Sites of Involvement
Monostotic and polyostotic forms arise more frequently in long bones (proximal femur, tibia, humerus) and craniofacial bones – especially in the jaws (the maxilla is more involved than the mandible) – and ribs. Any bone may be affected.
Monostotic disease affects a single bone usually with one focus. Rarely, more foci may be seen in the same bone.
Polyostotic fibrous dysplasia is more frequently monomelic, polyostotic monomelic fibrous dysplasia – involving homolateral lower limbs and ilium. The more severe cases are polymelic.
Spine is an infrequent location, more frequently associated with the polyostotic form.
Rib location is more frequent in older patients, probably due to older patients having more commonly chest radiography, and the lesions are incidentally discovered.
Clinical Symptoms and Signs
Incidental radiographic finding after a roentgenological examination for other reasons, most frequently in ribs.
Pain may appear depending on site and extent of the lesion.
Occasionally, localized swelling may appear, more frequently in the jaws and the skull.
Monostotic lesions may progress and enlarge, most commonly during skeletal growth. They tend to stabilize after puberty, but lesions may not be quiescent in adulthood.
Some lesions may grow during pregnancy.
Some lesions may produce deformity.
Pathologic fracture – frequently in the proximal femur – and deformity of weight-bearing bones are commonly seen.
Craniofacial lesions may produce asymmetry, exophthalmos, impingement of cranial nerves – like the optic nerve – affection of middle ear structures, and deformity. Chronic headache may be present.
Polyostotic form may involve a few bones of a single extremity or compromise more than 50 % of the skeleton. This presentation begins early in life – before 10 years of age – and the disease has a rapid progress.
Limb length discrepancies.
Polyostotic forms associated to McCune-Albright syndrome – which is more common in females – present cutaneous pigmentation in the form of yellow or brown patches frequently located on neck, back, shoulder, chest, and pelvis. Albright macules of fibrous dysplasia have irregular and dented borders like the coast of Maine, whereas in neurofibromatosis, the macules are irregular but with smooth borders resembling the coast of California, endocrine hyperactivity; precocious sexual development – sometimes starting as early as 3 years – vaginal bleeding; development of breasts; hyperthyroidism; frequently secondary hyperparathyroidism; Cushing’s syndrome; acromegalia; hyperprolactinemia; adrenal hyperplasia; early bone maturation with premature closure of the growth plates and consequent short stature; and renal phosphate wasting – in 50 % of the patients. Infrequently, excess FGF-23 is produced, determining osteomalacia as in the phosphaturic mesenchymal tumor. If not detected in time, these patients may present deformities and fractures, leading to a crippling disease.
Roentgenological appearances in monostotic or polyostotic form are similar.
Centered, intramedullary, elongated lesions arising in metaphysis or diaphysis of long bones. Epiphyseal location is uncommon, while the growing plates are open.
Well-circumscribed areas of rarefaction, commonly bordered by a sclerotic rim, frequently so thick that appear as a rind.
The upper or lower limit frequently shows a triangular shape.
The lesion has a ground-glass-like or lytic appearance. The roentgenological density varies in relation with the intralesion amounts of fibrous or osseous tissue.
The lesions may expand the affected bone – but have sharp circumscription – especially in rib, pelvis, fibula, and small tubular bones. Lesion scallops and erodes the endosteal surface, thinning the cortex, sometimes expanding bone and determining a newly formed cortex.
Periosteal reaction and soft-tissue extension are extremely infrequent, without a pathologic fracture occurring.
Rarely some lesions produce a large expansile mass that bulges into soft tissues. More frequent in craniofacial skeleton. Some authors classified this case as fibrous dysplasia protuberans.
Craniofacial lesions may be extremely dense, especially the maxilla and skull base.
Bone deformity – bowing – usually affects weight-bearing long bones such as the femur – “shepherd’s crook deformity” – and the tibia.
In rare lesions with cartilaginous component, a radiographic pattern of ring and spotty calcifications is frequently present. This pattern is especially frequent in the upper end of the femur and in polyostotic disease.
Pathologic fracture may occur. The fracture line is often transverse similar to what occurs in Paget’s disease fractures. This pattern of fracture reveals that the microstructure of bone is disturbed, like in Paget’s disease or osteopetrosis.
Rarely secondary ABC may give an aggressive appearance to the lesion that simulates a sarcoma.
Expansile well-circumscribed lesion.
Soft mineralization in relation with ground grass roentgenographic pattern.
The density is related with the mineralization of the lesion.
Helpful in delimiting the involvement especially in maxillary and craniofacial bones.
Well-circumscribed T2-weighted lesion.
The ground-glass pattern is seen as a soft mineralized lesion.
Cysts appear with fluid signal characteristics, with a homogeneous hyperintense signal on T2-weighted images.
Shows the scalloping on the endosteal surface of the cortex.
Image Differential Diagnosis
Radiologically similar to cortical fibrous defect
Different histological pattern
Cortical lesion arising in the tibia and fibula of children
Frequently produce an anterior bowing of the affected bone
Not a medullary lesion
Benign Fibrous Histiocytoma
Radiolucent with sclerotic margins
Epiphyseal and diaphyseal location
Giant Cell Tumor
Epiphyseal lesion involving metaphysis in its evolution.
Highest incidence in the third decade of life.
More frequent in females.
Inner border of the lesion is lytic. Extremely rare to be sclerotic.
Solitary Bone Cyst
Well-outlined centrally and symmetrically located metaphyseal-diaphyseal lucency, expanding, thinning, and scalloping the cortices.
The cyst has its greatest diameter near the epiphyseal plate and its conic end in the shaft.
A multilocular or trabeculated appearance may be seen due to prominent endosteal bony ridges in the inner cortical wall.
MRI usually confirms the cystic nature and its fluid content without solid components that can be bloody in fractured lesions.
Usually present spotty dense calcifications
Is usually a more radiolucent lesion in comparison with FD
Radiolucent, eccentrically elongated lesion in the metaphysis of long bones, juxtaposed to the endosteal surface of one cortex
Metaphyseal Fibrous Defect
In small bones or long bones of small diameter, the lesion may occupy the whole diameter of the medullary cavity and pose the differential with MFD.
Radiologically, MFD is a radiolucent, eccentric lesion juxtaposed to the endosteal surface of one cortex in contrast to FD, which is more centrally located in the medulla.
As long bone grows, it becomes separated from the epiphyseal plate, moving toward the diaphysis.
Lesions tend to thin and slightly expand one or both cortical surfaces.
The inner border is well defined and slightly sclerotic, suggestive of a nonaggressive lesion, producing a scalloped margin.
Infrequently, large lesions may involve the entire width of the bone.
Usually associated to cortical disruption
Area of lucency situated eccentrically in the medullary cavity in the metaphysis of a long bone.
Most ABCs are completely lytic, but a few contain traces of mineral.
Frequently presents a multiloculated appearance.
Less commonly, the lesion may be situated centrally.
Later on, “ballooned” or “aneurysmal” cystic expansion of the affected bone –“blowout” – is evident. Usually forms a thin sclerotic rim of ossification due to periosteal new bone formation.
CT and MRI show a cystic and radiolucent lesion, with internal septations and multiple fluid-fluid levels. Some FD, however, may show features of secondary ABC. In any case, both CT and MRI help to highlight the eventual presence of an underlying lesion.
Well-Differentiated Intramedullary Osteosarcoma
Lacks a sclerotic border
Frequently, the material is obtained by core needle biopsy.
In surgical biopsies, tissue is tan-gray, dense, and fibrous.
The number and size of immature osteoid or bone trabeculae present within the fibrous tissue give it a more or less gritty quality to the cutting procedure.
When the lesion is more mineralized, the appearance is white to yellow.
Sometimes the lesion may present a dense ossification, most common in craniofacial and jaws locations.
In surgical specimens, the lesions are intramedullar centrally located and well defined, the bone contour is expanded, and the cortex is thinned. In older lesions, an edge of reactive sclerotic bone may be seen.
Cyst formation is commonly present, and the cyst cavity is filled with yellowish clear fluid. Cyst formation may be slight or extensive.
Secondary aneurysmal bone cyst may sometimes appear with blood-filled multicameral cyst.
Yellowish areas are related to lipid-laden histiocytes – xanthoma cells.
Chondroid foci, when present, are seen as bluish translucent nodules. Rarely, the cartilaginous component may predominate.
Hemorrhages and hemosiderin pigment are found after pathologic fractures.
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Histologically, the lesion is composed by a bland proliferation of plump fibroblasts in a dense collagenous, well-vascularized matrix by thin capillary vessels.
The spindle cells present a storiform or whorled pattern without cytological atypia, hyperchromatism, or nuclear pleomorphism.
Mitoses are extremely infrequent. Mitoses are seen in the picture of pathologic fractures. Atypical mitoses are not seen.
Within the fibrous tissue, in typical fibrous dysplasia, a variable presence of discontinuous immature woven osteoid or bone trabeculae is seen, with flat or spindle-shaped osteoblast rimming.
Bony trabeculae of different sizes are arranged in a haphazard, nonfunctional fashion. They are peculiar with so-called “Chinese characters” shapes. They are curvilinear, in C, U, and sometimes circumferential shapes.
In more mature lesions, trabeculae may present reversal cement lines simulating the histological appearance of Paget’s disease.
The presence of a lamellar pattern in some trabeculae is extremely infrequent.
With silver stain for reticulin fibers or polarization microscopy, it is possible to see the continuity between newly formed reticulin fibrils of the dysplastic stroma and those that form the matrix of woven trabeculae. This corresponds to a preexisting compressed network of newly formed reticulin fibers, which later mineralize with calcium granules.
Typical fibrous dysplasia may show large areas without bone formation. These areas have a particular storiform pattern of the spindle cells.
The presence of rounded ossicle-like or cementicle-like structures resembling psammoma bodies or cementoid bodies is frequently found in fibrous dysplasia. This feature is most commonly seen in lesions arising in the base of the skull and sometimes is erroneously diagnosed as meningiomas. This pattern is also seen in other sites as long bones but rarely are a prominent component of the lesions.
Osteoclasts can be sometimes observed resorbing immature bony trabeculae.
Hemorrhagic areas and hemosiderin pigment deposits are found after pathologic fractures as well as areas of necrosis.
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