[Atraumatic instability of the anterior cruciate ligament caused by an intraligamentous cyst].

A 28-year-old man reported that he has a feeling of "giving away" in his knee while playing soccer. He has had no previous injury. At a physical examination, the results of Lachman's test and the pivot shift test were pathological. Magnetic resonance imaging revealed an intraligamentous cyst of the anterior cruciate ligament (ACL). During arthroscopy it became evident that the cyst had destroyed the ACL. After ACL reconstruction, physiological stability of the knee joint was achieved. Joint cysts are rare and frequently cause pain and a limited range of motion. An association between intraligamentous cysts and instabilities of the affected joint has not been described so far. Preventive resection of asymptomatic and incidentally found intraligamentous cysts should be discussed.