Hello, I am sorry to hear about Buster. I am a veterinarian and will do my best to help. I just came online and saw your question; I am sorry you did not get help from an expert before now.
The "knee" is in the back leg. Is it his carpus, or wrist, the joint below the elbow in the front leg?
This might be able to be cured, but first you have to find out what the problem is. The treatment would depend on what is wrong. It could be a muscle strain, a sprained wrist joint, a hairline fracture of one of the bones in or near the wrist joint. Is the joint swollen? Will he put much weight on it?
I hope you were able to see my response; I have not heard back from you! How is Buster doing?