General:
Developmental non-inherited skeletal anomaly
Abnormal differentiation of osteoblasts replace normal marrow and cancellous bone by immature bone and fibrous stroma.
Usually incidental
Generally do not require further investigation
Monostotic:
70-80%
Common sites - rib, femur, tibia, mandible, skull and humerus
Do not convert to polyostotic form, do not increase in size, become inactive by puberty
Polyostotic:
Association with McCune Albright's syndrome in 3%, Mazabraud syndrome
Common sites - skull, facial bones, pelvis, spine, shoulder.
Often unilateral, sometimes monomelic
Fractures and deformities are common
Inactive by puberty
McCune Albright's syndrome:
Mainly in girls
Triad - precocious puberty, polyostotic fibrous dysplasia, café au lait spots (coast of Maine - irregular margins)
More disabling than pure polyostotic disease
Mazabraud syndrome:
Fibrous dysplasia and soft-tissue myxomas
Higher incidence of osteosarcomatous change
Radiography:
Well defined, expansile, medullary lesion with hazy ground glass matrix; but may be lucent or sclerotic
May show endosteal scalloping, but cortical contour usually remains smooth
MRI:
Int-to-low on T1
Int-to-high on T2/ STIR, may show low signal rim
Heterogenous enhancement
Bone scan:
Nonspecific increased uptake
Complications:
Fracture
Malignant degeneration - l%
FD of jaw:
Teeth generally remians undisplaced with resorption (c.f in Cemento-ossifying fibroma teeth are displaced and/or resorbed
FD of rib:
Most common rib tumour
30% of benign rib tumours
Mmost common rib - second.
Approximately 20% of monoostotic and 50% of polyostotic FD involves ribs, most commonly posterior and lateral aspects
Typically asymtomatic
Enlargement, deformity, cortical thickening, increased trabeculations, lytic matrix
Ground glass pattern
Amorphous or irregular calcification
Low-to-int on T1 and low-to-high on T2
FD of femur:
Shepherd's crook deformity with varus angulation
FD skull and face:
May cause cranial nerve dysfunction, visual or hearing impairment
Face involvement = Leontiasis ossea
Cherubism - hereditary symmetrical involvement of mandible and often maxilla
Reference:
1. Hughes et al. Benign primary tumours of the ribs. Clin rad (2006): 61:314-322
2. Fitzpatrick KA et al. Imaging Findings of Fibrous Dysplasia with Histopathologic and Intraoperative Correlation. AJR 2004; 182:1389-1398
