Total joint arthroplasty involves the replacement of native articulating joints, such as the hip, knee, shoulder and ankle, with artificial components. In the U.S., these artificial joints are normally composed of plastic and metal. According to THE ORTHOPAEDIC INDUSTRY ANNUAL REPORT®, 2.9 million joint replacements were performed worldwide in 2010, with 1.4 million hip replacements, 1.1 million knee replacements and 100,000 shoulder replacements.1 This report estimates that by 2015, with the aging and more demanding demographics, there will be over 600,000 hip replacements and 1.4 million knee replacements performed in the U.S. alone.
The most commonly-used bearing surface is ultra-high molecular weight polyethylene (UHMWPE), usually articulating against a metal counterface. Since its first introduction in 1962 by Dr. John Charnley, researchers have been striving to improve the properties of UHMWPE to increase its efficacy in total joint replacement surgeries. This paper describes these efforts from the early off-the-shelf engineering material to the highly wear resistant and oxidatively-stable formulations used today.
History of UHMWPE
Starting from the position that friction was the most important parameter for a bearing surface, Dr. Charnley initially experimented with polytetrafluoroethylene, including glass-filled PTFE, which resulted in severe wear in most of his total hip replacement patients.2 He determined that wear properties, rather than friction, were key to a successful long-term implant. An opportune visit from a sales representative for gears fabricated from UHMWPE made him aware of this material and resulted in the first UHMWPE hip component being implanted in 1962.
For almost three decades, little change was made to Charnley’s original concept of UHMWPE articulating against stainless steel, although a few attempts were made to improve the wear resistance during this time. During the 1970s, scientists at Zimmer incorporated pyrolitic carbon fibers into the UHMWPE in an attempt to improve wear and reduce creep. Wear studies and impact testing demonstrated that the carbon fibers caused both roughening of the metal counterface, leading to greater wear and a reduction of the impact strength of the components.3,4 In another attempt in the 1990s, DePuy developed and marketed a hot, isostatically-consolidated UHMWPE that exhibited an extended chain crystal structure, as opposed to the folded-chain crystal structure normally found in UHMWPE. This material, named Hylamer™, exhibited a higher crystallinity and modulus compared to conventional UHMWPE. However, although initial wear tests showed equal wear to conventional UHMWPE and mechanical tests indicated improved properties, Hylamer was discovered to be susceptible to oxidation, which resulted in rapid degradation of these properties after shelf-storage or implantation.5
With increasing clinical implantation time, a better understanding of the modes of failure of the material was gathered. In particular, implant loosening was occasionally reported in patients having previously stable implants. The initial hypothesis placed the blame on the polymethyl methacrylate-based bone cement that was used to fixate a large number of implants, resulting in the phrase “bone cement disease.” When a new generation of cementless implants exhibited the same loosening behavior, scientists re-examined the evidence and concluded that particulate debris generated from the UHMWPE during articulation resulted in a foreign body immune response with subsequent tissue resorption, ultimately resulting in implant loosening in some cases. The industry recognized that improvements needed to be made to the wear resistance of UHMWPE to minimize particle formation and prolong in vivo duration and patient satisfaction.
First Generation Highly Crosslinked UHMWPE (1998-2007)
When polyethylene is exposed to high energy irradiation, hydrogen abstraction leaves behind unpaired electrons on the carbon atoms on the main chain. These unpaired electrons, or free radicals, can form covalent bonds, or crosslinks, if they combine with free radicals on adjacent polymer chains. It was recognized as early as the 1970s that crosslinking UHMWPE had a beneficial effect on wear behavior,6 when researchers used radiation doses up to 1000 kGy to crosslink the UHMWPE. However, due to the small patient population and poor clinical follow-up, this initial study was not widely disseminated, and the orthopaedic industry would wait another 20 to 30 years before rediscovering the benefits of highly crosslinked UHMWPE.