Locked thumb metacarpophalangeal joint caused by sesamoid entrapment.

We report an unusual case of locked thumb in a 24-year-old man, caused by entrapment of the radial sesamoid in the metacarpophalangeal joint. Tomography clearly revealed sesamoid entrapment in the joint. Surgical exploration and cadaver dissection demonstrated that avulsion of the proximal portion of the palmar plate and some dissociation of the sesamoid from the palmar plate with intraarticular displacement were necessary for the sesamoid to be trapped in the joint.