Osteoarthritis is a common condition in which changes in the joints lead to pain and disability. The cause of osteoarthritis is unknown. It is also unclear why people with osteoarthritis develop pain, because osteoarthritis mainly affects the cartilage in joints and cartilage does not contain any nerves responsible for conducting pain. Knowing more about when and why pain develops in osteoarthritis might help us to develop better treatments for this condition. Some patients with knee injuries or knee osteoarthritis develop abnormalities, called lesions, in the central portion of the bone (bone marrow) that can be seen on magnetic resonance imaging (MRI). This radiologic test uses magnetic field and radio waves to create pictures of internal organs. These bone marrow lesions are thought to represent areas of contusion (sort of like internal bruises). It is unknown whether bone marrow lesions seen on MRI are associated with the occurrence of pain in osteoarthritis.