A: Experts estimate that one in five dogs has canine arthritis or osteoarthritis. Osteoarthritis is a painful, degenerative joint disease that strikes dogs of any age, breed or size. Similar to arthritis of the knee or hip in humans, it breaks down the joints of both dogs and cats. Arthritis is as painful to dogs as it is to us. Read more in New Weapons in the War on Canine Arthritis.