Rib-within-a-rib appearance refers to a feature that may be present on plain radiographs of the patients with β thalassemia. As the name suggests, the ribs take on an appearance of having another rib superimposed. The phenomenon is usually visible in the anterior and middle segments of the ribs 1.
This appearance of the ribs occurs as the subperiosteal red bone marrow extends longitudinally within the cortex 1,2. This phenomenon happens as a result of inadequate hematopoiesis leading to a compensatory expansion of the bone marrow 2.