Oh dear

Hope you are better soon
I broke my ankle at the end of August 08 after losing about 42lb (broke tib, fib and one other bone). I did need surgery (2 long screws and a plate with 7 screws). Yes I wouldnt' move it too much - I remember I could feel the bones moving around in my ankle when waiting for surgery. You need ot be careful with it until they sort you out. My weight fluctated a lot by 7lb after the accident for months - mixture of water weight and building muscles again I guess. I didn't lose weight when I broke my ankle but aimed to eat at about maintaince levels. You do need to eat fairly well as it helps with the healing - lots of calcium rich foods and make sure you're getting all your vitamins, etc. I restarted proper exercise (as opposed to just physio exercises) (slowly) and lowered calories once an x-ray had showed the bone had healed. It still hurts now - a lot of activity on it, cold weather, hot weather etc. all irratate it and it does swell and retain water so I guess if you have surgery on your knee it may be similar. I was 273lb when I restarted losing weight just over a year ago so it certainly hasn't put an end to my weight loss journey and if you're determined it won't to yours either.
Hope your operation is organised and over with soon then you can get being in cast over with and move on. I could hear lots of noise from my ankle too

Not pleasent to do so much damage to a bone. There's a forum for broken legs called mybrokenleg where you will be ablee to get specific advice for your injury from people who have gone through similar.