Crosslinking stabilized UHMWPE

This chapter first summarizes MOM and COC alternative bearing designs and some of the unique risks associated with their use. The history of MOM bearings is particularly noteworthy, because it predates the use of UHMWPE in artificial hip joints. We also review the use of ceramics as a counter face in articulations with UHMWPE. For all practical purposes, however, highly crosslinked UHMWPE remains the most widely used alternative to conventional UHMWPE in orthopedics today. Thus, this chapter also summarizes the development of highly crosslinked and thermally stabilized UHMWPE and describes the characteristics of the most prevalent alternative to conventional UHMWPE in joint arthroplasty. 

Clinical introduction of first-generation highly crosslinked and thermally stabilized UHMWPEs forTHA. 

Vitamin E is an effective free radical scavenger, and its ability to stabilize UHMWPE at extremely low concentrations also results in its tendency to reduce the efficiency of crosslinking during irradiation. The ability of vitamin E to reduce crosslinking efficiency has been noted by several previous authors [2, 7, 8]. Because the crosslinking behavior of UHMWPE is related to its wear resistance, the development of vitamin-E-blended materials involves a potential tradeoff in wear resistance versus oxidation stabihty, as illustrated schematically by Figure 16.2, for concentrations of up to 3000 ppm. 

At 1.98 million cycles, the wear rates of crosshnked and crosslinked compatibilized UHMWPE/HA were not significantly different from the control (50kGy irradiated and stabilized GUR 1050) at 1.98 million cycles the total wear (i.e., mass lost) of crosslinked compatibilized UHMWPE/HA was 5.54 mg, which was not significantly different from the total wear of the control (50kGy irradiated and stabilized GUR 1050), which was 3.56mg. The lower wear (mg) and wear rates (mg/miUion cycles) of crosshnked and crosslinked compatibihzed UHMWPE/HA compared to UHMWPE/HA exhibited the positive effect of chemical crosshnking and compatibilization on the wear resistance of UHMWPE/HA materials [83]. 

The wear rates of the three types of samples shown in Figure 18.9 vary differently as a function of time [83]. The crosshnked UHMWPE/HA wear rate appears to decrease in the first several intervals and then increases after 0.99 mUhon cycles only to begin decreasing again. The wear rate of the crosslinked compatibilized UHMWPE/HA decreases up to 0.99 million cycles and then increases slightly. The 50kGy irradiated and stabilized GUR 1050 wear rate appears to slowly increase throughout time at least until the last interval. 

FIGURE 18.9 Wear rates to 1.98 million cycles for crosslinked UHMWPE/HA, crosslinked compatibilized UHMWPE/HA, and the 50kGy irradiated and stabilized UHMWPE control. 

In the Longevity process, the UHMWPE bars are warmed, placed in a carrier on a conveyor, and are exposed to electron beam radiation, with a total dose of 100 kGy. The UHMWPE does not heat above the melt transition during the crosslinking. After irradiation, the UHMWPE is heated above the melt temperature (>135°C) for stabilization of free radicals. Components are then machined from the Longevity material, enclosed in gas-permeable packaging, and sterilized by gas plasma. 

The oxidative stability of Marathon has been assessed by accelerated aging (DiMaio et al. 1998, McKellop, Shen, and Salovey 1998). hi both McKellop s and DiMaio s studies, the properties of the crosslinked and remelted UHMWPE were not significantly affected by accelerated aging. Although Marathon is currently not available for knee apphcations, this material has been evaluated extensively in knee simulator experiments (MdCnulty and Swope 2003). 

The developments of innovative bearing surfaces made of UHMWPE with optimized crosslink densities may provide improved in-vivo wear resistance and long-term structural stability. Such improvements are essential in avoiding peri-prosthetic osteolysis, the leading cause of implant failure, and may ultimately extend the life of total joint replacements. Such material improvements have the potential to make important contributions to the field of total joint arthroplasty. 

The rationale behind stabilizing the residual free radicals in highly crosslinked UHMWPE with the antioxidant vitamin E (a-tocopherol. Figure 15.1) [1, 2] was to provide oxidation and wear resistance to UHMWPE without... 

By using the model and method previously described, desired concentration profiles can be achieved through radiation crosslinked UHMWPE joint implant components. In the following sections, the wear resistance, mechanical properties, and oxidative stability of vitamin-E-diffused, radiation crosslinked UHMWPE prepared by using the diffusion technique previously discussed will be reported. 

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