What is a buckle wrist fracture? My four-year old has one of these.

Buckle or torus fractures are compression injuries. In children, at the end of the bone is a growth center that consists of several layers of growing bone. The force of the impact from a traumatic injury or fall that compresses this area can cause the area around it to buckle outward.

It's actually a subcategory of a larger classification of breaks called greenstick fractures. The bone bends and partially breaks. This kind of fracture only occurs in young, soft bone. Once bones become harder and more brittle with age, the bone may give a little, but it doesn't bend.

You've probably seen this phenomenon yourself if you've ever tried to break a stick of green wood in half. It bends and bends and bends but doesn't snap in half like older, dryer wood.