Fibrous Dysplasia
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Direct answer
Fibrous dysplasia is a benign fibro-osseous disorder in which normal bone and marrow are replaced by fibrous tissue and immature woven bone. It is caused by a post-zygotic activating GNAS mutation, and its radiological hallmark is the ground-glass lesion. Monostotic disease is common, while polyostotic disease may form part of McCune-Albright syndrome, which adds café-au-lait spots and endocrine overactivity.
What you must remember
- Genetics: a somatic, post-zygotic activating GNAS mutation raises cAMP signalling in osteoblast precursors, so affected cells lay down abnormal woven bone.
- Extent: monostotic disease involves one bone and is the commoner form, while polyostotic disease involves several bones and carries greater deformity risk.
- Radiology: a well-defined, expansile, ground-glass lesion in the medullary cavity, often with a thin sclerotic rim.
- Histology: fibrous stroma containing curvilinear woven bone trabeculae, classically described as a Chinese letter pattern.
- Shepherd crook deformity: coxa vara with bowing of the proximal femur from a lesion in the femoral neck, which may fracture pathologically.
- McCune-Albright syndrome: polyostotic dysplasia with café-au-lait skin pigmentation and endocrine overactivity, classically precocious puberty.
- Management: observation for asymptomatic lesions, curettage and grafting or fixation for fractures, and avoidance of radiotherapy because it may cause sarcoma.
Reading a ground-glass lesion in a child
A 10-year-old girl has a painless, expansile swelling of the thigh and a limp that began after a minor fall. X-ray shows a ground-glass lesion in the proximal femur, with a fracture running through it.
- Identify the lesion. A painless, expansile ground-glass lesion in a long bone or rib of a child or young adult points to fibrous dysplasia rather than a tumour.
- Define the extent. Skeletal survey and bone scintigraphy show whether disease is monostotic or polyostotic, and craniofacial involvement changes the plan.
- Look for the syndrome. Café-au-lait spots, precocious puberty or other endocrine excess in polyostotic disease suggest McCune-Albright syndrome and prompt an endocrine work-up.
- Assess fracture risk. A proximal femoral lesion with cortical thinning, pain or pathological fracture may need curettage with grafting or internal fixation.
- Observe the quiet lesion. Stable, asymptomatic lesions are followed with serial imaging, because growth usually slows after skeletal maturity.
- Avoid radiotherapy. Radiation can transform the lesion into a sarcoma, so surgery or observation is preferred in any young patient.
Keeping the lesion benign in the answer
Fibrous dysplasia questions reward recognising the syndrome and avoiding harmful treatment.
- Trap: choosing radiotherapy for a painful lesion in a young patient Correct: radiotherapy is avoided because it can cause malignant transformation into sarcoma.
- Trap: missing the skin and endocrine signs in polyostotic disease Correct: café-au-lait spots and precocious puberty point to McCune-Albright syndrome, which changes the work-up and the follow-up.
- Trap: labelling the shepherd crook as a congenital hip problem Correct: it is coxa vara caused by fibrous dysplasia of the proximal femur, and it often ends in pathological fracture.
Frequently asked questions
What is the Chinese letter pattern in fibrous dysplasia?
The Chinese letter pattern is the histological appearance of curved, irregular woven bone trabeculae scattered through a fibrous stroma. The trabeculae resemble the strokes of Chinese characters, and this pattern helps separate fibrous dysplasia from other fibro-osseous lesions on biopsy.
Which syndrome combines polyostotic fibrous dysplasia with café-au-lait spots?
McCune-Albright syndrome combines polyostotic fibrous dysplasia, café-au-lait skin macules and endocrine overactivity, classically precocious puberty. It is caused by the same post-zygotic GNAS activating mutation, and the distribution of the mutation is mosaic.
Why is radiotherapy avoided in fibrous dysplasia?
Radiotherapy in fibrous dysplasia can induce malignant transformation into osteosarcoma or fibrosarcoma, particularly in young patients. Curettage with grafting, fixation or observation is therefore preferred for symptomatic lesions.
What causes the shepherd crook deformity in fibrous dysplasia?
A lesion in the proximal femoral neck weakens the bone and causes coxa vara with bowing, which gives the shape of a shepherd's crook. The deformity often leads to limp and pathological fracture, and it is treated surgically when it is symptomatic.
Which gene is mutated in fibrous dysplasia?
Fibrous dysplasia is caused by activating mutations in the GNAS gene, which encodes the Gs-alpha subunit of a G-protein. The mutation is post-zygotic, so it affects only a mosaic subset of bone-forming cells in the body.