Paradoxical chest movement is most closely associated with which thoracic injury?

• A. Pneumothorax
• B. Rib fracture
• C. Flail chest
• D. Pulmonary contusion

Answer: (C)

Paradoxical chest movement is a hallmark of a flail chest. This happens when multiple adjacent ribs are fractured in two or more places, creating a free-floating segment of the chest wall. That unstable segment moves in the opposite direction to the rest of the chest during breathing: it draws inward during inspiration as the rest of the chest expands, and bulges outward during expiration as the chest compresses. This abnormal motion reflects mechanical instability of the chest wall.

Other thoracic injuries don’t produce this distinct paradoxical motion. Pneumothorax involves air in the pleural space and can cause breathing difficulty and chest pain but not the free-floating segment’s opposite motion. A rib fracture by itself causes local pain and splinting but not paradoxical movement. Pulmonary contusion is a lung tissue injury that leads to hypoxemia and respiratory distress without chest-wall instability.