I was discussing in another place about cloning, specifically ethics and law, but one commenter mentioned an upcoming hip replacement and how he would love to be able to use a cloned hip instead of a manufactured one.
That got me thinking about his predicament which led me to think about just how spare structural body parts might not be viable. Muscles, ligaments, organs? Why not? Eyes, liver, kidneys, even stomachs and bowel, sure, but not bones. Nothing structural.
Bones grow with you and are shaped by your environment over time, strengthened by impacts and weakened by diet, bent and twisted by circumstances, even lengthened and shortened as time goes on. It would be impossible (interesting word) to recreate that life while growing a clone structural part.
You couldn't just clone "a hip" for example as it consists of the pelvis, pubic ramus, and the femur, all of which need to fit together correctly with a bunch of ligaments and muscles around and through the joint.
A cloned femur, only the top portion of which you could use, wouldn't fit into the pelvis socket, which will have also been damaged over time, so a new pelvis would need to be grown in order for a part of it could be cut off and used to replace the damaged portion. That join, and the one in the femur, would be weak spots until bone had grown over them; a period of many years.
Cloned sinovial fluid would need to be reintroduced at the right pressure as well, not an easy task; this is not required for artificial joints.
It would be much easier and cheaper to replace the damaged parts with metal, polyethylene and ceramic. Turning us into cyborgs continues apace.
It's always interesting to chase these random ideas down!