An unique case is presented, never mentioned before the world specialized literature, concerning an intraarticular teratoid formation in the knee. Initially a sessile, pediculated formation 3/3 cm in size, on the interior aspect of the synovial sac, became free following an accident, and blocked the knee in the flexed position at an angle of 30 degrees, simulating rupture of the medial meniscus and intercondylar luxation.