hello,The elbow joint is made up of the upper arm bone called the humerus and two forearm bones called the radius and the ulna. There are two cartilage-covered parts of the end on the humerus: The trochlea on the inside of the elbow The capitellum on the outside of the elbow In the elbow, the capitellum and the radius touch to absorb some of the pressure transmitted from the wrist. In certain sports, like baseball and gymnastics, and in professions that require heavy lifting, more pressure than normal is transmitted to the outside of the elbow. This sometimes causes injury to the bone and cartilage of the capitellum called an osteochondral defect, they are thought to be due to repeated microtrauma,avascular necrosis,macrotrauma.