Anatomical Basis

The brachial plexus (Fig. 12.1) is composed of the anterior primary rami of the last cervical nerves (C5, C6, C7, C8) and of the first thoracic nerve (T1):

• Anterior roots of C5 and C6 join to form the superior trunk.

• Anterior root of C7 forms the middle trunk.

• Anterior roots of C8 and T1 join to form the inferior trunk.

Each trunk splits into an anterior and posterior division:

• The anterior divisions join to form two anterior cords (lateral and medial) and their terminal branches: musculocutaneous, median and ulnar nerves.

• The posterior divisions join to form the posterior cord and its terminal branches: axillary and radial nerve.

Several nerves arise proximal, at the level of the roots or trunks. Their examination offers valuable information for the level of the injury:

The long thoracic nerve arises very proximally from the roots C5 and C6 and sometimes C7 and C8. Lesion of this nerve is a poor prognostic factor in brachial plexus injuries.

The suprascapular nerve arises from the superior trunk: a normal activity of the muscles innervated by this nerve in a traumatic palsy indicates that the injury is situated distal to the origin of the suprascapular nerve in the superior trunk.

The phrenic nerve arises from the C4 root and C5 participates by an anastomosing branch. Hemidiaphragm palsy should be diagnosed by clinical and radiological examination.

The presence of Claude Bernard Horner syndrome indicates a proximal injury involving the communicating ramus from T1 to the stellate ganglion.

Posterior roots are more resistant than the anterior roots as they grip with more rootlets to the spinal cord and are larger. Posterior rootlets are protected by the posterior spinal ganglion and their arrangement on the spinal cord provides higher resistance to traction.

The upper roots are attached to the cervical spine by ligaments between the transverse processes and the epineurium of the roots. The stronger attachment of the upper roots in comparison to the lower roots explains the higher frequency of avulsions of the lower roots. Anatomic variations of the brachial plexus have been described: H. J. Seddon defined the plexus as prefixed when the contribution of C4 is significant and that of the T1 is lacking. On the contrary, the plexus is postfixed when the contribution of T1 is large and that from C4 is lacking or minute. Variations in the length of the roots and in the formation of the trunks have been also described.

The arrangement of the plexus on two different planes, anterior and posterior and the absence of connections between these planes, is very important, especially for the surgeon approaching the injured plexus and performing the nerve grafts:

• The posterior plane is constant and simple and is dedicated to the extension of the upper extremity.

• The anterior plane is complex and variable and is dedicated to the flexion of the upper extremity.

Fig. 12.1. Distal to the ganglions (ggls), the anterior roots of C5, C6, C7, C8 and T1 join to form the superior, middle and inferior trunks. The trunks split into anterior and posterior divisions that join to form the secondary trunks (sec) or cords: the anterior divisions of the superior, middle and inferior trunks form the anterior lateral (A) and anterior medial cord (B) and their terminal branches: musculocutaneous (mc), median (M) and ulnar (U) nerve, whereas the posterior divisions of the trunks form the posterior cord (C) and its terminal branches: axillary (ax) and radial (R) nerve.

Schematically, the anterior part of the root at the level of the scalene muscles corresponds to the anterior plane and the posterior part of the root to the posterior plane. But the distribution of each root to these two planes is variable:

• C8-T1 may have an extended territory in the posterior plane.

• C7 has a variable distribution in the secondary trunks in the anterior plane.

The mean number of surgical fascicular groups and nerve fibers varies among the roots. C7 contains the largest number of surgical fascicular groups, 6 or 7, whereas C5 contains only 1 or 2 and T1 contains 2 to 3. Each muscle of the upper extremity is innervated by two or more roots.

These variations explain the difficulties in elucidating the topographic level of the lesion and the inconsistency between clinical and pathological findings.

As the same nerve injury may lead to different clinical manifestations, the importance of a complete surgical exploration of the brachial plexus is easily realized.

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