Pigmented Villonodular Synovitis Presenting as a Popliteal Cyst

INTRODUCTION: Pigmented villonodular synovitis is an uncommon disease characterized by hyperplastic synovium, large effusions and bone erosions. It commonly mimics other conditions. As a result, the diagnosis may remain elusive, as the classic signs of tenderness and effusion are not specific. Occasionally, PVNS presents as a popliteal cyst, which may divert attention from intra-articular pathology. In this article, we describe a case of pigmented villonodular synovitis that presented with a popliteal cyst as the chief problem. CASE REPORT: A twelve year old boy presented to us with a history of a gradually enlarging mass in the left popliteal fossa since eighteen months. He complained of restriction of terminal knee flexion, but he had no pain. After clinical examination and imaging the mass was interpreted as being either a popliteal cyst or chronic hematoma without ruling out the possibility of soft tissue sarcoma. An open excisional biopsy of the lesion was performed which revealed a reddish-brown cystic-nodular mass communicating with the knee joint. The histology was consistent with that of pigmented villonodular synovitis. We performed an arthroscopic synovectomy 3 weeks later. Abnormal synovium found in all compartments of the knee was removed. Histology from the intra-articular lesion was also consistent with a diagnosis of pigmented villonodular synovitis. CONCLUSION: A swelling in the popliteal region may be due to various etiologies. The cause should be thoroughly investigated. Along with clinical examination and imaging the tissue must be subjected to histopathological examination. Pigmented villonodular synovitis can rarely present as a popliteal cyst. In such a situation, treatment consists of dealing with intra-articular and extra-articular pathology.