A flail chest occurs when a segment of the thoracic cage is separated from the rest of the chest wall. This is usually defined as at least two fractures per rib (producing a free segment), in at least two ribs. A segment of the chest wall that is flail is unable to contribute to lung expansion. Large flail segments will involve a much greater proportion of the chest wall and may extend bilaterally or involve the sternum. In these cases the disruption of normal pulmonary mechanics may be large enough to require mechanical ventilation.