Sorry to hear about Beta. I had the surgery done on my Lab, Louie and Pit Bull, Darlene, (4 knees altogether) and I will tell you that it is VERY important that you keep Beta from playing too hard before surgery, and after. Beta could damage the knee MUCH more. It is very painful, even if she seems to want to rough house.
Can you possibly leash walk Beta outside? I know it is a PITA, but well worth not having the knee get worse.