Working Diagnosis:
Minimally complex extra-articular lipoma with patellar tendinosis

Treatment:
In-office lipoma removal and subsequent rest and activity modification.

Outcome:
The patient underwent an in-office mass excision with blunt dissection. The lipoma was resected in three parts due to significant adhesions to surrounding tissue. Overall, the mass was excised without compromise of the joint capsule or other complication. The patient tolerated the procedure well. He was placed on prophylactic antibiotics due to the extent of debridement required for excision and the required procedure time. At the seven-day follow-up appointment, the wound was noted to be well healed, and sutures were removed without complication. Pathology results revealed the mass to be a benign lipoma.

Author's Comments:
Lipomas are common soft tissue lesions and most often present in the body's cephalic region, specifically in the head, neck, shoulders, and back. They occur predominantly in adults aged 40-60 without gender predisposition and are multiple in approximately 5% of cases. Lipoma of the knee, however, are exceedingly rare. When they do occur in the knee, they are mostly intra-articular. When considering an extra-articular lipoma of the knee, few reported cases are found. These lesions should be differentiated from malignant liposarcoma with MRI. In-office surgical removal is a curative solution, and prognosis is usually excellent with rare recurrence (1-2%) following excision.

Editor's Comments:
Lipoma arborescens is a rare cause of monoarticular joint pain. The etiology is not known, but it is characterized by villous proliferation of the synovial membrane. It has also been referred to as villous lipomatous proliferation of synovial membrane, diffuse lipoma of the joint, or diffuse synovial lipoma. The most common presentation is chronic, progressive, painless swelling of the involved joint, with the knee being the most common site of occurrence. MRI is considered the diagnostic modality of choice. Extraarticular lipoma arborescens are exceedingly rare with only a few cases reported in the literature.

References:
1. Minami S, Miyake Y, Kinoshita H. Lipoma arborescens arising in the extra-articular bursa of the knee joint. SICOT J. 2016;2:28. doi: 10.1051/sicotj/2016019. Epub 2016 Jul 6. PMID: 27382924; PMCID: PMC4935798.
2. M. De Vleeschhouwer, E. Van Den Steen, G. Vanderstraeten, W. Huysse, J. De Neve, L. Vanden Bossche, "Lipoma Arborescens: Review of an Uncommon Cause for Swelling of the Knee", Case Reports in Orthopedics, vol. 2016, Article ID 9538075, 5 pages, 2016. https://doi.org/10.1155/2016/9538075
3. Ioannis Tsifountoudis, Dimitrios Kapoutsis, Anastasios-Nektarios Tzavellas, Ioannis Kalaitzoglou, Apostolos Tsikes, George Gkouvas, "Lipoma Arborescens of the Knee: Report of Three Cases and Review of the Literature", Case Reports in Medicine, vol. 2017, Article ID 3569512, 9 pages, 2017. https://doi.org/10.1155/2017/3569512

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