To address the challenges of tomorrow and advance the state of the art, materials researchers must focus on the development of bioactive materials and an effective artificial articular cartilage.
Advances in medical technology are helping people to live longer, healthier lives, but some body 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 were 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.
In this article we will examine some of the man-made materials used for orthopaedic applications and highlight their advantages and disadvantages. The critical role of bioactive material in bone replacement technology also will be addressed.
Man-made orthopaedic materials
|Figure 1: The basic microstructure of articular cartilage. The calculations are based on data from Joseph Buckwalter’s AAOS Instructional Course Lectures, volume 54, 465-480 (2005).|
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 (ultrahigh 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 minimising 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 and 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. The question we must ask ourselves is: Why, after 60 years of development work, do we not have a better or, indeed, perfect implantable device?
Bone, a living material
It all comes down to biocompatibility, and the more advanced concept of bioactivity, in relation to implantable materials. Is the material or resultant debris compatible with the body? Is it really corrosion-resistant at the atomic, nano and micrometre 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 non-metal bone-replacement materials, let’s go over a few basics of natural bone.
|Figure 2: The basic components of a total hip replacement.|
Taking a long bone as an example, there are three basic structural layers: articular cartilage on the surface, followed by 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, since 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 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 must be very strong 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 minimise 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 to 24% of total weight and chondrocytes (cells) only account for about 1% by volume. Proteoglycan monomers account for 6 to 12% of cartilage weight.
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. While 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. By comparison with metals, collagen fibre 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. As it is a living material, understanding its renewable mechanisms and biological process is key to developing bioactive materials.
|Figure 3: Linear wear rates of different materials. (Me = metal; Ce = ceramic; XPE = cross-linked PE).|
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 used in total joint components have a molecular weight varying between 4 to 6 million grams per mole; the material is known as ultra-high molecular weight polyethylene (UHMWPE). UHMWPE is an extremely tough yet flexible material. Figure 2 shows the components of a total hip replacement, where PE is used as the liner. In theory, the PE liner was 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 did 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 it has a higher wear rate than MOM (see Figure 3). 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 irradiation 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 irradiation.3 4
| Figure 4: The microstructure of zirconia toughened alumina developed at Ceram. The
light phase is zirconia and the dark phase is alumina.
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 5–13 compared with metal or PE. The general trend and conclusions are persuasive, although variations exist in the reported data. Figure 3 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. Figure 4 shows microstructures of zirconia (light phases) toughened alumina (dark phases) developed at Ceram. This ceramic composite has 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:
|Figure 5: 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 can be used in combination with other materials, such as polymers, to form hybrid composites. Figure 5 shows one example of a ceramic hybrid. This ceramic compound is composed of a ceramic foam (light phases) and a mixture of polymers (dark phases) developed at Ceram. This hybrid material takes advantage of the ceramic’s bioactivity and hardness and the toughened polymer’s flexibility. 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, polyetherether-ketone (PEEK) has been recently 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× 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.
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Xiang Zhang, PhD, is
Division of Medical Materials and Devices, Ceram,
Queens Road, Penkhull,
Staffordshire ST4 7LQ, UK | Tel. +44 1782 764 428