Each year, approximately 1.4 million joint replacement procedures are performed worldwide. One of the main concerns in the development of implants for joint replacement procedures is the interaction of the implant's bearing surfaces�those materials that come in contact with each other while in motion. As the bearing couples rub against each other, tiny particles of debris are released into the body. As debris accumulates, the body activates macrophages that stimulates the production of antibodies, which attack the debris, the implant and the surrounding bone. The process can lead to osteolysis, possible implant loosening, and subsequent failure of the implant.
This is an increasing concern, as progressively younger individuals become candidates for joint replacement surgery. Implant recipients who are older can expect a total joint replacement to last a decade or more without needing significant changes to the technology. However, because younger implant recipients are more likely to resume physical activities, it is critical that medical device manufacturers use bearing materials that minimize implant wear, thus reducing the effects of wear debris on the body and increasing the implant's life span.

Medical device manufacturers have used several different bearing materials to minimize wear, including ultra-high-molecular-weight polyethylene (UHMWPE), metallic alloys, ceramics and cross-linked polyethylene (XLPE), all of which have certain benefits and drawbacks. There is no perfect wear couple. Designers select materials based on wear performance and additional properties such as the biological response to wear debris, ease and reproducibility of manufacture, creep performance, fatigue resistance, impact strength and cost. As researchers seek to expand the range of bearing materials, there has been growing interest in implantable-grade polyetheretherketone (PEEK) polymer because of its mechanical properties, biocompatibility and potentially low wear rates.

Early metal-on-metal joints (such as the McKee-Farrar prosthesis) were discarded in favour of the metal-on-UHMWPE prostheses because of the high frictional torques produced by large-diameter bearing, as well as impingement problems and poor surgical technique with subsequent failure. Despite a high incidence of failure, there have been a few reported cases of success after as many as 20 years of use with little wear. A new generation of metal-on-metal hip prostheses designed to closer tolerances and using superior metal compositions has greatly improved the lifetime of such implants. Although metal-on-metal bearings are associated with low wear rates, there are concerns that wear debris can trigger osteolysis. A further issue is the release of chemically active metal ions into the surrounding tissues. While these ions may stay bound to local tissues, metal ions may also bind to protein moieties that are transported into the bloodstream and lymphatics to remote organs.

The use of ceramic-on-ceramic joints avoids the issues of metal ions. The initial problems concerning the brittle failure of ceramic implants have largely been eliminated through improved fabrication techniques with higher-purity materials and finer grain sizes. Although ceramic-on-ceramic joints are associated with low wear rates, wear and fracture can be problems when ceramic joints are misaligned.

XLPE has been investigated as a bearing material to address some of the limitations of UHMWPE. XLPE has shown promising results in simulator studies and early clinical work. The process of cross-linking has been shown to reduce wear. But the process can result in residual free radicals, which can lead to oxidative degradation and subsequent embrittlement within the material.

Thermal treatments following irradiation have been employed to reduce the concentration of free radicals. However, annealing or remelting treatments can reduce tensile and yield strength of XLPE as well as promote fatigue-crack propagation resistance. This is of particular importance in knees where the articular surfaces are not completely congruent. It reinforces the notion that no ideal wear couple for all applications has yet been developed.

PEEK because of its versatility, mechanical strength, and biocompatibility, is now routinely used in long-term medical implant applications. One example of this potential is an orthopedics company's development of a carbon fiber�reinforced PEEK (CF-PEEK) polymer composite acetabular liner, which is used for articulation against a ceramic femoral head. Hip-joint simulator testing up to 10 million cycles showed that wear of the CF-PEEK polymer composite cups was about 1% that of UHMWPE cups.
Further work investigating use of a CF-PEEK acetabular cup against large (54 mm) alumina femoral heads has demonstrated that such wear couples approach the bearing performance of hard-on-hard surfaces. PEEK cups do not have the same reputations for brittleness or metal ion release. They offer greater design potential than polyethylene because they have thinner surfaces and can be injection molded. Subsequently, a clinical study of implantable-grade CF-PEEK acetabular inserts was initiated in 2001. To date, no complications following implant surgery or adverse reactions to the material have been reported.
Compared with carbon fiber�reinforced UHMWPE, implantable-grade PEEK polymer has a fiber-matrix interface at least a factor of 10 times stronger than carbon fiber�reinforced UHMWPE. So fiber release is essentially eliminated, enabling PEEK to sustain comparatively large stresses over long periods of time. Furthermore, the creep resistance of PEEK ensures that the interface between the fiber and the polymer matrix remains intact.

Although traditional materials continue to provide clinical benefits for patients requiring total joint replacement, the initial results from implantable-grade PEEK polymer as a bearing material are also promising. This has prompted further research into the potential bearing performance of PEEK polymer for different applications. Such research may lead to a more-diverse range of materials as candidates for applications, including shoulder prostheses or artificial spinal disks, for which wear is a concern.