Fibrous dysplasia

Fibrous dysplasia (FD) is a benign tumour-like congenital process, manifested as a defect in osteoblastic differentiation and maturation, with progressive replacement of normal bone with immature woven bone.

Demographics and clinical presentation

• 2^nd decade (highest incidence between 3 and 15 years)
• 75% before age 30
• progresses until growth ceases
• M:F = 1:1

FD is often an incidental finding. Morbidity may arises from compression and displacement of adjacent structures.

Pathology

Macroscopically the medullary cavity is filled by abnormal whitish fibrous tissue.

Histology

• fibrocellular matrix of immature collagen contains small irregularly shaped trabeculae of immature, inadequately mineralized bone ⁶.
• trabeculae not rimmed by osteoblasts (DDx from ossifying fibroma)
• cartilaginous islands present in 10% (DDx: chondrosarcoma)

Location and classification

Fibrous dysplasia can affect any bone, and can divided into four sub types ⁸ (although there is some overlap):

1. monoostotic : single bone
2. polyostotic : multiple bones
3. craniofacial : skull and facial bones alone
4. cherubism : mandible and maxilla alone

Monostotic form

The monostotic form (involving only one bone) is by far the most common and accounts for 70 - 80% of cases ⁶. It is usually asymptomatic until 2^nd-3^rd decade, but can be seen throughout adulthood ⁶. After puberty the disease becomes inactive, and monostotic form does not progress to polyostotic form.

Location

• ribs (28% - most common) ^6,7
• proximal femur (23%)
• tibia
• craniofacial bones (10-25%) ⁴
• humerus

Polyostotic form

In the remaining 20 - 30% of cases multiple bones are involved. As expected this presents earlier, typically in childhood (mean age of 8 years) with 2/3 having become symptomatic by the age 10. In 2 - 3% of cases there is an associated endocrinopathy (see McCune-Albright syndrome). It can also be associated with Mazabraud syndrome.

Presentation

• leg pain, limp, pathologic fracture (75%)
• abnormal vaginal bleeding (25%)
• coast of Maine cafe-au-lait spots
  • predominantly on back of trunk (30-50%)
  • often ipsilateral to bone lesions

Location

• often unilateral and monomelic (one limb) ⁶
• femur (91%)
• tibia (81%)
• pelvis (78%)
• foot (73%)
• ribs
• skull + facial bones (50%) ⁴
• upper extremities
• lumbar spine (14%)
• clavicle (10%)
• cervical spine (7%)

Craniofacial form

The craniofacial form is also known as leontiasis ossea, due to the face resembling that of a lion. True craniofacial form is isolated, although craniofacial invlovement may be seen in both monostotic  (in 10 - 25% of cases) and  polyostotic form (in 50% of cases) ⁴.

Presentation

• cranial asymmetry
• facial deformity
• nasal stuffiness
• proptosis
• visual impairment / unilateral blindness

Location

• sphenoid, frontal, maxillary, ethmoid bones > occipital, temporal bones

Cherubism

Cherubism is a hereditary fibrous lesion of bone and often considered a special variant of FD. It is also known as familial fibrous dysplasia, and may in fact represent a distinct form of a giant cell reparative granuloma. It is an autosomal dominant disorder with variable penetrance ². Onset is usually in childhood and is typically more severe in males. Fortunately it typically regresses after adolescence.

• bilateral jaw fullness and slight upward turning of eyes
• bilateral expansile multiloculated cystic masses with symmetric involvement of mandible and maxilla

May be associated with:

• endocrine disorders :
  • precocious puberty in girls
  • hyperthyroidism
  • hyperparathyroidism : renal stones, calcinosis
  • acromegaly
  • diabetes mellitus
  • Cushing syndrome : osteoporosis, acne
  • growth retardation
• soft-tissue myxomas (rare) = Mazabraud syndrome: typically multiple intramuscular lesions in vicinity of most severely affected bone

Radiographic features

Plain film

• ground-glass opacities
• may be completely lucent (cystic) or sclerotic
• well circumscribed lesions

Skull

• blistering / bubbling cystic skull vault lesions
• commonly cross sutures
• sclerotic skull base
• widened diploic space with displacement of outer table, inner table spared (this is in contrast to Paget disease, in which case the inner table is involved)
• obliteration of paranasal sinuses
• displacement of orbit

Pelvis + ribs

Ribs are the most common site of monostotic fibrous dysplasia. Fibrous dysplasia is the most common cause of a benign expansile lesion of a rib (see rib lesions)

• bubbly cystic lesions
• fusiform enlargement of ribs
• protrusio acetabuli

Extremities

• may lead to premature fusion of growth plates leading to short stature
• bowing deformities
• Shepherd's crook deformity of femoral neck
• discrepant limb length

CT

• ground-glass opacities (56%) ⁴
• homogenously sclerotic (23%)
• cystic (21%)
• well-defined borders
• expansion of bone, with intact overlying bone
• endosteal scalloping may be seen ⁶

MR

MRI is not useful in differentiating fibrous dysplasia from other entities as there is marked variability in the appearance of the bone lesions, and they can often resemble tumour or more aggressive lesions. 

• T1 : variable, may enhance brightly with contrast ⁴
• T2 : variable

Nuclear Medicine

increased tracer uptake on bone scans (lesions remain metabolically active into adulthood)

Differential diagnoses

• Paget disease (mosaic pattern bone histologically, radiographically similar to monostotic cranial lesion, outer table involved, sparing of facial bones)
• neurofibromatosis type 1 (rarely osseous lesions, vertebral column is primary target, ribbon ribs, cystic intraosseous neurofibroma rare, cafe-au-lait spots smooth, familial disease)
• hyperparathyroidism (principally histologic problem, chemical changes, generalized deossification, subperiosteal resorption)
• osteofibrous dysplasia (almost exclusively in tibia of children <10 years + anterior bowing, monostotic, lesion begins in cortex, spontaneous regression)
• nonossifying fibroma
• simple bone cyst
• giant cell tumour (no sclerotic margin)
• enchondromatosis
• eosinophilic granuloma (beveled margin in skull)
• osteoblastoma
• haemangioma
• meningioma
• adamantinoma : When an FD-like lesion is seen in the tibia, adamantinoma should be included in the differential diagnosis.

Treatment and prognosis

Usually no treatment is required as the bone lesions usually do not progress beyond puberty. If mass effect is severe then surgical decompression may be considered.

Not surprisingly FD bone is weaker than normal and thus susceptible to pathologic fractures.

Sarcomatous dedifferentiation (osteosarcoma, fibrosarcoma, malignant fibrous histiocytoma or rarely chondrosarcoma) is occasionally seen (less than 1%) and is more common in the polyostotic form. It should be noted that many reported cases may relate to previous treatment with radiation therapy ⁶.