BASIC SCIENCES- Artificial cartilage in Joint Replacement

NEW AVENEUS IN JOINT REPLACEMENT – THE CUTTING EDGE

What is the best alternative to joint cartilage…. Metal on polyethylene, metal on metal, ceramic on ceramic, metal on ceramic??

How about artificial cartilage!!

A search for new options has begun in early 21st century to have a more natural and biological bearing surface having better durability than the present ones. Present bearing surfaces still have problems of debris and osteolysis although recent advances have surely decreased the incidence [1].

A new direction was unveiled when a Japanese team at Hokkaido University in Japan discovered what is called a double network hydrogels [2]. Hydrogels are materials that are 80% - 90% water held in a polymer network. These can be easily broken apart like gelatin. The Japanese team added a second polymer to the gel and to their surprise the new material called double network gels had incredible strength that rivaled natural cartilage [3,4,5]. The gel had the pliancy of gelatin but would not break even when deformed more than 1000%.

Work using the national institute of standard and technology neutron scattering technology is exploring the structure of the gel to discover the molecular level toughening mechanism, to allow for more precise designing of next generation of these hydrogels being both rigid and tough at the same time. The key feature of these interpenetrating gels is a strong but easily compressed fibrous network, which is filled with a weak but viscous and difficult-to compress “gel” that traps water in the fiber matrix. This feature is akin to the structure of cartilage, ligament, tendon, and other soft connective tissue. As a result of the similarity between these manufactured gels and natural cartilage, they are sometimes called "Biomimetic gels"—that is, of a structure that mimics the natural tissue [1]. Researchers hope to design a synthetic cartilage that could endure year after year under the rigors of body before needing to be replaced. However there is still much time required to explore this option and to study practicality of in-vivo use.

A second interesting research comes from field of nanotechnology. Nano materials are typically between 0.1 and 100 Nanometers in size with 1 nm being equivalent to 1 billionth of a meter. This is a scale at which the basic functions of the biological world operate. Increased surface area and weird quantum effects at atomic scale provide substances with unusual physical and chemical properties. Considering that the healthy human joint cartilage surface is covered with a nanometer-scaled phospholipid layer [6], grafting a phospholipid-like layer on the liner surface may realize an ideal lubricity resembling the physiological joint surface.Chen et al [7] have developed biocompatible molecular nanobrushes that slide past each other with friction coefficients that match those of cartilage. In some respects, they perform even better: the brushes remain highly effective even at pressures of 7.5 megapascals. Cartilage performs well only up to around 5 megapascals – a natural limit because joint pressure only rarely exceeds that level.

Each 60-nanometre-long brush filament has a polymer backbone from which small molecular groups stick out. Those synthetic groups are very similar to the lipids found in cell membranes and although they're neutral overall, they are positively charged at one end and negatively charged at the other. In a watery environment, each of these molecular groups attracts up to 25 water molecules through electrostatic forces, so the filament as a whole develops a slick watery sheath. These sheathes ensure that the brushes are lubricated as they rub past each other, even when firmly pressed together to mimic the pressures at bone joints. This extreme lubrication is attributed primarily to the strong hydration of the phosphorylcholine-like monomers that make up the robustly attached brushes, and may have relevance to a wide range of human-made aqueous lubrication situations.

Jennifer Elisseeff, a professor of engineering and orthopaedic surgery at Johns Hopkins University in Baltimore, who was not involved with the study, says the new material is an "important step forward" for joint lubrication studies.

A team led by Hiroshi Kawaguchi at the University of Tokyo in Japan has already tried earlier versions of these nanobrushes to coat the polyethylene surfaces of artificial joints 2004 [8]. The team found that the hydrophilic molecules creating very effective lubricating water layer in the artificial joints. Tests in a hip-joint simulator found that the coated polyethylene showed an astonishing 40 times less wear than the uncoated version. Later in 2006 the team investigated the nanobrushes for clinical application [9]. They investigated the stability of the 2-methacryloyloxyethyl phosphorylcholine grafting during sterilization and the wear resistance of the sterilized liner during longer loading comparable to clinical usage. Radiographic spectroscopy confirmed the stability of the 2-methacryloyloxyethyl phosphorylcholine polymer on the liner surface after the gamma irradiation. They used a hip wear simulator up to 1 x 10(7) cycles to test sterilized cross-linked polyethylene liners with and without 2-methacryloyloxyethyl phosphorylcholine grafting. The 2-methacryloyloxyethyl phosphorylcholine grafting markedly decreased the friction, the production of wear particles, and the wear of the liner surface.

In 2009 the same Japanese team has used a better version of nanobrushes called poly(2 methacryloyloxyethyl phosphorylcholine (MPC)) (PMPC) to coat the joints [10]. They studied the effect of cross linking the polyethylene with or without PMPC coating and also studied PMPC coated Co–Cr alloy and alumina ceramic femoral heads in the hip joint simulator. They concluded that the PMPC grafting for obtaining super-lubrication on the PE liner is more efficient than the cross-linking of the PE liner and the change of the femoral head materials for extending longevity of artificial hip joints. The development of this nanotechnology in the biomaterials science would improve the quality of care of patients having joint replacement and have a substantial public health impact. A large-scale clinical trial is now underway to further study this HORIZON.


References
1. Muehleman C, Connor D, Fyhrie DP, Marsh JL, Anderson D. On the Horizon From the ORS. J Am Acad Orthop Surg. 2009 Jul;17(7):473-6.

2. http://www.arthritis.org/gel-artificial-cartilage.php.


3.Yasuda K, Ping Gong J, Katsuyama Y, et al: Biomechanical properties of hightoughness double network hydrogels. Biomaterials 2005;26:4468-4475.


4. Yang-Ho N, Katsuyama Y, Kuwabara R, et al: Toughening of hydrogels with double network structure. e-Journal of SurfaceScience and Nanotechnology 2005;3:8-11.


5. Azuma C, Yasuda K, Tanabe Y, et al: Biodegradation of high-toughness double network hydrogels as potential materials for artificial cartilage. J Biomed Mater Res A 2007;81:373-380. Medline


6. Kirk TB, Wilson AS, Stachowiak GW. The morphology and composition of the superficial zone of mammalian articular cartilage. J Orthopaedic Rheumatol 1993;6:21–8.


7. Chen M, Briscoe WH, Armes SP, Klein J. Lubrication at physiological pressures by polyzwitterionic brushes. Science. 2009 Mar 27;323(5922):1698-701.

8. Moro T, Takatori Y, Ishihara K, Konno T, Takigawa Y, Matsushita T, Chung UI, Nakamura K, Kawaguchi H. Surface grafting of artificial joints with a biocompatible polymer for preventing periprosthetic osteolysis. Nature Materials. 2004 Nov;3(11):829-36


9. Moro T, Takatori Y, Ishihara K, Nakamura K, Kawaguchi H. 2006 Frank Stinchfield Award: grafting of biocompatible polymer for longevity of artificial hip joints. Clin Orthop Relat Res. 2006 Dec;453:58-63.


10. Moro T, Kawaguchi H, Ishihara K, Kyomoto M, Karita T, Ito H, Nakamura K, Takatori Y. Wear resistance of artificial hip joints with poly(2-methacryloyloxyethyl phosphorylcholine) grafted polyethylene: comparisons with the effect of polyethylene cross-linking and ceramic femoral heads. Biomaterials. 2009 Jun;30(16):2995-3001