I'm working an appeal on ankle and knee issues, so I have some experience. Mine is different, in that they were found to be service connected, but the ankle rating was 10% and the knee was 0% (despite an MRI reading that is word for word what is required for 20% when it comes to torn ligaments).
SO....the first thing you have to battle is to get the disability rated as "service connected". It sounds like it is, but you basically need to show that you currently have a disability that is the result of an injury that happened while you were in the service. Might be as simple as using your military medical records. Wouldn't hurt to have a current medical diagnosis from a doctor that states that your "current condition is the result of an injury suffered in 19___ while in the Army". After that, it's a matter of what exactly your disability is......be aware, that it doesn't matter that you broke you leg or rolled your ankle 100 times.....if you can't show that something is wrong NOW, you get nothing. SO.....it sounds like you have issues with mobility or "limited/range of motion". Get your doctor to do an exam and complete a DBQ which will show exactly how much limitation of motion you have. There are places on the form for them to mark that you have pain, weakness, tenderness, swelling and whatever else is wrong. Make sure they include the frequency of the issue. You don't want the VA trying to claim that you only have issues every once in while, if you having them daily or weekly. It would be GREAT if the doctor could use the term "severe" or "marked", because the VA is going to rate the level of this disability as either "moderate" or "marked" and since they don't really define either, they normally go with "moderate" which is 10%. Marked will get you 20%. Hope this helps. Good luck my friend.