Flail Chest. 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.lail Chest. 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.