After working in the O.R. for 40 years, I can offer some insight into this "souvenir of surgery" issue.
A long time ago, it was common practice to give patients the parts that had been cut out, if they wanted them. Lots of kids took their tonsils to Show & Tell at school. Back when cataract surgery involved removing the lens whole, it was given to the patient in a little jar of formalin. When one of our patients thought the nurse had delivered a pill and swallowed her lens, formalin and all, that practice stopped. Increasing awareness of the hazard of formalin, the rise of diseases like hepatitis and AIDS, and the increasing activity of lawyers caused the hospital I worked at to set a blanket rule banning giving any tissue to patients or their family. I suppose an exception would be made if a limb were delivered to a funeral home. "Hardware" is a different matter. Metal screws and plates can be cleaned and sterilized before being returned to the patient. Sometimes the patient's request for these parts doesn't get communicated, sometimes the parts go to the surgeon, who fails to get them to the patient.
Implant metals include stainless steel, chrome/cobalt/molybdenum alloy, or titanium alloy.
As for using bone from a total knee surgery, it would be difficult to find something to do with what gets cut off. At the joint, the bone is a thin, hard shell over spongy "cancellous" bone. If they cut off a leg or arm, there might be enough of the thick, hard "cortical" bone of the shaft to fashion an inlay or small knife handle.