Top of knee joint. 
This is the top of an artificial knee joint: I've removed the plastic block that attaches just underneath this metal part, and which mates with the double-ball of the other half, which you can see under aluminum.  Unfortunately they seem to be a different brand or something because the parts don't fit each other. 
I originally thought these were titanium, but the source, Paul Wellin's Brother, reports as follows: 
"The top of knee joint and the hip joint ball attachment are probably made of a different alloy, cobalt-chrome, for increased strength and decreased flexibility as compared to titanium." 
I don't know whether there is more cobalt or more chromium in the alloy, so I've somewhat arbitrarily chosen to put them under cobalt. 
Source: Paul Wellin 
Contributor: Paul Wellin 
Acquired: 14 June, 2002 
Text Updated: 11 March, 2007 
Price: Donated 
Size: 2.5" 
Purity: <60% 
Sample Group: Body Parts+Medical
 |  
  |  
 |  
  |