Tennis elbow, also known as lateral epicondylitis, is an overuse injury resulting in lateral elbow pain. The condition is caused by small tears of the extensor carpi radialis brevis (ECRB) tendon at its origin in the lateral epicondyle of the humerus (Figure 1).
The diagnosis of lateral epicondylitis can be made on physical examination. The patient is asked to actively extend the wrist against resistance; pain and tenderness near the origin of the wrist extensor muscles is a positive response (Figure 2). This exam maneuver suggests why a wrist brace may be helpful; lateral epicondylitis is an elbow disorder with symptoms that are provoked by wrist action.
The ECRB inserts on the posterior base of the third metacarpal bone and serves as an active extensor of the hand at the wrist.
More importantly, the ECRB also stabilizes the wrist by resisting wrist flexion. This is done with a so-called isometric contraction: the muscle fires but does not actually shorten. (It stays the same length; hence the term ‘iso-metric’.) The ECRB provides a counterforce to stabilize the wrist, which otherwise would palmar flex under the sway of the finger flexor muscles. (Think about it: the wrist joint cannot be “aware” of the destination of a tendon that crosses it; it only “knows” that something is pulling it into flexion.)
Because the ECRB is active when the fingers grasp, a patient cannot rest the ECRB by simply avoiding wrist extension; he or she must also avoid wrist flexion too! And that means not grasping anything.
Thus, in order to allow the ECRB to rest without banning the use of the fingers altogether, the wrist must be externally stabilized.
By immobilizing the wrist with a brace (such as the one shown in Figure 3), a patient is free to grasp items in the hand while resting the ECRB and allowing the tendon to heal.