The costal hook: an indicator of occult flail segment in chest trauma.

The presence of a hook-like configuration at the site of rib fracture reflects significant rotational displacement. Such displacement can occur only as the result of a further fracture at another site within the same rib and is therefore indicative of a flail segment injury even if a second fracture site is not clearly identified. This appearance is illustrated and termed the 'costal hook' sign.