3rd UHMWPE International Meeting Presentation: On the Mechanical Properties of UHMWPE
Ultra-high molecular weight polyethylene (UHMWPE) is a member of the polyethylene family of polymers. The International Standards Organization (ISO 11542) defines UHMWPE as having a molecular weight of at least 1 million g/mole and the American Society for Testing and Materials (ASTM D 4020) specifies that UHMPWE have an average molecular weight greater than 3.1 million. The UHMWPE formulations used in orthopaedic applications typically havea molecular weight between 2 to 6 million g/mole. UHMWPE is a linear (non-branching) semicrystalline polymer. It is initially manufactured as powder with a percent crystallinity on the order of 60-75%, depending on the resin. Orthopaedic components may be made by directc ompression molding, or by machining from ram extruded rod or compression molded sheets. The physical and mechanical properties of UHMWPE are influenced by resin variations andconsolidation process. Native UHMWPE typically has a density of 0.930-0.945 g/ml, elastic modulus of 0.8-1.5 GPa, tensile yield strength of 19.3-23 MPa, elongation at fracture of 200-350%, and an ultimate stress of 30.4-48.6 MPa (1). As with any polymer, the mechanicalproperties are both rate and temperature dependent.
The physical and mechanical properties of UHMWPE are altered by ionizing radiation, such as may be used for sterilization or for the purposes of deliberate crosslinking (to improve wear resistance) (1,2). When UHMWPE is radiation sterilized in the presence of oxygen, chain scission predominates over crosslinking. Chain scission leads to a decrease in molecular weight and an alteration of mechanical properties. In addition to the immediate radiation effects, oxidative degradation of UHMWPE components radiation sterilized in air-permeable packaging will occur during shelf-storage prior to implantation and will continue to occur during in vivo use (1-3). Oxidation embrittles UHMWPE, leading to a decrease in the elongation to failure, an increase elastic modulus, and a decrease in fatigue crack propagation resistance. These changes in mechanical properties have resulted in the premature failure of some UHMWPE components in vivo.
Crosslinking improves the wear resistance of UHMWPE compared to conventional UHMWPE (non-crosslinked, or lightly crosslinked during radiation sterilization). In the United States, highly crosslinked UHMWPEs are produced using 50 to 105 kGy of either gamma or electron beam radiation, depending on the manufacturer and the process. In general, crosslinking adversely affects uniaxial ductility, fracture toughness, and fatigue crack propagation resistance (4-6). Radiation-induced crosslinked UHMWPE materials still contain free radicals that can lead to oxidative degradation; thus, post-processing to reduce or eliminate free radicals (remelting above the melt transition or annealing below the melt transition) is usually conducted. While effective at eliminating entrapped free radicals, heating above the melt temperature also leads to a reduction in crystal size, which leads to a reduction in yield stress and ultimate stress (1). In contrast, annealing preserves the original crystal structure and better retains mechanical properties, but reduces free radicals less effectively than remelting; thus, oxidation is possible.
Methods to more effectively reduce free radicals without having to remelt the material (e.g, doping with vitamin E) have been explored as a means to maintain crystallinity, while simultaneously maintaining better mechanical properties of crosslinked UHMWPE (7). The combined effects of radiation-induced crosslinking followed by remelting or annealing on mechanical properties of UHMWPE can be complex. For example, using a crosslinking dose of 100 kGy, the elastic modulus, yield stress, and ultimate stress of a remelted material was significantly lower than an annealed material (4). However, though the estimated fracture toughness, KC, was found to decrease with increasing radiation dose, no significant difference in KC was found between the annealed and the remelted highly crosslinked materials (5). The crosslinked materials were also found to be more sensitive to notches under uniaxial tensile loading as compared with non-crosslinked UHMWPE, though, again, no pronounced difference was found between the remelted and annealed UHMWPE materials (8).
References1.Kurtz, S.M.: The UHMWPE Handbook: Ultra-High Molecular Weight Polyethylene in Total Joint Replacement (2005) Elsevier Academic Press, New York, NY.
2. Rimnac, C.M., Kurtz, S.M.: Nuclear Instruments and Methods in Physics Research, B (2005) 236:30.
3.Kurtz S.M., Rimnac C.M., Hozack, W., et al.: J. Bone Joint Surg. (2005) 87A:815.
4.Kurtz, S.M., Villarraga, M.L., Herr, M.P., et al.: Biomaterials (2002) 23:3681.
5.Gencur, S.J., Rimnac, C.M., Kurtz, S.M.: Biomaterials (2003) 24:3947.
6.Goldman, M., Pruitt, L.: J. Biomed. Mater. Res. (1998) 40:378.
7.Oral, E., Christensen, S.D., Malhi, A.S., et al.: J. Arthroplasty (2006) 21:580.
8.Sobieraj, M.C., Kurtz, S.M., Rimnac, C.M.: Biomaterials (2005) 26:3411.
AcknowledgementsNIH AR 47192, NIH AR 47904, NIH T 32 GM07250, Strkyer Orthopaedics, Zimmer, Inc., Sulzer, Wilbert J. Austin Chair.
Fatigue crack propagation is reduced by crosslinking and 37°C PBS environment.
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