A flail chest is a life-threatening medical condition that occurs when a segment of the rib cage

Human rib cage

-See also:*Terms for bones*Terms for anatomical location*Articulation of head of rib-References:* Clinically Oriented Anatomy, 4th ed. Keith L. Moore and Robert F. Dalley. pp. 62–64...

 breaks under extreme stress and becomes detached from the rest of the chest wall

Thoracic wall

The thoracic wall is the boundary of the thoracic cavity.The bony portion is known as the thoracic cage. However, the wall also includes muscle, skin, and fascia....

. It occurs when multiple adjacent ribs are broken in multiple places, separating a segment, so a part of the chest wall moves independently. The number of ribs that must be broken varies by differing definitions: some sources say at least two adjacent ribs are broken in at least two places, some require three or more ribs in two or more places.