Luxating patella, a painful condition in which the kneecap slips out of position, is common in smaller-breed dogs but can also occur in larger dogs and cats, according to veterinarian Roger Grothaus. Four grades are used to evaluate the severity of the condition, notes Dr. Grothaus. Surgical repair is indicated for grades two through four, the most severe. Owners may not realize the knee is the source of their dog's pain, but a veterinarian can make the diagnosis with a physical exam. "A lot of times an owner will come in and they will think it's the dog's hips that are bothering them, but it's actually the knees," Dr. Grothaus said.