Ligament · Hand

Collateral ligaments of the metacarpophalangeal joints (hand)

The side ligaments of the knuckles, one on each side of every joint where a finger meets its metacarpal. Each runs backwards and downwards from a dimple on the side of the metacarpal head to the side of the base of the finger's first bone — an oblique line, not a straight one, and that obliquity is the whole story. The metacarpal head is not round but cam-shaped, wider and deeper at the front, so bending the knuckle drags the ligament over a larger surface and pulls it tight, while straightening lets it slacken. This is why you can spread your fingers wide apart with the hand flat and almost not at all once you close it into a fist.