The characteristic paradoxical motion of the flail segment occurs due to pressure changes associated with respiration that the rib cage normally resists:

• During normal inspiration, the diaphragm contracts and intercostal muscles push the rib cage out. Pressure in the thorax decreases below atmospheric pressure, and air rushes in through the trachea. However, a flail segment will not resist the decreased pressure and will appear to push in while the rest of the rib cage expands.
• During normal expiration, the diaphragm and intercostal muscles relax, allowing the abdominal organs to push air upwards and out of the thorax. However, a flail segment will also be pushed out while the rest of the rib cage contracts.

The constant motion of the ribs in the flail segment at the site of the fracture is exquisitely painful, and, untreated, the sharp broken edges of the ribs are likely to eventually puncture the pleural sac and lung, which may be fatal.

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