Fibrous dysplasia, including McCune-Albright syndrome, is a genetic, non-inheritable benign bone disorder that may involve a single or multiple bone, typically occurring in the diaphysis or the metaphysis of long bones. In very rare instances polyostotic fibrous dysplasia present involvement of the epiphysis in long bones. Aneurysmal bone cysts are benign, expansile, lytic bone lesions formed by cystic cavities containing blood, that may occur de novo or secondary to other lesions of bone, including fibrous dysplasia. We report a case of an 18-year-old female with polyostotic fibrous dysplasia (McCune-Albright syndrome) with diaphyseal and unusual multiple foci of epiphyseal involvement of long bones as well as in the patella, and a simultaneous aneurysmal bone cyst of the left femoral neck with pathologic fracture. This is the first report of a simultaneous aneurysmal bone cyst in a patient with polyostotic fibrous dysplasia (McCune-Albright syndrome) with involvement of diaphysis and epiphysis of long bones, highlighting that fibrous dysplasia should be included in the differential diagnosis of polyostotic tumors involving the diaphysis as well as the epiphysis. In patients with polyostotic fibrous dysplasia there should be an active search for lesions in the epiphysis.Fibrous dysplasia (FD) is a developmental benign bone disorder, with fibroblasts proliferation resulting in excessive fibrous tissue replacing normal calcium hydroxyapatite of the osteoid matrix, with distortion and overgrowth of the affected bone.

FD is a genetic, non-inheritable disorder caused by mutations occurring post-zygotically in the guanine nucleotide alpha stimulating gene - GNAS1 gene- of the osteoblastic lineage cells  with consequent mutation of the α-subunit of the Gs stimulatory protein leading to activation and inappropriate overproduction of cyclic adenosine monophosphate (cAMP) and c-fos overexpression causing defects in the osteoblastic differentiation. The fibroblasts-like cells at histology correspond to poorly differentiated osteoblasts, leading to extensive proliferation of fibrous tissue. In the mutated cells the secretion of interleukin-6 is elevated, with consequent activation of osteoclasts, allowing osteolysis and expansion of the FD lesion. The combination of these pathways leads to the known radiographic appearance of FD, as an expansile lesion, with endosteal scalloping, without periostal reaction, and a ground-glass density within the lesion. FD arises sporadically, and there are no confirmed cases of vertical transmission. FD is a rare disorder representing 2.5% of osseous tumors overall and 5-7% of all benign bone tumors  found equally in both sexes.

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Mishita
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Journal of Bone Research and Reports