This saddle-shaped articulation is partially responsible for the thumb's opposable motion.

At its upper part is a deep groove, running from above obliquely downward and medialward; it transmits the tendon of the Flexor carpi radialis, and is bounded laterally by an oblique ridge.

The medial surface presents two facets; the upper, large and concave, articulates with the trapezoid bone; the lower, small and oval, with the base of the second metacarpal.

The carpal bones function as a unit to provide a bony superstructure for the hand.

[2] The trapezium is susceptible to arthritis at the joint with the metacarpal bone of the thumb, due to overuse.