828 - Pigmented Villonodular Synovitis in a 14-Year-Old: An Uncommon Cause of Post-Traumatic Knee Swelling

Case Diagnosis: Pigmented villonodular synovitis (PVNS) is a rare, benign proliferative disorder of the synovium characterized by hemosiderin deposition, synovial hypertrophy, and villonodular architecture. It most commonly affects adults in their third to fifth decades and typically involves large joints. In pediatric patients, PVNS is uncommon and can often be misdiagnosed.

Case Description: A 14-year-old female presented with acute left knee pain, swelling, and limping after she fell onto her flexed anterolateral knee. Days later, while squatting, she experienced a sudden pop with increased pain and swelling in her knee. There was no prior history of joint issues. On exam, she had an antalgic gait, decreased left knee range of motion, and a gross effusion. MRI was ordered to evaluate for internal derangement, but revealed intact ligaments and menisci. Instead, significant focal synovial thickening with nodularity was observed in the anterior compartment, suggestive of localized PVNS. The patient began a home-based rehabilitation program and later underwent left knee arthroscopy with partial synovectomy. Postoperative recovery was favorable, and by three months, she returned to sports without pain.

Discussions: The patient’s mechanism of injury initially raised suspicion for internal derangement, and PVNS was not suspected. PVNS often presents with an insidious onset and recurrent atraumatic hemarthrosis, but its rarity and overlap with more common pathologies can delay diagnosis. The unusual age of onset in this case, along with its presentation after an acute injury, makes it notable. Additionally, the patient’s rapid recovery could be attributed to early preoperative physical therapy, suggesting a role for prehabilitation in synovial disorders.

Conclusions: This case underscores the importance of maintaining a broad differential diagnosis in adolescents with joint swelling post-trauma. MRI is essential in evaluating atypical presentations, and early physical therapy may support improved surgical outcomes in rare diseases, such as PVNS.