The common extensor tendon is a soft tissue of the human forearm. It gets its name from the fact that it is a shared tendon, with four muscles of the posterior forearm arising from its distal or lower end near the elbow. Likewise, it is so named because it is the common tendon of the muscles that extend the wrist joint, meaning that they act to bend the dorsal or back side of the hand backward toward the posterior side of the forearm. The common extensor tendon can commonly become inflamed from repetitive stress as a consequence of activity, which is known as tennis elbow.
At the lower end of the humerus, the bone of the upper arm, is a pair of rounded prominences known as the medial and lateral epicondyles. Arising from the lateral epicondyle, the prominence on the outside aspect of the bone on the thumb side of the arm, is the common extensor tendon. Like any tendon, it is composed of densely packed collagen and elastin fibers, which give it a white, ropelike appearance. Narrow at the top where it attaches to the lateral epicondyle, it broadens as it descends, dividing into four separate bundles of fibers.
Each of these fibrous divisions attaches to the proximal or upper end of one of four forearm extensor muscles: the extensor carpi radialis brevis, the extensor carpi ulnaris, the extensor digitorum, and the extensor digiti minimi. The outermost of these is the extensor carpi radialis brevis, which runs down the thumb side of the posterior forearm, crosses through the wrist joint, and attaches to the proximal end of the third metacarpal, the long bone of the palm beneath the middle finger. Next to this is the extensor digitorum, a larger muscle that runs down the center of the posterior forearm and attaches to all four fingers. It extends or straightens the hand and fingers as well as the wrist joint.
Alongside the extensor digitorum is a thin muscle known as the extensor digiti minimi. Nearer to the ulnar or pinky-finger side of the arm, it inserts along the base of the little finger, extending that finger only. The muscle of the common extensor tendon that is nearest this side of the arm is the extensor carpi ulnaris, which attaches to the proximal end of the fifth metacarpal, or the palm bone beneath the pinky. In addition to extending the wrist, it adducts the hand on the wrist, meaning that it bends it sideway in the direction of the pinky as in waving. Similarly, the extensor carpi radialis brevis abducts the hand, or bends it to the side of the thumb.
All four muscles are especially active when the elbow and wrist are simultaneously extended, or when the elbow is straightened at the same time as the wrist is bent backward. An example of this motion is a tennis player performing a backhand. Performing this motion frequently and repetitively can lead to a condition known as tennis elbow, or lateral epicondylitis. It is characterized by inflammation of the common extensor tendon where it attaches to the humerus, often as a result of tiny tears in the tendon that are not given sufficient time to heal, and is felt as pain that can radiate all the way to the back of the hand. In more severe cases, the tendon may suffer a partial tear away from the bone. Most cases of tennis elbow are treated non-surgically with rest, icing, and anti-inflammatory medication.