Taking a PEEK at Material Options for Orthopedics
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.
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 firstname.lastname@example.org .
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 ConcernsWhat 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.
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
Advanced Acetabular Cup DesignA 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 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
ConclusionCFR PEEK has a number of desirable properties for orthopedic prosthesis design requirements. Among these are:
•Low wear against metal and ceramic counter faces
•High strength-to-weight ratio
•Production via injection molding
•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.
References1 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 cobaltchromiummolybdenum femoral heads in a hip simulator. Part H Technical Note; Proc Instn Mech Engrs Vol. 213; 1999: 475484.
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.