Dislocation of os trigonum presenting as a loose body in the ankle.

UNLABELLED: Dislocation of the os trigonum presenting as a loose body is a rare clinical entity. In this report, we describe the case of a 34-year-old male who presented with symptoms of acute left ankle pain that was aggravated by motion and weight bearing. He also displayed lateral collateral ligamentous laxity, without the presence of an effusion. Radiographic inspection revealed a 10 x 15-mm loose body within the ankle joint cavity. Arthroscopic intervention showed the loose body to be round and smooth, and it could not be deformed with the tip of a probe, although it could be displaced up to 5 mm with ankle manipulation, in particular with inversion stress of the ankle. The loose body was removed with an arthroscopic forceps. Based on its location within the ankle, and the lack of any articular cartilage on the removed joint fragment, we determined the loose body to represent a dislocated os trigonum. The rarity of this case presentation makes one consider the association of symptomatic os trigonum with ankle instability. LEVEL OF CLINICAL EVIDENCE: 4.