Bone · Hand

Trapezium

Os trapezium

The distal-row carpal at the base of the thumb, and the reason the human thumb can do what it does. Its lower surface is not flat but saddle-shaped — concave in one direction and convex in the other — and the base of the first metacarpal sits on it the way a rider sits in a saddle. Its palm-side surface carries a tubercle and, beside it, a deep groove through which the tendon of flexor carpi radialis runs on its way into the hand. It is the most mobile foundation in the wrist, sitting immediately beside the most rigid one.