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.hest Trauma Rib fractures & Flail Chest Chest wall injury is a extremely common following blunt trauma. It varies in severity from minor bruising or an isolated rib fracture to servere crush injuries of both hemithoraces leading to respiratory compromise.