The Effect of Metal Phase on Wear
The metal phase is also very critical. Unlike a mechanical device, the hip cannot have lubrication forced into it, nor can it run constantly. It, by its nature, starts and stops regularly. Each time this occurs, lubrication diminishes and the metal surfaces rub against each other. The study of early metal on metal total hips showed better performance from the parts that were cast from high carbon metal without further heating. The BIRMINGHAM HIP is intentionally left in the metal phase created from the casting process. This process leaves large blocks of hard material, called carbides, made from a mix of carbon and the metal. These blocks are significantly harder than the metal surrounding them, and they provide a significant amount of wear resistance. Heat treating the material causes the carbon to dissolve back into the metal, losing the wear resistance benefits.
Many competitive resurfacings have been heat treated to make it easier to polish the bearing or to attach porous material for bone ingrowth. These products do not maintain the metal phase that has been proven in both the long track record of the early metal-on-metal total hip implants and the decade long track record of the BIRMINGHAM HIP.
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