Originally Published MDDI May 2005

Plastics

PEEK, which has significant benefits in vivo, may be what OEMs want for implant devices.

Stuart Green

Polyetheretherketone (PEEK) polymer is an exceptionally strong engineering thermoplastic that retains its mechanical properties even at very high temperatures. The material is tough and abrasion resistant with high-impact strength and excellent flexural and tensile properties. It has a low coefficient of friction and resists attack by a wide range of organic and inorganic chemicals and solvents.

Implantable-grade PEEK polymer, which is suitable for long-term (greater than 30 days) implantation, shows significant benefits over traditional materials, such as polyethylenes, metallic alloys, and ceramics.
Because of its unique properties, implantable-grade PEEK polymer is used in a variety of components for the medical market, including applications for human-implantable medical devices.

Implantable-grade PEEK polymer is characterized by its biocompatibility—the suitability of a material for exposure to the body or bodily fluids, and its biostability—the ability of a material to maintain its physical and chemical integrity after implantation in living tissue. The combination of strength, stiffness, and toughness, along with the ability to be repeatedly sterilized without the degradation of its mechanical properties, makes it suitable for implantable medical device applications. This article discusses the mechanical properties of PEEK in detail.

Traditionally, metals or ceramics are chosen for hard-tissue applications, and polymeric materials are selected for soft-tissue applications. One of the major problems in orthopedic surgery is the mismatch of stiffness between the bone and metallic or ceramic implant. The moduli of metals and ceramics are fixed at their inherently high levels, whereas the modulus of implantable-grade PEEK can be adapted. This adaptability reduces stress concentrations that can be transferred to the bone and stimulates the healing process. Bone remodeling occurs in response to physical stress, or lack thereof. Bone is deposited in sites that are subject to stress and resorbed from areas where there is little stress.¹

PEEK Qualitie
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Implantable-grade PEEK polymer is one of the most chemically resistant polymers available. It displays chemical resistance, confirmed by 30-day exposure to simulated body fluid environments utilizing a sodium chloride solution, glycerol, vegetable oil, and an alcohol, with no adverse influence on the material's mechanical properties. Similarly, compression tests after soaking in physiological saline for up to 5000 hours have confirmed the stability of the polymer under these exposure conditions.

The natural unfilled polymer is characterized by high strength, extreme resistance to hydrolysis, and resistance to ionizing radiation. Therefore, it can be repeatedly sterilized using conventional gamma irradiation, steam, or ethylene oxide. It has been exposed for 2500 hours at 200°C to 14-bar steam without significant deterioration of its mechanical properties.
Its chemical structure makes it very tolerant to gamma irradiation. In contrast, gamma irradiation of other polymeric materials induces cross-linking or chain scission, which leads to weakening and embrittlement (see Figure 1).

The material's chemical structure ensures extreme stability against hydrolysis, even at elevated temperatures. The relative thermal index (or continuous VSE temperature) against VL 746 B for PEEK is 260°C. Implantable-grade PEEK polymer can be steam sterilized repeatedly without reduction or deterioration in mechanical properties. In addition, EtO residues are within the limits specified in ISO 10993-7, even following three repeated sterilization cycles.

Manufacturing Processes and Biocompatibility Testing

The development of implantable-grade PEEK polymer has been achieved using enhanced manufacturing procedures, supported by physical, chemical, and mechanical testing conducted at key stages of production. Independent laboratories have performed biocompatibility and biostability testing following ISO 10993 and USP Class VI procedures.

The material is manufactured to the highest level of purity, with complete material history traceability. Device and drug master files containing traceability information, as well as additional testing results and extensive data concerning the polymer and its manufacturing methods, are on file with FDA.

Formulation with Additives

Implantable-grade PEEK polymer compounds can be formulated using a variety of additives including carbon fibers, barium sulfate, and glass fibers. The additives satisfy various application-specific requirements.

Glass fibers may be compounded with the polymer to enhance its mechanical properties without substantially changing the color of the base material.

Because the polymer is naturally radiolucent, adding barium sulfate at varying concentrations enables the x-ray density of devices to be tailored from mild to strong radiopacity.

By compounding implantable-grade PEEK polymer with short carbon fibers, the strength of the natural unfilled polymer can be increased significantly. With higher strength, the polymers can address higher-stress applications.

Carbon-Fiber Compounds

Short-fiber-reinforced materials are produced by combining carbon fibers with the polymer in a twin-screw compound extruder under controlled conditions suited to medical polymer production. The resulting compound may be injection molded using a system with process temperature capability approaching 400°C.

Typical fiber loading may be 30– 35% by weight for short-fiber compounds, increasing the material's modulus from 3.5 to approximately 18 GPa and its tensile strength from 100 to 230 MPa.

With a stiffness close to that of cortical bone, carbon-fiber-reinforced implantable-grade PEEK polymer compounds are used in applications for which stress shielding may have a critical effect on the lifespan of an implant. For example, if a hip implant is manufactured from metallic components with a significantly higher stiffness than cortical bone, the higher stiffness may lead to bone resorption and weakening at the implant site as the device sustains a greater proportion of the applied load.

This example is in contrast to hip stems made from implantable-grade carbon-filled PEEK compounds that demonstrate elastic properties similar to the surrounding bone and that reduce the effects of stress shielding.

Carbon-Fiber Composites

In certain applications for which superior mechanical properties are required, implantable-grade PEEK polymer may be used as the matrix polymer in combination with continuous carbon fibers to form reinforced composite materials. This fiber reinforcement significantly enhances the mechanical properties of the polymer.

In one example, continuous-fiber-reinforced implantable-grade PEEK polymer is made using a proprietary pultrusion process in which carbon fibers and PEEK are processed into a filament, brought together into bundles, and shaped into a rod.

For a continuous-fiber material in pultruded rod form, the fiber loading is approximately 70% by weight (60– 62% by volume), which significantly increases the mechanical properties of the material in the fiber direction. Flexural strength is increased from approximately 150 to more than 1000 MPa, and stiffness is increased from 3.5 to 150 GPa.

The mechanical properties of continuous-fiber-reinforced rod products are comparable to those of metallic materials such as cobalt-chrome, titanium, alloys, and stainless steel (see Figure 2). Such composites are also available as preimpregnated tapes, comprising uniaxially aligned carbon fibers and a PEEK matrix. The tapes are used for hand lay-up compression-molded or filament-wound parts. Components, or successive plies of reinforcing materials, or resin- impregnated reinforcements are applied to the mold and the composite is built up by hand. Compression molding then cures the material.
Carbon-fiber-reinforced implantable-grade PEEK polymer compounds and composites have been tested according to the ISO 10993-1 guidelines.

Fiber Reinforcement for Load-Bearing Applications

Medical device manufacturers have processed continuous-fiber-reinforced PEEK rods by composite flow molding to make load-bearing structural components and fastening elements such as pins for spinal surgery or bone plates for trauma fixation.
The use of composite flow molding has enabled medical device manufacturers to develop significantly improved implants that reduce operation trauma, shorten in-patient hospital stays, increase patient comfort, enable less-invasive applications, and increase tissue tolerance.

Aside from mechanical properties that compare favorably with metals, implantable-grade PEEK polymer composites offer superior medical imaging compatibility under magnetic resonance imaging (MRI). They are radiolucent, meaning they are almost invisible under x-ray inspection, allowing uncluttered image visualization. Traditional metallic implants are not radiolucent, which prevents a complete inspection of tissue and bone.

Carbon-Fiber Reinforcement for Wear Applications

Adding short carbon fibers to implantable-grade PEEK polymer increases its tribological wear properties. Tribology, or how a material interacts with the surface of another material while in motion, is a critical concern in the development of orthopedic implants, because such implants need to retain their structure while moving against other surfaces and components.

Recently, the ability of bearing surfaces to minimize wear in orthopedic implants has been investigated. Often the selection of an alternative bearing surface may change how a joint is affixed to the bone, or it may lead to structural consequences in terms of stiffness and load transfer to the bone. Two alternative wear materials that have been investigated are metal-on-metal and ceramic-on-ceramic bearings.

Highly cross-linked polyethylene was developed to reduce wear and has shown promising results in simulator studies and early clinical work.² Although the initial problems concerning oxidative degradation of highly cross-linked polyethylene have been rectified, the cross-linking process can reduce static mechanical properties such as tensile and yield strength as well as fatigue-crack propagation resistance. Such reductions are of particular importance in the knees, where articular surfaces are not completely congruent. As a result, highly cross-linked polyethylene is not used for knee implants.

Some historical uses of carbon fibers in combination with ultra-high-molecular-weight polyethylene were not entirely successful.³ This may be attributed to lower bond strength between the fiber surface and the surrounding polymer matrix. Implantable-grade PEEK has a very strong bond with carbon fibers so fiber release is significantly reduced or eliminated.^4,5

Additionally, because of its creep resistance, implantable-grade PEEK polymer can sustain comparatively large stresses over long periods of time without significant time-induced extension, and with good fiber-matrix interfacial bond strength.

A clinical study of carbon-fiber-reinforced implantable-grade PEEK polymer acetabular inserts was initiated in 2001.⁶ To date, of the 20 procedures performed, no complications following implant surgery or adverse reactions to the material have been reported.

Wear Debris

In addition to linear wear rates, the size, shape, and number of wear particles also affect the life of an implant. When micron-sized wear particles are released into the surrounding tissues, the macrophage cells are provoked, resulting in osteolysis (the dissolution or degeneration of bone tissue) and possible implant loosening.

Osteolysis tends to become evident during the second decade of an implant's lifespan. While it is claimed that highly cross-linked polyethylene may afford lower wear rates, simulator studies indicate that the wear debris may be slightly smaller, resulting in a greater number of wear particles per year.⁷ The short history of the current cross-linked polyethylene implants precludes a conclusion regarding the potential for osteolysis.

Simulator tests indicate that acetabular inserts made from implantable-grade PEEK polymer with carbon fibers yield lower wear rates and a smaller amount of wear particles compared with other materials.^8,9 The particles were biologically tested and results showed that the material was well tolerated biologically. Implantable-grade PEEK polymer particles extracted from the simulator test for acetabular inserts were found to be smaller than 15 µm. The particles were biologically tested in concentrations of 0.5 and 1.0 mg/ml with human fibroblasts. These tests showed that the material was well tolerated biologically.⁶

Unlimited Design Solutions

In addition to benefiting from the mechanical, chemical, and biological characteristics of implantable-grade PEEK, many medical device manufacturers use the polymer because of the varied design solutions it provides.

The polymer may be processed using conventional thermoplastic processing equipment and techniques such as injection molding, extrusion, compression molding, and powder coating.

It is often used in injection molding operations to economically mass-produce high-performance components without the need for postprocess annealing or machining. Extrusion may be used to produce film and sheet and monofilament tubing, rods, and compounds with pigments or fillers. The viscosities of these materials are comparable to commodity polymers at their melt temperatures. Implantable-grade PEEK polymer is available in three grades: standard viscosity, medium viscosity, and low viscosity.

Medical Applications

Because of implantable-grade PEEK polymer's versatility, it is being used in the development of many applications for long-term implants. It is now being used in joint-replacement systems and in spine surgery, especially for cages used in vertebral fusion surgery.

It is also being used in the development of cardiovascular applications such as heart valves and intracardiac pumps; for suture anchors for arthroscopy; and for dental applications such as permanent dentures, bridges, abutments, and healing caps.

Implantable-grade PEEK polymer is being investigated for a wide range of medical devices for both long-term implantation and short-term body contact. It possesses a unique combination of biocompatibility, x-ray and computed tomography translucency, and MRI compatibility. Furthermore, its adjustable mechanical performance, chemical resistance, sterilization options, and ability to be thermally processed easily make this thermoplastic an increasingly popular choice for implantable devices.

Reference

1. RB Salter, Textbook of Disorders and Injuries of the Musculoskeletal System, 1st ed. (Baltimore: Williams and Wilkins, 1970), 7.
2. HA McKellop et al., “Development of an Extremely Wear Resistant UHMWPE for Total Hip Replacement,” Journal of Orthopedic Research 17 (1999): 157–167.
3. TM Wright et al., “Analysis of Surface Damage in Retrieved Carbon Fibre Reinforced and Plain Polyethylene Tibial Components from Posterior Stabilised Total Knee Replacements,” Journal of Bone and Joint Surgery 70-A (1998): 1312–1319.
4. H Kobayashi et al., “Effect of Quenching and Annealing on Fiber Pull Out from Crystalline Polymer Matrices,” Advances in Composite Materials 1 (1991): 155–168.
5. RA Latour and MR Meyer, “Fiber Reinforcement of Ultrahigh Molecular Weight Polyethylene,” in Transactions of the Annual Meeting of the Society for Biomaterials (Scottsdale, AZ: International Biomaterials Symposium, 1991), 14, 285.
6. 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.
7. Kengo Yamamoto et al., “Microwear Phenomena of Ultrahigh Molecular Weight Polyethylene Cups and Debris Morphology Related to Y Radiation Dose in Simulator Study,” Journal of Biomedical Materials Research 56, no. 1 (2001): 65–73.
8. VK Polineni et al., “Characterization of Carbon Fiber Reinforced PEEK Composite for Use as a Bearing Material in Total Hip Replacements,” in Alternative Bearing Surfaces in Total Joint Replacement, ASTM STP 1346, (West Conshohocken, PA: ASTM, 1998).
9. G Maharaj et al., “Characterization of Wear in Composite Material Orthopaedic Implants,” Biomedical Materials and Engineering 3 (1994): 193–198.

Stuart Green, PhD, is the technical manager for Invibio Ltd. (Thornton Cleveleys, Lancashire, UK). His PhD is in short-fiber composite materials.

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