"Do Not Touch": An Uncommon Benign Fatty Bone Tumour.

Answer: E A 27-year-old male with no medical illness presented to the orthopaedic clinic with left knee pain since 1 week. There was no history of trauma. There was no tenderness or swelling on physical examination and the range of motion of the left knee was normal. Frontal and lateral radiographs of the left knee (Fig. 1) were obtained, which showed an eccentric and well defined radiolucent lesion with a thin sclerotic rim located in the proximal metadiaphysis of the left tibia. The lesion had a narrow zone of transition. There was no cortical destruction, periosteal reaction or any soft tissue swelling. The left knee joint was normal. The radiographic findings were in keeping with a non-aggressive bone lesion of the tibia. What is the most likely diagnosis? A. Fibrous dysplasia B. Non-ossifying fibroma C. Simple bone cyst D. Chondroid tumour E. Intraosseous lipoma Findings and Diagnosis Radiographically, the common differential diagnosis of non-aggressive lucent lesion affecting the tibia in patients less than 40 years of age includes fibrous dysplasia, non-ossifying fibroma, simple bone cyst, aneurysmal bone cyst, and chondroid tumours like chondromyxoid fibroma and enchondroma. Subsequently, magnetic resonance imaging (MRI) of the left knee was performed on the patient. The MRI showed that the lesion was hyperintense, with signal intensity similar to that of the subcutaneous fat on coronal T1-weighted MRI image (Fig. 2A). It had a thin hypointense rim that corresponded to the sclerotic margin seen on radiographs. On the coronal T2-weighted fat-suppressed MRI image, the lesion appeared largely hypointense, consistent with the suppression of fat signal (Fig. 2B). The lesion also showed an irregular internal low signal intensity area on the T1-weighted image which appeared hyperintense on T2-weighted fat-suppressed image. The diagnosis was intraosseous lipoma. The patient has been asymptomatic since then. Hence, no follow-up investigation was done.