Bone · Hand

Trapezoid

Os trapezoideum

The smallest bone of the distal row, wedged between the trapezium and the capitate and shaped like a boot — broad across its back surface and narrow on the palm side. Its entire lower surface is taken up by a deep notch that receives the base of the second metacarpal, gripping it like a tenon in a mortise. That interlock is unusually tight, and it is what makes the index finger's ray the most immobile in the hand. It is also the carpal you are least likely to be able to find by touch, buried as it is behind the base of the index finger.