ClinicalNarrologV and Neurosurgery, 95 (Suppl.) (1993) S12-Sl6 0 1993 Elsevier Science Publishers B.V. All rights reserved 0303-8467193fiO6.00
anatomy of the brachial plexus P.M. Rozing
Department of Orthopaedic Surgery, University Hospital, Leiden, The Netherlands
Key words: Brachial plexus; Shoulder joint; MRI; Shoulder muscles Summary The anatomy of the shoulder region is discussed focusing on the course of the bra&al enables one to visualize the topographical anatomy of this area.
Introduction The purpose of this study is to discuss those aspects of the anatomy that are important for the understanding of the mechanism of the brachial plexus injuries. Magnetic resonance imaging (MRI) appears to be an excellent tool to visualize the topographic anatomy of the shoulder region. Spinal nerves, trunks and cords can be identified and distinguished from the surrounding structures [ 11. Topographical
The brachial plexus extends between the cervical spine and the axilla (Fig. 1). For a detailed and precise description of the local anatomy, the neck region has been divided into different areas. The sterno(cleido)mastoid muscle divides the neck into an anterior and posterior part (Fig. 2). The posterior part is a large triangular area bordered by the clavicle, the anterior rim of the trapezius muscle and the sternomastoid muscle, and is called regio colli lateralis. This posterior triangle is likewise subdivided by the omohyoid muscle into two unequally-sized triangles. The small triangle medial to the omohyoid muscle is called the omoclavicular triangle and contains the lower trunk and subclavian vessels. The large upper triangle is contains the upper and middle trunks with their divisions, and the three scalenus muscles. The scalenus anterior muscle arises from the cervical transverse processes of the C3-C6 vertebrae and inserts to the scalene tubercle of the first rib. The scalenus medius muscle
Correrpondence to: Prof. Dr. P.M. Razing, Department of Orthopaedic Surgery, Leiden University Hospital, Rijnsburgerweg lo,2333 AA Leiden, The Netherlands. Phone: (+31)-71263605; FAX: (+31)-71-152588.
plexus. Magnetic resonance imaging
lies dorsally to the scalenus anterior and extends from all the cervical transverse processes to the first rib behind the groove for the subclavian artery. The scalenus posterior muscle is often closely attached to the scalenus medius and runs from C.5 and C6 to the second rib. One does well to realize that the scalenus anterior muscle is attached to the anterior tubercles of the cervical transverse processes and the medius and posterior muscles to the posterior tubercles of the transverse processes. The cervical spinal nerves emerging from the intervertebral foramen and lying in the sulcus between the anterior and posterior tubercles are therefore sandwiched between the scalenus anterior and medius muscles. These two muscles are also separated by the subclavian artery at the level of the first rib. Occasionally, the nerves C5 and C6 traverse the scalenus anterior. The two inferior nerves G? and Tl are present in the omoclavicular triangle and in the retroclavicular space, inferiorly bounded by the pleura. They cross the first rib posteriorly to the subclavian artery between the scalenus anterior and medius. The subclavian vein runs at the medial margin of the scalenus anterior muscle in a triangular area bordered by the medial margin of the scalenus muscle, the first rib and the sternomastoid muscle. In the retroclavicular area, the funiculi or cords are formed and woven around the axillary artery before they enter the axillary space. This space is bounded posteriorly by the subscapularis, teres major and latissimus dorsi muscles and anteriorly by the pectoralis major and minor and the subclavius muscles. The terminal branches of the plexus originate below the level of the pectoralis minor. Behind the posterior wall of the axilla, the long head of the triceps descends from the scapula forming two triangular areas and one quadrangular area. One triangular area is bordered by the teres major and the humerus through which the radial nerve with accompanying vessels reaches the back of
lat. N. thoracicus
N. cut. antebrachii N. medianus
Fig. 1. Schematic drawing of the bra&al plexus (from the Leiden BP Working Group).
the humerus. The quadrangular area, more generally known as the quadrilateral tunnel, and its superior wall are formed by the subscapular muscle, the glenohumeral joint capsule and the teres minor muscle. Its lateral wall is constituted by
the coracobrachialis muscle and the humeral neck, its medial wall by the inferior glenoid rim and the long head of the triceps muscle, and its inferior wall by the teres major muscle. The axillary nerve accompanied by the posterior circumflex vessels runs as a neurovascular bundle through this tunnel.
Fig. 2. Artist’s view of the supraclavicular bra&al plexus in a human specimen. The stemocleidomastoid muscle has been transected and reflected. The external jugular vein, and omohyoid and anterior scale& muscles have been removed.
of the plexus
The spinal nerves CS, C5 and C7 of the supraclavicular part of the plexus are fixed to the vertebral column by small fibrous attachments of the epineurium to the transverse processes and in addition to the cervical fascia which extends as a fibrous sheath around the neurovascular bundle. Those reinforcements, in combination with the dural sleeve, protect the roots from avulsion [3,4]. C8 and Tl do not have these additional attachments and are more prone to avulsion by traction. On its course down, the plexus is stabilized by the different layers of the cervical fascia. At the level of the clavicle the plexus is relatively fixed to the surrounding structures because it lies, together with the bra&al artery and vein, embedded in a fascial sheath. More distally, the plexus is fixed by terminal branches, such as the musculocutaneous nerve which is tightly attached to the coracobrachialis muscle, the axillary nerve to the quadrilateral tunnel, the median nerve to the pronator teres and the ulnar nerve to the cubital sulcus.
Fig. 4. Coronal oblique view. Arrow indicates the suptascapular
nerve and accompanying
Fig. 3. Sagittal oblique view. Arrow points to the quadrilateral tunnel. vessels.
s15 Mobility of neck and shoulder
The wide range of motion of the cervical spine and shoulder joint makes high demands on the mobility of the brachial plexus. If the limits of elongation of the plexus are surpassed, a so-called brachial plexus traction injury will result. The violence and the mechanism of the trauma determine the location and extent of the lesion. The cervical spine is the most mobile part of the vertebral column, allowing flexion-extension, lateral inclination and axial rotation. The C&C7 area permits 40 degrees of flexion and 25 degrees of extension, a total of 98 degrees of lateral bending and 90 degrees of rotation. Lateral bending is coupled with axial rotation due to the oblique angles of the facet joints [5,6]. A full range of motion of the shoulder requires well functioning sternoclavicular, glenohumeral and acromioclavicular joints and a proper scapulothoracic gliding plane. Maximal elevation in the scapulohumeral joint can only be achieved when the scapula rotates with regard to the scapulothoracic wall and at the same time the humerus exorotates. These are coupled movements and when, during abduction in the frontal plane, the exorotation of the humerus is prevented, the shoulder is blocked in 90-120 degrees of abduction . This is caused by the capsular reinforcements or ligaments of the glenohumeral joint capsule and in particular the inferior glenohumeral ligament. In this blocked position (abduction with endorotation) the neurovascular bundle is pulled in a ventro-caudal position to the humeral head by the overlying coracobrachialis muscle and the short head of the biceps and runs over the protruding humeral head. During exorotation of the arm the neurovascular bundle glides to occupy a ventral position relative to the protruding humeral head. In the ventro-caudal position, the nerves are prone to injury when the humeral head dislocates anteriorly.
The spatial relations of the brachial plexus in the shoulder region can be studied and visualized with different imaging techniques such as computed tomography (CT) and MRI [l&10]. The latter allows high-contrast, multiplanar imaging with high spatial resolution. In the shoulder, a surface coil with specific design can be used. It is important to select the following appropriate imaging planes to obtain optimal information while keeping the imaging time within acceptable limits. The axialplane is optimal for displaying the biceps tendon, the glenohumeral joint and the glenoid labrum. The neurovascular bundle can be visualized in detail as well. The
spinal nerves are easily identified
at their emergence
the intervertebral foramen. The coronal plane is useful in visualization of the supraclavicular part of the brachial plexus. In the shoulder region it is well-suited for imaging of the acromioclavicular joint and its relation to the supraspinatus muscle. The sugdtulplane offers the best view to depict the relation between the brachial plexus (as it traverses the axillary fat body) and the surrounding vessels and muscles. A disadvantage of the sagittal and coronal planes is the variability of these planes to the recognizable anatomical landmarks of the shoulder. Therefore, the interpretation of these pictures for evaluation of pathologic conditions in this region may be very difficult. This is due to the position of the scapula with regard to the frontal plane and the variation in this position in different patients. For this reason two oblique planes (both parallel to and longitudinally perpendicular to the glenoid cavity) are used if visualization of specific shoulder structures is wanted, the so-called coronal oblique and the sagittd oblique images [ll]. These views can be interpreted more easily and the terminal plexus branches can be followed in relation to the surrounding structures. In the sagittal oblique view the quadrilateral tunnel with the axillary nerve and accompanying vessels can be seen (Fig. 3). The corona2 oblique view is useful in the evaluation of the suprascapular nerve as it enters through the suprascapular notch (Fig. 4). In the upper arm, the terminal branches like the ulnar, radial and median nerves can be clearly seen. In conclusion, MRI is a useful tool to study the spatial relations in the shoulder region. Its value as a diagnostic procedure in traumatic brachial plexus injuries is subject to ongoing research.
The author is grateful to Mr. H. Wetselaar, medical artist, for preparing Figure 2.
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