Cobalt Prosthetic Alloys
The use of metals to repair and replace parts in the human body, goes back a long way. In 1775, iron wire is reported as having been used to fix a fractured bone. It became known however that not all materials were what we would now term biocompatible.
Several metals were used – platinum, gold, etc. – but in 1924, it was concluded by Zierold that a cobalt-chrome alloy had the best combination of properties. In 1937, Vitallium arrived on the scene, this being a CoCrMo alloy which had good strength, corrosion resistance and above all, was tolerated by the body.
Cobalt-chrome alloys of the Vitallium type are still used in the production of knees and hips, though they are only used when wear resistance is paramount as they are relatively heavy.
Typical alloys are 62%Co, 30%Cr, 5%Mo and 52%Co, 26%Cr, 14%Ni, 4%Mo. They are therefore basically Stellites® being close to HS21 and 31.
In recent years, Ti.318 (6Al4V) has moved into the prosthetic field but the Co/Cr alloys have maintained their place and one now sees composite hip joints with CoCr balls and titanium stems. This is also an expanding market with replacement joints being fitted more and more routinely to younger and younger people.
Prosthetic devices were traditionally cast using the lost wax process but the need for higher strength and better fatigue properties led to the development of wrought versions of the alloys. The lower carbon content of these alloys resulted in improved hot workability/forgeability but lower strength than cast alloys. Thermo-mechanical forging techniques were developed that allowed the required strength of the as-forged component to be achieved.
In response for the need of improved properties, alloys produced by powder metallurgy have been developed in the past few years.