Locked Thumb Metacarpophalangeal Joint Caused by Entrapment of the Sesamoid Bone

Locked thumb metacarpophalangeal joint caused by entrapment of the sesamoid bone is an uncommon entity. The intact volar plate, strong ligamentous attachment between the sesamoid bone and the joint, and altered axis of inserted muscle all contribute to failure of closed reduction. The patient's history, clinical presentation, and plain radiographic findings are all important clues to diagnosing this rare disease. We describe 2 patients with a similar mechanism of injury, where the sesamoid bones were displaced into the thumb metacarpophalangeal joint causing lock thumb. The anatomy, pathophysiology, and surgical management of the patients are also described and discussed. Both patients were successfully treated with meticulous open reduction.