What causes ribbon ribs on the chest radiograph?
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Answer:
Ribbon ribs are abnormally thin, gracile, wavy or twisted ribs with an irregular, undulating contour and reduced cross-sectional diameter, giving the appearance of ribbons rather than of normal rounded bone. The best known association is neurofibromatosis type 1. The classic explanation was pressure erosion by intercostal neurofibromas, but ribbon ribs occur in NF1 patients with no demonstrable intercostal tumour, so the deformity is now understood as part of the generalised mesodermal and bone dysplasia of the condition, of a piece with dural ectasia, posterior vertebral scalloping, sphenoid wing dysplasia and tibial pseudarthrosis. Other causes are Melnick-Needles syndrome, osteogenesis imperfecta, trisomy 18 and Gorham disease. Where an intercostal neurofibroma is present, it can usually be shown directly on MRI as a fusiform T2-hyperintense lesion along the neurovascular bundle, often with a target sign, and it should be looked for rather than assumed.
Why is it called so?
The thin, flattened, undulating ribs look like ribbons rather than bones.
Pathophysiology
Two mechanisms operate. In the dysplastic mechanism, an intrinsic mesodermal defect impairs normal periosteal bone formation and modelling, so the rib fails to acquire normal cortical thickness and diameter and remains thin and mechanically weak, deforming into a wavy contour under normal respiratory and postural loads. In the pressure mechanism, a slow-growing intercostal neurofibroma sitting along the inferior rib margin causes chronic remodelling and erosion of the adjacent cortex, thinning it locally. Because ribbon ribs are seen in NF1 without tumours, the dysplastic mechanism is now regarded as primary and the pressure effect as an additional contributor where a tumour is present.
Alternative names: Twisted ribbon ribs, ribbon-like ribs
Other associated named signs: Posterior vertebral scalloping and dural ectasia, sphenoid wing dysplasia, tibial pseudarthrosis, and the target sign of a neurofibroma on MRI
References
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