I have had Aussies that regularly injured herself playing. It seems like there are so many reasons for a dog to hold her leg up. How long has your dog been doing this? Somethings resolve naturally in a week or so. And how have you isolated it to the knee? Did the vet have no idea what could be causing it? My Aussies have had torn ligaments, sprains, broken toe, etc. Usually with massaging the area I could isolate the area of injury as I watched which areas they felt discomfort. Sometimes it was as simple as a thorn they had stepped on, a cut on their paw, or a sprain. Usually I just used massage, anti-inflammatory herbs (like turmeric), and healing poultice (comfrey). Hope your girl heals up quickly!