The arch in the back is compensation for an injury or other damage. Our Newfie with bilateral patellar luxation has the same arch and shakes from muscle spasms, not pain. One knee is grade three and the other was a four until the surgery. He was about the same age when he started getting symptoms.
Have you seen the Vet. yet? They can tell a lot just manipulating the dog's legs. We had a full diagnosis before the X-rays were done. If they sedate the dog for x-rays, especially of the hips, you will get a much better view of the bones.
Got pictures of the dog? If there is an angulation problem, one of the other OEM owners might notice it.
