Joint · Hand

Metacarpophalangeal joints (hand)

The knuckles — where each finger meets the long bone of the palm behind it, and the hand's most versatile joints. The rounded metacarpal head sits in a shallow socket on the base of the finger, an arrangement that allows two directions of movement instead of one: bending and straightening, plus spreading the fingers apart and drawing them together. The head is not a sphere but a cam, wider and deeper on its palm side, so the side ligaments tighten as the knuckle bends — which is why the fingers can splay when the hand is flat and are locked into line once it is closed. The thumb's joint of the same name is shaped differently, with a flatter head and two small sesamoid bones beneath, and behaves much more like a hinge.