Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Fibrous dyplasia

Fibrous dysplasia is a benign, developmental anomaly of the bone-forming mesenchyme in which the medullary cavity is replaced with fibrous material, woven bone, and spindle cells. Virtually any bone in the body can be affected. It is a nonhereditary disorder of unknown cause. Fibrous dysplasia does not spread or proliferate, and malignant transformation is rare (0.5%).

Pathophysiology

Fibrous dysplasia is caused by abnormal proliferation and maturation of fibroblasts; it has been likened to a localized developmental arrest because all the pregenitors of normal bone are present, but they do not differentiate into their mature structures. Benign fibrous tissue with abnormally arranged, dysplastic trabeculae of immature woven bone replaces normal bone. The lesions are medullary but may replace both cancellous and cortical bone.

Epidemiology

• Age:The peak age of diagnosis is 5 to 20 years
  • Two thirds of patients with polyostostic disease are asymptomatic before the age of 10
  • With monostotic disease, patients as old as 20 – 30 years are asymptomatic
• Race: No specific racial predilection
• Sex: The incidence is equal in males and females.

Clinical Findings

The followling four disease patterns are recognized:

1. Monostotic form
2. Polyostostic form
3. Craniofacial form
4. Cherubism

Monostotic form

Approximately 80% of cases of fibrous dysplasia are monostotic. The typical sites of involvement are the proximal femur, tibia, ribs, and facial bones. This form most frequently occurs in those aged 10–30 years. The degree of bone deformity is relatively less severe compared with that of the polyostotic form.

Polyostotic form

About 20% of cases of fibrous dysplasias are polyostotic. In polyostotic fibrous dysplasia, the distribution of lesions is unilateral in up to 90% of cases; monomelic and widespread distribution are less frequent. Most bony lesions occur in the lower extremities, including the pelvis, legs, and feet. Rib, spine, or skull lesions are also often seen. Common deformities include leg-length discrepancy (lesions are in the longer leg) and bowing of the weight-bearing bones due to the weakened structural integrity of the bone. This bowing phenomenon can lead to a varus angulation of the proximal femur, the shephard's crook deformity.

The polyosttotic form is associated with McCune-Albright syndrome - sexual precocity in girls, polyostotic fibrous dysplasia, and cafe-au-lait spots with serpiginous borders ("rocky coast of Maine"). Up to 30% of women with polyostotic fibrous dysplasia may have the full spectrum of the McCune-Albright syndrome.

The clinical course ranges from asymptomatic to multiple pathologic fractures, rapidly progressive deformities, severe pain, and debilitation. Malignant transformation has been documented but is rare.

Craniofacial form

This pattern of the disease occurs in 10-25% of patients with the monostotic form and in 50% with the polyostotic form. It also occurs in an isolated craniofacial form. Sites of involvement most commonly include the frontal, sphenoid, maxillary, and ethmoidal bones. The clinical manifestations include frontal bossing, facial asymmetry, inferolateral orbital displacement, and blindness.

Cherubism

This special variant of fibrous dysplasia is an autosomal-dominant disorder of variable penetrance. It occurs in children and is more severe in boys. Regression may occur after adolescence. The jaw is broad and protruding. Involvement of maxilla and mandible is symmetric.

Other features

Fibrous dysplasia may be associated with endocrinopathies in 2-3% of cases - hyperthyoridism, hyperparathyroidism, and precocious puberty. The polyostotic fibrous dysplasia can be associated with intramuscular myxomas (usually adjacent to the affected bone). This association is known as Mazabraud syndrome.

Radiologic Findings

Plain radiography is the first line study. CT may be required to assess complex regions such as the spine, pelvis, chest, and facial skeleton. Bone scintigraphy has a limited role in the detection of subtle pathologic fractures. MRI may be necessary to assess cord compression when the spine is involved.

• Radiolucent expansile medullary lesions. The lesions are medullary but may replace both cancellous and cortical bone.
• The abnormal area may be sharply circumscribed and marginated by a thick layer of reactive bone, or it may blend gradually with the adjacent normal bone.
• Degree of lucency depends on the amount of osteoid
• "Ground-glass" appearance is due to dysplastic microtrabeculae, which are no thicker than 0.1 mm.
• Bowing deformities: biomechanically insufficient bone - shepherd's crook deformity of the femur (coxa vara).
• Base-of-skull lesions tend to be sclerotic (in contrast to lucent lesions elsewhere)

Fibrous dyplasia of the skull

Fig. 1 Fibrous dyplasia of the skull.

Fig. 2 Fibrous dyplasia is the most common etiology of benign rib lesions (left first rib).

Fig. 3 Fibrous dyplasia of the left iliac, radiograph and CT.

Fibrous dysplasia of the right femur with "shepherd's crook" (coxa vara) deformity

Fig. 4 Fibrous dysplasia of the right femur with "shepherd's crook" (coxa vara) deformity.

Fibrous dysplasia of the tibia ("long lesion in a long bone")

Fig. 5 Fibrous dysplasia of the tibia ("long lesion in a long bone").

Fig. 6 Fibrous dysplasia of the humerus.

Treatment

Therapy is often restricted to the orthopedic management of complications.

References

• Fibrous dysplasia by Jonathan Swanson, M.D. & David Coy, M.D., University of Washington Department of Radiology
• MacDonald-Jankowski DS, Yeung R, Li TK, Lee KM. Computed tomography of fibrous dysplasia. Dentomaxillofac Radiol. 2004 Mar;33(2):114-8.