1 Introduction

A number of metallic alloys have found extensive use as biomaterials. They include cobalt-chromium alloys, which usually have small amounts of molybdenum in them, stainless steel, and titanium and their alloys, all of which have been used in orthopaedics. In addition, the complex silver-tin-copper amalgams have been used for well over 150 years in dentistry as filling materials, and gold and other precious metal alloys are used to fabricate crowns. In addition, titanium and its alloys are used as dental implants.

For hip replacement surgery, the standard materials for the femoral stem have been one of the following: (i) cobalt-chromium with small amounts of molybdenum and possibly nickel, (ii) 316L stainless steel or (iii) titanium with 6% aluminium and 4% vanadium, generally written Ti-6Al-4V. Cobalt-chromium and 316L stainless steel were the first metals to be used for such applications,1 and were for a long time considered to be well tolerated by the body, even though corrosion products were frequently found in tissues surrounding the implant, or even at sites somewhat removed from the location of the implant.2 ^,3 These days, however, there is an increase in use of titanium alloys, and new alloys are being developed in attempts to improve on the excellent biocompatibility of the well established alloys such as Ti-6Al-4V. Their biocompatibility in bone contact applications arises partly from their relative lack of corrosion and of the low cytotoxicity of any corrosion products released, and also because their moduli are much lower than those of...