Osteoarthritis—the most common form of arthritis—is a degenerative joint disease, and the major cause of physical limitations and disability in older people. Today, 35 million people (13 percent of the U.S. population) are 65 and older, and more than half of them have clear evidence of osteoarthritis in at least one joint. By 2030, 20 percent of Americans (about 70 million people) will have passed their 65th birthday and will be at risk for osteoarthritis.