Ultra High Molecular Weight Polyethylene: Mechanics, Morphology, and Clinical Behavior

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 1 st 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 to the virgin material. Therefore, a 2 nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1 st generation while also seeking to provide oxidative stability and improved mechanical properties.

Introduction

For over four decades, ultra high molecular weight polyethylene (UHMWPE) has been used as one-half of the metal- or ceramic-on-plastic bearing couple in total joint replacement (TJR) components due to its toughness, durability, and biological inertness (Kurtz 2004). Though there are metal-on-metal and ceramic-on-ceramic bearing couples, the majority of joint replacement designs utilize UHMWPE. In 2004, over 700,000 total hip and total knee replacements were performed (AAOS 2008) and lifetimes of 15-20 years can often be attained (Karuppiah, Sundararajana et al. 2006). Projections show that the overall number of total joint replacements will greatly increase by 2030, reaching 850,000 to 4.3 million hip and knee replacement procedures (Kurtz, Ong et al. 2007; AAOS 2008). The most current failure rates from theCanadian Joint Replacement Registry (2007) found that aseptic loosening (48%), followed by osteolysis (27%), UHMWPE wear (26%) and instability (14%) were the leading reasons reported for revisions of a primary hip replacements in 2005–2006. The same report stated that the reasons for revisions of primary total knees were aseptic loosening (33%), followed by UHMWPE wear (30%) and instability (17%). Thus failure of UHMWPE is still a leading contributor to failure in total joint replacements. It is not yet known how modern formulations of UHMWPE will affect these revision rates.

However, a larger percentage of these replacements are expected to be put into younger, more active patients which does merit concern (Kurtz, Lau et al. 2008). Part of this changing paradigm in the patient population may be may be related to the obesity epidemic in the USA. One study has shown that the percentage of patients needing TJR that are obese (52% in 2005) is greater than the percentage of the general population that is obese (24% in 2005) (Fehring, Odum et al. 2007). Regardless of the cause, the necessity to develop longer lasting more resilient formulations of UHMWPE is clear in light of these increasing demands on TJRs.

Conclusions

UHMWPE is a complex material and its material, morphological, and mechanical properties are potentially temporal and dependent on functional loading and environmental conditions. It is desirable to understand the material and mechanical properties for individual formulations of UHMWPE and also how the temporal evolution of morphology affects the mechanical properties of these materials. Further elucidation of these basic relationships should allow for development of even more effective formulations of UHMWPE.

UHMWPE has been a clinically successful material in total joint replacements. However, with rising patient demands it is of paramount importance that the UHMWPE materials available continue to be improved. The orthopaedic community has a strong history of altering techniques related to the manufacture of UHMWPE components to improve performance. Advances in sterilization methods have somewhat assuaged the oxidative degradation that UHMWPE experienced during shelf aging. First generation highly crosslinked materials significantly lowered the in vivo wear rates of total joint replacements, which should lead to a decrease in aseptic loosening due to wear particle induced osteolysis. Second generation highly crosslinked materials have been developed that not only preserve these advances in wear resistance, but have improved on the maintenance of other mechanical properties and have also improved on the oxidative stability of highly crosslinked UHMWPE. It is of great interest to the orthopaedic community to see how these materials will perform clinically.