Primary synovial chondromatosis mimicking medial meniscal tear in a young man.

Magnetic resonance imaging has recently become a noninvasive and cost-effective diagnostic tool for meniscal pathology in the knee. We report an unusual case of primary synovial chondromatosis for which the clinical diagnosis was medial meniscal tear but magnetic resonance imaging also showed a displaced torn meniscus, causing a so-called "double meniscus sign." The patient was definitely diagnosed and successfully treated with arthroscopy. We discuss the clinical diagnostic and therapeutic challenges, and the possible pathogenesis.