The CT examination is somewhat superfluous. It confirms the multiple left sided rib expansion, with intermediate density medullary cavity. Although the cortex is indistinct in areas this is a not uncommon appearance. There is no gross osseous cortical margin disruption by soft tissue. These features again suggest fibrous dysplasia. For completeness the CT examination also demonstrates fatty infiltration of the liver.
Fibrous dysplasia is a developmental condition of the bones where there is interrupted osteoblastic activity and replacement of the medullary cavity by an immature poorly woven fibro-osseous matrix; this weakens the bone which is then prone to expansion. The appearance may be uni-ostotic (including instances of several adjacent ribs), polyostotic (often associated with café au lait patches and metabolic bone disorders, e.g. McCune Albright). Craniofacial fibrous dysplasia and cherubism are considered a special variant of mono-ostotic disease although the former also occurs in polyostotic disease. More recently cherubism has been proposed as a different entity as the histology is more suggestive of a giant cell granuloma. Finally, an association with local intramuscular myxomas is termed Mazabraud's syndrome. Lesions are frequently completely asymptomatic. Most lesions have formed between the ages of 10-15.
The appearances on plain radiographs and hence other imaging modalities can be variable. A ground glass matrix is very typical and can be seen in polyostotic cases in combination with undertubulation of the long bones, indicative of the effect on bone maturation. However, completely “bubbly” cystic and sclerotic lesions can be seen. Periosteal reactions are rare. In the pelvis bone softening may result in protrusio acetabuli. Bowing deformities, “shepherd’s crook” deformity and asymmetric limb shortening are recognised effects. Heterogeneity of signal is often present at MRI which is not particularly helpful, predominantly low T2 signal and enhancement may be present. Bone isotope imaging is usually positive and can be helpful in determining the extent of disease.