Developing PEEK Polymer as a Bearing Material for Implants

PLASTICS

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 an 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 stimulate 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.

Wear Couples

By definition, joints have bearing surfaces that rub against each other while in motion. Therefore, it is not enough to understand the wear properties of one material when multiple materials are used in artificial joints. It is critical that the wear properties of all bearing materials be considered. This is extremely important in the hip, knee, and shoulder for loading and range of motion. However, wear performance is also key when selecting materials for ankle, finger, and elbow joints, in addition to new applications such as motion preservation devices in spinal applications.

In the hip, forces can reach six times a person's body weight, and in the knee forces can reach 17 times a person's body weight when rising from a seated position. Therefore, the ability of a material to produce good wear performance must be assessed with respect to the relevant loads and motions.

Traditional Bearing Materials

The first successful wear couple was developed in 1962 and involved a combination of hard and soft bearing materials in the form of metal in combination with UHMWPE.

This combination of bearing materials has proved extremely successful in terms of wear performance and cost. It remains the most commonly selected wear couple for total joint replacement in hips, knees, and shoulders.

Despite the success of these restorative procedures, UHMWPE-on-metal or UHMWPE-on-ceramic prostheses have a limited implant life because of the wear and damage that the metal component, and to a lesser extent the ceramic, inflicts on the UHMWPE surface. This can lead to the production of small particles of UHMWPE debris, which can trigger osteolysis.

Wear must decrease to reduce bone resorption. One way of reducing wear volume is to incorporate hard bearing surfaces such as metal-on-metal or
ceramic-on-ceramic prostheses.

Early metal-on-metal joints (such as the McKee-Farrar prosthesis) were discarded in favor 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.^1,2 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 concerns 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.³

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 alleviated 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 malpositioned.

Development of Bearing Materials

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.⁴

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 Polymer

Because of its versatility, mechanical strength, and biocompatibility, medical device manufacturers have routinely used implantable-grade PEEK polymer in long-term medical implant applications. Manufacturers are beginning to take note of the material's potential as a bearing material.

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.⁵

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.⁶ 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.⁷

Surgeons and device engineers may be familiar with the attempts to improve the wear performance of UHMWPE through the inclusion of carbon fibers. 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.^8-12 Furthermore, the creep resistance of PEEK ensures that the interface between the fiber and the polymer matrix remains intact.

All-Polymer Wear Couples

Material combinations selected for new implantable bearing devices have traditionally been governed by those materials selected for hip, knee, and shoulder prostheses. A notable exception to this was McKellop's investigation into all-polymer hip prostheses in the early 1980s. The study demonstrated that a combination of UHMWPE and polyacetal produced 39% lower wear than a UHMWPE and cobalt-chromium couple.¹³

With the development of an alternative bearing material in the form of implantable-grade PEEK polymer, device designers can explore alternative design principles and wear couples. In motion-preservation devices, it is possible to have a two-component, injection-molded device where the polymer acts both as the endplate and as the bearing surface. Such a design has been implemented in a commercially available finger joint. The device is manufactured by injection molding, and it features a hinged joint. The wear volume from the PEEK-on-PEEK articulating surfaces has been measured and found to be lower than those of previous materials.

Wear and Osteolysis

While alternative biomaterials must demonstrate wear performance, the biological response to the wear debris is also extremely important. Osteolysis can become evident as early as five to seven years after implantation. Although it is claimed that XLPE may afford a lower volume of wear, the short implant history of the current XLPE implants limits any conclusion regarding the potential for osteolysis.

Use of alternative bearing materials will therefore be dependent on the size, shape, and biological response to wear debris. In light of the historical problems with carbon fiber–reinforced UHMWPE tibial components, several groups have studied the biological response to CF-PEEK. Particles of CF-PEEK extracted from simulator testing for acetabular cups were biologically tested in concentrations of 0.5 and 1.0 mg/ml with human fibroblasts. These tests showed the particulate debris to be biocompatible, with no adverse affects.⁷

Alternative materials may also offer benefits when hemiarthroplasty is employed, because using a lower-modulus material may be preferable when articulating against cartilage. The wear performance of a cobalt-chromium-molybdenum femoral head was compared with a femoral head in which a polyurethanecarbonate cap was used. Most of the cartilage was worn away with the metal head, and there was evidence of necrosis of subchondreal bone. With the polyurethanecarbonate head, the remaining cartilage was very similar to the natural acetabulum, with no evidence of damage to the polyurethanecarbonate.¹⁴

Additional Material Requirements

Not only must a new material provide exceptional wear performance and have a preferential biological response to wear debris, it must also offer the same possibilities with respect to primary and secondary fixation. The selection of implantable-grade PEEK does not prevent the device engineer from carrying out hydroxapatite (HA) or titanium coating of the implant to aid secondary fixation, or from developing porosity at the surface of the implant to aid bony ingrowth.
Commercial applications of HA and titanium-coated PEEK implants are currently available. This can again reduce the number of components needed to make up the artificial joint since a combination of high strength, impact resistance, and low wear can eliminate the need for a metal backing.

Conclusion

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 with a number of different counterfaces. 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.

References

1. SA Jacobsson, K Djerf, and O Wahlstrom, “Twenty-Year Results of McKee-Farrar versus Charnley Prosthesis,” Clinical Orthopaedics and Related Research 329S (1996): S60–S68.

2. TP Schmalzreid et al., “Long-Duration Metal-on-Metal Total Hip Arthroplasties with Low Wear of the Articulating Surfaces,” Journal of Arthroplasty 11, no. 3 (1996): 322–331.

3. L Savarino et al., “Ion Release in Patients with Metal-on-Metal Hip Bearings in Total Joint Replacement: A Comparison with Metal-on-Polyethylene Bearings,” Journal of Biomedical Materials Research 63, no. 5 (2002): 467–474.

4. OK Muratoglu et al., “Unified Wear Model for Highly Crosslinked Ultra-High Molecular Weight Polyethylenes (UHMWPE),” Biomaterials 20, no. 16 (1999): 1463–1470.

5. Wang et al., “Suitability and Limitations of Carbon Fiber Reinforced PEEK Composites as Bearing Surfaces for Total Joint Replacement,” Wear 225-229 (1999): 724–727.

6. A Unsworth and SC Scholes, “Long-Term Wear Behaviour of a Flexible, Anatomically Loaded Hip Cup Design” (paper presented at the 12th International Conference on Biomedical Engineering, Singapore, December 7–10, 2005).

7. N Pace et al., “Clinical Trial of a New CF-PEEK Acetabular Insert in Hip Arthroplasty,” in Abstracts from the European Hip Society 2002 Domestic Meeting (Baveno, Italy: European Hip Society), 212.

8. H Kobayashi et al., “Effect of Quenching and Annealing on Fiber Pull Out from Crystalline Polymer Matrices,” Advanced Composite Materials 1, no. 2 (1991): 155–168.

9. RA Latour Jr. and MR Meyer, “Fiber Reinforcement of Ultrahigh Molecular Weight Polyethylene,” in Transactions from the Annual Meeting for the Society for Biomaterials, International Biomaterials Symposium 14 (1991): 285.

10. MR Meyer, RA Latour, and HD Shutte, “Long-Term Durability of Fibre/Matrix Interfacial Bonding in Hygrothermal Environments,” Journal of Thermoplastic Composite Materials 7 (1994).

11. LA Zhang and MR Piggott, “Water Absorption and Fibre/Matrix Interface Durability in Carbon-PEEK,” Journal of Thermoplastic Composite Materials 13 (2000).

12. MR Piggott, “Why the Fibre/Polymer Interface Can Appear to Be Stronger than the Polymer Matrix,” Composites Science and Technology 57, no. 8 (1997): 853–857.

13. McKellop et al., “Evaluation of Wear in an All-Polymer Total Knee Replacement. Part 1: Laboratory Testing of Polyethylene on Polyacetal Bearing Surfaces,” Clinical Materials 14, no. 2 (1993): 117–126.

14. TM Turner, “Reduced Articular Cartilage Wear Using an Elastomer Compared to a CoCr Femoral Head in Canine Hemiarthroplasty” (paper presented at the 30th Annual Meeting, Society for Biomaterials, Memphis, April 27–30, 2005).

John Devine, PhD, is senior product development scientist for Invibio Ltd. (Thornton Cleveleys, Lancanshire, UK).

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