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What determines if a metal is suitable for transplantation such as in the hip? What I am most interested in however is why might some metals be toxic to animals once in the body?

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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

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If the question is why a transplant implant like an artificial hip may be made from a metal, then toxicity issues of the metals are not the main question.

I think it would be better to focus on metal sensitivity.

Metal sensitivity is not a specific immune response as with transplanted organs, but a non specific response to a foreign substance in contact with the body. While Surgical stainless steel and similar hardened alloys which contain iron, nickel, chromium and molybdenum are popular for metal components of implants, even plastic coated pieces can cause scarring and irritation.

This reaction is highly variable from one individual to the next and can have no effect or cause further surgery or rejection of the implant. the cause is poorly understood and so diagnosis is still in an exploratory state.

Since oxidation and rust seem to cause more irritation, it seems that corrosion resistance is an important part of implant metal choices. Hardness, durability and Cost is also an issue since I'm sure gold and platinum would also be great choices from a cost perspective.

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I think the question is specifically interested in what makes some metals better or worse than others. –  Amory Oct 3 '13 at 19:11
    
i agree, though I feel that within the the set of metals that might realistically be chosen for implants might be a more focused answer. –  shigeta Oct 4 '13 at 14:02
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I think @Amory answer is a good one but I would like to add a new point for the last part of your question.

Our normal body cells can recruit the immune system when there is infection but the transplants can not. Some findings suggest that bacteria can form a biofilm over that part and so can become difficult to treat with antibiotics.

Source : I read this in a book called Good Germs, Bad Germs by Jessica Snyder Sachs

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yes this is i think more to the point. why is surgical steel better for transplants than say, iron or copper? immune rejection is a big part of the problem, but i don't know the answer myself –  shigeta Oct 2 '13 at 2:07
    
@shigeta I don't know why surgical steel is better but I can only say that whatever artificial material we use, biofilms are going to be a problem . –  biogirl Oct 2 '13 at 13:09
    
Biofilms can form on materials within body cavities exposed to fluids. But can it form on implant materials within tissues surrounded by tissue fluids? Are there any references? –  Ram Manohar M Oct 2 '13 at 18:36
    
then why do they bother to use some alloys of metal at all? its not for the looks... lmgtfy emedicine.medscape.com/article/1230696-overview –  shigeta Oct 2 '13 at 21:12
    
@shigeta I really don't know why. –  biogirl Oct 3 '13 at 17:31
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