A Rare Presentation of a True Intra-articular Lipoma: A Case Report and Review of Imaging Findings.

This report describes the unique case of an intra-articular lipoma in a 34-year-old male. The patient presented with a history of chronic knee pain associated with an intermittent sensation of the knee giving way. Physical examination and initial radiographic imaging were unremarkable. Magnetic resonance imaging (MRI) revealed a 9.2 x 6.7 mm ovoid mass posterior to the posterior cruciate ligament (PCL) exhibiting hyperintense signals on T1-weighted images and intermediate-to-high intensity signals on T2. On subsequent proton density fat suppression sequences, the mass demonstrated homogenous signal suppression and was confirmed as being a lipoma. To the best of our knowledge, this is only the second reported case of an intra-articular lipoma arising posterior to the PCL. Intra-articular lipomas, albeit rare, should be considered in the differential diagnosis for chronic knee pain with associated joint motion abnormalities. MRI remains the gold standard in imaging intra-articular soft tissue pathology and should be the study of choice in differentiating intra-articular lipomas from similar conditions such as pigmented villonodular synovitis and lipoma arborescens.