Ultra high molecular weight polyethylene: mechanics, morphology, and clinical behavior

Abstract

Ultra high molecular weight polyethylene (UHMWPE) is a semicrystalline polymer that has been used for over four decades as a bearing surface in total joint replacements. The mechanical properties and wear properties of UHMWPE are of interest with respect to the in vivo performance of UHMWPE joint replacement components. The mechanical properties of the polymer are dependent on both its crystalline and amorphous phases. Altering either phase (i.e., changing overall crystallinity, crystalline morphology, or crosslinking the amorphous phase) can affect the mechanical behavior of the material. There is also evidence that the morphology of UHMWPE, and, hence, its mechanical properties evolve with loading. UHMWPE has also been shown to be susceptible to oxidative degradation following gamma radiation sterilization with subsequent loss of mechanical properties. Contemporary UHMWPE sterilization methods have been developed to reduce or eliminate oxidative degradation. Also, crosslinking of UHMWPE has been pursued to improve the wear resistance of UHMWPE joint components. The 1st generation of highly crosslinked UHMWPEs have resulted in clinically reduced wear; however, the mechanical properties of these materials, such as ductility and fracture toughness, are reduced when compared with the virgin material. Therefore, a 2nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1st generation while also seeking to provide oxidative stability and improved mechanical properties.

Figure 1

Adapted from (Wright and Goodman 2001) and (Kurtz, Rimnac et al. 2000). Schematic of the plastic damage layer in a component and an SEM showing the aligned lamellae in the damage layer.

Figure 2

Bar chart showing the r² values for several failure criteria for the four formulations of UHMWPE tested in (Bergstrom, Rimnac et al. 2005). The chain stretch criterion is concluded to be the best for UHMWPE. Courtesy of (Bergstrom, Rimnac et al. 2005).

Figure 3

Retrieved tibial component with a fractured stabilizing post. Courtesy of (Mauerhan 2003).

Figure 4

Engineering stress-strain curves for a conventional (25kGy irradiation sterilized in N₂) and several first generation highly crosslinked materials. Annealing was performed at 130°C and remelting was performed at 150°C (Murphy, Sobieraj et al. 2008).

Figure 5

Fatigue crack propagation results (in ambient temperature and pressure) for a conventional UHMWPE and for first generation highly crosslinked remelted and annealed formulations. Courtesy of (Gencur, Rimnac et al. 2006).

Figure 6

S-N fatigue results for non-irradiated (Virgin), as-irradiated (β50 and β150), irradiated and remelted (β50R and β150R), and irradiated and annealed (β50A and β150A) formulations of UHMWPE. Adapted from (Medel, Pena et al. 2007).