I was born without a right kneecap and my leg was hyperextended over my shoulder. The only treatment I've had was some strategic breaking shortly after birth and I have very limited motion. After some childhood falls, it's basically bone rubbing on bone with a bit of scar tissue mixed in.
Oddly enough it's my left knee that has the most pain issues because it's had to compensate my whole life. For a 25 year old, I have some nasty old person level arthritis. I have a slightly off gate and limp sometimes. I have a lot of grinding and achey pain, but I'm not really sure what level it would count as because it's been around my whole life. I can't go over 4mph without my knee dislocating. Doctors find me very novel but are never much help. I'm going to need a knee replacement on both sides within the next few years, but first I have to convince a doctor not to make me wait until I'm 45.
Also, flat feet and severe big toe/little toe bunions don't help the situation.