For the reconstruction of thumb and finger extension in C7–T1 root injuries of the brachial plexus, and for patients with quadriplegia with Grades 3 through 5 of the international classification of muscle function.

Surgery should be performed within 12 months of trauma, ideally roughly 6 months postinjury. However, in patients with lower root avulsion of the brachial plexus, surgery can be performed as early as 3 months after injury. In patients with a spinal cord injury, the nature of the paralysis (i.e., is it an upper or lower motoneuron type of palsy?) should be determined. In the presence of muscle denervation, the need for surgery can be anticipated. Otherwise, the limits of spontaneous recovery should be exhausted, which generally is ~ 6 months posttrauma.

Radial wrist extensors and the brachioradialis muscle should be functional, because these muscles share the same spinal level of innervation (i.e., myelomere) as the supinator muscle.

Paralysis of the supinator muscle

Long-standing lower-type injuries of the brachial plexus (in such palsies, a free muscle transfer reinnervated by the supinator nerve is preferable)

Long-standing spinal cord injury with an upper motoneuron syndrome (a relative contraindication because, in such palsies, muscles are not actually denervated; however, there is no clinical evidence that nerve transfers can successfully reinnervate these chronically paralyzed muscles)

Injury that is more than 1 year old

Paralysis of the thumb and finger extensors is evident. Not evident is testing of the supinator muscle, because the biceps can produce supination either with the elbow flexed or extended. We have found it useful to flex the elbow and, while in full pronation, ask the patient to supinate against resistance created by the examiner. The examiner places one hand over the upper third of the proximal dorsal aspect of the patient′s forearm to palpate for supinator contraction. The hand is placed over the extensor digitorum communis, because this muscle is paralyzed; hence, muscle contractions in this position should stem from the supinator muscle.

Electromyograms of the extensor digitorum communis help to identify denervation and thereby determine the urgency of surgery, particularly with a spinal cord injury. In general, electromyograms of the supinator muscle are not easily obtained. However, in the presence of paralysis of the extensor digitorum communis, recording of electrical activity in the supinator muscle is more reliable. Unfortunately, muscle strength does not correlate with electromyographic findings.

It is very important to assess the strength of wrist extension. If wrist extension can be accomplished against resistance, the supinator muscle should be normal.

Magnetic resonance imaging of the brachial plexus is important to document the presence of any pseudomeningoceles in the lower roots of the brachial plexus. These indicate root avulsion, thereby excluding the possibility of spontaneous recovery.

The posterior interosseous nerve passes from the anterior aspect of the elbow to the dorsal side of the forearm, by running between the two layers of the supinator muscle.

The proximal tendinous margin of the supinator muscle is known as Frohse′s arcade. At this point or slightly proximal to it, the main branches to the supinator muscle arise. In general, there is a lateral branch to the superficial head and a medial branch to the deep head of the supinator muscle. For an approximate distance of 20 mm, both branches travel parallel to the posterior interosseous nerve ( Fig. 21.1a ).

Near the distal margin of the supinator muscle, the posterior interosseous nerve branch divides into (a) a short superficial division to the extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris; and (b) a deep division to the abductor pollicis longus, extensor pollicis brevis and longus, and extensor index proprius.

The number of myelinated fibers within the branches of the supinator muscle corresponds to 70% of that of the posterior interosseous nerve distal to the arcade of Frohse ( Fig. 21.1b ). Detailed biometric data are given in Table 21.1 .