The =fourth carpal= bone (Os carpale quartum, unciform) is somewhat
wedge-shaped, and is readily distinguished from the second by its
greater size and its posterior tubercle. The =superior= or =proximal
surface= articulates with the intermediate and ulnar; it is convex and
curves outward, backward, and downward, encroaching on the external and
posterior surfaces. The =inferior= or =distal surface= bears two inner
facets for the large (third) metacarpal and an outer one for the
external (fourth) metacarpal bone. The =internal surface= has two or
three facets for articulation with the third carpal, between which it is
excavated and rough. The =anterior= or =dorsal surface= is convex and
rough. The =external surface= is small, being encroached upon by the
superior articular surface. The =posterior= or =volar surface= bears a
tubercle on its lower part.[21]