Introduction
FD is a rare,
congenital,
non-heritable entity,
regarding benign intramedullary abnormalities either presenting as an isolated skeletal finding (monostotic type,
M-FD),
accounting for 70-80% of cases,
or affecting multiple bones (polyostotic type,
P-FD).
P-FD can occasionally exhibit a monomelic pattern.
The ribs are one of the most common sites of M-FD,
while the femur and tibia are most commonly involved in P-FD.
Craniofacial FD affects only the skull and facial bones,
with the frontal,
sphenoid,
ethmoid,
orbit,
zygoma,
maxilla,
mandible,
and temporal bones being the common sites.
M-FD of the clivus is extremely rare.
FD can affect all age groups,
most predominantly children and young adults,
with no recognized gender predilection.
FD may manifest with a variety of clinical features,
mostly depending on the age of onset,
type of the disease,
site(s) of involvement in the skeleton and associated endocrine disorders.
It can often be asymptomatic,
detected as an incidental finding on a radiological examination,
particularly the monostotic type.
When symptomatic,
FD usually presents with pain,
edema,
bone deformities and/or fracture,
causing morbidity.
Clinical symptoms like recurrent sinusitis,
epiphora,
proptosis,
central neurologic signs and neurovascular compression symptoms,
can be seen in case of craniofacial FD.
P-FD may be associated with precocious puberty (McCune-Albright Syndrome/MAS),
exostoses (fibrocartilaginous dysplasia) or intramuscular myxomas (Mazabraund’s syndrome).
The most frequent extra-skeletal manifestations are skin hyperpigmentation (café-au-lait spots) and precocious puberty,
commonly seen in MAS.
A histologically similar condition,
osteofibrous dysplasia (OFD),
is characterized by fibro-osseous intracortical lesions extending to the medullary bone,
almost exclusively occurring in the tibia or fibula of young children.
Imaging features
FD is usually diagnosed with plain X-rays,
computed tomography (CT) and/or magnetic resonance imaging (MRI).
Bone scintigraphy may be used to establish the extent and activity of the disease.
MRI is the imaging study of choice in the follow-up process of young patients,
owing to lack of radiation.
Bone biopsy can be effective in case of unclear findings.
- The classic radiographic appearance of FD is a well-circumscribed lesion,
usually of ground-glass matrix but also radiolucent or mixed lytic/sclerotic,
surrounded by a layer of sclerotic reactive bone (rind sign).
This pattern results from defective mineralization of immature dysplastic bone.
The classic rind sign is most commonly seen in the proximal femur.
FD also causes expansion and remodeling of the involved bone,
as well as bowing deformities associated with lesions in the long bones.
A coxa varus angulation of the proximal femur is frequently seen in femoral involvement of FD (shepherd crook deformity).
Shepherd crook deformity may also be seen in other disorders such as Paget disease of the bone and osteogenesis imperfecta.
Typically,
there is no periosteal reaction associated with FD lesions.
Wide,
transverse lucencies (looser zones) or true fractures may be seen in the deformed bone.
- CT depicts FD lesions mostly as ground-glass opacities but they can also appear homogeneously sclerotic or mixed sclerotic/lytic,
with well-defined borders and varying degrees of bony expansion and endosteal scalloping.
CT scanning may be required to assess difficult regions such as the skull base,
the spine and the pelvis.
CT can better define the extent of the lesion and the presence of secondary aneurysmal bone cysts and is often used for quantifying the optic canal diameter in cases of craniofacial involvement.
- On MRI,
lesions usually appear as hypointense on T1-WI (isointense to muscle) while on T2-WI there is considerable variability in signal intensity (low,
intermediate,
high),
mostly depending on the trabecular bone volume and the amount of collagen within the lesion.
Contrast MRI typically shows delayed enhancement,
with variable patterns (homogeneous,
heterogeneous,
central,
ring-like).
Contrast MRI is particularly useful for detecting cystic degeneration,
aneurysmal bone cysts and soft-tissue involvement.
It is also indispensable for the assessment of nervous structures in case of craniofacial FD.
Although FD is usually a straightforward radiographic diagnosis,
on occasion it may be confused with malignant entities,
because it has a rare potential of malignant transformation,
with an increased frequency in the polyostotic form,
especially in patients with Albright or Mazabraund’s syndrome.
Malignancies in M-FD are extremely rare.
The most frequent entities to be considered in the differential diagnosis are central osteosarcoma and chondrosarcoma,
especially in the case of excessive calcification and soft-tissue ossification.
Other controversial situations include increase in the size and/or osteolytic areas of an FD lesion and the presence of an expansile lytic M-FD lesion.
Although imaging diagnosis may be easier in cases of P-FD or typical M-FD forms,
false-positive and false-negative diagnoses are not uncommon.
Treatment
Conservative therapy includes administration of analgesics,
calcium and vitamin-D supplements,
and treatment of associated endocrine abnormalities in case of MAS.
Surgery (intramedullary nails,
bone resection,
osteotomy) is indicated in case pain does not respond to the conservative treatment,
especially in extended bone damage and presence of fracture or increased fracture risk.
In the event of hearing/vision loss or optic nerve compression,
craniofacial surgery is the treatment of choice.