Working Diagnosis:
Pigmented Villonodular Synovitis

Treatment:
The patient experienced an immediate functional improvement after the arthrocentesis and was referred to orthopedic oncology for long-term treatment planning. The orthopedic oncologist recommended a follow-up MRI with contrast in 3 months with consideration for left knee arthroscopy.

Outcome:
The patient was lost to follow-up.

Author's Comments:
This case demonstrates an uncommon case of a spontaneous monoarticular effusion due to the localized form of pigmented villonodular synovitis, or tenosynovial giant cell tumor. Pigmented villonodular synovitis is a benign proliferative disorder of synovium characterized by lipid-laden macrophages, giant cells, hemosiderin, and stromal and fibroblast proliferation4. It affects primarily the knee in young adults and presents as atraumatic effusion or swelling.
Pathognomonic features of pigmented villonodular synovitis are an atraumatic serosanguinous effusion and low signal intensity on T1 and T2 weighted MR imaging due to hemosiderin deposition3. There are two forms of pigmented villonodular synovitis: Localized and diffuse. Localized pigmented villonodular synovitis involved a single synovial area and has a lobular appearance on MRI4. Diffuse pigmented villonodular synovitis encompasses most or all the involved joint, bursa, or tendon sheath.
MRI monitoring is recommended for asymptomatic patients1. Resection plus synovectomy is the preferred treatment due to the risk of local destruction and continued symptoms. Recurrence following resection is more common with diffuse pigmented villonodular synovitis.

Editor's Comments:
Awareness of pigmented villonodular synovitis as a diagnosis presents the most difficult challenge in making the diagnosis. In a 16-year study by Willimon, Busch, and Perkins5, the average time to diagnosis was 16 months after symptom onset. Although pigmented villonodular synovitis was considered 70% of the time in the initial presentation, misdiagnosis occurred more than half the time.
Return to activity in individuals with pigmented villonodular synovitis has been difficult to study and assess due to the rarity of the condition. Individualized treatment plans to maximize functional outcomes have been the mainstay. These plans include surgical versus non-surgical therapy as well as open versus laparoscopic technique. Patient-reported outcomes via the Patient-Reported Outcomes Measurement Information System (PROMIS) have been studied with 80% of patients endorsing return to or near pre-symptomatic activities2.

References:
DeLaney TF. Treatment for tenosynovial giant cell tumor and other benign neoplasms affecting soft tissue and bone. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA, February 14, 2021.
Gelhorn HL, Ye X, Speck RM, Tong S, Healey JH, Bukata SV, Lackman RD, Murray L, Maclaine G, Lenderking WR, Hsu HH, Lin PS, Tap WD. The measurement of physicial functioning among patients with tenosynovial giant cell tumor (TGCT) using the patient-reported outcomes measurement information system (PROMIS). Journal of Patient-Reported Outcomes. 2019; 3: 6. https://doi.org/10.1186/s41687-019-0099-0
Nigrovic PA. Overview of hemarthrosis. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, November 3, 2020.
White N. Pigmented Villonodular Synovitis (PVNS). In: The 5-Minute Sports Medicine Consult. 2nd Ed, Bracker MD (Ed), Lippincott Williams & Wilkins, Philadelphia, PA, 2011.
Willimon, SC, Busch MT, Perkins, CA. Pigmented Villonodular synovitis of the knee: An underappreciated source of pain in children and adolescents. Journal of Pediatric Orthopaedics. 2018; 38 (8): e482-5. https://doi.org/10.1097/BPO.0000000000001213.

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