Atraumatic hemarthrosis caused by a large mediopatellar plica

Hemarthrosis of the knee has various etiologies and is classified into atraumatic or post-traumatic. Among atraumatic factors, hemarthrosis due to synovial plica is extremely rare. We report a case of atraumatic hemarthrosis caused by the mediopatellar plica. A 21-year-old male truck driver was referred to our hospital, because of swelling and pain of the right knee without history of trauma. Bloody synovial fluid was aspirated by arthrocentesis. However, his symptoms recurred and persisted. The range of motion was normal, but the patient complained of anteromedial knee pain during maximum flexion. Routine biochemical analyses were within normal limits. Plain radiographs were normal. Magnetic resonance imaging (MRI) of the knee showed the hypertrophic mediopatellar plica and an irregular signal of the infrapatellar fat pad. Arthroscopy revealed a voluminous mediopatellar plica trapped between the patella and the medial femoral condyle. It attached to the center of the infrapatellar fat pad, the so-called tongue. When the tourniquet was released, fresh bleeding was observed from the region between the mediopatellar plica and the tongue. Because that region was considered to be the cause of the hemarthrosis, the mediopatellar plica and the tongue were excised. A histologic examination of the tongue showed evidence of bleeding with nonspecific synovitis. After the procedure, the patient was asymptomatic and there were no clinical signs of recurrence.