What makes a metal safe to use for an artificial joint?
There are a number of reasons why a given metal may be toxic to an animal:


Radioactive metals are the easy ones.
Some metals can affect the normal biosynthetic pathways within the body.  Lead is a good example of this, as it can take the place of calcium; I believe beryllium does the same for magnesium.
Some metals themselves are very reactive.  Hexavalent chromium ($Cr(VI)$) is highly reactive, and will cause damage through severe oxidative reactions, whereas $Cr(III)$ is largely okay.
A lot of metals are okay, even required, at some level, but become toxic at too high a concentration.  Iron, for example.
People can simply have allergies to metals.  Nickel in silver jewelry is a good example, although in my experience it can be overcome.


The basic gist is that some metals are required for life, some aren't but aren't too terrible, and some are completely toxic to it.  Different organisms can vary a bit in which metals are useful or not.  A good implant will of course not be toxic at all, and will be strong and long-lasting.  Titanium is often used, given its strength and general nonreactiveness, but so is cobalt, despite being toxic to some degree.



Ref: Metal Sensitivity in Patients with Orthopaedic Implants.  Hallab, et al. J Bone Joint Surg Am, 2001 Mar 01;83(3):428-428