The exact cause of osteoarthritis remains unknown, but it is linked to several factors, including being overweight. Osteoarthritis occurs when cartilage, the cushioning material that covers the ends of bones at joints, deteriorates. The joint space narrows and bone can rub on bone, causing pain. Excess weight places added stress on weight-bearing joints (like the hips and knees), possibly accelerating the breakdown of cartilage. Body mass index (BMI) is a measure of body fat.