Crosslinking highly crosslinked UHMWPE

Highly crosslinked UHMWPE can be produced by irradiation of a blank UHMWPE with ionizing radiation, in particular by X-rays, y-rays or electron beams, in order to produce radicals. The subsequent treatment of the irradiated material consists in exciting free radicals, which have not recombined, by means of microwave radiation or ultrasound. The process is claimed to ensure a substantially complete recombination of the free radicals. In addition, the crosslinking of the UHMWPE is also further optimized (30). 

Bergstrom, J. S., Rimnac, C. M., and Kurtz, S. M., Prediction of Multiaxial Behavior for Conventional and Highly Crosslinked UHMWPE using a Hybrid Constitutive Model, Biomaterials, 24 1365-1380 (2003)... 

In the late 1990s, highly crosslinked UHMWPE was found to have an effectively zero wear rate. Muratoglu et al. (2001) described how 40 mm thick discs of UHMWPE were irradiated in air using a lOMeV electron... 

Wang (2001) found that irradiation crosslinked UHMWPE had a significantly lower wear rate than un-crosslinked material. The radiation dose must be high to obtain the optimum effect (Fig. 15.19). Rieker et al. (2003) showed that the wear surfaces of highly crosslinked UHMWPE implants after 18 months in vivo, consisted of folds (Fig. 15.20). Such folds are also found in conventional UHMWPE, but fatigue leads to their detachment from the surface. The folds on the surface of the crosslinked polymer appear to stay in place. Crosslinking leads to a reduction in crystallinity, hence a... 

Figure 15.20 Folds on the wear surface of highly crosslinked UHMWPE (Rieker, C. B. et al., j. Arthoplasty, 18(7) (Suppl. I), 48, 2003) Elsevier.

Pruitt, L. A., Deformation, Yield and Fracture and Fatigue of Conventional and Highly Crosslinked UHMWPE, Biomaterials, 26, 905, 2005. 

Contemporary gas-permeable packaging for ethylene sterilization of Dunasul highly crosslinked UHMWPE components, used by Centerpulse, Inc. (Austin, TX). 

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. 

Although a variety of metallic biomaterials have been employed in joint arthroplasty, alloys of cobalt, 28% chromium, and 6% molybdenum (CoCr) are viewed as the gold standard for use in MOM bearings. CoCr alloy is also considered the gold standard as a femoral head material for articulations against conventional as well as highly crosslinked UHMWPE (Muratoglu and Kurtz 2002, Sauer and Anthony 1998). CoCr alloys (e.g., Vitallium ) have been used for hip replacements since 1938, when the biomaterial was employed in the Smith-Petersen mold arthroplasty (Smith-Petersen 1948). 

UHMWPE was introduced as a total hip bearing surface. The high wear resistance of highly crosslinked UHMWPE offers the possibility of avoiding osteolysis related to the accumulation of wear particles. 

In the next section, we first briefly review the historical experience of highly crosslinked UHMWPE for hip replacement and summarize the general characteristics of contemporary materials in current clinical use. The next section also describes the effect of thermal treatment on the properties of fhis family of materials. In the final part of this section, we summarize the latest short-term clinical results using highly crosslinked and thermally treated UHMWPE. For more detailed information about specific highly crosslinked formulafions, see Chapter 15. 

Historical Clinical Experience with Highly Crosslinked UHMWPE... 

Starting in 1998, orthopedic manufacturers introduced highly crosslinked UHMWPEs for THR. These materials are processed with a total dose ranging from 50 to 105 kGy, depending on the manufacturer. Besides choice of dosage, each manufacturer adopted a different route for production that includes a proprietary combination of three important factors 1) an irradiation step, 2) a postirradiation thermal processing step, and 3) a sterilization step (Figure 6.10). 

The choice of thermal treatment has a significant impact on the crystallinity and mechanical properties of highly crosslinked UHMWPE (Kurtz et al., 2002). At a dosage 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 (Table 6.3). 

For the two highly crosslinked UHMWPEs shown in Figure 6.11, 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 with remelted material. Therefore, the... 

Current Clinical Outlook for Highly Crosslinked UHMWPEs... 

The use of alfernafive bearings entails potential risks for the patient. With MOM bearings, the concern is the potential for cancer associated with longterm elevated metal ion exposure. With COC, the concern is the risk of fracture for the femoral head and/or the acetabular liner. With highly crosslinked UHMWPE, following extensive multi-institutional testing, researchers have not yet been able to determine the risks relative to conventional UHMWPE for hip replacements. 

Kurtz S.M., C. Cooper, R. Siskey, and N. Hubbard. 2003. 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... 

Highly crosslinked UHMWPE materials were first clinically introduced in the... 

One of the reasons for thermally treating highly crosslinked UHMWPE is to... 

Comparison of GUR 1050 in three conditions as described by the respective load-displacement curves. Longevity and DURASUL are commercially available formulations of highly crosslinked UHMWPE, which exhibit the characteristic geometric strain hardening in the drawing phase of the small punch test. Free radical quenching by remelting UHMWPE above its f>eak melt transition typically reduces the observed peak load as exhibited here (Edidin and Kurtz 2001). 

Villarraga M.L., S.M. Kurtz, M.R Herr, and A.A. Edidin. 2002. Multiaxial fatigue behavior of conventional and highly crosslinked UHMWPE during small punch testing. 7 Biomed Mater Res in press. 

Conventional and highly crosslinked UHMWPE materials exhibit similar qualitative behavior as a result of the underlying similarities of the material microstructures. In this section we examine the behavior of four different UHMWPE materials to illustrate the characteristic material response of UHMWPE. This data will also be used as the basis for the development of material models in the rest of the review. Two of the four materials were conventional UHMWPE and two were highly crosslinked UHMWPE. All materials were created from the same lot of ram-extruded GUR 1050 (Eigure 14.1). 

The first conventional UHMWPE material was a control, virgin, unirradiated material. The second conventional UHMWPE material was gamma-radiation sterilized in nitrogen with a dose of 30 kGy (referred to as 30 kGy, Y-N2). The two highly crosslinked UHMWPEs were both gamma irradiated with an absorbed dose of 100 kGy. One of the crosslinked materials was heat treated until the specimen center reached 110°C for 2 hours (referred to as 100 kGy, 110°G). The second was heat treated at 150°G for 2 hours (referred to as 100 kGy, 150°G). The degree of crystallinity of the four material types was determined by differential scanning calorimetry (DSC), see Table 14.2. 

To address these limitations, a new constitutive model was developed for conventional and highly crosslinked UHMWPEs (Bergstrom, Rimnac, and Kurtz 2003). This model, which is inspired by the physical micromechanisms governing the deformation resistance of polymeric materials, is an extension of specialized constitutive theories for glassy polymers that have been developed during the last 10 years, is discussed later. 

The most advanced material model presently available for UHMWPE is the HM. This model focuses on creating a mathematical representation of the deformation resistance and flow characteristics for conventional and highly crosslinked UHMWPE at the molecular level. The physics of the deformation mechanisms establish the framework and equations necessary to model the behavior on the macroscale. As already mentioned, to use the constitutive model for a given material requires a calibration step where material-specific parameters are determined. A variety of numerical methods may be used to determine the material-specific parameters for a constitutive theory. In the previous section we employed a numerical optimization technique to identify the material parameters for the constitutive theory. 

Of greater importance is how well the physics-inspired model framework represents the governing micromechanisms, and ultimately, how well the model can predict the behavior of a given material under different loading conditions than that for which the model was originally calibrated. The simulations of the small punch test demonstrate that the HM provides satisfactory and valid predictions of large-deformation multiaxial behavior of conventional and highly crosslinked UHMWPEs. 

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