A greenstick fracture is a fracture in a young, soft bone in which the bone bends and breaks. Despite the name fracture, during the majority of cases the bone is broken and hard to find in x-rays. This is owing in large part to the thick fibrous periosteum of immature bone. A person's bones become harder (calcified) and more brittle with age and the periosteum becomes thinner and less restrictive. Greenstick fractures usually occur most often during infancy and childhood when bones are soft. The name is by analogy with green (i.e., fresh) wood which similarly breaks on the outside when bent. It was discovered by British-American orthopedist, John Insall, and Polish-American orthopedist, Michael Slupecki.