**3.1.1 Metal-on-polyethylene bearings**

176 Recent Advances in Arthroplasty

mm. Interestingly, in the metal-on-metal group with large femoral heads (52 mm), all patients experienced very high separation values after heel-strike in the first 30% of stance phase, which decreased to a no-separation condition during midstance. In the metal-onmetal group with small-size femoral heads (38 mm), the trend was similar to the other bearings with small femoral heads.(Komistek 2002) For all patients, the sound signals were examined and compared with the kinematic findings. Interestingly, there was a distinct correlation of a high-frequency sound occurring at the time when the femoral head slid back into the acetabular component. As the femoral component impacted the acetabular cup, the sound sensor revealed a high-frequency sound, representing impact conditions. A thud-like "clicking" sound was detected for the subject having a metal-on-polyethylene bearing. Similar, but much more accentuated, was the sound recorded for the subjects with a ceramic-on-polyethylene prosthesis. Clear and rich "clicking," combined with some crepitus, was observed for the subjects having a metal-on-metal polyethylene sandwich total hip arthroplasty. The subjects with a metal-on-metal total hip prosthesis experienced a sound similar to a "rusty door hinge". Ceramic-on-ceramic total hip arthroplasty subjects experienced a "squeaking" sound of variable degree, which was present throughout the entire gait cycle. The ceramic-on-ceramic articulations were considered to be the noisiest.(Rodriguez 2008) In all patients with separation, a knocking sound was observed

Mismatched ceramic couples, acetabular component malposition and impingement have been proposed as factors in the development of squeaking. However, not all mismatched and malpositioned components lead to squeaking. (Restrepo 2008) Additionally, squeaking has been observed in properly matched and positioned implants and when no evidence of neck-to-rim impingement is present. Audible squeaking of a hip replacement remains a still unexplained phenomenon. The sliding motion within the acetabular cup could lead to the induction of vibrational propagation across the interface of the femoral head and acetabular cup, possibly leading to audible interactions. It is also hypothesized that the "squeaking" sound, which mainly occurs in ceramic-on-ceramic total hip prostheses, may be due to separation of the femoral head. This movement of the femoral head from the acetabular component and the impact conditions generated from sliding back can be a source for the

During total hip arthroplasty, both the femoral and acetabular bearing surfaces are surgically replaced with metallic, polymeric, and/or ceramic components.(Fig.6) Throughout the twentieth century, many different combinations of these materials have been explored as candidate bearing surfaces for total hip arthroplasty. Today the most widely accepted bearing couple (i.e., combination of bearing materials for the hip joint) consists of a femoral head fabricated from cobalt chromium molybdenum (CoCr) alloy articulating against a polymeric component fabricated from ultrahigh molecular weight polyethylene (UHMWPE). The use of the CoCr/UHMWPE bearing couple has provided consistent results in total hip arthroplasties around the world for the past four decades. In 1998, an estimated 1.4 million UHMWPE components were implanted worldwide, with approximately half of these bearings being implanted in the hip. At most 200,000 metal-onmetal or ceramic-on-ceramic components have been implanted in patients worldwide between 1988 and 2000, corresponding to less than 10% of total hip replacements during the

when the femoral head contacted the acetabulum.(Glaser 2008)

acoustic emission observed in total hip prostheses. (Jarrett 2009)

**3. Alternate bearing surfaces** 

The metals used in conjunction with polyethylene principally have included stainless steel, cobalt-chromium alloy (in the vast majority), and titanium alloy. In some cases, the metal components have been surface hardened; for example, by nitriding or ion-implanting. In general, the wear rate of polyethylene against stainless steel has been comparable to that against cobalt-chrome alloy in laboratory tests and in clinical use. In contrast, although the wear rate of polyethylene against titanium alloy under clean conditions appears to be comparable to that with the other metals, the greater vulnerability of titanium alloy to abrasion by entrapped third-body particles can cause severe, runaway wear. Hardening of the surface of the titanium alloy by techniques such as gas nitriding, solution nitriding, or ion implanting can markedly improve its resistance to abrasion by third-body particles, and good 10-year results have been reported for titanium nitride-hardened TiAlNb alloy balls used with polyethylene cups. Nevertheless, if a hardened surface eventually is penetrated, severe wear of the underlying alloy still can be triggered. Consequently, even hardened titanium alloys have seen limited clinical use as bearing surfaces. The vast majority of metalon-polyethylene bearings used in hip prostheses have involved cobalt-chrome alloy femoral balls, including cast or forged alloys, and the wear rate of this combination now forms the clinical baseline against which potentially improved bearing combinations are evaluated. As

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 179

historical arthroplasty literature, in which what is now considered UHMWPE was historically referred to as high-density polyethylene (HDPE) or polythene. Today, highdensity polyethylene refers to a material with a molecular weight of 100 to 250,000 daltons and is suitable for milk jugs, not artificial joints. In a hip simulator, HDPE has a wear rate

The ultra-high molecular weight polyethylenes (UHMWPEs) that were used for acetabular cups implanted during the past 30 years were fabricated from raw powder (also called resin). The powder is converted to solid form using one of three distinct methods. In the extrusion process, the polyethylene powder is driven by a ram through a heated nozzle, fusing the flakes into a continuous bar, typically several inches in diameter. The components are machined from the bar stock. In bulk compression molding, the powder is placed in a large mold and heated under pressure to fuse it into a block or sheet from which the final components are machined. In net-shape molding, the powder is placed in a metal mold having the desired shape of the implant and then fused under heat and pressure, such that little or no final machining is required. The final step in any fabrication process was

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

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

of the lamella and may emerge to connect with adjacent lamellae.(Fig.7)

Fig. 7. Schematic crystalline structure of polyethylene

that is four times higher than that of UHMWPE.

sterilization.

**3.2.2 Crystallinity** 

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

### **3.1.2 Ceramic-on-polyethylene bearings**

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

#### **3.2 Polyethylenes**

#### **3.2.1 Chemical structure and molecular weight**

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 historical arthroplasty literature, in which what is now considered UHMWPE was historically referred to as high-density polyethylene (HDPE) or polythene. Today, highdensity polyethylene refers to a material with a molecular weight of 100 to 250,000 daltons and is suitable for milk jugs, not artificial joints. In a hip simulator, HDPE has a wear rate that is four times higher than that of UHMWPE.

The ultra-high molecular weight polyethylenes (UHMWPEs) that were used for acetabular cups implanted during the past 30 years were fabricated from raw powder (also called resin). The powder is converted to solid form using one of three distinct methods. In the extrusion process, the polyethylene powder is driven by a ram through a heated nozzle, fusing the flakes into a continuous bar, typically several inches in diameter. The components are machined from the bar stock. In bulk compression molding, the powder is placed in a large mold and heated under pressure to fuse it into a block or sheet from which the final components are machined. In net-shape molding, the powder is placed in a metal mold having the desired shape of the implant and then fused under heat and pressure, such that little or no final machining is required. The final step in any fabrication process was sterilization.
