Fractured arm. Two coloured X-rays of a spiral fracture (yellow and purple) of the upper arm bone (humerus). A spiral fracture is a diagonal break across a bone and is caused by a twisting movement about the long axis of the bone, for example during a fall when ski-ing or ice-skating. This type of fracture is unstable and tends to slip even if the bone is splinted. The tips of the broken bone can damage blood vessels, nerves and skin. Two X-rays were taken at different angles to investigate the structure of the break.