If it makes you feel any better, I have a few with "loose knees" in my home and have had many others! Your vet is talking about luxating patella and it is very common in yorkies, many of which never do need surgery.
Not one of my pups has ever been referred by my vet to a specialist for their knees. Matter of fact, I have spoken with a board certified surgeon about them and he has said unless there is a real problem with them walking, there is no need to rush into surgery. I recently spoke with him and he said that they do repair these knees in puppies where the growth plates are not yet closed. My guess is that Bella's are, but you might want to be sure. The surgeon I use is excellent, and you could go to him for an opinion if it might make you feel better. His name is Dr. Brian Beale and he is at Gulf Coast Veterinary Specialists at Post Oak and the Loop.
Home He is not surgery happy and would only suggest surgery if absolutely necessary. He is excellent about education....an appointment with him is amazing and well worth the time and money!
I would advise you to keep Bella's weight within normal limits as excess weight is hard on them and definitely try to keep her from jumping on and off furniture. I have steps to my bed. Some people use ramps.