polyethylene wear was reported with alumina balls, but less with zirconia balls.(Yoshitomi 2009) The greater hardness of ceramic balls renders them more resistant than metal balls to scratching by entrapped abrasive contaminants that can, in turn, accelerate the wear of the opposing polyethylene cup (wear mode 3). The differences in the relative wear rates in the various clinical studies might reflect differences in the amount of third-body contamination, with those studies having relatively little such contamination showing comparable polyethylene wear rates for ceramic or metal balls. Nevertheless, contamination by metal particles may be detrimental even with a ceramic ball, because the particles can adhere to the surface of the ceramic, effectively roughening it and, thereby, increasing abrasion of the polyethylene. Metal also can be transferred to the ceramic by contact against metallic components or instruments during surgery.(Garvin 2009) Regardless of the bearing material used, care must be taken to minimize the formation of abrasive contaminants in vivo, for example, by

the advantage in general has limited its clinical application.(Kim 2005)

avoiding those porous coatings that are prone to shed particles. (Clarke 2000)

Polyethylene is a polymer of ethylene and consists of a carbon backbone chain with pendant hydrogen atoms. It is the simplest of polymer molecules chemically, but as the length of the polymer chain increases, so too does the complexity of the material. UHMWPE, used in orthopedic hip and knee applications since 1962, has a molecular weight ranging from 2 to 6 million daltons. By virtue of its molecular weight, UHMWPE has the desirable attributes of wear and impact resistance, together with ductility and toughness. These attributes make UHMWPE highly suitable as a bearing material. There has been some confusion in the

**3.2.1 Chemical structure and molecular weight** 

**3.1.2 Ceramic-on-polyethylene bearings** 

**3.2 Polyethylenes** 

Crystallinity is an important attribute of all polyethylenes, including cross-linked polyethylene. The molecular chains in polyethylene have a natural tendency (driven by thermodynamics) to preferentially fold up against themselves whenever possible, hindered by the considerable crowding and thermal jostling presented by adjacent molecules. Regions of the polymer with folded chains are referred to as crystallites, whereas the regions with randomly oriented polymer chains are referred to as the amorphous regions. In polyethylene, the crystallites have a particular "lamella" shape. If we were to dissolve away the amorphous regions, the crystalline lamellae in polyethylene would look something like twisted, interconnected sheets. The molecular chains are oriented perpendicular to the plane of the lamella and may emerge to connect with adjacent lamellae.(Fig.7)

Fig. 7. Schematic crystalline structure of polyethylene

These connective polymer chains (not shown) are referred to as tie molecules. In particular, it is thought that tie molecules contribute greatly to the inherent wear resistance of polyethylene. The elastic modulus and yield stress of polyethylene will increase in direct

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 181

radicals are retained in the crystal domains, but the number is substantially reduced by the elevated temperature. When thermal treatment is conducted below the melt transition of 135°C, it is referred to as annealing, and above the melt transition, it is called remelting.

Fig. 8. Diagrammatic representation of the manufacturing process of highly cross-linked

The changes in the mechanical properties of the radiation- and heat-treated polyethylene are primarily dominated by changes in the crosslink density and crystallinity. Under multiaxial loading conditions, it is more difficult to separate effects of crystallinity and cross-linking, since both appear to influence the large-strain mechanical behavior in a more complex, synergistic manner. The crystallinity of polyethylene is a function of the radiation dose level and thermal treatment history. Irradiation generates smaller chains with increased mobility, leading to recrystallization and a slight increase in the crystallinity of the polymer. The changes in the crystallinity during the postirradiation thermal treatment depend on temperature. If the thermal treatment is carried out below the melting transition (<135oC), the chain mobility increases, which, in turn, increases the crystallinity of the polymer. When the thermal treatment is performed at temperatures above the melting transition (>135oC), during cool-down to room temperature, the crystallization of the polymer takes place in the presence of the crosslinks. This leads to a decrease in the crystallinity of the polymer. The radiation dose level used in the irradiation step determines the crosslink density. The crosslink density of the polyethylene limits the ultimate elongation that can be achieved during plastic deformation prior to failure. Therefore, at higher radiation dose levels, the cross-linked polymer exhibits reduced ultimate tensile strength and elongation at break under uniaxial tension. As a result, the work to failure also decreases. As the crosslinking reduces the chain mobility, it also inhibits the active energy-absorbing mechanisms. Therefore, at high uniaxial deformation rates, such as impact loading, the energy absorption before failure decreases, leading to a decrease in the toughness.(Baker 2003) Another important variable that affects the mechanical properties of the radiation- and heat-treated polyethylene is the irradiation temperature. When the polymer is irradiated at an elevated temperature (90oC < T < 135oC) the effect of the crosslink density on the large strain mechanical properties decreases significantly. This may be explained by a nonstatistical distribution of the crosslinks resulting at increased irradiation temperatures. As a result, the low crosslink-density matrix controls the large-strain mechanical properties.(Dumbleton 2006) The polyethylenes irradiated at increased temperatures have been reported to exhibit

**3.2.5 The effect of crosslinking on mechanical properties of polyethylene** 

polyethylene; the effects of radiation and heat treatment

relation to the number of crystals present. Many of the processing steps for clinical polyethylenes are tailored specifically to optimize the crystalline structure and thereby tune its material properties. Polyethylene typically has a crystalline content of about 50%. The thermal processing alters the basic organization of molecular chains in polyethylene by modifying the size and shape of the crystals.
