February 2001 Bulletin

Research focuses on polyethylene wear

By Clare M. Rimnac, PhD

Ultra high molecular weight polyethylene (UHMWP) has been successfully used as a bearing surface against a metallic or ceramic counterface in total joint replacements for more than 30 years. However, failure of total joint replacements associated with the production of wear debris from repeated contact between the articulating surfaces is a clinical concern.

The debris that is generated can promote a biological response that can lead to osteolysis, implant loosening, and infection. When polyethylene is one of the bearing materials, the majority of the debris that is generated is polyethylene debris. Thus, it has been a general premise that material (and design) modifications that reduce the wear damage of polyethylene joint components will improve the long-term performance of the joint replacement.

There have been several efforts in the past to modify the polyethylene material to reduce wear. Some of the efforts to modify polyethylene have included adding carbon fibers for reinforcement (Poly II™, Zimmer) and high temperature/high pressure processing to form an extended-chain crystalline morphology (Hylamer™, DePuy-DuPont Orthopaedics). These modified polyethylene materials were found to have equivalent wear resistance to conventional polyethylene.

Modifications in the sterilization methods of polyethylene components may contribute to improved wear resistance. In the 1990s, researchers and manufacturers became increasingly more aware of the effects of gamma radiation sterilization on the mechanical properties and wear resistance of polyethylene. It was found that polyethylene components that were radiation sterilized in air could undergo oxidative degradation of the material, leading to deleterious changes in the physical and mechanical properties of the material. More importantly, it was found that some components could subsequently continue to oxidatively degrade, or age, such that the components could become progressively more brittle and less resistant to fracture and fracture-related wear damage modes, such as delamination.

The clinical consequences of oxidative degradation of polyethylene are still being investigated. In the meantime, today, all of the orthopaedic implant manufacturers in the United States sterilize polyethylene components by much less damaging methods, including gamma radiation in a reduced oxygen environment, gas plasma, or ethylene oxide gas.

Recently, the idea of improving wear resistance in total hip acetabular components by inducing crosslinks into the polyethylene morphology has been intensely investigated. Crosslinked polyethylene acetabular hip components have become available for clinical use as well. The motivation for this approach was based on the overall favorable clinical experience of crosslinked polyethylene acetabular hip components in Japan and in the United Kingdom. In addition, crosslinking of UHMWP had been known for many years in industry to be a viable means by which to improve its abrasion resistance.

With respect to total hip replacements, several investigators have reported on exciting results from multidirectional hip wear simulator studies that demonstrated substantially reduced rates of wear of crosslinked polyethylene acetabular components. Crosslinking of polyethylene can be accomplished by ionizing radiation or by chemical methods. Exposure to ionizing radiation leads to the formation of free radicals in the polyethylene. Some of the free radicals combine to form crosslinks (or tie points between the long chain molecules). Other free radicals remain reactive and could possibly lead to oxidative degradation, unless specifically extinguished.

The commercially available crosslinked polyethylene acetabular hip components are crosslinked by either gamma radiation or electron beam radiation. The process is conducted in such a manner so as to minimize or inhibit oxidative degradation. For example, in one manufacturer’s process, oxidative degradation of the crosslinked polyethylene is inhibited by irradiating the polyethylene in a low oxygen environment and by following the irradiation step with a heat treatment that effectively extinguishes the free radicals by allowing them to recombine.

The dose level of ionizing radiation for the new crosslinked polyethylenes is greater than that which is used for sterilization. This higher dose induces a greater degree of crosslinking than occurs at the sterilization dose level. It has been shown in multi-directional hip wear simulator studies that the wear rate is a function of the molecular weight between crosslinks, such that a decrease in molecular weight between crosslinks leads to an improved resistance to wear. It is thought that the molecular ties arising from crosslinking reduces abrasive and adhesive wear of the polyethylene by improving the resistance of the polymer molecules to plastic deformation at the articulating surface.

One potential area of concern is that the new crosslinked polyethylenes generally have reduced ductility and fracture resistance as compared with conventional polyethylenes. Thus, while resistance to adhesive and abrasive wear appear to have been significantly improved by crosslinking, it is not yet clear how the wear modes that are related to fracture resistance may be influenced in components manufactured from crosslinked polyethylene. For this reason, the introduction of crosslinked polyethylene into tibial knee components, for which the fracture-related wear damage modes of pitting and delamination are more prevalent, has proceeded more slowly than for acetabular hip components.

Clare Rimnac, PhD, is an associate professor and director of the Musculoskeletal Biomechanics and Orthopaedic Engineering Laboratories in the Departments of Mechanical and Aerospace Engineering and Orthopaedics at Case Western Reserve University in Cleveland, Ohio.

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