Anterior Interosseous Nerve- The Benedictine and O.K. Sign

A patient with a complete anterior interosseous nerve injury or a high medial nerve injury should be asked to make a fist. The first and second digits will have difficulty in flexing, while the other digits will flex. The third digit will be weak, while the fourth and fifth digits are normal. This position of the hand is similar to the position taken during a hand blessing. The Benedictine sign is different from an “ulnar claw hand”. Ulnar claw hand refers to damage to the ULNAR nerve and is seen when attempting to extend all the digits (leaving the 4^th and 5^th digits flexed). The O.K. sign is used to check for paralysis of the anterior interosseous nerve due to entrapment or compression injury. A patient with paralysis of the anterior interosseous nerve will be unable to make the O.K. sign. This is due to weakness of the flexor pollicis longus and flexor digitorum profundus muscles. A typical pinch attitude is associated with anterior interosseous nerve injury.

The anterior interosseous nerve arises from the median nerve about 4-6cm distal to the elbow, which is about 1/3 of the way down the forearm. It exits from the anterolateral aspect of the median nerve and it runs between the radius and the ulna on the interosseous membrane between and below the muscles of the flexor digitorum profundus and the flexor pollicis longus.

The anterior interosseous nerve supplies the flexor digitorum profundus muscle for the index and long fingers. It also supplies the flexor pollicis longus and the pronator quadratus muscles. The flexor digitorum profundus muscle for the index and long fingers is supplied by the anterior interosseous nerve. The medial part of the FDP is supplied by the ulnar nerve (FDP has dual innervation). The anterior interosseous nerve passes dorsal to the pronator quadratus with the anterior interosseous artery and provides innervation to the volar wrist capsule. The terminal branch of the anterior interosseous nerve innervates the carpal joint capsule.

In patients with Martin-Gruber Connection, the median nerve, or anterior interosseous nerve to the ulnar nerve in the forearm may present with intrinsic muscle weakness. It may be differentiated also from Parsonage-Turner Synrome (acute brachial plexus neuritis) and patient may have pain in the affected extremity. In anterior interosseous nerve entrapment, the median nerve conduction study result will be normal, however the needle EMG of the anterior interosseous innervated muscles will be abnormal.