The extensor digiti minimi is a muscle of the posterior forearm that inserts on the back of the hand. Long and narrow in shape, it extends from just above the elbow all the way to the base of the little finger. This muscle is considered an extrinsic extensor of the hand, meaning that the body of the muscle is found not in the hand but in the forearm, with only its tendon crossing the wrist joint. As its name suggests, the extensor digiti minimi is responsible for extending or straightening the little finger.
Arising from the lateral epicondyle of the humerus, the outermost of the two knobby bony protrusions at the bottom of the humerus bone in the upper arm, this muscle originates via the common extensor tendon. The common extensor tendon is the tendon that attaches several of the muscles of the posterior compartment of the forearm to the lateral epicondyle. When the arm is at one’s side and the palm is facing forward, this muscle is found on the lateral or thumb side of the arm, just to the outside of the hinge of the elbow. The extensor digiti minimi begins here alongside the extensor digitorum communis, extensor carpi radialis brevis, and extensor carpi ulnaris muscles, all extensors of the wrist and fingers.
From here the extensor digiti minimi muscle runs slightly obliquely down the center of the back of the forearm, forming a small tendon just before crossing the wrist joint. This tendon passes posterior to the distal radioulnar joint, which is where the radius and ulna bones in the forearm converge at the wrist, and then passes beneath the dorsal carpal ligament and curves toward the pinky finger. The dorsal carpal ligament is a broad, band-like ligament that wraps the wrist horizontally, holding all the tendons entering the hand together in a bundle.
As the tendon of the extensor digiti minimi crosses the hand, it divides into two parallel tendons that together approach the first phalangeal bone, the nearest of the three small bones making up the little finger. Just before they insert on the phalanx, they are joined by a small section of the tendon of the extensor digitorum communis muscle, known as the extensor expansion, which crosses obliquely from just below the ring finger to converge with the two. Together, they attach to the base of the first phalanx on the dorsal or back side. This attachment site means that any contraction of the extensor digiti minimi will pull upward and backward on the little finger, extending the entire digit posteriorly.