Implantable-grade polyetheretherketone (PEEK) high-performance thermoplastic has been extensively used in the development of spinal-fusion cages and plates since its introduction in 1999. Interest in the material continues to spread throughout the orthopaedic market.
Its properties can be tailored to meet specific requirements. For example, chopped carbon-fibre (CF) additives can be used to enhance the material’s mechanical strength and stiffness. Tensile strength can be raised from 100–230 MPa and the modulus increased from 3.5–18 GPa to make PEEK more similar to the properties of cortical bone (Figure 1). This may have implications for the prevention of bone resorption due to stress shielding.PEEK is naturally radiolucent and allows clinicians to assess healing and bone formation around an implant without the interference of X-ray scatter or the generation of magnetic resonance imaging or computer tomography artefacts. Where visibility is an advantage, the polymer can be tailored by adding variable amounts of barium sulphate to allow the desired level of implant visibility for confirmation of positioning in patients.
Figure 1. (click to enlarge) Flexural modulus comparison chart.
Widening application range
Newer spinal technologies such as motion preservation have adopted PEEK. One area being developed is dynamic stabilisation where vertebral support and motion can be achieved with PEEK pedicle-screw-based devices. An articulating PEEK-on-PEEK design has been used in intradiscal arthoplasty.
Next-generation joint prostheses are another potential application area. PEEK polymer has been employed as an acetabular liner for articulation against a ceramic femoral head. Hip-joint simulator testing up to 10 million cycles showed that the wear of the CF-reinforced PEEK polymer composite cups was approximately 1% that of ultra-high molecular weight polyethylene (UHMWPE) cups.1 Further work using CF-reinforced PEEK acetabular cups wearing against large (54 mm) alumina femoral heads has demonstrated that this combination of materials approaches the bearing performance of hard-on-hard metal or ceramic bearing surfaces.2 This avoids the concerns of ceramic brittleness or metal ion release and offers greater design potential than polyethylene: the cups can be injection moulded and the harder wearing surfaces can be made thinner.
Figure 2. (click to enlarge) Wear comparison chart. (Source: A. Unsworth et al. 2)
The wear properties of PEEK polymer (Figure 2) suggest it has the potential for increasing implant lifespan. Also, because the wear-debris particles are of a different number, size and shape, it may help reduce osteolysis. A clinical implant study using CF-reinforced implantable-grade PEEK polymer acetabular inserts has reported no complications or adverse reactions to the material since its start in 2001.3 Wear-debris particles of CF-reinforced PEEK extracted from acetabular cup simulator tests were biocompatible with no adverse affect on human fibroblasts at particle concentrations of 0.5 and 1.0mg/mL.3
The historical concerns highlighted with CF-reinforced UHMWPE tibial components, including poor fibre and polymer adhesion and delamination, appear to have been addressed by replacing the polyethylene with PEEK.4 Studies have shown that the interfacial shear strength ranges from 72.8–202 MPa with CF-PEEK composites. 5,6,7,8 This is at least an order of magnitude stronger than with UHMWPE (4.5–7.1 MPa).9 This, combined with the better mechanical stability of PEEK over extended loading periods, means that the fibre-polymer interface is less prone to de-bonding.
PEEK polymer is also being used to develop arthroscopic applications such as small, high-strength bone screws or anchors required for anterior cruciate ligament fixation and soft-tissue attachments. Its ability to be injection moulded enables the mass production of small components with complex geometry, including recesses, holes and gripping points.
For high load bearing applications, PEEK polymer can be reinforced with continuous CF to yield a composite material with different properties than those present in unreinforced or chopped CF PEEK. The composite offers mechanical properties comparable to traditionally used metallic materials and the added benefits of imaging compatibility, better fatigue behaviour and light weight. For example, compared with unreinforced PEEK, continuous CF-PEEK composite has increased flexural strength from 100–1000 MPa and stiffness from 3.5–150 GPa, which makes the material suitable for load bearing trauma applications such as bone fracture plates and translaminar fixation pins.
Future implantable materials
The industry’s attention is on the development of more bioactive materials. Enhancing long-term implantable materials to evoke a positive interaction with the surrounding biological environment is a reality achieved through coatings, surface modifications, topography and different physical forms such as porosity. Many companies associated with the medical device industry have developed products such as attachment peptides, stimulatory growth factors and anti-thrombogenic agents in the cardiovascular arena. As these complementary technologies extend to include the processing of polymers such as PEEK, it will allow many beneficial properties to be pulled together and exploited in the next generation of devices.
1. Wang et al., “Suitability and Limitations of Carbon Fibre Reinforced PEEK Composite Bearing Surfaces for Total Joint Replacement,” Wear, 225–229, p 724–727 (1999).
2. A. Unsworth and S.C. Scholes, ICBME Congress, Singapore, December 2005.
3. N. Pace et al., “Clinical Trial of a New CF-PEEK Acetabular Insert in Hip Arthroplasty,” Abstracts, European Hip Society 2002 Domestic Meeting p 212.
4. L.M.O. Birken et al., “Composites as Bearing Partner in Total Knee Replacement: Failure Analysis of Poly II Components With Respect to Material Design,” European Society for Biomaterials conference proceedings 1998.
5. M.R. Meyer et al., “Long-Term Durability of Fibre–Matrix Interfacial Bonding in Hygrothermal Environments,” Journal of Thermoplastic Composite Materials, 7, July 1994.
6. L.A. Zhang ad M.R. Piggott, “Water Absorption and Fibre–Matrix Interface Durability in Carbon-PEEK,” Journal of Thermoplastic Composite Materials, 13, March 2000.
7. H. Kobayashi et al., “Effect of Quenching and Annealing on Fibre Pull-Out From Crystalline Polymer Matrices,” Adv. Composite Materials, 1, 2, 155–168 (1991).
8. M.R. Piggott, “Why the Fibre–Polymer Interface Can Appear to be Stronger Than the Polymer Matrix,” Composites Science and Technology, 5,7, 853–857 (1997).
9. R.A. Latour and M.R. Meyer, “Fibre Reinforcement of Ultra High Molecular Weight Polyethylene,” 17th Society for Biomaterials, May 1991, Scottsdale, Arizona, USA.
Marcus Jarman-Smith is Bioscience Project Manager for Invibio Ltd, Technology Centre, Hillhouse International, Thornton Cleveleys FY5 4QD, UK,
tel. +44 1253 866 812, e-mail: email@example.com, www.invibio.com.