Working Diagnosis:
Pigmented Villonodular Synovitis (PVNS)

Treatment:
Left knee arthroscopic limited synovectomy

Outcome:
Immediate post-operative, the patient was non-weight bearing. He was off crutches to weight bear as tolerated on post-operative day 2.
2nd week: The patient performed home exercises as per rehabilitation protocol 2x/day
4th week: Progressed to low impact exercise and physical therapy with no residual symptoms of pain and locking of left knee.
1 month follow up: Well healed knee incisions, small effusion, ROM 0 to 135 degree, not cleared to play.
6 month follow up: The patient was cleared for noncontact sports, no residual symptoms.

Author's Comments:
Pigmented villonodular synovitis (PVNS) is an uncommon, benign, proliferative disorder of the synovium, primarily affecting adults between the ages of 20 and 50 years. PVNS is a type of Tenosynovial Giant Cell tumor that affects the synovium of joint and tendon sheath. It is marked by excessive growth of synovial tissue leading to formation of villous or nodular structure and presence of hemosiderin deposit which gives the tissue a distinctive pigmented appearance. While PVNS can involve any joint, it typically presents as monoarticular, with the knee being the most frequent site of involvement in the pediatric population. Cardinal clinical features of PVNS include pain, localized swelling, joint tenderness, and a palpable soft-tissue mass.
Due to its insidious onset, radiographic imaging often appears unremarkable in the early stages of the disease. Consequently, MRI is considered the most accurate diagnostic modality. PVNS is frequently misdiagnosed for other joint conditions, such as juvenile idiopathic arthritis (JIA), hemophilic arthropathies, subacute bacterial infections, and tuberculosis.
Surgery is the mainstay with arthroscopic partial synovectomy being the primary choice to prevent disease progression and protect joint function. Post-operative physical therapy is vital to regain joint strength, range of motion, and return to normal activities.

Editor's Comments:
This is an important case to illustrate that not all locking in the knee is meniscus related. Mostly PVNS has an insidious onset of symptoms (swelling and mechanical locking) that can be intermittent but progressive with about 18 months from symptom onset to diagnosis. Although less common in the pediatric population, and therefore potentially overlooked, in this particular 17 yr old patient, the care team appropriately had PVNS on the differential diagnosis and proceeded with MRI and biopsy. This allowed for definitive diagnosis and treatment plan which included ressection followed by rehabilitation and eventual return to play.

References:
Tyler, Wakenda K. MD, MPH; Vidal, Armando F. MD; Williams, Riley J. MD; Healey, John H. MD. Pigmented Villonodular Synovitis. Journal of the American Academy of Orthopaedic Surgeons 14(6):p 376-385, June 2006.
Hong Hoa, Doan Van Ngoc, Nguyen Minh Chau, Tran Phan Ninh, Nguyen Van Sang, Vu Tri Long,Pigmented villonodular synovitis of the knee in a child: a case report,Radiology Case Reports,Volume 17, Issue 5,2022. https://doi.org/10.1016/j.radcr.2022.03.006

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