Was this diagnosed with x-rays done under anesthesia? They stress the knee joint to find out how much it luxates. Something to keep in mind, LP can cause pain even though the dog does not show it. Gracie had a grade 2 and grade 3. I chose to have them both fixed at once before they progressed to painful, debilitating arthritis. Grade 4 is as bad as it gets and by then the knee is completely dislocated. Grade 3 is serious business. Here is a link to info about the grades
OFA: Patellar Luxation Grades