A 51-year-old woman presented with moderate knee pain refractory to conservative measures. Radiographs revealed a well-defined, but irregularly-shaped ovoid soft tissue density at the posterior superior aspect of the infrapatellar fat pad of Hoffa. On magnetic resonance imaging, the mass was mildly heterogeneous and had intermediate signal in the proton density series and mixed signal intensity on the T2-weighted images. A low-signal rim could be seen around a portion of the lesion. Arthroscopic resection was performed and a vascular stalk was encountered. The differential diagnosis included: ganglion cyst, meniscal cyst, intra-articular lipoma, villous proliferation of the synovial membrane, Hoffa disease, and intracapsular chondroma. Histologic examination revealed a fibrous capsule partially surrounding a benign lipomatous neoplasm containing an abundance of thin- and thick-walled blood vessels with periadventitial myxoid stroma. To our knowledge, this is the first reported case of an intra-articular angiomyxolipoma in the literature. At 8-month follow-up, the patient was asymptomatic with no sign of recurrence. We postulate a low recurrence rate based on the insidious growth rate and benign histological appearance of such lesions. To our knowledge, no other intra-articular lipomatous lesion of the knee has been removed solely with arthroscopic techniques. Further follow-up information is needed to better understand the natural course of these lesions.
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