Ligament · Hand

Collateral ligaments of the interphalangeal joints (hand)

A pair of short, tough cords running down either side of every finger joint beyond the knuckle — the middle joint and the fingertip joint of each finger, and the single joint of the thumb. Each runs obliquely from a small pit on the side of the head of the bone above to the side of the base of the bone below, and together they are the reason a finger bends only forwards and cannot be waggled sideways once it leaves the knuckle. Unlike the knuckle's collaterals, which slacken when the finger is straight, these stay close to taut through the whole range, because the head of a phalanx is a symmetrical pulley rather than a lopsided cam. That constant tension is what makes these joints stable in every position rather than only in one.