a 17-year - old man visited our hospital because of an abnormal finding on a simple radiograph of the right knee that was taken in a primary care clinic for constitutional varus knee . physical examination revealed no tenderness , swelling , effusion , or limitation of motion of the knee . plain radiographs of the right knee demonstrated a 252520 mm well - defined multi - septated osteolytic lesion in the epiphysis of the distal femur with sclerotic rim . 1 ) . gadolinium - enhanced magnetic resonance imaging ( mri ) was performed ( fig . the mri showed an eccentric lobulated contoured mass in the lateral femoral condyle , which displayed heterogeneous intermediate signal intensity on a t1-weighted image and heterogeneous high signal intensity on a t2-weighted image . based on the radiologic findings , chondroblastoma , giant cell tumor , chondromyxoid fibroma , and fibrous dysplasia were considered as differential diagnoses . curettage and autogenous bone graft were performed through a cortical fenestration to prevent collapse of the articular surface . histologic findings showed curvilinear trabeculae of metaplastic woven bone in a hypocellular , fibroblastic stroma ( fig . the fibroblastic stroma showed low cellularity and consisted of myxoid material in a collagenous matrix . fibroblastic proliferation and some foamy macrophages with disorganized bony trabeculae were histological features corresponding to fibrous dysplasia . fibrous dysplasia accounts for 5%7% of all benign bone tumors1 ) and may involve one bone ( monostotic ) or multiple bones ( polyostotic ) . although any bone may be affected , fibrous dysplasia is typically found in the diaphyses or the metaphyses in cases affecting long bones . some authors have reported cases of polyostotic fibrous dysplasia involving the epiphyses2,3 ) . however , epiphyseal involvement in monostotic fibrous dysplasia is extremely rare . to our knowledge , involvement of the distal femoral epiphysis , as in our subject , has not been previously reported . monostotic fibrous dysplasia usually manifests between 20 and 30 years of age and most commonly affects the femurs or the ribs4,5 ) . diagnosis is usually based on clinical , radiographic , and histopathologic features7 ) . on simple radiographs , fibrous dysplasia presents as a well - marginated peripheral sclerotic bone lesion ; it shows a variety of patterns , which may be lucent , sclerotic , or mixed , or may have the appearance of ground glass , depending on the amount of bone trabeculae and fibrous elements . differential diagnoses for such lesions should include epiphyseal lesions such as chondroblastoma , chondomyxoid fibroma , or giant cell tumor . chondroblastoma is usually seen as a well - defined radiolucent lesion , with a margin that is either smooth or lobulated , and a thin sclerotic rim . chondromyxoid fibroma is often seen as a lobulated , eccentric radiolucent lesion , mostly with a well - defined sclerotic margin . septation is seen in about 50% of cases and matrix calcification may also be present10 ) . giant cell tumor is usually lytic , eccentric , and often lacks a sclerotic rim . however , unusual variants may render radiographic diagnosis difficult , definite a diagnosis should be based on pathologic findings , especially in cases such as this one . although most cases of fibrous dysplasia are asymptomatic and do not progress with age , and thus do not require surgical treatment , juxta - articular lesions in weight - bearing joints , as in our case , may need a surgical treatment to prevent articular surface collapse leading to deterioration of joint function .
fibrous dysplasia is a common benign skeletal lesion that may involve a single bone or multiple bones . although fibrous dysplasia can affect any bone , monostotic fibrous dysplasia of the long bone typically occurs in the diaphysis or metaphysis . 
 we report a very rare case of monostotic fibrous dysplasia involving the epiphysis of the distal femur in a young man .