**3.2.3 Cross-linking**

Cross-linking is the foundation of all modern polyethylene total hip bearings. Cross-linking is defined as the joining of two independent polymer molecules by a chemical covalent bond. Only radiation cross-linking has been commercialized by orthopedic device manufacturers. The first step involves irradiation of the polyethylene molecule. Next, irradiation produces a hydrogen radical, leaving a so-called "free" radical on the polyethylene molecule. Actually, the radical on the polymer chain has extremely limited mobility and is hindered by the adjacent molecule. For cross-linking to occur, free radicals must be present on adjacent polyethylene molecules, and the molecules must be mobile. Free radical recombination takes place primarily in the amorphous phase of the polymer, where the molecules are in close enough proximity to allow the formation of the interchain carbon-carbon bond that constitutes the cross-link. When the adjacent radicals react, a covalent bond, or cross-link, is formed between the two polyethylene molecules. In the crystalline phase, because of the increased distance between the molecules, crosslinking is not favored. As a result, the free radicals generated in the crystalline regions are postulated not to take part in the crosslinking reactions and become trapped. These residual free radicals are the known precursors of oxidation-induced embrittlement secondary to gamma sterilization. (Muratoglu 2001)

The extent of cross-linking in polyethylene is proportional to the absorbed dose of radiation. Historically, polyethylene bearings were gamma sterilized at a dose of 25 to 40 kGy. This dose resulted in the formation of some cross-links. Saturation of cross-linking was achieved only at approximately 100 kGy of absorbed dose. Today, cross-linked polyethylenes are processed with a total dose ranging from 50 to 105 kGy. In general, increasing the dose provides a proportional improvement in wear resistance, with diminished benefits observed above 100 kGy. (Geedink 2009)

### **3.2.4 Thermal processing: Annealing and remelting**

In the production of a highly cross-linked UHMWPE, the material is subjected to a thermal treatment step to reduce the level of free radicals via further cross-linking reactions. At higher temperatures the polymer molecules have increased mobility, thereby increasing the probability of free radicals on adjacent chains to react and form cross-links.(Fig.8) For the thermal treatment to be effective at eliminating all free radicals, it must be conducted above the melting temperature of the material at 150°C. Heating above the melting temperature destroys the crystalline regions of the material, thus making the free radicals that were in the crystals available for cross-linking. The disadvantage of melting is the reduction in polymer crystal size and in mechanical properties (e.g., material yield and ultimate strength) that ensues when the material returns to room temperature. A compromise solution is to heat the material to 130°C to 135°C, just below the melting temperature. This solution preserves the original crystal structure, retains mechanical properties, and makes more free radicals available for cross-linking than would be available without thermal treatment. Some free

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

Cross-linking is the foundation of all modern polyethylene total hip bearings. Cross-linking is defined as the joining of two independent polymer molecules by a chemical covalent bond. Only radiation cross-linking has been commercialized by orthopedic device manufacturers. The first step involves irradiation of the polyethylene molecule. Next, irradiation produces a hydrogen radical, leaving a so-called "free" radical on the polyethylene molecule. Actually, the radical on the polymer chain has extremely limited mobility and is hindered by the adjacent molecule. For cross-linking to occur, free radicals must be present on adjacent polyethylene molecules, and the molecules must be mobile. Free radical recombination takes place primarily in the amorphous phase of the polymer, where the molecules are in close enough proximity to allow the formation of the interchain carbon-carbon bond that constitutes the cross-link. When the adjacent radicals react, a covalent bond, or cross-link, is formed between the two polyethylene molecules. In the crystalline phase, because of the increased distance between the molecules, crosslinking is not favored. As a result, the free radicals generated in the crystalline regions are postulated not to take part in the crosslinking reactions and become trapped. These residual free radicals are the known precursors of oxidation-induced embrittlement secondary to gamma

The extent of cross-linking in polyethylene is proportional to the absorbed dose of radiation. Historically, polyethylene bearings were gamma sterilized at a dose of 25 to 40 kGy. This dose resulted in the formation of some cross-links. Saturation of cross-linking was achieved only at approximately 100 kGy of absorbed dose. Today, cross-linked polyethylenes are processed with a total dose ranging from 50 to 105 kGy. In general, increasing the dose provides a proportional improvement in wear resistance, with diminished benefits observed

In the production of a highly cross-linked UHMWPE, the material is subjected to a thermal treatment step to reduce the level of free radicals via further cross-linking reactions. At higher temperatures the polymer molecules have increased mobility, thereby increasing the probability of free radicals on adjacent chains to react and form cross-links.(Fig.8) For the thermal treatment to be effective at eliminating all free radicals, it must be conducted above the melting temperature of the material at 150°C. Heating above the melting temperature destroys the crystalline regions of the material, thus making the free radicals that were in the crystals available for cross-linking. The disadvantage of melting is the reduction in polymer crystal size and in mechanical properties (e.g., material yield and ultimate strength) that ensues when the material returns to room temperature. A compromise solution is to heat the material to 130°C to 135°C, just below the melting temperature. This solution preserves the original crystal structure, retains mechanical properties, and makes more free radicals available for cross-linking than would be available without thermal treatment. Some free

modifying the size and shape of the crystals.

**3.2.3 Cross-linking** 

sterilization. (Muratoglu 2001)

above 100 kGy. (Geedink 2009)

**3.2.4 Thermal processing: Annealing and remelting** 

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 polyethylene; the effects of radiation and heat treatment

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

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

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 183

during gamma sterilization and subsequent shelf-storage, and the remaining free radicals may induce some oxidation during long-term use in vivo. The moderate level of crosslinking that was present in the vast majority of UHMWPE components implanted over the past three decades was an unintentional byproduct of the 2.5 to 4 Mrads of gamma radiation used to sterilize the components. The improvement in wear resistance in these polyethylenes relative to non-cross-linked polyethylene ranges from about 30% to 50%, compared to the 85% or more reduction achieved with intentionally elevated cross-linking doses. A number of laboratory wear simulations have demonstrated that the wear rate of UHMWPE cups decreases markedly with an increasing level of radiation-induced crosslinking. The greatest reduction per Mrad occurs as the dose increased from zero to about 8 Mrads, with progressively less improvement at higher doses and no additional benefit after 15 to 20 Mrads. While this dose-wear relationship was the basis for the recent development of a variety of intentionally-cross-linked polyethylenes, the developers have arrived at very different opinions regarding the appropriate dose and other processing parameters for

In addition to the optimum method of cross-linking, the optimum amount of cross-linking to use is also a subject of current debate. Because increasing the level of crosslinking causes a progressively greater reduction in some mechanical properties, such as ultimate strength, ductility, fracture toughness, and fatigue strength, one extreme is to avoid cross-linking altogether (for example, by simply sterilizing with ethylene oxide or gas plasma). This avoidance is to retain the maximum values of strength, elongation, and fracture toughness, despite the fact that it results in substantially higher wear of the polyethylene. Among those who advocate crosslinking, the particular dose used represents that manufacturer's approach to balancing reduced wear against the need to maintain other mechanical properties well above that needed for acceptable clinical performance. Those using the high levels of cross-linking (9.5 to 11 Mrads), about 3 to 4 times the typical dose used historically to sterilize polyethylene components, maintain that these high levels are justified to obtain the additional 5% to 10% improvement in wear over that provided by a moderate dose, despite the corresponding reduction in other physical properties. In contrast, advocates of a moderate cross-linking dose, such as 5 Mrads, maintain that the corresponding reduction in wear to 85% below that of a noncross-linked polyethylene will be sufficient to avoid an osteolytic reaction in even the most active patients, without unnecessarily reducing other physical properties. Clearly, it is not desirable to use a dose that will result in mechanical

The first widely used total hip replacements featured cobalt-chromium alloy bearing against itself, primarily the McKee-Farrar (Howmedica, Limerick, Ireland) design, along with the Mueller (Sulzer AG, Winterthur, Switzerland), Ring (Downs, Ltd, Mitchham, England) and others. Because of a relatively high rate of early failure, the first-generation metal-on-metal hips were largely supplanted by the Charnley prosthesis, which featured a stainless steel ball and a polyethylene socket. Disregarding the early failures, the long-term survivorship of the early metal-on-metal designs has been comparable to that of the metal-onpolyethylene Charnley. In particular, the steady-state wear rates have been on the order of a

optimizing the clinical performance of a polyethylene implant.

failure of the polyethylene components.(Moore 2008; Bradford 2004)

**3.2.9 Optimum method for cross-linking** 

**3.3 Metal-on-metal bearings** 

better large-strain mechanical properties than those irradiated at room temperature with identical radiation dose levels.(Affatato 2005)

### **3.2.6 Conventional polyethylene surfaces**

Although ultra-high-molecular-weight polyethylene (UHMWPE) has low friction and dampening properties, it has one major disadvantage, which is adhesive and abrasive wear. The UHMWPE particles produced from cyclical loading are thought to play a major role in particle-induced osteolysis and secondary implant failure and loosening. Osteolysis results from the intrusion of polyethylene debris between the implant surface and bone, inducing a macrophage response. Polyethylene also can fail as a result of third-body wear secondary to polyethylene degradation. A polyethylene wear rate of 0.10 mm per year is the threshold for the development of osteolysis. In addition, conventional polyethylene thickness should not decrease below 6 to 8 mm, or accelerated wear polyethylene failure and osteolysis will develop.(Lundberga 2006)

### **3.2.7 Highly cross-linked polyethylene surfaces**

Highly cross-linked polyethylene was introduced to reduce the polyethylene wear rate in THA.(McKellop 1999) It is well documented that young age, male gender, and high activity level increase the risk of wear, osteolysis, and mechanical failure. However, these different factors that influence wear rate in conventional polyethylene have no significant influence when using highly cross-linked polyethylene.(Rohrl 2007) In vitro studies on highly crosslinked polyethylene have shown that wear can be reduced by 42% to 100% compared with that of conventional polyethylene, and, multiple in vivo studies support many of these in vitro results. The annual linear wear for highly cross-linked polyethylene has been reported to be 45% that of the conventional liner at 5 years after implantation. This wear rate is well below the threshold for lysis. Long-term results (10 to 20 years) of highly cross-linked polyethylene support that the low wear rates reported approach that of MOM and ceramicon-ceramic bearings.(Digas 2007; Atienza 2008)

### **3.2.8 Sterilization techniques**

Today, polyethylenes are sterilized by different techniques with regard to the fact that sterilization process can affect the physical properties of polyethylene. Some brands of the polyethylene components are now sterilized without irradiation, using either ethylene oxide or gas plasma, in order to minimize the oxidative degradation. Because these methods do not generate free radicals in the polyethylene, they completely avoid the potential for immediate and long-term oxidative degradation of the mechanical properties and wear resistance. However, because ethylene oxide or gas-plasma do not induce cross-linking, these polyethylenes do not take advantage of improving the wear resistance following crosslinking.(Digas 2003) Other brands of polyethylenes are still sterilized with gamma radiation, but for doing so, the polyethylene components are sealed in some type of low-oxygen atmosphere, including vacuum, inert gas, or with an oxygen scavenger. In addition, one manufacturer anneals the polyethylene acetabular cups after sterilization by heating them in the nitrogen packaging at 37° to 50°C (well below the melt temperature of 135°C to avoid distorting the components) for about 6 days to reduce the level of residual free radicals that were induced by the gamma radiation. These modifications in sterilization techniques can markedly reduce but not necessarily eliminate the oxidation that would otherwise occur during gamma sterilization and subsequent shelf-storage, and the remaining free radicals may induce some oxidation during long-term use in vivo. The moderate level of crosslinking that was present in the vast majority of UHMWPE components implanted over the past three decades was an unintentional byproduct of the 2.5 to 4 Mrads of gamma radiation used to sterilize the components. The improvement in wear resistance in these polyethylenes relative to non-cross-linked polyethylene ranges from about 30% to 50%, compared to the 85% or more reduction achieved with intentionally elevated cross-linking doses. A number of laboratory wear simulations have demonstrated that the wear rate of UHMWPE cups decreases markedly with an increasing level of radiation-induced crosslinking. The greatest reduction per Mrad occurs as the dose increased from zero to about 8 Mrads, with progressively less improvement at higher doses and no additional benefit after 15 to 20 Mrads. While this dose-wear relationship was the basis for the recent development of a variety of intentionally-cross-linked polyethylenes, the developers have arrived at very different opinions regarding the appropriate dose and other processing parameters for optimizing the clinical performance of a polyethylene implant.
