Failure and Tribology of Biomaterials
As discussed in the previous chapters, materials that are used in the fabrication of medical implants or devices must not only be biologically compatible, but also mechanically strong enough to endure in vivo mechanical stimulus upon implantation, particularly to suit them in load-bearing orthopedic and dental applications. An understanding of the basic mechanics of biomaterials is therefore important to both the development of new biomaterials and the selection of appropriate materials for a specic structural application. Mechanical behaviors, in particular deformation, fracture, fatigue, and tribological aspects, of structural biomaterials are considered the most important characteristics responsible for the success of long-term biomedical implants or devices. The implants or devices may be of metal, ceramic, polymer, or their composites. Among them, metals are often used in loadbearing applications, either in monolithic or in composite form, primarily due to their superior strength. For instance, titanium (Ti) and Ti-based alloys are extensively used in the manufacture of prosthetic devices for use in humans, owing to their tissue compatibility and mechanical reliability. The mechanical properties of pure Ti and its alloys are given in Table 4.1 (Niinomi 1998). Although several Ti-based alloys have been developed in recent years, Ti-6Al-4V is one of the well-established alloys widely used in orthopedic applications, particularly hip replacement surgery, owing to its high strength, high resistance to fatigue and wear, low specic weight, excellent corrosion resistance, exceptional biocompatibility, and it also develops a good interfacial strength with host tissues. Some of the prosthetic devices, such as hip joints, bone plates, screws, and dental post, made from Ti-based biomaterials are shown in Figure 4.1. However, the biomedical community often experiences failure of biomaterials to a variety of loading conditions upon implantation, which will be discussed in Section 4.13 with clinical examples. This is because biomaterials are subject to forces (loads) when they were implanted at the load-bearing sites. For instance, mechanical mismatch between the metallic implant and bone tissue could result in uneven stress distribution at the bone-metal interface. This is called stress-shielding effect.
The part of the bone not stressed is subject to necrosis that eventually causes implant loosening and more susceptibility to fracture. In this regard, one has to be well versed in the fundamental aspects of biomechanics while designing the structural biomaterials for a given application, as it is an important tool in improving the mechanical performance and durability of biomedical implants. The following sections describe the fundamental aspects of deformation, fracture, fatigue, and tribological behaviors from the biomaterials point of view.