The orthopedics market is dominated by metal materials used for implants. While these options offer an excellent level of healthcare for patients requiring such devices, they certainly are not the absolute best answer to replicate natural bone. A carbon fiber-reinforced polymer alternative is being used to replace metals in many applications and is achieving exceptional results.

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Amy Kinbrum is a product development scientist with Invibio Ltd. She has recently completed her PhD in biomedical engineering. Kinbrum's current focus is in the tribology of artificial joints and analysis of the particle debris produced. She can be reached at +44 1253 898000 or akinbrum@invibio.com.

Made with PITCH CFR PEEK polymer, the anatomically shaped Stryker MITCH PCR acetabular cup is only 3.0 mm thick and results in efficient stress distribution.

The commonplace and serious problem of osteoarthritis among the ageing population presents serious challenges for health professionals that have prompted a variety of novel engineering solutions to address the deep aching hip pain of bone grating against bone. Thanks to the availability of new biomaterials options, a host of improvements in performance and innovations in device design are increasingly possible.

Since the 1960s, there has been the option of arthroplasty. The early Charnley prostheses used metal against polymer, but the market has since expanded, providing a great deal of choice for the osteoarthritic patient and their health providers.

Addressing Material Wear Concerns

What remains, however, is the problem that caused the initial prostheses to fail; osteolysis and subsequent implant loosening have a severe effect on the survivorship of a prosthesis. There are several ways to reduce wear rates and thus, the onset of osteolysis. For example, wear rates could be reduced by using a harder material, or one with higher wear resistance, or by creating a lubrication regime within the prosthesis that causes very little wear to occur.

Finite element meshes of the (clockwise from top left) resurfaced hip, Cambridge cup, MITCH cup, MITCH pelvis, and Cambridge pelvis.

In addition to addressing the need for lower wearing prostheses, device companies are also seeking materials that may encourage development of new designs intended to conserve patient bone and more efficiently transfer stress to bone surrounding the implant.

Implantable polyetheretherketone (PEEK) polymers have a history of widespread use in implantable devices, including spinal, orthopedic, dental, and cardiovascular applications. Their application in hip joint prostheses may provide the industry with a tool to address stress shielding while maintaining low wear rates. For example, carbon fiber-reinforced (CFR) PEEK polymer is produced by twin screw compounding of implantable PEEK polymer with carbon fibers; resulting in a carbon fiber-reinforced granule that can be used to direct injection mold final devices and near net shapes, or it can be extruded into stock shapes for machining. The incorporation of fibers provides a Young's modulus of 12 GPa, matching the modulus of cortical bone and providing sufficient strength to permit its use in very thin implant designs which distribute the stress more efficiently to the bone.

Simple pin on plate screening tests have shown that CFR PEEK bearing against alumina produces a wear factor of 0.153 mm³N^-1m^-1 × 10^-6 under conditions which would not create a favorable lubrication regime; against a metallic counter face, the wear factor was 0.129 mm³N^-1m^-1 × 10^-6. These figures compare favorably with an ultra-high molecular weight polyethylene (UHMWPE) wear test on the same material testing device which shows a wear factor of 1.1 mm³N^-1m^-1 × 10^-6.¹

Figure 1² demonstrates the wear factors for various combinations of PEEK materials against both Biolox Delta and Biolox Forte materials. The lowest wear rates were found for PITCH based CFR material bearing against Biolox Forte. This combination was investigated by Stryker Orthopedics using a 28 mm ceramic femoral head and CFR PEEK as a liner material. The simulator testing of this wear couple is shown in figure 2³.

These results led to a small clinical trial of CFR PEEK acetabular liners which commenced in 2001. To date, there have been no reports of adverse wear or biological response.⁴

Reduction of Stress Shielding

Figure 1: Wear factor comparison for various combinations of PITCH CFR-PEEK material against ceramic counterfaces.2

As stated previously, the goal of the device manufacturer is not only to reduce wear but to do so with a design that addresses additional issues such as stress shielding. The strength of CFR PEEK allows thin, bone conserving parts to be produced. These thinner parts combat stress shielding, a problem that occurs with materials that have a higher density than bone. When stress is shielded, the natural strength of the bone is depleted according to Wolff's Law and the supporting bone is therefore prone to fracture. Thinner parts allow the natural stresses of the joint to travel through the prosthesis, permitting the natural bone to take on the normal load and, because no bone loss occurs, reducing the tendency of the bone to fracture. It also reduces the tendency for radiolucency (a fibrous tissue that separates the prosthesis or cement from the bone; resulting from either biological reaction to PMMA or mechanical loosening) around the backside of the prosthesis.

Advanced Acetabular Cup Design

A thin profile was also a key objective of the most recent design of an acetabular cup, which exploits the excellent mechanical strength, creep, and fatigue resistance of the CFR PEEK material. At 3.0 mm in thickness, the very thin but strong cup is anatomically shaped. Modeling has shown that the combination of the design and stiffness of the material results in semi-lunar periprosthetic stress distributions, consistent with contact regions measured in vitro.⁵

Figure 2: Simulator wear testing results of various femoral head and acetabular cup combinations.3

This device has undergone extensive wear testing to 25 million cycles using a 54 mm femoral head. These tests demonstrated an average wear rate of 1.16 mm³/million. This compares with 48.2 ±3.7 mm³/million cycles for a conventional 28 mm metal on UHMWPE joint.⁶ Just as importantly, this device is also one of the first to incorporate a direct coating of both porous titanium and hydroxyapatite directly onto the PEEK cup in order to enhance fixation to bone.

Figure 3⁷ shows how the wear rate of PITCH CFR PEEK/alumina compares with other common materials on the market. These results are extremely promising for medical device designers and also offer the advantage of eliminating concerns surrounding metal ion release.

Wear debris of CFR PEEK has been cytotoxically analyzed and was revealed to be non-toxic to cells. Studies on L929 or U937 cell lines by John Fisher's group at Leeds University showed no cytotoxic effects.⁸ Due to the low wearing properties of PITCH CFR PEEK, fewer particles are produced than from other implantable polymers, such as UHMWPE. There have also been additional studies comparing the biological response of CFR PEEK and UHMWPE particles in a rat pouch model where a similar biological response was recorded.⁹

Injection Molding

Figure 3: Wear rate comparison of PITCH CFR-PEEK against alumina to other common hip arthroplasty wear couples.6,7

CFR PEEK is easily manufactured into components using injection molding, a key benefit that not only eliminates machining costs but cuts down on scrap. Flexibility in manufacturing and reduced material usage contributes to reduced total manufacturing costs. An obvious challenge in injection molding is the ability to predict and control shrinkage to produce the tight tolerances that are necessary within the medical device industry. There is, however, considerable experience in designing tooling, gating, and heating such that tolerance and high part-to-part consistency can be maintained.

Conclusion

CFR PEEK has a number of desirable properties for orthopedic prosthesis design requirements. Among these are:

•Low wear against metal and ceramic counter faces

•High durability

•High strength-to-weight ratio

•Low creep

•Production via injection molding

•Proven biocompatibility

•Gamma and steam sterilization compatible

These properties translate into significant benefits for orthopedic device design, including:

•Low wearing and bone conserving designs

•Good load distribution that prevents stress shielding

•Innovative designs not possible with metals, ceramics, or other polymers

These material properties, the result of key advances in the development of polymeric biomaterials such as PEEK, offer the potential for new innovation in the design of arthroplasty devices. Early stage in vitro testing of these materials is extremely promising and reveals the potential to provide real clinical benefits for even the more active arthroplasty patients of today.

References

¹ Joyce TJ, et al. A multi-directional wear screening device and preliminary results of UHMWPE articulating against stainless steel. Biomedical Materials and Engineering. 10 (3-4); 2000: 241-249.

² Scholes SC and Unsworth A. The wear properties of CFR-PEEK-OPTIMA articulating against ceramic assessed on a multidirectional pin-on-plate machine. Proc. I Mech E Part H: J. Engineering in Medicine. Vol. 221; 2006: 281-289.

³ Berger T. Biospine 2 Congress. Liepzig: Sept. 2007.

⁴ Pace N, et al. Clinical Trial of a New CF-PEEK Acetabular Insert in Hip Arthroscopy. Abstracts from the European Hip Society: 2002 Domestic Meeting.

⁵ Manley M, et al. Biomechanics of a PEEK horseshoe-shaped cup: Comparisons with a predicate deformable cup. Paper C655/058. Institution of Mechanical Engineers, "Engineers & Surgeons: Joined at the Hip" London, April 19?21, 2007.

⁶ Smith S and Unsworth A. A comparison between gravimetric and volumetric techniques of wear measurement of UHMWPE acetabular cups against zirconia and cobalt–chromium–molybdenum femoral heads in a hip simulator. Part H Technical Note; Proc Instn Mech Engrs Vol. 213; 1999: 475–484.

⁷ Scholes SC, Unsworth A, and Jones E. Long term wear behaviour of a flexible, anatomically loaded hip cup design. International Society for Technology in Arthroplasty.

⁸ Howling, et al. Biological response to wear debris generated in carbon based composites as potential bearing surfaces for artificial hip joints. J. Biomed Mater Res Part B Appl Biomater. 67B; 2003: 758-764.

⁹ Latif AMH, et al. Pre-clinical studies to validate the MITCH PCR Cup: a flexible and anatomically shaped acetabular component with novel bearing characteristics; J Mater Sci. Mater Med: Online.

Online

For additional information on the technologies and products discussed in this article, see MDT online at www.mdtmag.com or Invibio Ltd. at www.invibio.com.