The incomplete anterior interosseous nerve syndrome.

The anterior interosseous nerve syndrome involves paralysis of the flexor pollicis longus, flexor digitorum profundus of the index and long fingers, and the pronator quadratus. We have encountered 33 cases of an incomplete syndrome in which only the flexor pollicis longus or the flexor digitorum profundus of the index finger is either paretic or paralyzed. This entity must be distinguished from flexor tendon rupture, flexor tendon adherence or adhesion, and stenosing tenosynovitis. The nerve is usually compressed by fibrous bands that most commonly originate from the deep head of the pronator teres and to the brachialis fascia. Less common causes of compression are: fibrous bands from the superficial head of the pronator teres; bands from the superficialis arcade; the nerve running deep to both heads of the pronator; and compression by a double lacertus fibrosus. Patients presenting with paresis should be observed. Most will improve spontaneously without surgery. We recommend exploration and neurolysis of the anterior interosseous nerve in patients who present with complete paralysis of either muscle-tendon unit and who have shown no improvement as determined by physical examination or repeat electromyography after 12 weeks of observation. Recovery after neurolysis is often rapid and complete.