as many as 50% of children and young adults aged 2–25 years. It is synonymous with fibrous cortical defect or metaphyseal fibrous defect if localized to the cortex or medulla, respectively. The vast majority arises eccentrically in the metaphysis of the distal femur and proximal tibia. Plain radiographs show a sharply demarcated oval radiolucency with the long axis parallel to the cortex (
Fig.
21.29 ). The findings are sufficiently specific on plain radiography that biopsy is rarely necessary. NOFs form gray to yellow-brown cellular lesions containing fibroblasts and macrophages. The cytologically bland fibroblasts are frequently arranged in a storiform (pinwheel) pattern, and the macrophages may take the form of clustered cells with foamy cytoplasm or multinucleated giant cells (
Fig. 21.30 ). Hemosiderin is commonly present. Most small NOFs undergo spontaneous resolution within several years.
Fig.21.28Aneurysmalbonecystwithblood-filledcysticspacesurroundedbyafibrouswallcontainingproliferatingfibroblasts,reactivewovenbone,andosteoclast-typegiantcells.
Fibrous dysplasia is a benign tumor that has been likened to a localized developmental arrest of bone constituents.
The lesions arise during skeletal development, and they appear in several distinctive but sometimes overlapping clinical patterns:
Monostotic: involvement of a single bone
Polyostotic: involvement of multiple bones
Fig.21.29Nonossifyingfibromaofthedistaltibiametaphysisproducinganeccentriclobulatedradiolucencysurroundedbyascleroticmargin.
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Fig.21.30Storiformpatterncreatedbybenignspindlecellswithscatteredosteoclast-typegiantcellscharacteristicofafibrouscorticaldefect.
Mazabraud syndrome: fibrous dysplasia and soft tissue myxoma
McCune-Albright syndrome: polyostotic fibrous dysplasia, café-au-lait skin pigmentations, and endocrine abnormalities, especially precocious puberty
All of the aforementioned variants result from a somatic gain-of-function mutation in GNAS1, the gene that is also mutated in pituitary adenomas (Chapter 20). The mutations produce a constitutively active Gs-protein that promotes cellular proliferation by increasing cellular levels of cAMP. The phenotype depends on the stage of embryo-genesis when the mutation is acquired and on the fate of the cell harboring the mutation. A mutation during early embryogenesis produces the McCune-Albright syndrome, whereas a mutation during or after formation of the skeleton in an osteoblast precursor results in monostotic fibrous dysplasia. The skeletal manifestations arise from a cAMP–mediated interruption of normal osteoblast differentiation.
MORPHOLOGYThelesionsoffibrousdysplasiaarewellcircumscribed,intramedullary,andvarygreatlyinsize.Largerlesionsexpandanddistortthebone.Thelesionaltissueiscomposedofcurvilineartrabeculaeofwovenbonesurroundedbyamoderatelycellularfibroblasticproliferation.Thetrabeculaelackprominentosteoblasticrimming(
Fig.21.31 ).Cysticdegeneration,hemorrhage,andfoamymacrophagesareothercommonfindings.
Monostotic fibrous dysplasia often stops enlarging at the time of growth plate closure. The lesion is frequently asymptomatic and is usually discovered incidentally, but it may cause pain, fracture, and discrepancies in limb length. Symptomatic lesions are cured by curettage.