Working Diagnosis:
Lipoma arborescens

Treatment:
The patient was encouraged to take anti-inflammatories as needed and was referred to orthopedic surgery. He elected for knee arthroscopy with complete synovectomy and synovial biopsy. The biopsy showed synovial and fibroadipose tissue with focal mild chronic inflammation.

Outcome:
His pain and swelling improved following the surgery. He underwent rehabilitation with the athletic training staff for several weeks after surgery with further improvement of his pain and eventual resolution of his swelling. At that point, he was able to participate in offseason workouts with the team.

Author's Comments:
Lipoma arborescens is a rare condition involving villous synovial proliferation with replacement of subsynovial tissue with adipose cells. It most commonly affects the knee but can involve other joints. Because symptoms, course, onset, and laboratory studies are variable, diagnosis is based on MRI findings and biopsy results (if applicable). This condition should be considered in the differential diagnosis of acute or chronic monoarticular joint effusion.

Editor's Comments:
This case is a great example of lipoma arborescens, which is a rare intra-articular tumor like lesion that is a cause of slowly progressive and painless joint effusion which can persist for many years. It can cause persistent pain later in its course. Etiology is unknown but may be associated with osteoarthritis or chronic inflammation or chronic traumatic stimuli of synovium causing subsynovial villous fat proliferation usually involving the suprapatellar pouch of the knee. Labs and synovial fluid analysis is generally benign. This patient had high CRP and high WBC count in the synovial fluid with high percentage of neutrophils suggesting inflammatory cause of the lipoma arborescens. It would be beneficial to have an inflammatory lab work-up searching for an etiology. Close follow-up in this patient may be necessary to see if the effusion returns so that work-up for an etiology can be performed.

Lipoma aborescens should be strongly considered when a knee joint aspiration yields a dry tap or no fluid aspirated. It is most common in men, in the knee joint and in fourth and fifth decades of life. Though other articles state that it is equally common in men and women. It is more commonly secondary as seen in elderly from chronic irritation. Less commonly is idiopathic and younger age group in second or third decades as in this case. Diagnosis is based on MRI findings of villous synovial masses with high fat signal intensity. Radiographs may show fullness of soft tissue opacity particularly at suprapatellar pouch giving a clue to the diagnosis. Ultrasound shows frond like mass of echopattern like subcutaneous fat that undulates in the effusion which is soft and compressible distinguishing it from PVNS which is firm and noncompressible. Primary treatment is directed toward treating underlying conditions to reduce further progression. Secondary treatment is arthroscopy with debridement if continues to be symptomatic with reported recurrence rate of 2.8% in one study.

References:
de Souza TP, Carneiro JBP, Dos Reis MF, Batista BB, Gama FAS, Ribeiro SLE. Primary lipoma arborescens of the knee. Eur J Rheumatol. 2017;4(3):219-221. doi: 10.5152/eurjrheum.2017.17014
D’Mello Z, Neogi DS, Punit AS, Sathe S. Lipoma Arborescens of the Knee Joint after Anterior Cruciate Ligament Injury. Orthop Surg. 2013;5(2):142-145. doi: 10.1111/os.12043
Garnaoui H, Rahmi A, Messoudi A, et al. Intra-articular lipoma arborescens of the knee: A report of two cases with bilateral localization. Int J Surg Case Rep. 2018;51:224-227. doi: 10.1016/j.ijscr.2018.08.052
Khalid S, Asif N, Afrose R, Faizan M, Khalid M, Sherwani RK. Lipoma arborescens: is it the cause or effect? Egypt Rheumatol Rehabil. 2015;42:45-48. doi: 10.4103/1110-161X.155652
Khodaee M, Roy D, VanBaak K, Bafus BT. An Unusual Knee Mass in a Soccer Player. Asian J Sports Med. 2014;5(3):e23187. doi: 10.5812/asjsm.23187
Natera L, Gelber PE, Erquicia JI, Monllau JC. Primary lipoma arborescens of the knee may involve the development of early osteoarthritis if prompt synovectomy is not performed. J Orthop Traumatol. 2015;16:47-53. doi: 10.1007/s10195-014-0295-x
Sanamandra SK, Oon Ong K. Lipoma arborescens. Singapore Med J. 2014;55(1):5-11. doi: 10.11622/smedj.2014003
Wang et al. JBJS Rev. 2019 Apr;7(4):e8.
Tsifountoudis et al. Case Rep Med. 2017; 2017: 3569512. Published online 2017 Jan 24. doi: 10.1155/2017/3569512

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