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Aortic Coarctation....The most common cause of rib notching

By far the most common cause of rib notching. Usually involves the posterior 4th - 8th ribs and rarely develops before age 6. Aortic narrowing typically occurs at or just distal to the level of the ductus arteriosus.
Characteristic double bulge in the region of the aortic knob represents prestenotic and poststenotic dilatation. Collateral flow bypassing the aortic constriction to reach the abdomen and lower extremities comes almost entirely from the two subclavian arteries via the thyrocervical, costocervical, and internal mammary arteries and their subdivisions to the posterior intercostals and then into the descending aorta.
Explanation :
The large volume of blood traversing this route causes dilatation, tortuosity, and increased pulsation of the intercostal arteries, which result in gradual erosion of the adjacent bones.  

Unilateral rib notching in coarctation occasionally occurs on the left side when the constriction is located proximal to an anomalous right subclavian artery and on the right side when the coarctation occurs proximal to the left subclavian artery (only the subclavian artery that arises proximal to the aortic obstruction transmits the collateral blood to the intercostals). Notching of the first two ribs does not occur because the first two intercostal arteries, arising from the supreme intercostals, do not convey blood directly to the postcoarctation segment of the aorta. The last three intercostal arteries conduct blood away from the postcoarctation aortic segment and thus do not greatly enlarge or cause rib notching.
Coarctation of the aorta. Notching of the posterior fourth through eighth ribs (arrows).