A 24-year-old male presented with history of pain in the knee following trauma. Radiograph and MRI images have been provided.
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Diagnosis: Aneurysmal bone cyst
Plain radiograph shows a well-defined lytic lesion involving the distal metaphysis of the femur with a narrow zone of transition. The lesion reaches into the epiphyses; however, it does not reach up to the cortical surface. There is no periosteal reaction or cortical break. MRI reveals a well-defined multiloculated cystic lesion with fluid-fluid levels in the distal femur. The lesion does not reach up to the subarticular surface and there is no periosteal reaction/cortical break.
Aneurysmal bone cyst (ABC) is a non-neoplastic solitary lesion of bone consisting of a cystic cavity filled with blood. It is a misnomer as it is neither an aneurysm nor a true cyst but is made of blood filled channels. It occurs in patients between 5 and 20 years. The common symptom is pain. It affects the long tubular bones and spine. Lesions are located in the epiphyses and are eccentric.
• It is seen as an expanding, rapidly growing, saccular lytic, sharply demarcated by a thin subperiosteal shell
• It has multiple fine septae internally and sharply demarcated bulging, scalloped borders
• Most lesions are metaphyseal; however, they extend to the epiphyseal end of the bone after the growth plate has closed
• It is associated with ballooning and thinning of the cortex (known as the "blown-out" appearance)
CT and MRI:
• Fluid-fluid levels in multicystic lesions are characteristic for ABC and are probably owing to settling of degraded blood products. Although this is classic for an ABC, these can also be seen in osteosarcoma, giant cell tumor, and chondroblastoma
• Individual chambers are separated by septae surrounded by a thin peripheral rim of low signal intensity on T1- and T2-weighted images. The septae may show enhancement on postcontrast scans; however, the lesion itself does not enhance
• Bone scan may show increased update peripherally with a photopenic center (doughnut sign)
1. Yochum T, Rowe L. Yochum and Rowe's essentials of skeletal radiology. Philadelphia, PA: Lippincott WIlliams & Wilkins; 2005.
2. Gailard F. Aneurysmal bone cust. Radiopaedia.org. Available from http://radiopaedia.org/articles/aneurysmal-bone-cyst. Accessed July 31, 2015