Elisea,
You brought up some great points in your post about metal allergies. Unfortunetly, I do not have a good answer for those questions. There are many complex factors that are considered when creating a metal alloy that needs to be implanted in a person. Traditionally joint replacements combine colbalt, chromium, nickle and other metals. The choice for these metals have to do with bio-compatabilty, resistance to corrosion, strength, elasticity, resistance to deformation, hardness, and roughness. I am not sure specifically how the nickle composition specifically effects each of these factors.
There are other metal alloys used in orthopaedics such as titanium. Titanium is frequently used for plating systems, nails and screws. Titanium alloys have virtually no nickle, but is not used for joint replacements. The reason for this has to do with its bio-mechanical properties.
Orthodoc