Osteoarthritis is a type of joint disease that occurs when the cartilage that pads the bones in a joint wears out. As a result, it causes bones to rub together, leading to pain and limited movement. This joint disease most commonly affects the hands, neck, lower back, or the large weight-bearing joints of your body (such as knees and hips). As the most common type of arthritis, osteoarthritis affects more than 20 million people in the United States.