Ultrahigh-Molecular-Weight Polyethylene
Weihong Jin, Paul K. Chu, in Encyclopedia of Biomedical Engineering, 2019
UHMWPE is a linear polyolefin with a repeating unit of − CH2CH2 −. Medical-grade UHMWPE has long chains with a molecular mass of 2 × 106–6 × 106 g mol− 1 and is a semicrystalline polymer with a set of ordered regions embedded in a disordered amorphous phase (Turell and Bellare, 2004). UHMWPE has low friction, high wear resistance, good toughness, high impact strength, high resistance to corrosive chemicals, excellent biocompatibility, and low cost.
UHMWPE has been used clinically in joint implants for over 40 years, particularly as an articular liner in a total hip replacements and tibial insert in total knee replacements. In 1962, UHMWPE was first used as acetabular components and has become the dominant bearing materials in total hip replacements since the 1970s. However, the wear of UHMWPE in contact with harder components made of metals or ceramics was a major problem in orthopedics in the 1980s mainly due to continuous reorientation of the polymer chains. The wear debris may induce osteolysis leading to loosening of implants and weakening of the bone structure.
There was a major breakthrough in the development of highly cross-linked UHMWPE in the late 1990s. Cross-linking of UHMWPE is widely implemented by radicalizing the side chains with radiation such as gamma ray, electron beam, or chemicals such as peroxide to improve the wear resistance due to the decreased mobility of the polymer chains after cross-linking (Lewis, 2001). To improve the oxidation resistance, the cross-linked UHMWPE is thermally treated. Highly cross-linked UHMWPE has been successfully used in load-bearing joints and becomes the standard in total hip replacements.
Prior to implantation, orthopedic implants are generally sterilized by gamma irradiation in ambient air. Gamma ray induces the formation of free radicals through chain cleavage. After gamma irradiation, free radicals may still exist in the polymer and react with available O species during storage or in vivo induce detrimental oxidation of UHMWPE (Premnath et al., 1996). Although highly cross-linked UHMWPE has enhanced wear resistance, other properties such as the ductility, fracture toughness, fatigue resistance, and tensile strength may be compromised by gamma irradiation (Lewis, 2001; Premnath et al., 1996).
Nonionizing methods such as sterilization using ethylene oxide gas or gas plasma emerge, and some stabilization treatment has also been conducted after cross-linking to eliminate the deleterious influence mentioned earlier (Kurtz et al., 1999). The antioxidant vitamin E is also incorporated into cross-linked UHMWPE to suppress oxidation by reacting with free radicals (Bracco and Oral, 2011).
There is still no clinical history in joint replacement components even though vitamin E exhibits safety and biocompatibility. Therefore, methods to enhance the wear resistance without impairing any other essential properties of UHMWPE and long-term clinical application are desired for UHMWPE in orthopedic applications.
Post time: Jun-26-2023
