It was not until materials became available that promoted an osseointegrated or biointegrated interface and surgical techniques were developed to reliably obtain such interfaces that endosseous implants became a viable treatment option for tooth replacement (Craig, Powers and Wataha, 2004, Ch.15). Commercially-pure titanium is actually an alloy, containing 99 wt percent titanium and small amounts (0.18 wt percent to 0.40 wt percent) of oxygen with trace amounts (less than 0.25 percent) of iron, carbon, nitrogen, and hydrogen. The amount of oxygen determines the grade of the alloy. Increasing amounts of oxygen increase the strength but decrease the ductility of the alloys. The concentrations of the other trace elements are critical to the strength, phase structure, and corrosion resistance of titanium alloys. All the alloys are equally able to osseointegrate with bone.
Titanium alloy contains roughly six wt percent of aluminum and four wt percent of vanadium, which doubles its tensile strength relative to commercially-pure titanium, but reduces its ductility (craig, Powers and Wataha, 2004, 316). However, it promotes good osseointegration. A complex mixture of adherent oxides of titanium and oxygen are used to coat either titanium or titanium alloy, and the presence of these oxides appears to be necessary for the osseointegration process to occur. The alloys reform these oxides rapidly after any damage to the surface of the alloy (in 1 msec) because the titanium is too reactive to exist in its native state in an oxygen atmosphere. The thickness of the oxide layer is usually from 20 to 100 Ao, with the oxygen-rich oxides nearer to the surface, and the oxygen-poor oxides nearer the alloy. It is necessary for oxides to form in a controlled environment because contaminants from the air and the metal will be incorporated into the oxide layer and adversely affect the ability of the alloy to osseointegrate with the bone.
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