| Titanium and some of its alloys are used as biomaterials for dental and orthopaedic applications. The most common grades used are commercially pure titanium and the Ti6Al4V alloy, derived from aerospace applications. Physiological Behaviour These materials are classified as biologically inert biomaterials or bioinert. As such, they remain essentially unchanged when implanted into human bodies. This is no doubt a result of their excellent corrosion resistance. The human body is able to recognise these materials as foreign, and tries to isolate them by encasing them in fibrous tissues. However, they do not illicit any adverse reactions and are tolerated well by the human body. Furthermore, they do not induce allergic reactions such as has been observed on occasion with some stainless steels, which have induced nickel hypersensitivity in surrounding tissues. The surface of titanium is often modified by coating it with hydroxyapatite. Plasma spraying is the only commercially accepted technique for depositing such coatings. The hydroxyapatite provides a bioactive surface (i.e. it actively participates in bone bonding), such that bone cements and other mechanical fixation devices are often not required. Mechanical Suitability Titanium and its alloys possess suitable mechanical properties such as strength, bend strength and fatigue resistance to be used in orthopaedics and dental applications. This is part of the reason why they have been employed in load-bearing biomedical applications in stead of materials such as hydroxyapatite, which displays bioactive behaviour. Other specific properties that make it a desirable biomaterial are density and elastic modulus. In terms of density, it has a significantly lower density (table 1) than other metallic biomaterials, meaning that the implants will be lighter than similar items fabricated out of stainless steel or cobalt chrome alloys. Table 1. Densities of selected biomaterials and cortical bone. | | | Cortical Bone | ~2.0 g.cm-3 | 7-30 | | Cobalt-Chrome alloy | ~8.5 g.cm-3 | 230 | | 316L Stainless Steel | 8.0 g.cm-3 | 200 | | CP Titanium | 4.51 g.cm-3 | 110 | | Ti6Al4V | 4.40 g.cm-3 | 106 | Having a lower elastic modulus compared to the other metals is desirable as the metal tends to behave a little bit more like bone itself, which is desirable from a biomechanical perspective. This property means that the bone hosting the biomaterial is less likely to atrophy and resorb. |