Knees do not necessarily wear out evenly, sometimes one part of the knee is perfectly fine while another part is completely destroyed. If only part of the knee joint is worn out, why replace all of it?
The dreaded “hammy” is the tearing of the muscle or tendon fibers of the powerful hamstring muscles at the back of the leg. It’s dreaded because the pain is sharp, and the recovery can be long. Here is what’s known and what’s new: