Titanium as a Biomaterial for Implants

An ideal biomaterial is expected to exhibit properties such as a very high biocompatibility, that is, no adverse tissue response. Also, it must have a density as low as that of bone, high mechanical strength and fatigue resistance, low elastic modulus and good wear resistance. It is very difficult to combine all these properties in only one material. Some metals are used as biomaterials due to their excellent mechanical properties and good biocompatibility. Since the metallic bonds in these materials are essentially non-directional, the position of the metals ions can be altered without destroying the crystal structure, resulting in a plastically deformable solid. This is also an advantage when thinking about the device manufacture technology. The principal disadvantage of metals is its corrosion tendency in an in-vivo environment. Most metals can only be tolerated by the human body in small amounts even as metallic ions. The consequences of corrosion are the disintegration of the material implant, which will weaken the implant and the harmful effect of corrosion products on the surrounding tissues and organs. Some metals are used as passive substitutes for hard tissue replacement such as total hip and knee joints, for fracture healing aids as bone plates and screws, spinal fixation devices and dental implants. Some metallic alloys are used for more active roles, as actuators such as vascular stents, and orthodontic archwires. Metallic biomaterials can be conveniently grouped in the following categories:  Stainless steel  Cobalt base alloys  Titanium base alloys  Specialty metallic alloys Examples of ASTM standards for some of these metallic biomaterials are shown in Table 1. The first metal alloy developed specifically for human use was the “vanadium steel” but it was no longer used in implants because its corrosion resistance is inadequate in vivo. Later in the 1950s, 18-8sMo with very low carbon content (known as 316L) stainless steel was introduced and is actually widely used for implant fabrication. This alloy has a very good resistance to chloride solutions and poor sensitization. The castable CoCrMo alloy has been used for many decades in dentistry and, relatively recently, in making artificial joints. The wrought CoNiCrMo alloy is relatively new, now used for making the stems of prostheses for heavily loaded joints such as the knee and hip. Both alloys have excellent corrosion resistance.