epiphysis

The epiphysis is the end of a long bone, the part that meets the next bone in a joint. It is built differently from the shaft: spongy bone inside a thin shell of compact bone, because the load it carries arrives at a joint surface and spreads out rather than running straight down. The spongy architecture follows the lines the load takes, so the bone is thick where the force is and thin where it is not. The joint surface is covered with cartilage, which has no blood supply of its own and therefore heals poorly. In a growing bone the epiphysis is separated from the shaft by a plate of cartilage where length is added; when that plate closes, growth in length stops. In an adult the closed line is still visible on the bone, and it is one marker a radiographer reads.