Pigmented Villonodular Synovitis Mimicking an Acute Septic Hip in an Eight-Year-Old Male.

Pigmented villonodular synovitis (PVNS) is a less common but known cause of joint pain in the adult population. PVNS in pediatric patients is even more rare, with only case reports of occurrence in persons under the age of 18 years. Presentation is typically that of more insidious pain and limited range of motion, and is primarily seen in the knee joint. Diagnosis can be suspected with imaging, but ultimately surgical intervention is needed for tissue confirmation. We present a case of PVNS in a pediatric patient with acute symptoms concerning for a septic joint. The patient's workup revealed a large effusion on hip ultrasound, with operative intervention pursued and further imaging deferred given the patient's symptom burden. A 4 × 1 × 1.5 cm intra-articular pigmented mass excised from the synovium in the operating room. The patient's symptoms improved after the procedure, with pathology showing sheets of plump mononuclear cells in a collagenized stroma with hemosiderin deposits, confirming the diagnosis. This case highlights the importance of keeping non-infectious etiologies in the differential diagnosis of acute onset joint pain.