Metals: structure and properties

Abstract

Goals After reading this chapter the student will understand the following. Structure and properties of metals commonly used for making biomedical implants and devices. Use of different metals as biomaterials. Metals are extensively used as materials for biomedical implants, devices, and surgical tools. Some of the implants made from metals are shown in Figure 5.1. For example, metals are used for orthopedic reconstructions (implants for artificial hip, knee, shoulder, and elbow joints), fracture fixation (plates, pins, screws, rods, and nails), oral and maxillofacial reconstructions (dental implants and mini-plates), and cardiovascular interventions (stents, heart valves, and pacemakers). In general, metals used for biomedical applications should exhibit the following properties: high corrosion resistance, biocompatibility, high wear resistance, excellent mechanical properties. Most metallic biomaterials have a stable surface oxide layer that enhances their corrosion resistance properties. It is believed that the presence of this stable surface oxide layer is key to the biocompatibility of metals. The mechanical properties of the metal are important and should satisfy the requirements of the specific application in the body. For instance, when a metal is used to augment a bone, the elastic modulus of the metal should be ideally equivalent to that of the bone. If the elastic modulus of the metal is greater than that of bone, then the load experienced by the bone is reduced due to a phenomenon known as stress shielding. This can cause the bone to remodel to adjust to the lower load and eventually result in the loss of bone quality. In another example, stainless steel is commonly used for making coronary stents due to its well-suited mechanical properties. Stainless steel has good radial strength (due to its high elastic modulus of ~190 GPa), low recoil, good expandability, and sufficient flexibility, which makes it a highly preferred metal for making stents. Several metals such as titanium, stainless steel, cobalt–chromium alloys, nitinol (nickel–titanium alloy), tantalum, and magnesium have been used for a variety of clinical applications, with titanium, stainless steel, and cobalt–chromium alloys being the most commonly used metals. This chapter describes the structure and properties of these metals.