Advances in medical technology are helping people to live longer, healthier lives, but some parts do wear out in the process. The American Academy of Orthopaedic Surgeons (AAOS) reports that 478,000 knee and 234,000 total hip replacements are performed in 2008 in the United States.¹ The AAOS also predicts that knee replacements will reach 3.4 million by 2030 and hip replacements will more than double. Furthermore, instances of revision arthroplasty remain high: in the United States, 40,000 knee and 46,000 hip revisions were performed in 2004. EU statistics are in the same range. Clearly, the orthopaedics sector, which is valued at €28 billion today, has many years of growth ahead.
 

This article examines some of the man-made materials used for orthopaedic applications and highlights their advantages and disadvantages. The critical role of bioactive material in bone replacement technology also will be addressed.

Figure 1. The simple microstructure of articular cartilage is novel in that it exhibits strength, flexibility, and shock absorption.

Materials: A History

Man-made materials have been used in orthopaedic applications for several decades. The first metallic hip replacement surgery was performed in 1940. PMMA cement was first used for the same purpose in 1951. Modern ceramic total hips came into use in 1995. To date, metals, polymers (ultra high molecular weight polyethylene, or UHMWPE), ceramics and combinations of these materials have been used in orthopaedic applications, and materials development is ongoing. Choosing an optimal material for the design and development of orthopaedic devices has been challenging, and it remains so as manufacturers strive to develop high-performance products while minimizing failure associated with material selection.
 

Metals, such as stainless steels, were the first materials used to fabricate implantable medical devices. Metal-on-metal (MOM) implants have been around since the 1950s and continue to work well in certain applications. During the last 60 years, more metallic devices have been introduced for medical applications, but not without some performance and quality issues. Concerns associated with orthopaedic metal implants include the release of metal particulates into the bloodstream, which may affect kidney function. The particulates also may travel across the placenta during pregnancy, and there is the potential of metal particles, mainly cobalt chromium, leading to cancer. After 60 years of development work, some may question why there is not a better or, indeed, perfect implantable device.
 

Biocompatibility, and the more advanced concept of bioactivity, in relation to implantable materials are key attributes in the latest advances. Is the material or resultant debris compatible with the body? Is it corrosion-resistant at the atomic, nanometer, and micrometer levels? How strong is it? What is its modulus of elasticity? Above all, have we learned something significant about the body’s natural biomaterials? Have we understood that all of the body’s components, including bones, are living materials? Before examining nonmetal bone-replacement materials, it’s important to go over a few basics of natural bone.

The basic components of a total hip replacement show a PE liner, which was supposed to act as articular cartilage.

Bone, a Living Material

Taking a long bone as an example, there are three basic structural layers: articular cartilage on the surface, compact bone close to the surface, and spongy bone. Of course, the makeup of artificial orthopaedic devices is not comparable to the microstructure of natural bone, because bone is a living material. Bone cells carry out the healing process through resorption and deposition. As healthy bone is subject to the wear and tear of use, the bone develops nano- or microfractures that weaken it. The bone reacts to this weakening in the same way it does to other repair tasks. When resorption takes place, osteoclast cells are dispatched to attack the weak bone and, with the aid of a variety of systemic hormones, the deteriorating bone is dissolved and the collagen and mineral phase is reabsorbed. The bone particles are absorbed by the body as calcium, leaving tiny areas that have been essentially excavated by the osteoclast. Next, osteoblasts arrive on the scene and collect around the damaged sites. The osteoblasts fill in the damaged area with new bone. The new bone absorbs systemic calcium and becomes healthy bone tissue.

Articular Cartilage

Articular cartilage is a biomaterial that has not received sufficient attention with regard to the design and development of artificial hips and knees. Bone joint surfaces are covered with a strong but lubricated layer of articular cartilage. In most large joints, it is about 5-mm thick and allows the surfaces of the joints to slide against each other without causing damage. The material itself is strong enough to bear and transfer load from one part of the body to another. Articular cartilage also acts as a shock absorber and eases stress concentration to minimize peak pressures on the subchondral bone. The main constituents of articular cartilage are illustrated in Figure 1.
 

The predominant single component of articular cartilage is interstitial fluid, which counts for up to 80% of total cartilage weight, depending on the origin and integrity of the cartilage. Collagen makes up 12–24% of total weight, chondrocytes (cells) only accounts about 1% by volume. Proteoglycan monomers account for 6–12% of cartilage weight.
 

Figure 2. Shown are the linear wear rates of different materials (Me is metal, Ce is ceramic and XPE is cross-linked polyethylene).

Articular cartilage is a very porous material. The structure is simple, consisting only of collagen fibrils that crosslink to form networks. Figure 1 shows a model of the network. Although collagen represents a small proportion of the articular cartilage, it imparts dynamic strength to the overall structure. Without other tissue constituents working collectively and synergistically with the collagen fibrils, this simple structure would collapse upon loading. In comparison with metals, collagen fiber is neither strong nor durable. Nevertheless, we have yet to find an artificial material that is as active and effective as articular cartilage. Part of the reason for this is the difficulty of developing materials. But it is also rooted in a lack of understanding of how cartilage can work for more than 100 years in a healthy body. It must be a living material like in other parts of the body. Understanding its renewable mechanisms and biological processes is key to developing bioactive materials.

Polyethylene

Polyethylene (PE) was introduced by Sir John Charnley, a British orthopaedic surgeon, in the late 1960s. PE has a simple polymeric structure, consisting of repeating CH2 units along the polymer long chain. Medical grades of PE are used in total joint components have a molecular weight between 4 million and 6 million g/mol. UHMWPE is an extremely tough yet flexible material. In theory, the PE liner is expected to act, at least partially, as articular cartilage, allowing hard metal surfaces at the joints to slide against one another without causing damage (assuming that PE provides superior mechanical performance). Unfortunately, the material does not live up to its promise. Standard UHMWPE is not a perfect material, particularly when it comes to long-term performance. As observed in MOM implants, PE also produces particles because of its higher wear rate than MOM (see Figure 2). PE is also subject to fatigue failure, especially if it is exposed to high-energy irradiation to produce cross-linked UHMWPE.² The effect of high-energy radiation on the chemical, physical, and mechanical properties of polymers is extremely complicated. The author of this paper has investigated and used a model polymer (polypropylene) obtaining a range of mechanical properties from ductile to brittle by varying doses of gamma radiation.^3,4

Microstructures of zirconia (light phases) and toughened alumina (dark phases) were developed at CERAM to enable advances in medical applications.

The microstructure of ceramic and polymer hybrids developed at CERAM. The porous ceramic zirconia is the light phase and the polymer hybrids are the dark phase.

Ceramics

The most important breakthrough in materials for orthopaedic applications in the last 25 years is the use of ceramics for hip joints. Ceramics have several advantages over metals and polymers. They are the most chemically and biologically inert of all materials. They are also strong and hard. Thus, ceramics are resistant to scratches from the tiny particles (bone cement or metal debris, for example) that occasionally land between the artificial joint surfaces. To date, almost all reported results demonstrate that ceramics produce the lowest rate of wear particles compared with metal or PE.5–13 The general trend and conclusions are persuasive, although variations exist in the reported data. Figure 2 compares the linear wear rate of different materials compiled and published by CeramTec. Metal on PE (Me/PE in the figure) shows the highest wear rate, while ceramic on ceramic (Ce/Ce) has the lowest.
 

The main disadvantage of medical-grade ceramics resides in their fragility. Unlike metals and polymers, ceramic materials cannot deform under stress. When the stress acting on a medical ceramic material exceeds a certain limit, the ceramic bursts. Burst fractures of ceramic components of total hips observed in the past were caused by the poor quality of the ceramic material at that time. However, even current medical ceramics remain fragile. Therefore, developing toughened ceramics is and will continue to be a focus. Further development of microfracture mechanics and the design of micro- and nanoceramic composites will lead to more advanced ceramics for medical applications. Microstructures of zirconia (light phases) and toughened alumina (dark phases have achieved fracture toughness as high as 7.2 MPa m1/2, the highest fracture toughness result achieved to date.
Material selection for orthopaedic device development and design. The benefits of employing ceramics for orthopaedic applications include the following:

■ Long life with the lowest wear rate compared with a polymer or metal alloy.
■ Bioinert properties, so the body doesn’t react to particles.
■ Bioactive properties, if the material is designed in a way that stimulates new bone growth.

Ceramics can be used in combination with other materials, such as polymers, to form hybrid composites. Various forms of ceramic hybrid compounds can be made into different microstructures for different applications including, but not limited to, spinal fusion, suture anchors, fixation and trauma screws, femoral implants, dental implants, and partial and total joint replacement. All follow the basic principle of design and development: a specific material for a specific implant device. Again, at the start of the project and prior to design control, evaluation, and selection of a material is the key to success.
 

Besides polyethylene, polyetheretherketone (PEEK) has been introduced into the orthopaedic market. It is an organic polymer thermoplastic used as an engineering material. One good example of it is the Infuse bone graft developed by Medtronic.

The Future of Bioactive Materials

So, what does the future hold for materials for orthopaedic applications? It is clear that the development of new bioactive materials will be a basic requirement and that this will be a challenge to materials experts. Orthopaedic medical devices must be designed with bioactivity in mind.
 

Another challenge is to develop new artificial articular cartilage. The material must not only be bioactive but it must be able to bear loads as great as 6.5 times the body weight in the knee and act as a lubricated layer at the joints. With reference to Figure 1, the design and development of the new materials must allow the inclusion of a large amount of interstitial fluid, some chondrocytes (cells), proteoglycan and so forth to be strong and active.
 

References

1. “The Value of the Home Health Solution for Osteoarthritis, Joint Replacement Surgeries and Therapy Solutions,” Alliance Whitepaper Joint Replacement Osteoarthritis, July 2010.
2. J Furmanski et al., “Clinical Fracture of Cross-linked UHMWPE Acetabular Liners,’’ Biomaterials 30, no. 29 (2009): 5572–5582.
3. XC Zhang et al., “The Ductile—Brittle Transition of Irradiated Isotactic Polypropylene Studies Using Small Angle X-ray Scattering and Tensile Deformation,” Polymer 41 (2000): 3797–3807.
4. XC Zhang et al., “The Morphology of Gamma-ray Irradiated Isotatic Polypropylene,” Journal Appliance Polymaterials Science 74 (1999): 2234–2242.
5. C Heisel et al., “Bearing Surface Options for Total Hip Replacement in Young Patients,” Journal of Bone and Joint Surgery 85-A (2003): 1366–1379.
6. H Warashina et al., “Biological Reaction to Alumina, Zirconia, Titanium and Polyethylene Particles Implanted onto Murine Calvaria,” Biomaterials 24, no. 21(2003): 3655–3661.
7. AS Greenwald, JP Garino, “Alternative Bearing Surfaces: The Good, The Bad, and The Ugly,” Journal of Bone and Joint Surgery 83-A, supplement 2, part 2 (2001): 68–72.
8. C Hendrich, et al., “Highly Crosslinked Ultra Molecular Weight Polyethylene- (UHMWPE-) Acetabular Liners in Combination with 28 mm BIOLOX Heads,” eds. F Benazzo, F Falez, M Dietrich, Bioceramics and Alternative Bearings in Joint Arthroplasty, 11th BIOLOX Symposium Proceedings, Steinkopff Verlag, Darmstadt, Germany (2006).
9. JM Martell, JJ Verner, and SJ Invaco, “Clinical Performance of a Highly Cross-linked Polyethylene at Two Years in Total Hip Arthroplasty: A Randomized Prospective Trial,” Journal of Arthroplasty 18, no. 7 suppl. 1 (2003): 55–59.
10. LP Zichner, HG Willert, “Comparison of Alumina Polyethylene and Metal Polyethylene in Clinical Trials,” Clinical Orthopaedics and Related Research, 282 (1992): 86–94.
11. LP Zichner, T Lindenfeld, “In-vivo-Verschleiß der Gleitpaarungen Keramik-Polyetyhlen gegen Metall-Polyethylen” (In vivo wear of ceramics-polyethylene in comparison with metal-polyethylene),” Orthopäde 26 (1997): 129-134.
12. CR Bragdon et al., “Steady-State Penetration Rates of Electron Beam–Irradiated, Highly Cross-Linked Polyethylene at an Average 45-Month Follow-Up,” Journal of Arthroplasty 21, no. 7 (2006): 935–943.
13. D.W. Manning, et al., “In Vivo Comparative Wear Study of Traditional and Highly Cross-linked Polyethylene in Total Hip Arthroplasty,” Journal of Arthroplasty 20, no. 7 (2005): 880–886.

Xiang Zhang is principal consultant, division of medical materials and devices at CERAM (Stoke-on-Trent, Staffordshire).

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