Osteoarthritis is the most common form of arthritis, affecting millions of people worldwide. It occurs when the protective cartilage that cushions the ends of your bones wears down over time. Although osteoarthritis can damage any joint, the disorder most commonly affects joints in your hands, knees, hips and spine. A type of arthritis that occurs when flexible tissue at the ends of bones wears down. The wearing down of the protective tissue at the ends of bones (cartilage) occurs gradually and worsens over time.