how to model an artificial knee joint?
I think you would model this item in the same way that you would model any item.
1. Come up with a design solution.
2. Model each part that you think you need (using approximate dimensions)
3. Assemble the parts into an assembly (using mates).
4. Modify the parts so they fit together and work together properly.
5. Start again and do a better job of it, now that you've learned a bit more about what you're doing.
This might not be the best method but it works for me.
P.S. Is the artificial knee meant to be part of a prosthetic leg or is it to be a surgical implant?