By the year 2030, the number of individuals age 65 or older is projected to double in the United States, where those age 85 and older are predicted to become the fastest growing segment of the population.1 One in two adults older than 65 has some form of arthritis, and according to the American
The biomet Oxford Partial Knee with a CFR-PEEK mensical bearing. Photo courtesy of Biomet. |
Academy of Orthopaedic Surgeons (AAOS), osteoarthritis is among the leading causes of disability in the elderly. The pain and loss of activity resulting from osteoarthritis can be profound, often necessitating a partial or total joint replacement; in 2004, two out of three patients hospitalized for osteoarthritis underwent a joint replacement procedure. 1
Then, as now, joint replacement remains the treatment of choice to restore function to painful arthritic joints, and demand is predicted to keep pace with the rapidly aging U.S. population. During the next quarter of a century, the incidents of arthritis are projected to soar by 40 percent; by the year 2033, approximately one-quarter of all Americans, roughly 67 million people, will be impacted.2
Although the overall number of joint replacement procedures performed annually in the United States3 has risen steadily since the early 1990s, the area of knee replacement has shown the greatest growth. According to AAOS, in 2006, more than 540,000 total knee replacements procedures were performed in the United States, where, by 2030, it is projected that nearly 3.5 million primary total knee replacements will be performed annually. 2
As the population ages while remaining more active, the performance expectations for joint replacements are rising, and more advanced materials are required to meet the increasing demands.
One such material that has shown promise for joint arthroplasty applications is implantable-grade polyetheretherketone (PEEK) polymer.
The first implantable PEEK polymer, PEEK-OPTIMA, was introduced by London, United Kingdom-based Invibio Biomaterial Solutions in 1999. The material has a proven history of use and biocompatibility supported by U.S. Food & Drug Administration (FDA) Drug & Device Master Files, and is used in thousands of approved devices in the United States, Europe and elsewhere.
To date, more than 2 million medical devices manufactured with the material have been implanted in patients globally.
The combined mechanical properties and performance characteristics that PEEK polymer provides are exceptionally well-suited to joint replacement and specifically can be tailored to the precise and exacting demands of load-bearing, articulating applications against a hard counterface, such as in hip and knee arthroplasty. Implantable PEEK is a semi-crystalline polymer that offers a unique combination of material properties such as strength, stiffness, creep and fatigue resistance and has a chemical structure that confers biostability and tolerance to modern sterilization techniques.
This combination of properties has provided a versatile and value-added material platform for the medical device community, which first began to use the material in spinal interbody fusion devices.
Limitations to Traditional Material Couplings
The traditional material coupling used in total knee replacement, metal on ultra-high molecular weight polyethylene (UHMWPE) has served to alleviate the pain and debilitation of injury or arthritis. However, wear particle-induced osteolysis and oxidative degradation and delamination are the most frequent causes of failure. To minimize this risk, device manufacturers must recognize the volume of wear generated and the immunological impact of the wear particles that are produced.
Implant manufacturers now are investigating the tribological performance (friction, lubrication and wear) of new biomaterials and material combinations in an effort to overcome the long-standing wear limitations associated with conventional biomaterials and material couplings. PEEK on PEEK, for example, has been used successfully in spinal arthroplasty, providing low wear rates and advantages related to improved imaging.
The mechanical and tribological properties of PEEK polymer can be modified by the inclusion of reinforcing carbon fiber of various lengths. Control of the fiber content allows the flexural modulus, wear properties and strength of the carbon fiber-reinforced (CFR) PEEK material to be tailored to a particular application, enabling advanced device designs that confer both surgeon and patient benefits.
Importantly, CFR-PEEK can result in a modulus closely matched to that of cortical bone, and is shown to encourage load sharing and minimize stress shielding, necessary for healthy bone. The inherent mechanical strength of CFR-PEEK enables the design and production of extremely thin and flexible device elements that provide physiological stress transfer, thereby minimizing bone loss, fostering bone strength and reducing the tendency of fracture, research has shown.
A Growing Body of Evidence
There is a growing body of evidence supporting the use of an advanced CFR-PEEK compound in joint arthro-scopy. An example of this is a new acetabular cup design that exploits the excellent mechanical strength, creep properties and fatigue resistance of CFR-PEEK. The result is an extremely thin (3 millimeter thickness), anatomically shaped device.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 per million cycles. This compares with 48.2 +/- 3.7 mm3 per million cycles for a conventional 28 mm metal-on- UHMWPE joints.5 CFR-PEEK wear rates for hip bearings compare favorably to that of cross-linked polyethylene, which shows an in vivo wear rate of 4.5 mm3 per million cycles for a 28 mm diameter bearing.6 This device also is noteworthy for promoting enhanced fixation to bone, the result of direct coating of the CFR-PEEK acetabular cup with both porous titanium and hydroxyapatite.
CFR-PEEK has been extensively assessed in terms of mechanical strength, wear performance (measured against metal and ceramic counterfaces), bony fixation, and immunological/biological response to wear debris generated by the material. In comparison to UHMWPE and other implantable grade polymers, CFR-PEEK produced fewer wear particles.7 Cell line studies conducted on CFR-PEEK wear particles demonstrated their biocompatibility and showed no adverse cytotoxicological impact. Separately conducted rat pouch model studies demonstrated a similar biological response to that of UHMWPE. 8
These results show the material's value and significant promise in hip replacement and encourage exploration of its enormous potential in knee arthroplasty.
Implications for Knee-Bearing Material
The possibilities afforded by the novel use of CFR-PEEK as a bearing material within the knee could provide new approaches to knee arthroplasty. Durable, flexible, high creep and wear resistant, and injection moldable, the material offers a unique combination of attributes that may prove ideally suited to the next generation of knee prosthesis.
In knee surgery, as in other forms of joint replacement, there is a move toward more conservative surgery-more bone sparing and allowing the preservation of the non-damaged parts of the joint. This has led to greater interest in the concept of unicondylar knee replacement. The knee joint is made up of two condyles, or compartments, and joint damage/joint wear often is limited to a single condyle (the medial condyle is most commonly affected). Unlike total knee replacement that removes all the knee joint surfaces, a unicondylar knee replacement replaces only the damaged part (condyle) of the knee joint. The smaller-sized prosthesis allows implantation through a reduced incision compared with total knee replacement. This less-invasive surgery shortens recovery time and leaves the door open for total knee arthroplasty, if it is needed in the future. Most unicondylar prostheses are made from metal against a polyethylene. However, as patient expectations outstrip the capabilities of currently available prostheses, a gap in the market for a more durable material may exist.
A PEEK-based version of the Orthoglide medial knee implant was tested by Minnetonka, Minn.-based Advanced Biosurfaces Inc., which also manufactures the implant in a range of materials including PEEK-OPTIMA against polyacetal femurs and frozen femurs. The results showed early promise. Wear rates were 5.6 milligrams per million cycles for roughened polyacetal against PEEK-OPTIMA and for frozen femur tests the wear rate was 11.3 mg per year. Further tests on CFR-PEEK demonstrated the potential for even greater reductions in wear rate.
Testing of CFR-PEEK polymer against a hard counterface provides positive indications for its potential to meet such demands for knee arthroplasty. The device tested was an exact replica of the Oxford Partial Knee from Biomet (Warsaw, Ind.), but with CFR-PEEK in place of the standard UHMWPE mensical bearing. A five million-cycle in vitro wear simulator test was carried out at Durham University in the United Kingdom to test the wear properties of CFR-PEEK bearing articulating against the cobalt chrome femoral and tibial counterfaces. 9
A wear simulator allows an initial wear assessment of different materials under similar loading and motion that occurs during the walking cycle. The results show that CFR-PEEK against the cobalt chrome counterface is lower wearing than the literature reports of similar UHMWPE components against cobalt chrome counterfaces. The medial and lateral components of this particular design have slightly different bearing surfaces to allow movement that more closely resembles the natural knee joint; wear rates, therefore, are treated separately.
The CFR-PEEK insert produced medial and lateral wear rates of 1.70 and 1.02 mm3 per million cycles, respectively. This compares with the published values of 6.69 mm3 per million cycles and 2.98 mm3 per million cycles for UHMWPE bearings articulating against cobalt-chromium-molybdenum (CoCrMo) obtained in another wear simulator study investigating unicondylar knees10 and 6mm3 per year obtained in a study investigating medial Oxford Partial Knee bearings retrieved from patients. 11
The friction factor was measured during the tests and ranged from 0.14 to 0.24, and the lubrication regime indicated by the friction data was boundary to mixed, meaning that there would be no point within the walking cycle that the two surfaces would be completely separated by a film of lubricant. Significantly, the CFR-PEEK polymer surfaces got smoother as the test progressed. As a surface gets smoother and more negatively skewed there is a higher possibility that the lubrication acting between the two bearing surfaces will completely separate the surfaces leading to fluid film lubrication. Since the surfaces are no longer touching, wear will be minimal.
For both medial and lateral devices, the wear rates of CFR-PEEK against CoCrMo in this knee prosthesis design showed very promising results which could benefit future designs and performance. The promising results highlight future possibilities, including overcoming limitations associated with traditional material couplings, and reducing the knee prosthesis from three parts to just two. Simplifying the total knee prosthesis will enable easier surgical procedures and overcoming wear issues associated with traditional material couplings will likely overcome the problem of "backside" wear.
With a traditional metal on UHMWPE knee, for example, polymer will creep and bone will not adhere to it directly.It either must be cemented or a tibial tray must be used as a base for the UHMWPE to "lock into" and for bone to grow onto the backside. By comparison, if CFR-PEEK was used, the backside could be coated with a bone-bonding material such as hydroxyapatite; therefore, only one tibial component would be needed resulting in a total knee replacement consisting of just two parts.
The flexibility and durability of a CFR-PEEK allows novel concepts such as an all-polymer two-part knee. This idea has been tried previously with an alternative polymer and showed promise that a PEEK version may be beneficial.
An all-polymer (DuPont Delrin acetal resin) knee has been evaluated in clinical trials with noteworthy results. A cumulative survivorship of 77 percent at 10 years has been observed; this compares with a cumulative survivorship of 86 percent for a metal total knee replacement. 12
CFR-PEEK can be injection molded easily to produce near-net shapes and finished components; this key benefit eliminates machining costs and minimizes scrap.
Manufacturing flexibility and reduced material usage contributes to reduced total manufacturing costs. A challenge of injection molding is the ability to predict and control shrinkage to produce the tight tolerances that are necessary within the medical device industry. It is possibile, however, to design tooling, gating, and heating such that tolerance and high part-to-part consistency can be maintained.
Summary
These results and ongoing developments combine to demonstrate that CFR-PEEK is well suited to the design demands of articulating joint devices and opens the door to its use in new designs and approaches to total knee arthroplasty, including:
- * Conservation of bone and, therefore,thinner components
- * Resurfacing of the knee
- * CFR-PEEK single tibial tray/bearing surface
- * CFR-PEEK as an injection moulded femoral component
The most successful PEEK-based devices will be those that make full use of the polymer's properties to change the design of the device to provide patient benefits.
These may include:
- * Improved designs for fixation
- * Reducedstress shielding
- * Improved visualization for placement
- * Thinner components where space is compromised
- * Alternative to metal on metal to address metal ion concerns
- * New designs for conservation of bone
- * Improved wear performance in hemi-arthroplasty devices
- * More efficient processing and component manufacturing
Specific to evolving approaches to knee arthroscopy and component design advances, CFR-PEEK has shown significant promise to confer surgeon/patient benefits that include:
- * Reducing total knee replacement to two components, therefore, reducing the possibility of wear (i.e., wear on the back side of the tibial component no longer occurs)
- * Molding flexibility of PEEK allows the intricate and highly patient-specific designs necessary for knee devices.
Amy Kinbrum is a product development scientist with Invibio, Ltd. She recently completed her Ph.D. 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 akinbrum@invibio.com.
References
1. The Burden of Musculoskeletal Diseases in the United States, 2008, United States Bone and Joint Decade, page 1. (The Burden of Musculoskel-etal Diseases in the United States is a joint project of a number of musculoskeletal organizations under the auspices of the United States Bone and Joint Decade. The full publication can be viewed at: www. boneandjointburden.org)
2. Executive Summary, The Burden of Musculoskeletal Diseases in the United States, 2008, United States Bone and Joint Decade.
3. Web site, American Academy of Orthopaedic Surgeons, as viewed on Jan. 6, www.aaos.org/Research/stats/ Knee-Facts.pdf.
4. Manley M, Ong K, Kurtz S, Rushton N, and Field R. Biomechanics of a PEEK horseshoe-shaped cup: Com-parisons with a predicate deformable cup. Paper C655/058. Institution of Mechanical Engineers, "Engineers & Surgeons: Joined at the Hip," London, April 19-21, 2007.
5. 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. Proc Instn Mech Engrs1999 Vol. 213 Part H Technical Note, p. 475-484.
6. Sakoda H., Voice A.M., McEwen J., Isaac G.H., Hardaker, C., Wroblewski, B.M., Fisher, J. A comparison of the wear and physical properties of silane cross-linked polyethylene and ultra-high molecular weight polyethylene. Journal of Arthroplasty, Vol. 16, Issue 8, December 2001, p. 1018-1023.
7. 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: p. 758-764 (2003).
8. Latif AMH,Mehats A., Elcocks M., Rushton N., Field R.E., and Jones E.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 November 2007.
9. Scholes, S.C. and Unsworth, A. Pitch-based carbon-fibre-reinforced poly (etheretherketone) OPTIMA assessed as a bearing material in a mobile bearing unicondylar knee joint.Proc. Instn. Mech. Engrs. Part H, 2009, Vol. 223, H1, p. 13-26.
10. Laurent, M.P., Johnson, T.S., Yao, J.Q., Blanchard, C.R. and Crownin-shield, R.D. In vitro lateral versus medial wear of a knee prosthesis. Wear, 2003. 255: p. 1101-1106.
11. Psychoyios V., Crawford R.W., O'Connor J.J., Murray D.W. Wear of congruent meniscal bearings in unicompartmental knee arthroplasty: a retrieval study of 16 specimens. J Bone Joint Surg [Br] 1998;80-B: p. 976-82.
12. Moore D.J., Freeman M.A.R., Revell P.A., Bradley G.W., Tuke M. Can a total knee replacement prosthesis be made entirely of polymers? Journal of Arthroplasty Vol. 13 No. 4 June 1998, p. 388-393.