= entity previously mistaken for fibrous dysplasia

Age: newborn up to 5 years

Histo: fibrous tissue surrounding trabeculae in a whorled storiform pattern

Location: normally confined to tibia (middiaphysis in 50%), lesion begins in anterior cortex; ipsilateral fibula affected in 20%

• enlargement of tibia with anterior bowing
• cortex thin / invisible
• periosteal expansion
• sclerotic margin (DDx: nonosteogenic fibroma, chondromyxoid fibroma)
• spontaneous regression in ⅓

Cx: pathologic fracture in 25%, fractures will heal with immobilization; infrequently complicated by pseudarthrosis

DDx: fibrous dysplasia, Paget disease