I haven't done that so I don't have a pony in that race. However, having read from others, I would think it would be more beneficial for you to stick with (raw/fresh) juices straight through instead of throwing fasting into the mix.
Your body needs the continual vitamins and minerals to help HEAL your knees. You may want to do a search on the orange juice diet/fast.
Juicing alone will also mean you're not eating the foods that are detrimental to your body's joints.
Out of respect for this board, (it is solely to be about water fasting) you may continue to "PM" me if you wish, although I've posted here at CZ for over six years, so there's a lot of stuff I've already posted and it can probably be found through a simple search.
I'm pulling for you!
Don't forget, until you really set your mind to total health, you can STILL make little changes along the way; for example, you can still remove the BREAD part of your sandwich.