The case illustrates the characteristic imaging appearance of unicameral bone cyst in pediatric age group. Patients with unicameral bone cyst may present with pathological fractures giving the classic "fallen fragment sign". These cyst usually will heal by itself, although injections or aspiration may enhance the healing process.
MRI in this case helped in narrowing the differential diagnoses. Unicameral bone cyst will appear as a fluid-filled cyst in the center of the medullary cavity, whereas an aneurysmal bone cyst typically will be eccentric in location and shows multiple fluid-fluid levels and internal septations. Fibrous dysplasia, on the other hand, will demonstrates intralesional matrix with heterogeneous intermediate T1 signal and heterogenous low T2 signal intensity.