Interosseous Nerve Compression

Anterior interosseous nerve

The anterior interosseous nerve arises from the median nerve about 5cm above the medial epicondyle of the elbow. It runs along the volar surface of flexor digitorum profundus and along the interosseous membrane between the ulna and radius. It is largely a motor nerve. It supplies the flexor pollicis longus, lateral half of flexor digitorum profundus, and pronator quadratus.
A variation called the Martin Gruber anastomosis¹ occurs in 10 to15% of people. This is when the motor nerve crosses over from the median to the ulnar nerve. In half of these, palsy of the anterior interosseous nerve could lead to palsy of the intrinsic muscles of the hand that are normally supplied by the ulnar nerve.

Posterior interosseous nerve

It is a deep motor branch of the radial nerve and supplies all the extrinsic wrist extensors except for the extensor carpi radialis longus. The radial nerve enters the anterior compartment of the arm between brachialis medially and brachioradialis and extensor carpi radialis longus laterally. The posterior interosseous nerve passes through the supinator muscle as is goes from the anterior to the posterior surface of the forearm. It supples extensor carpi radialis brevis and supinator before entering the arcade of Froshe. This is the commonest site for entrapment.

Lesions of either anterior or posterior interosseous nerves are uncommon.

Anterior interosseous nerve palsy

Anterior interosseous nerve (AIN) compression is also known as Kiloh Nevin syndrome.
The main problem with this lesion is weakness and difficulty moving the index and middle fingers. There is weakness of the flexors of the interphalangeal joint of the thumb due to weakness of flexor pollicis longus and the distal interphalangeal joints of the index and middle fingers due to weakness of flexor digitorum profundus.
Normally when someone pinches something between the index finger and thumb, metacarpo-phalangeal and interphalangeal joints of the thumb and index finger are flexed. With a nerve deficit, the terminal phalanges of the thumb and index finger are extended or hyperextended.

Posterior interosseous nerve palsy

The presenting complaint is often pain rather than weakness. Active supination from a pronated position along with flexion of the wrist may reproduce the patient's symptoms.
The patient will be unable to extend the thumb or other digits at the metacarpophalangeal joints. With complete palsy, it will still be possible to extend the wrist with the extensor carpi ulnaris but they are unable to extend the wrist from the neutral position or in ulnar deviation. They can extend the digits at the interphalangeal joints, but not at the metacarpophalangeal joints.
With posterior interosseous nerve palsy the brachioradialis, extensor carpi radialis longus and of the extensor carpi radialis brevis and supinator are intact.
There is often tenderness over the lateral epicondyle of the elbow and almost always they have tenderness more distally over the arcade of Froshe. Resisted supination of the forearm will cause pain and often resisted pronation will too. Full pronation of the forearm produces pressure on the nerve. Most patients will have pain with resisted extension of the middle finger.
Pain is relieved by blocking the posterior interosseous nerve 3 cm proximal to the wrist joint, injecting about 1 cm ulnar to Lister's tubercle.

Anterior interosseous nerve

Causes of compression of the anterior interosseous nerve include:

• The commonest is a tendinous origin of the deep head of pronator teres
• An enlarged bicipital tendon bursa may impinge on the nerve
• An aberrant or thrombosed radial artery in the mid-forearm
• A thrombosed ulnar artery
• A fascial band at the origin of flexor digitorum superficialis

Posterior interosseous nerve

• Radiocapitellar joint ganglions and synovitis
• Congenital tightness of ligamentous arcade of Frohse
• Poorly placed screws for fracture fixation
• Radial recurrent vessels
• The arcade of Froshe is the commonest location of nerve compression in radial tunnel syndrome
• Radial nerve entrapment at the elbow
• Peripheral nerve compression

Anterior interosseous nerve

• A lesion of the lateral cord of the brachial plexus
• Avulsion of flexor digitorum profundus or of index profundus tendons
• Tendon ruptures are noted by placing digits in different positions and applying tension to the flexor tendons
• In patients with low ulnar nerve injury, some interosseous muscle intrinsic function may be maintained due to the Martin Gruber anastomosis between anterior interosseous nerve and the ulnar nerve
• C-8 radiculopathy is rare

Posterior interosseous nerve

• C7 radiculopathy will produce weakness of triceps and wrist flexors, unlike a PIN lesion
• Lateral epicondylitis will not show tenderness about 4 cm distal to the lateral humeral epicondyle
• Rupture of extensor tendon

The anterior interosseous nerve is deep as are the muscles it supplies. Hence neurophysiology and EMG studies are difficult.
Injection of local anaesthetic 4 fingers breadth distal to the lateral epicondyle will result in temporary PIN palsy and, in the case of PIN syndrome, will result in temporary relief of pain.

Rest and immobilisation may help. The value of NSAIDs and steroid injection is dubious. Surgical treatment should not be delayed for more than 8 to 12 weeks in anterior interosseous syndrome.² Tendon transplantation may be required.³Posterior interosseous nerve lesions also tend to require surgical exploration to get good results.⁴

Early operation tends to produce good recovery.