Metals and Alloys in alloplastic implant materials
Most of the dental implant systems are made of metals or their alloys. Currently commercially pure titanium and its alloy Ti-6Al-4V are most popular. Earlier alloys such as stainless steel, Tantulum, the Co-Cr alloys e.g. Co-Cr-Mo or Co-Cr-W-Ni were used. Of these, only the Co-Cr alloys with Mo are cast, the remainders are used in the form of prefabricated structures. Cobalt chromium alloys containing Berylium are not suitable for implantation as the ions of this metal cause an extremely toxic reaction into tissue.
Martensitic and Ferritic stainless steels are disregarded because of their magnetic property which is undesirable. Austenitic stainless steels are non-magnetic non heat- treatable steels, have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. Of these, the chromium-nickel steels are the most widely used steels and are also known as 18-8(Cr-Ni) steels. However, stainless steel undergoes more corrosion than Co-Cr alloys when embedded in the tissues. Pitting, galvanic corrosion, crevicular and stress corrosion are all observedin metallic implants. This corrosion is enhanced by the piezoelectric properties of bone which causes the release of metallic ions into the surrounding tissue. A tissue reaction occurs when the ion concentration reaches a high value.