The Required Mechanical Properties. of Hip and Knee Components

The Required Mechanical Properties of Hip and Knee Components S. M. Kurtz, Ph.D. Drexel University and Exponent, Inc., Philadelphia PA, USA Correspondence addresses: Implant Research Center, School of Biomedical Engineering, Science and health Systems Drexel University and Exponent, Inc., 2300Chestnut St., Suite 150, Philadelphia PA, 19103, USA skurtz@exponent.com

Introduction Voluntary national and international standards have been relied on by the orthopedic community as guidelines for the mechanical properties of medical grade UHMWPE. The two most widely used standards for medical grade UHMWPE, ASTM F-648 and ISO 5834, include specifications for the properties of the unconsolidated resin powder, as well as the properties of the consolidated stock material. For many years now, both the ASTM and ISO standards have been developed by the same the industrial participants and researchers. Thus, the ASTM and ISO standards are considered to be harmonized and consequently reflect a unified and international perspective on the properties of medical grade UHMWPE. The goal of this paper is to first review the material property guidelines for medical grade UHMWPE, and to describe how these properties may be altered by radiation crosslinking and thermal processing. Material Property Guidelines for UHMWPE The material property limits for medical grade UHMWPE are essentially the same in both ASTM F648 and ISO 5834 (Table 1), with the exception of different types of impact testing being specified in ASTM F648 and ISO 5434. Currently Ticona (Oberhausen, Germany) produces a Type 1 and Type 2 resin with the trade names of GUR 1020 and 1050, respectively. Prior to 2002, Basell Polyolefins (Wilmington, Delaware, USA) produced Type 3 resin with the 1900 trade name. This resin was discontinued in January 2002 and is no longer produced. Two orthopedic manufacturers have maintained large stockpiles of this resin, so orthopedic implants will continue to be fabricated from this resin, at least in the near future. Although these UHMWPE standards apply only to unirradiated stock material, some investigators have also adopted these guidelines for radiation crosslinked and thermally treated UHMWPE as well [1]. It should be noted, however, that none of the existing standards for medical grade UHMWPE material properties were originally conceived for polymer that had been subjected to sterilization, irradiation, or other processing steps. 52

Table 1. Specifications for Medical Grade UHMWPE Resins in ASTM F648-00 Type 1 Resin Type 2 Resin Type 3 Resin Density (kg/m3) 927-944 927-944 927-944 Yield Stress (MPa), 21 19 19 minimum Ultimate Tensile Stress 35 27 27 (MPa), minimum Elongation to Failure (%), 300 300 250 minimum Izod Impact Strength, double notched (kj/m 2 ), minimum 140 90 30 Tradeoffs in Material Properties with Irradiation and Thermal Treatment Three important processing steps are necessary to produce highly crosslinked polyethylene for hip and knee bearings. These steps are an irradiation step to promote crosslinking, an intra or postirradiation thermal processing step to increase the level of crosslinking and remove residual stress, and a sterilization step. The properties of UHMWPE are influenced by processes of irradiation, thermal processing, and sterilization. In the irradiation step, gamma and electron beam radiation produce free radicals (unpaired electrons) in the polyethylene, which in secondary chemical reactions leads to a combination of crosslinking and chain scission. Crosslinking is beneficial for reducing wear. Chain scission produces a decrease in molecular weight, with concomitant reduction of wear resistance and mechanical properties. When irradiation is conducted in the presence of oxygen, scission predominates over crosslinking. However, when conducted in an inert environment, such as nitrogen, crosslinking predominates over scission. Regardless of whether irradiation is conducted in air or in an inert environment, some of the free radicals will remain entrapped within the crystalline phase of the UHMWPE. Over time, these entrapped free radicals can migrate to the surface of crystals. If irradiation is done in air, these free radicals react with available oxygen causing further time-dependent chemical degradation. 53

Increased crosslinking improves the wear performance of UHMWPE compared to conventional material. However, the presence of the crosslinks adversely affects uniaxial ductility [2], and the uniaxial failure strain of UHMWPE decreases linearly with increasing radiation dose [2]. During irradiation, the loss of ductility depends on the crystalline microstructure of the UHMWPE, because crosslinking occurs primarily in the amorphous phase, where the molecular chains are in sufficient proximity such that a covalent bond can be created between adjacent polymer molecules by the applied energy [3]. Unirradiated UHMWPE typically has a crystallinity in the region of 50% [4], so some 50% of the material is amorphous content that may be crosslinked during irradiation. If the temperature of the UHMWPE changes during the crosslinking process, this can influence the distribution of crosslinking in the polymer and, hence, influence its ability to accommodate large strains prior to failure. The first choice an implant designer has to make is the method of crosslinking (e.g., gamma vs. electron beam). If irradiation is to be carried out using electron beam irradiation, the designer must consider the additional factor of irradiation temperature, since the rate of energy dissipation increases the temperature above the melting temperature. Of the six orthopedic manufacturers currently producing highly crosslinked UHMWPE implants, two have chosen electron beam irradiation, whereas the other four use gamma radiation crosslinking. In this review, we will restrict our attention to gamma radiation crosslinking at room temperature, as it is the most widely used crosslinking modality. For more information about the differences between electron beam and gamma irradiation of UHMWPE, the reader is referred to a recent review [3]. In the production of a highly crosslinked UHMWPE, the material is subjected to a thermal treatment step to reduce the level of free radicals via further crosslinking reactions. At higher temperatures the polymer molecules have increased mobility, thereby increasing the probability of free radicals on adjacent chains reacting to form crosslinks. For the thermal treatment to be effective at eliminating all free radicals, it must be conducted at 150 C, above the melt temperature of the material. Heating above the melting temperature destroys the crystalline regions of the material thus making the free radicals that were in the crystals available for crosslinking. The disadvantage of melting is the reduction crystal size and in material yield and the ultimate strength that ensues. A compromise solution is to heat the material to just below the melting temperature. This solution preserves the original crystal structure, retains mechanical properties, and makes more free radicals available for crosslinking than would be available without thermal treatment while still retaining some free radicals in the crystal domains. When thermal treatment is conducted below the 54

melt transition of 135 o C, it is referred to as annealing, and above the melt transition, it is called remelting. Typically, annealing is carried out at 130 o C and does not eliminate all free radicals, although the number is substantially reduced by the elevated temperature. The choice of thermal treatment has a significant impact on the crystallinity and mechanical properties of highly crosslinked UHMWPE [2]. At a dose of 100 kgy, the elastic modulus, yield stress, and ultimate stress of a remelted material is significantly lower than the respective properties for an annealed material (Figure 1, Table 2). Figure 1 compares the uniaxial tensile behavior of unirradiated UHMWPE material with conventionally sterilized (30 kgy, in N2) polyethylene, and with both annealed and remelted highly crosslinked polyethylenes (100 kgy). Table 2. Effect of Post-irradiation thermal treatment on uniaxial mechanical properties [5]. Note that these irradiation treatments were achieved with a single dose. Properties were determined from treated rods of GUR 1050. Dose (Gamma) Heat Treatment Yield Stress (MPa) Ultimate Stress (MPa) Elongation to Failure (%) 100 kgy None 23.2 ± 0.2 47.6 ± 2.0 238 ± 13 100 kgy 110 C Anneal 23.0 ± 0.3 47.3 ± 1.5 230 ± 12 100 kgy 130 C Anneal 22.6 ± 0.2 48.5 ± 1.5 231 ± 13 100 kgy 150 C Remelt 19.5 ± 0.3 43.9 ± 3.9 246 ± 12 55

Figure 1. Stress v. strain curves for conventional, nitrogen-sterilized UHMWPE (30 kgy, N2) and two forms of highly crosslinked UHMWPE. The highly crosslinked materials were both irradiated with 100 kgy in a single dose and either annealed (110 C) or remelted (150 C). For the two highly crosslinked UHMWPEs shown in Figure 1, the annealed material has an average degree of crystallinity of 60%, whereas the remelted material has a crystallinity of 43%. Throughout the entire stress-strain curve, the higher crystallinity of the annealed material results in a greater resistance to plastic deformation when compared to remelted material. Therefore, the selection of post-irradiation thermal treatment is the second most important decision for an implant designer, as it will influence not only the free radical content, but also the crystallinity, yield strength, and ultimate tensile strength of the highly crosslinked polyethylene. These reduced mechanical properties may not influence wear but will certainly influence the resistance of the material to damage caused by impingement or bearing lift off. Functional Fatigue Performance Based on contemporary hip simulator studies, adhesive/abrasive wear is no longer expected to be the primary limiting factor in the longevity of hip replacements incorporating highly crosslinked 56

UHMWPE. The improved wear resistance of highly crosslinked UHMWPE has recently prompted some implant designers to consider thin metal-backed liner designs and larger femoral heads to reduce the incidence of dislocation [6]. However, with the incorporation of highly crosslinked UHMWPE into new large-diameter cup designs, other modes of clinical failure, such as component fracture associated with rim loading and thin liners [7, 8], as well as impingement-related damage due to component malpositioning [9], may become new limiting factors for the long-term clinical performance of hip replacements. The clinical introduction of thin acetabular liners incorporating highly crosslinked UHMWPE raises new questions regarding the ability of these thin liner designs to withstand structural fatigue loading. To address functional fatigue loading of acetabular liners, researchers have recently suggested evaluating contemporary highly crosslinked UHMWPE materials in historical designs, such as the ACS design [10, 11], which in some circumstances has shown evidence of rim loading and fracture [8]. Walsh and colleagues [10], for example, found that remelting highly crosslinked UHMWPE significantly increased the fatigue crack growth rate in the ACS design, as compared with highly crosslinked cups that were not heat treated. Wang et al. [11] have evaluated conventional gamma sterilized, remelted, and annealed highly crosslinked UHMWPE in functional fatigue simulations of the ACS design. In contrast with the study by Walsh [10], the liners in Wang s study were not prenotched so that crack initiation (as opposed to propagation) could be evaluated. The radiation crosslinked and remelted materials studied by Wang et al. [11] exhibited significantly increased risk for structural fatigue failure, whereas the highly crosslinked and thermally annealed (below melt) material showed no increased risk. Conclusion Increased radiation crosslinking, beyond the 25 to 40 kgy dose needed for sterilization, is now recognized to substantially improve the wear resistance of UHMWPE. However, there are several additional decisions to be made when developing a highly crosslinked UHMWPE for joint replacement, and these decisions may strongly influence the mechanical properties of the bearing. The effect of UHMWPE mechanical properties on clinical performance is not fully appreciated at the present time. The proliferation of crosslinking technology into hip replacements, and more recently knee replacements, has resulted in six new proprietary UHMWPEs, with trade names like Crossfire, Durasul, Longevity, and Marathon. It will be many years until sufficient 57

evidence has been collected to test the hypothesis that these UHMWPEs all with different material properties significantly reduce the incidence of revision in hip and knee replacement. References [1]. Muratoglu OK, Bragdon CR, O'Connor DO, Jasty M, Harris WH. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. Recipient of the 1999 HAP Paul Award. J Arthroplasty 2001; 16: 149-60. [2]. Martell J, Verner JJ, Incavo SJ. Clinical performance of a highly crosslinked polyethylene at two years in total hip arthroplasty: a randomized prospective trial. Trans. American Association of Hip and Knee Surgeons 2002; 12: 24. [3]. Muratoglu OK, Kurtz SM. Alternative bearing surfaces in hip replacement. In Hip Replacement: Current Trends and Controversies. Eds. R. Sinha. New York: Marcel Dekker, 2002. [4]. Kurtz SM, Muratoglu OK, Evans M, Edidin AA. Advances in the processing, sterilization, and crosslinking of ultra- high molecular weight polyethylene for total joint arthroplasty. Biomaterials 1999; 20: 1659-88. [5]. Kurtz SM, Cooper C, Siskey R, Hubbard N. Effects of dose rate and thermal treatment on the physical and mechanical properties of highly crosslinked UHMWPE used in total joint replacements. Transactions of the 49th Orthopedic Research Society 2003; 28. [6]. Muratoglu OK, Bragdon CR, O'Connor D, Perinchief RS, Estok DM, 2nd, Jasty M, Harris WH. Larger diameter femoral heads used in conjunction with a highly cross- linked ultra-high molecular weight polyethylene: A new concept. J Arthroplasty 2001; 16: 24-30. [7]. Suh KT, Chang JW, Suh YH, Yoo CI. Catastrophic progression of the disassembly of a modular acetabular component. J Arthroplasty 1998; 13: 950-2. [8]. Bono JV, Sanford L, Toussaint JT. Severe polyethylene wear in total hip arthroplasty. Observations from retrieved AML PLUS hip implants with an ACS polyethylene liner. J Arthroplasty 1994; 9: 119-25. 58

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