An unusual loose body in the knee.

In a 30-year-old man, the movement of a loose body between the knee joint and a popliteal cyst caused difficulty in locating the loose body. The first roentgenogram showed a posteromedial opacity, which was thought to be a sesamoid. On a subsequent film, a loose body appeared in the intercondylar notch and the posteromedial opacity was absent. At arthroscopy, the loose body could not be found within the joint and a peroperative radiograph showed that the opacity had returned to its original position. The loose body was visualized with the arthroscope inserted via a posteromedial portal and passed into the popliteal cyst. After manipulation of the loose body into the posteromedial compartment, it was removed via arthrotomy. The possibility that a loose body may lie in a popliteal cyst should be considered if all other attempts have failed to locate it.