12 Locomotor system

ANATOMY

OVERVIEW OF CONTENT

The anatomy will be described on a mainly regional basis, dealing with the vertebral column, girdles and limbs. Transitional zones between these and other major anatomical regions will be included. For each region the organisational pattern will follow: osteology, arthrology, transitional zones, clinical points.

VERTEBRAL COLUMN

The vertebral column is made up of 32–34 vertebra:

• 7 cervical vertebrae;

• 12 thoracic vertebrae;

• 5 lumbar vertebrae;

• sacrum (5 fused vertebrae); and

• coccyx (3–5 fused vertebrae).

The column may be considered as a longitudinal series of bones, linkages and ‘holes’. The bones are the individual vertebrae, the linkages are the joints (including the intervertebral discs), ligaments, muscles and fasciae, and the ‘holes’ are the foramina, vertebral and intervertebral. The serially linked vertebral foramina constitute the vertebral canal. The intervertebral discs constitute about a quarter of the length of the adult column.

The vertebral column as a whole is:

• a protective segmented duct for neural and vascular tissue;

• a ‘mast’ guyed and supported by muscles, ligaments and fasciae;

• a weight-bearing supportive strut; and

• behaves mechanically as a cantilever or as an arch.

Viewed from its lateral aspect the column has a series of curves:

• the primary curves, thoracic and pelvic, are present throughout development, and are both convex posteriorly (kyphoses). The curvature is due chiefly to the shape of the component vertebrae.

• the secondary curves, cervical and lumbar, are lordoses (convex anteriorly), and appear as the column starts to bear the weight of the head (cervical) and of the trunk and limbs (lumbar). In these regions the intervertebral discs contribute to the curve, being thicker anteriorly than posteriorly.

The presence of these curves increases the ability of the column to withstand axial compression. The line of weight in the standing position intersects the curves at the cervicothoracic, thoracolumbar and lumbosacral junctions, then passes through the centre of gravity of the body at about the level of the second sacral vertebra. The varying curvature of the vertebral column determines the changing cross-sectional profile of the trunk. At a lordosis, as in the lumbar region, the anteroposterior diameter of the body cavity decreases markedly.

From the functional standpoint, the occipitoatlantoaxial region may be considered separately from the remainder of the mobile vertebral column (C3–L5). The majority of cervical rotation occurs at atlantoaxial level, while nodding movements and fine positioning (tilting) of the head on the neck occur mainly between atlas and occiput. In the rest of the mobile column, the final range of movement results from the summation of small movements at individual intervertebral levels (‘motion segments’). The regions with the most movement, and hence the least stability, are the junctions of the more ‘fixed’ levels (thoracic and sacral) with the more mobile levels (cervical and lumbar). The commonest forms of the unstable spinal injury occur at these junctional regions. At mid- and lower cervical levels and in the lumbar region the configuration of the facet joints, the arrangement of the muscles and the topography of the neighbouring soft tissues allow (forward) flexion, extension and lateral flexion (side-bending) movements which are largely prevented at thoracic level by the presence of the ribcage. Axial rotation (about the vertical axis of thecolumn) occurs maximally at thoracic level.

Osteology

Basic vertebral pattern

The basic vertebral pattern consists of a body and a neural arch surrounding a vertebral canal (Fig. 12.1).

Fig. 12.1 The parts of a vertebra. superior view. lateral view.

Source: Rogers A W, Textbook of anatomy; Churchill Livingstone, Edinburgh (1992).

Source: Rogers op. cit.

The atlas (C1):

• has no body;

• bears a kidney-shaped superior articular facet on a thick lateral mass which articulates with the occipital condyles of the skull; and

• posterior to this facet the upper part of the posterior arch is grooved by the vertebral artery.

The axis (C2):

• bears the dens (odontoid process) on the superior aspect of its body;

• nodding (agreement) and lateral flexion occur at the atlantooccipital joint; and

• rotation of the skull (disagreement) occurs at the atlantoaxial joint.

The vertebra prominens (C7) is so-called because it is the first clearly palpable spine of the vertebral column (T1 below it is in fact the most prominent one). Occasionally the foramen transversarium is absent on C7 (when present it does not transmit the vertebral artery).

Thoracic (Fig. 12.3)

These are characterized by demifacets on the sides of the body of the vertebra for articulation with the heads of the ribs. They are characterized also by facets on their transverse processes for the rib tubercles apart from the lower two thoracic vertebrae. They are also characterized by long and downward sloping spines and their articular facets are relatively vertical.

Fig. 12.3 A typical thoracic vertebra: thoracic 7. superior view. lateral view.

Source: Rogers op. cit.

Lumbar (Fig. 12.4)

Lumbar vertebrae are large with strong, square horizontal spines. The articular facets lie in the sagittal plane. L5 has a massive transverse process which connects with the whole of the lateral aspect of its pedicle and encroaches on the body. The transverse processes of the lumbar vertebrae attach solely to the junction of the pedicle with the lamina.

Fig. 12.4 A typical lumbar vertebra, L4. superior view. lateral view.

Sacral and coccygeal

There are usually five fused vertebrae in the sacrum, but segmentation anomalies are not unusual, giving four or six sacral vertebrae. The coccyx typically consists of four fused vertebrae, but the demarcation may be difficult to see. The fused lateral masses of the upper sacral vertebrae are massive, those of S1 forming the alae. The vertebral foramina of the fused vertebrae form the sacral canal, whose inferior opening is the sacral hiatus, flanked by the cornua.

Arthrology

The mobile vertebral column can be considered functionally as a series of linked pairs of vertebrae. Each pair of vertebrae and the linkage between its members constitute a motion segment. These linkages include joints, ligaments and muscles. Although movement between adjacent vertebra is slight, the additive affect is considerable. Movement particularly occurs at the cervico-thoracic and thoraco-lumbar junctions and hence these are the two common sites of vertebral injury. A typical intervertebral joint is shown in Fig. 12.5.

Fig. 12.5 The joints between adjacent vertebrae: lumbar 3 and 4. The left half of the L2–L3 disc has been cut away to show the plate of hyaline cartilage at the upper surface of L3. This, also, has been cut away to the left of the midline, exposing the upper surface of L3. superior view. lateral view.

Source: Rogers op. cit.

The vertebral laminae are linked by the ligamentum flavum of elastic tissue. The vertebral spines are linked by the tough supraspinous and weak interspinous ligaments. The articular facets are linked by articular ligaments around synovial joints.

The tough anterior and posterior longitudinal ligaments run the whole length of the vertebral bodies along anterior and posterior aspects respectively.

The vertebral bodies are joined by strong intervertebral discs. Each intervertebral disc consists of:

• a peripheral annulus fibrosus, which is adherent to the thin cartilaginous plate on the vertebral body above and below; and

• nucleus pulposus, which is gelatinous fluid surrounded by the annulus fibrosus.

The intervertebral discs constitute approximately one-quarter of the length of the spine, as well as accounting for its secondary curvatures. In old age the intervertebral discs atrophy resulting in shrinking and return of the curvature of the spine to the C-shape of the new born.

The joints between atlas, axis and the skull are shown in Fig. 12.6.

Fig. 12.6 The ligaments joining the axis, atlas and skull. midline sagittal section. posterior surface. The posterior arch of the atlas and the vertebral arch of the axis have been removed.

Movements and muscles

Large movements involve the bigger muscles, more distant from the vertebral column; finer movements involve muscles acting on few segments. Gravity plays a very important rôle in spinal movements: apparent antagonists to the movement often contract eccentrically as antigravity muscles, e.g. the spinal extensors ‘pay out rope’ to control trunk flexion from the standing position.

• Flexion (‘bending forward’): bilateral action of all muscles running anterior to the spinal segment(s) involved, e.g. longus colli, sternocleidomastoid, psoas, rectus abdominis.

• Extension (‘bending backward’): bilateral action of all muscles running posterior to the involved segments, e.g. semispinalis capitis, splenius, trapezius, erector spinae, multifidus.

• Lateral flexion (‘bending sideways’): balanced unilateral action of the long flexors and extensors; oblique abdominal muscles on the side of flexion.

• Rotation: contralateral sternocleidomastoid; ipsilateral splenius; oblique abdominals; rotatores.

Major anatomical relations

These are best appreciated by the examination of axial (transverse) sections of the body at serial levels. The list below is not comprehensive, and excludes many muscles.

• Posterior and posterolateral: spinal dural sheath (theca) in the vertebral canal; spinal nerves (loosely called the ‘nerve roots’) and their accompanying vessels in the intervertebral foramina; internal vertebral venous plexus.

• Anterior and lateral: pharynx; oesophagus; thoracic duct; major longitudinal vessels at all levels (e.g. vertebral, descending aorta, azygos; IVC); crura of diaphragm, psoas muscles; sympathetic trunk.

Transitional zones

With the increasing use of anterior approaches to the vertebral column at all levels, the anterior relations of the column are becoming more surgically relevant. The interface between the musculoskeletal axial structures and the viscera anterior to them is a good example of an anatomical ‘transitional zone’. Such areas tend to be neglected, both by anatomists and by clinicians.

In the neck, the interface is demarcated by the prevertebral fascia: the visceral ‘column’ of the neck moves actively and passively on the musculoskeletal ‘column’ in the plane of this fascia. The fascial anatomy of the neck is of great surgical importance, and is best seen in transverse (axial) sections (Fig. 12.7). The prevertebral fascia of the neck extends inferiorly as the endothoracic fascia and subsequently the retroperitoneal fasciaof the abdomen.

Fig. 12.7 Transverse section of the neck at the level of vertebra C7.

Source: Rogers op. cit.

Neurovascular patterns and relationships: the ‘holes’ and what they contain

The ‘holes’ are:

• the serial vertebral foramina (together constituting the vertebral canal);

• the intervertebral foramina; and

• in the cervical spine only, the foramina transversaria.

The vertebral canal contains the spinal cord and cauda equina in their meningeal coverings, an incomplete epidural layer of fat containing the internal venous plexus, and some tenuous connective tissue connecting the dural tube or theca with the posterior longitudinal ligament. The most lateral parts of the canal contain the roots of the spinal nerves in their dural sheaths, within which each pair of dorsal and ventral roots joins, immediately lateral to the dorsal root ganglion. The epidural ‘space’ is only ‘potential’ until defined by air or liquid let into it as a result of pathology or medical intervention (e.g. epidural injection, epidural anaesthesia, laminectomy). The cross-sectional size and shape of the vertebral canal varies with vertebral level, as does the proportion of its cross-sectional area occupied by the spinal cord and its coverings. The normal canal is large in the upper cervical and lower lumbar regions, and smallest in the midthoracic region.

Each typical intervertebral foramen (Fig. 12.5) is bounded superiorly and inferiorly by the pedicles, anteriorly mainly by the intervertebral disc and posteriorly mainly by the facet joint. The foramen contains the foraminal segment of the exiting ‘nerve root’ (spinal nerve), together with the radicular vessels and finerecurrent meningeal (sinuvertebral) nerves. Owing to the varying obliquity and length of the nerve roots, which run increasingly distally as the spine is descended, the dorsal root ganglia may lie in the lateral zone of the vertebral canal or in the intervertebral foramen.

The foramina transversaria in the lateral masses of the cervical vertebrae contain the vertebral vessels and a plexus of sympathetic nerves. The foramina of the seventh vertebra do not contain vertebral arteries.

The arterial supply of the cord is longitudinal via anterior and posterior spinal arteries lying on its surface. These arteries branch from the vertebral arteries, with segmental reinforcement at many levels via radicular arteries entering through the intervertebral foramina. The most important and largest of these radicular arteries, the arteria radicularis magna, usually occurs in the thoracolumbar junctional region on the left and may itself be responsible for the supply of a large part of the lower cord. The veins of the cord also run longitudinally on its surface, and communicate with each other and with the cerebellar veins and cranial venous sinuses.

The arterial supply of the vertebrae themselves is segmental. Paired arteries arise from the deep arteries of the posterior neck, from the aorta and from the internal iliac arteries. The radicular arteries arise from these segmental arteries to the vertebrae. The venous drainage from the vertebrae is mainly via large basivertebral veins into venous plexuses. In addition to the internal venous plexus described above, there is an external plexus, best developed in the cervical region. These valveless plexuses communicate freely with each other and with the caval systems, and constitute important routes for the spread of infection and malignancy.

The main clinically relevant variations in the neurovascular patterns concern the effect of vertebral segmentation anomalies on the number and position of nerve roots (spinal nerves) exiting the vertebral column. Such segmentation anomalies are not uncommon, and chiefly affect the lumbosacral region. The lumbar spine may be shortened to four vertebrae (‘sacralised L5’), or lengthened to six (‘lumbarised S1’), with accompanying rearrangement of the nerve roots leading to possible confusion in the clinical assessment of neurological signs in the lower limbs.

In spinal injuries, the spinal cord, the nerve roots and the spinal nerves may be involved separately or together. The site of injury may be within the vertebral canal or in the intervertebral foramen and ‘root canal’. Both cord and roots may be involved at any level above L1, where the cord stops in an adult. Below L1 only the roots of the cauda equina or the spinal nerves can be affected. The motor signs in spinal injury may be those of an upper motor neuron lesion, a lower motor neuron lesion, or a mixed picture may be seen. The whole spectrum of severity of nerve injury is possible, including the effects of acute or chronic nerve compression. The injury may be open (rarely) or closed, and the cause may be spinal fracture-dislocation, disc prolapse, or bony compression (spinal stenosis, which can occur at one or several spinal levels and which may affect the whole vertebral canal or only its lateral recesses). Clinical examination should include testing for both motor and sensory signs of root lesions: familiarity with dermatomes andmuscle ‘root values’ is essential (Chapter 8).

Clinical points

The posterior part of the annulus fibrosus is relatively thin and prone to rupture due to degeneration or injury. The nucleus pulposus protrudes posteriorly into the vertebral canal or intervertebral foramen, i.e. ‘slipped disc’. The commonest sites for ‘slipped disc’ are L4/5, L5/S1 or in the neck C5/6 or C6/7. A prolapsed L4/5 disc produces pressure on the root of L5 and that of L5/S1 on S1 root. Pain is referred to the back of the leg and the foot along the distribution of the sciatic nerves (sciatica). There is weakness of dorsiflexion of the ankle. With L5 lesions there will be numbness over the lower and lateral part of the leg and medial side of the foot. With S1 lesions there will be numbness over the lateral side of the foot and the ankle jerk may be diminished or absent. Direct posterior prolapse of the disc may compress the cauda equina.

PECTORAL GIRDLE

Though the term ‘girdle’ implies a complete, circumferential structure, the human pectoral girdle is usually described as consisting of two bones rather than four. Each separate ‘hemi-girdle’ comprises the scapula posteriorly and the clavicle anteriorly, and articulates with the axial skeleton by a single synovial joint, the sternoclavicular joint. Unlike the pelvic girdle, the pectoral girdle articulates neither with the vertebral column nor with its own contralateral fellow. Each pectoral girdle can, therefore, move independently of the other and of the vertebral column, thus giving the prehensile upper limb a versatile fixation platform. The clavicle also functions as a prop, maintaining the necessary relative separation of the limb from the trunk.

Osteology

Clavicle

This is one of the most frequently fractured bones, and has important muscle and ligament attachments and soft-tissue relations. It is subcutaneous throughout its length, but the overlying skin is very mobile, and open fracture is uncommon. The lateral half of the bone is concave forwards and flattened, with prominent anterior and posterior borders, while the medial half is convex forwards and almost cylindrical. The inferior surface is roughened laterally for the attachment of the coracoclavicular ligament and medially for that of the costoclavicular ligament (Fig. 12.8). These ligaments are very strong: the bone always fractures between their attachments, in the segment on the inferior surface of which the subclavius muscle is attached. Deltoid and pectoralis major are attached to the anterior surface, while trapezius attaches posterolaterally. The clavicular head of sternocleidomastoid attaches superomedially. The most important soft-tissue relations lie posteriorly, particularly at the medial end where the subclavian and brachiocephalic vessels are near. The divisions of the brachial plexus lie behind the medial two-thirds of the bone, separated from it by anoften surprisingly large suprascapular vein. The clavicle is crossed anteriorly and subcutaneously by the palpable supraclavicular nerves from the cervical plexus.

Fig. 12.8 Ligaments of the clavicle (the joint capsules are not shown).

The clavicle ossifies mainly in membrane, and is the first bone in the body to ossify. It is the only long bone to ossify in membrane.

Scapula (Fig. 12.9)

The scapula lies posterolaterally over the second to seventh ribs. It consists of a flat, thin triangular body whose lateral border is thickened for force transmission and to act as a lever for the action of the attached teres muscles. A notch at the lateral end of the superior border transmits the suprascapular nerve, which may become entrapped here, and demarcates the bases of the coracoid process and ‘neck’ of the scapula from the remainder of the bone. The coracoid process projects mainly anteriorly in the anatomical position, and gives attachment to three muscles (coracobrachialis, short head of biceps and pectoralis minor) and three ligaments (coracoclavicular, coracoacromial and coracohumeral). The thick, cylindrical neck, to which the suprascapular nerve and vessels are very closely related posteriorly, bears the shallow articular surface for the humerus, the glenoid cavity. A tubercle immediately above the glenoid gives attachment to the long head of biceps, while the long head of triceps attaches in the corresponding position below. The spine of the scapula projects from its posterior surface, and ends laterally in the flattened acromion process. The spine is a lever for muscle attachment, with trapezius superiorly and deltoid inferiorly. The posterior surface of the body gives origin to the supraspinatus and infraspinatus muscles, both of which are tightly bound down by thick fascia. Subscapularis fills the concavity of the body anteriorly, ‘padding’ the bone against the chest wall and its covering muscles. Serratus anterior, ‘slinging’ the scapula from the upper eight ribs, attaches to the inner aspect of the medial border of the scapula. The edge of the medial border gives attachment to the rhomboids and levator scapulae, which together with trapezius connect the pectoral girdle, and thus the upper limb, with the axial skeleton. Intra-articular fractures involving the glenoid may require internal fixation, but fractures of the body of the scapula displace little and unite well, owing to the close coverage, firm fascial binding to bone, and excellent blood supply of the overlying muscles.

Fig. 12.9 The scapula. posterior view. anterior view.

Source: Rogers op. cit.

Arthrology

The ‘shoulder joint’ is not the ‘shoulder’ joint. When a patient is asked to move the shoulder, or complains of pain in the shoulder, they are not concerned only with the glenohumeral joint. The shoulder complex – the mechanism by which the upper limb is positioned and fixed in space – includes all the joints of the pectoral girdle as well as the important ligaments and muscles which link the limb to the trunk. The pectoral girdle as a whole is very mobile, movements of the ‘shoulder complex’ conferring wide versatility of function on the upper limb. The limb skeleton is linked to the trunk by a chain of synovial joints: this comprises the glenohumeral joint (see p. 355), the acromioclavicular joint and the sternoclavicular joint. The pectoral girdle is ‘slung from’ the trunk and the vertebral axis by the numerous muscles attaching to scapula and clavicle and mentioned above, while the limb is in turn connected to the girdle by a further series of muscles attaching mainly to the scapula but including pectoralis major and deltoid, attached also to the clavicle. Both pectoralis major and a further large, ‘migrant’ limb muscle, latissimus dorsi, attach the limb directly to the trunk. Note that both these muscles can act as accessory muscles of respiration if their attachment to the limb is fixed.

Neither the sternoclavicular nor the acromioclavicular joint has inherent bony stability, though both contain fibrous intra-articular discs which aid congruity. Both these synovial joints rely on their capsules and on strong, closely adjacent ligaments to provide the stability necessary for effective action of the limb (Fig. 12.8).

In the sternoclavicular joint:

• the capsule is strong, with anterior and posterior thickenings;

• the fibrous disc, passing obliquely across the joint from the clavicle superiorly to the sternum inferiorly, acts as an intra-articular ligament; and

• the accessory ligaments are the very strong costoclavicular (rhomboid) ligament, passing to the first rib and its cartilage, and the weaker interclavicular ligament passing between the medial ends of the clavicles.

In the acromioclavicular joint:

• the capsule is weaker, though it is strengthened superiorly by the acromioclavicular ligament;

• the strongest ligament is the bipartite coracoclavicular; and

• the coracoacromial ligament, connecting the tips of the bony processes, lies at some distance from the joint. It may be considered ‘accessory’ both to the acromioclavicular and to the glenohumeral joints.

Transitional zones

Area enclosed by the pectoral girdle(s) (Fig. 12.7)

The thoracic spine, the axioscapular muscles (rhomboids and trapezii), the scapulae, clavicles and manubrium sternae form a ‘ring’ which may be considered as the complete pectoral ‘girdle’. This outer ring encompasses an inner ring made by the first ribs and their cartilages, the thoracic inlet. Thus in addition to including the major anatomical transitional zone between the neck and the thorax, this outer ring also encloses structures on their way between the root of the neck and the upper limbs (brachial plexus, subclavian vessels). There are also several surgically important named fasciae related and attached to the girdle(s) and to the boundaries of these inner and outer rings.

• The apices of the lungs lie beneath the suprapleural membranes (Sibson’s fascia) which attach around the medial borders of the first ribs.

• The clavipectoral fascia runs inferiorly from the clavicle, from which it suspends the concave fascial ‘floor’ of the axilla (axillary fascia), splitting to enclose both subclavius and pectoralis minor muscles and forming part of the anterior axillary ‘wall’ during its course.

• The investing layer of the deep cervical fascia (Fig. 12.7) attaches to the clavicle, as does the fascial sling from the intermediate tendon of the omohyoid muscle.

• The prevertebral fascia (Fig. 12.7) overlies the roots of the brachial plexus then runs posterolaterally between inner and outer rings and becomes the axillary sheath around the axillary vessels. Both the prevertebral and the pretracheal fasciae of the neck pass inferiorly through the inlet into the superior mediastinum.

The patterns and attachments of these fasciae, as well as acting as surgical guides and landmarks, may determine the direction and extent of spread of infection and haemorrhage. Note also that the anatomy of the inner ring – the thoracic inlet – may be compromised in the presence of a complete or partial cervical rib or band.

Between girdle and limb – the axilla

The axilla is a mobile conduit, protected when the upper limb is at rest but particularly vulnerable when the limb is abducted. The range and versatility of movement of the shoulder complex increases the vulnerability of the axillary structures. The shape of the axilla is maintained by bones, muscles and fasciae. It has walls, borders (the lower edges of the anterior and posterior walls), a ‘floor’, and a truncated apex bounded by the clavicle anteriorly, the first rib medially and the superior border of the scapula posteriorly. The four walls of the axilla are:

• anterior: consisting of the lateral part of pectoralis major, pectoralis minor, subclavius and the clavipectoral fascia;

• medial: formed by the upper chest wall and the overlying serratus anterior. It is crossed by the descending long thoracic nerve, and the intercostobrachial nerve emerges from it;

• lateral: the bicipital groove, between the humeral attachments of the anterior and posterior wall muscles. Here the axillary nerve and circumflex humeral vessels are close;

• posterior: includes subscapularis, the distal portions of teres major and latissimus dorsi and the proximal end of the long head of triceps. Between these muscles lie three named openings: the axillary nerve and posterior circumflex humeral vessels pass through the quadrangular space, the circumflex scapular vessels through the triangular space, and the radial nerve and profunda brachii vessels through the triangular interval.

Fasciae involved in the axilla include the clavipectoral fascia and axillary fascia and the axillary sheath. The main transitional structures to consider are the brachial plexus and the axillary vessels. It is tempting to imagine the brachial plexus as a flat, two-dimensional ‘railway junction’ structure, as usually illustrated in diagrams. Before it is dissected, however, the infraclavicular part of the plexus is closely grouped around and parallel with the axillary artery. The relationship of the cords of the plexus to the second part of the artery gives them their names. The distinction between supraclavicular and infraclavicular parts of the plexus is infrequently made in anatomical texts, but is very useful in the clinical approach to disorders and injuries of the plexus. The roots of the plexus emerge into the neck between the scalene muscles, the trunks lie in the posterior triangle, the divisions behind the clavicle, and the cords group around the axillary artery to reach their definitive lateral, medial and posterior positions behind pectoralis minor.

In addition to the plexus and vessels, the axilla contains fat, lymph nodes (see p. 482), the tail of the breast, the proximal parts of coracobrachialis and biceps brachii, and the intercostobrachial nerve. The key structures in the surgical anatomy of the axilla are the axillary vein (more vulnerable than the artery, and more closely related to the lymph nodes), pectoralis minor, the infraclavicular brachial plexus and the various fasciae.

Roots (5)

• anterior primary rami of C5, 6, 7, 8, T1.

Trunks (3)

• upper (C5, 6);

• middle (C7); and

• lower (C8, T1).

Divisions (6)

• Each trunk divides into anterior and posterior divisions.

Cords (3)

• lateral: fused anterior divisions of upper and middle trunks;

• medial: anterior division of lower trunk; and

• posterior: fusion of all three posterior divisions.

Branches of the brachial plexus

Roots

• nerve to rhomboids;

• nerves to scaleni

• nerve to serratus anterior (long thoracic nerve of Bell, C5, 6, 7).

Trunks

• suprascapular: upper trunk; supplies supraspinatus and infraspinatus;

• nerve to subclavius.

Cords

Lateral

• musculocutaneous;

• lateral pectoral; and

• lateral root of median.

Medial

• medial pectoral nerve;

• medial cutaneous nerve of the arm and forearm;

• ulnar nerve; and

• medial root of median.

Posterior

• thoracodorsal nerve (to latissimus dorsi);

• subscapular nerve;

• axillary nerve; and

• radial nerve.

The brachial plexus can be pre-fixed or post-fixed on rare occasions. A pre-fixed plexus has a contribution from C4; a post-fixed from T2.

Note that the plexus also has an autonomic component. This is entirely sympathetic. All cervical ventral rami receive grey rami communicantes from the cervical sympathetic ganglia. The first and second thoracic ventral rami send white rami communicantes to the sympathetic chain; these white rami contain the sympathetic outflow to the head and neck. Thus sympathetic autonomic deficit is an important component of brachial plexus injuries at all levels.

Clinical points

Brachial plexus injuries are not uncommon. The plexus and its branches may be injured supraclavicularly or infraclavicularly; it is rarely involved in clavicular fractures. The supraclavicular plexus is usually affected by closed traction injury, whereas the infraclavicular part may be stretched over the displaced humeral head in antero-inferior shoulder dislocation. Open injury of the plexus may occur at any level. The site of injury may be localised clinically, using knowledge of the points of branching. For example, sparing of serratus anterior would indicate a lesion distal to the roots, while if the spinati muscles were also spared the lesion could be placed more distally. The extent of the injury can also be assessed by careful clinical examination: complete paralysis of both heads of pectoralis major would indicate damage to all roots. Traction lesions of the upper roots (C5,6) produce the well-known Erb’s palsy (paralysis) with motor signs throughout the limb proximal to the wrist.

• Erb’s palsy. The muscles primarily supplied by C5 and C6 are affected. The shoulder is adducted because of paralysis of deltoid and supraspinatus, and medially rotated because of that of infraspinatus, teres minor and the posterior fibres of deltoid. Paralysis of the elbow flexors biceps, brachialis and brachioradialis leaves the elbow fully extended, and that of biceps and supinator leave the forearm pronated.

Lesions of the lower roots (C8,T1) produce the less common Klumpke’s palsy.

• Klumpke’s palsy. This mainly affects the small muscles of the hand, particularly the intrinsics (interossei and lumbricals) which are almost entirely supplied by T1. The wrist flexors may also be weakened. Thus the hand is flattened, extended at the metacarpophalangeal (MCP) joints because the active long extensors are unopposed by the intrinsics, and possibly extended at the wrist. The interphalangeal (IP) joints are in flexion, as the long finger flexors remain active. Damage to T1 may also cause a Horner’s syndrome.

In both types of injury it is important not to forget the sensory component: there will be numbness in the dermatomal distribution of the affected roots. Compression lesions of the lower plexus may occur in the ‘thoracic inlet syndrome’ in the presence of a normal plexus and an incompletely ossifed cervical rib. Associated vascular compression or stenosis may predispose to thrombosis at the subclavian-axillary transition. Complete cervical ribs may be associated with a pre-fixed plexus, and thus cause no neurological problem.

Another site of possible nerve entrapment in the pectoral girdle involves the suprascapular nerve, as it passes beneath the transverse scapular ligament into the suprascapular notch. A lesion here may present with weakness and wasting of both supraspinatus and infraspinatus muscles. Non-traumatic lesions of the brachial plexus include tumours of the plexus itself such as neurofibromata, which may present as a supraclavicular mass or as more distal neurological impairment in the limb, as well as malignant lesions which may involve the plexus by local spread. An example of the latter is the Pancoast syndrome, in which anapical carcinoma of the lung involves the lower root(s) of the plexus and produces a neural deficit similar to that described in Klumpke’s palsy. Remember that autonomic (sympathetic) deficit may also be present and clinically detectable in the assessment of brachial plexus lesions.

UPPER LIMB

Osteology

Humerus (Fig. 12.11)

The proximal expanded end of the humerus has three major components: the head and the two tuberosities or tubercles. The head, covered with articular cartilage, lies immediately above the anatomical neck. The greater tuberosity, less obviously the larger of the two when viewed from the front, lies lateral and posterior to the head. The lesser tuberosity lies anteriorly, and is separated from the greater by the bicipital (intertubercular) groove in which runs the tendon of the long head of biceps. The tendon lies in a synovial sheath which is continuous with the synovium of the glenohumeral joint. The proximal metaphysis of the humerus, below the head and tuberosities, is commonly fractured and is known as the ‘surgical neck’ of the bone. The tuberosities are processes for the attachment of the muscles of the rotator cuff, a very important group of four short muscles whose function as stabilisers of the shoulder joint overrides their function as motors of the limb. All four pass across the joint from scapula to humerus; only one lies anteriorly, and is thus a medial rotator of the humerus. This is subscapularis, attaching to the lesser tuberosity and reinforcing the anterior aspect of the joint. The other three attach to the greater tuberosity: supraspinatus, an abductor, lies above the joint and reinforces the superior capsule, while infraspinatus and teres minor lie posteriorly, are external rotators, and reinforce the posterior part of the capsule. Three other muscles pass anterior to the vertical axis of the shoulder and are thus medial rotators; all attach to the anterior aspect of the upper shaft in the region of the bicipital groove. Pectoralis major crosses the groove to its lateral lip, latissimus dorsi attaches in the floor of the groove, and teres major to its medial lip. The lineof attachment of the joint capsule excludes the tuberosities but includes the medial metaphysis, crossing the growth plate and making its medial end intracapsular. The axillary nerve lies just medial to this point, just below the capsule. The circumflex humeral arteries lie in close circumferential relationship with the bone at the surgical neck. The first important feature of the shaft of the humerus is the spiral groove, in which the radial nerve and the profunda brachii vessels run between the lateral and medial heads of triceps and in direct contact with bone. The other main feature is the tubercle for the attachment of the deltoid muscle, almost half way down the lateral border of the bone. The triangular shape of deltoid gives it its name: its broad base lies proximally on the pectoral girdle, so that it has fibres running anteriorly, superiorly and posteriorly to the shoulder joint. It can thus flex and extend the joint, in addition to its main function as an abductor. The motor supply of deltoid is the axillary nerve (C5,6), vulnerable just below the joint capsule at the surgical neck. The distal half of the front of the shaft is covered anteriorly by the attachment of brachialis and posteriorly by that of the medial head of triceps. Coracobrachialis attaches medially opposite the deltoid attachment.

Fig. 12.11 The humerus. anterior view. posterior view.

Source: Rogers op. cit.

The distal expanded end of the humerus is formed by the two condyles, medial and lateral. The complex articular surface comprises elements of both condyles: the lateral condyle includes the rounded capitulum, which articulates with the radius, and the lateral part of the pulley-like trochlea which articulates with the ulna. The medial part of the trochlea belongs to the medial condyle. The peripheral projections on each condyle are the epicondyles, medial and lateral. The ulnar nerve is directly related to bone behind the medial epicondyle, and anconeus muscle attaches behind the lateral. The cross-sectional profile of the shaft changes from tubular to flattened from front to back at the distal metaphysis. This is the supracondylar region, where fracture is common in childhood. All three major nerves of the arm and forearm lie on or close to bone here, the ulnar posteromedially, the radial anterolaterally and the median anteriorly with the brachial artery. The sharp medial and lateral borders of the metaphysis are the supracondylar ridges, to which pronator teres and extensor carpi radialis longus (ECRL) attach respectively. The main group of forearm flexors attaches to the medial epicondyle (common flexor origin), and the extensors to the lateral (common extensor origin). Brachioradialis attaches to the lateral and distal shaft just proximal to ECRL; the radial nerve appears in the anterior compartment between brachioradialis and brachialis at this level.

The humerus is connected to the deep fascia of the arm by the medial and lateral intermuscular septa. The medial septum extends distally from the teres major attachment to the medial epicondyle, and the lateral similarly from the deltoid attachment to the lateral epicondyle. These septa divide the upper arm into flexor and extensor osteofascial compartments.

The line of capsular attachment for the elbow includes the trochlea and capitulum but excludes both epicondyles. There are two definitive growth plates for the distal humerus: that for the medial epicondyle is entirely extracapsular, while that for the trochlea and lateral condyle crosses the capsular attachment and is extracapsular only posterolaterally.

Radius (Fig. 12.12)

The radius has two expanded ends of which the distal is by far the larger. The cylindrical proximal end or head forms part both of the elbow joint and of the superior radioulnar joint, its articular surfaces for both these joints being in continuity. Immediately distal to the circumferential radioulnar articular surface of the head is the narrower neck of the bone. The annular ligament runs around this circumferential articular surface of the head, not around the neck. The main muscles attaching to the proximal radius are the biceps medially, to the bicipital tuberosity, and the supinator laterally, wrapping around the neck and proximal shaft.

Fig. 12.12 The radius and ulna. anterior view. posterior view.

Source: Rogers op. cit.

The shaft of the radius is convex laterally; it is narrow and cylindrical in its proximal part, and has a sharp medial border to which attaches the interosseous membrane. Muscles attaching to the shaft anteriorly include flexor digitorum superficialis proximally, flexor pollicis longus over most of the middle third, and pronator teres in the distal third. Posteriorly, the lateral attachments of abductor pollicis longus and extensor pollicis brevis occupy the middle third, while pronator teres attaches posterolaterally at midshaft level.

The distal expanded end is smooth and concave anteriorly but grooved and convex posteriorly (dorsally). It is prolonged laterally into the radial styloid process, and its concave distal surface articulates with the scaphoid and lunate bones of the proximal carpus. The dorsal grooves bear the extensor tendons in their sheaths, most importantly that of extensor pollicis longus lying just medial to the dorsal tubercle (of Lister). Attrition rupture of this tendon can occur here when the wrist is immobilised in a cast. The lateral surface of the distal end is grooved by the tendons of abductor pollicis longus and extensor pollicis brevis in their sheaths, while the medial surface is concave and bears an articular surface for the inferior radioulnar joint. The distal growth plate of the radius lies entirely outside the capsule of the wrist joint. The interosseous membrane between radius and ulna divides the forearm into its flexor and extensor osteofascial compartments.

The posterior interosseous nerve (the deep branch of the radial) is vulnerable where it winds around the neck of the radius within the supinator muscle. The cutaneous terminal portion of the radial nerve runs quite close to the lateral aspect of the distal radius, in the favoured site for insertion of intravenous cannulae and crossing the common distal radial fracture lines.

Ulna (Fig. 12.12)

In the ulna the proximal expanded end is far larger than the distal. The proximal end terminates in the olecranon, the bony process for attachment of the triceps tendon. Anteriorly lies the trochlear notch, for articulation with the humerus, and just distal to this the coronoid process, to which brachialis attaches anteriorly, flexor digitorum superficialis and pronator teres medially, and supinator laterally. Anconeus attaches to the posterior surface of the olecranon and of the proximal metaphysis. The lateral side of the metaphysis bears the concave articular surface of the superior radioulnar joint (radial notch), which is in continuity with that of the trochlear notch. The annular ligament is attached anteriorly and posteriorly to the radial notch. The proximal growth plate of the ulna lies outside the capsular attachment of the elbow joint. The shaft of the ulna ‘mirrors’ in shape and cross-sectional profile that of the radius, with a sharp lateral border for the attachment of the interosseous membrane. The ulnar shaft is cylindrical and narrower distally, and is slightly convex medially. Flexor digitorum profundus attaches widely to its middle two fourths, and pronator quadratus to the distal fourth. Flexor digitorum profundus also shares an aponeurotic attachment to the posterior border of the ulna with flexor and extensor carpi ulnaris. This posterior border is subcutaneous throughout its length. Distal and medial to this aponeurosis, abductor pollicis longus, extensor pollicis longus and extensor indicis attach to the medial part of the posterior surface of the shaft. The distal expanded end of the ulna is prolonged medially as the ulnar styloid process, and bears a groove dorsally for extensor carpi ulnaris tendon. There is an articular surface laterally for the inferior radioulnar joint, and distally for the triangular cartilage whose distal surface is part of the wrist joint. The ulna does not articulate directly with any carpal bone. The distal growth plate of the ulna lies outside the line of attachment of the wrist joint capsule.

The ulnar nerve lies close to the medial aspect of the olecranon as it enters the forearm within flexor carpi ulnaris. Its dorsal branch runs closely around the distal shaft about 4 cm proximal to the styloid process.

Wrist

The carpal bones (Fig. 12.13) are arranged in two rows. The proximal row, from radial (lateral) to ulnar (medial) side, comprises scaphoid, lunate and triquetral, with the pisiform, a sesamoid bone in the tendon of flexor carpi ulnaris, located anteromedially. Some features of the scaphoid, the most commonly fractured carpal bone, should be recognised, in particular the waist of the bone and the way in which the blood supply enters mainly from the distal end, making avascular necrosis of the proximal end likely after a displaced waist fracture. The distal row, again from radial to ulnar, is made up of the trapezium, trapezoid, capitate and hamate bones. The carpal bones form a shallow arch, convex dorsally. The height of this arch is increased by ventral (palmar) bony processes, the hook of the hamate, the ridge of the trapezium and the tubercle of the scaphoid, and by the pisiform medially. These ‘pillars’ of the arch give attachment to the flexor retinaculum so that the concavity of the arch forms the carpal tunnel.

Fig. 12.13 The carpals, metacarpals and phalanges. anterior view. posterior view.

Source: Rogers op. cit.

Metacarpals and phalanges

These are structurally ‘mini’ long bones. Individual muscle attachments will not be dealt with here, but you should know those of the major flexors and extensors of the wrist and the digits.

Arthrology

Glenohumeral joint

This is a ball-and-socket joint, the relative shape and size of whose articular surfaces make it totally reliant on soft tissue structures for static and dynamic stability (Fig. 12.14). The capsule is attached around the margin of the glenoid cavity of the scapula, extending onto the base of the coracoid superiorly to include the biceps attachment. The glenoid labrum, deepening the concavity of the glenoid fossa, is entirely intracapsular. On the humerus, the capsule is attached around the anatomical neck except where it passes onto the medial metaphysis inferiorly. The latter attachment brings the inferior capsule into close relation with the axillary nerve, rendering the nerve vulnerable in anteroinferior dislocations. It also means that a metaphyseal osteomyelitic lesion of the proximal humerus may be intracapsular, leading to the possibility of septic arthritis as a sequel. The capsule is reinforced by the tendons of the rotator cuff muscles, which blend with it everywhere except inferiorly, and additionally by the coracohumeral ligament superiorly. There are also variable thickenings in the capsule anteriorly: these are the glenohumeral ligaments (GHL), whose detailed anatomy is of importance to the shoulder arthroscopist. The capsule is lax inferiorly, allowing wide abduction of the joint. The tendon of the long head of biceps passes through the joint over the head of the humerus, within its synovial sheath: it is intracapsular but extrasynovial (cf. the cruciate ligaments of the knee) and is a major arthroscopic landmark. The synovial sheath extends distally beneath the transverse ligament of the humerus into the bicipital groove. The two major bursae associated with the joint are the subacromial/subdeltoid bursa superiorly and the subscapular bursa anteriorly. The former lies between the ‘layers’ of the abductor mechanism of the shoulder (the acromion, with the attached deltoid muscle and coracoacromial ligament, and supraspinatus attaching to the greater tuberosity). It does not normally communicate with the synovial cavity of the glenohumeral joint. The latter is an extension of the glenohumeral synovium passing between the glenohumeral ligaments and deep to subscapularis muscle. The muscles crossing the glenohumeral joint and their actions upon it are summarised below. The joint is innervated by the nerves which supply those muscles, mainly by the axillary and suprascapular. The suprascapular artery, from the subclavian, and the subscapular and circumflex humeral arteries, from the axillary, are the main participants in the anastomosis around the scapula and the head of the humerus. The glenohumeral capsule is said to be at its tightest when the joint is abducted and externally rotated, though this position is often that which produces anteroinferior dislocation. The joint may be aspirated or injected anteriorly or posteriorly, the posterior subacromial approach being somewhat easier and also being that usually used for arthroscopy. Diagnostic arthrography of the shoulder has been largely superseded by MRI. The common approach for open shoulder surgery is anterior, passing between deltoid and pectoralis major. The muscles attaching to the coracoid are displaced medially, protecting axillary neurovascular structures, and the capsule is entered after dividing subscapularis.

Fig. 12.14 The right shoulder joint. anterior view. in section.

Source: Rogers op. cit.

Elbow joint

This is a modified hinge joint between the humerus and the forearm bones (Fig. 12.15). The main part of the joint is the humero-ulnar, between the trochlea of the humerus and the trochlear notch of the ulna. The trochlear diameter is greater medially than laterally, thus creating the valgus ‘carrying angle’ of the elbow. The trochlear articular surface of the humerus is continuous laterally with that of the rounded capitulum, which is confined to the anterior aspect of the bone and which articulates with the concavity of the head of the radius. The trochlear articular surface of the ulna continues laterally and distally over the radial notch, which articulates with the circumferential part of the radial articular surface. The superior radio-ulnar joint is thus continuous with the elbow joint, sharing a synovial ‘cavity’. The humeral attachment of the capsule of the elbow joint leaves the articular margins of trochlea and capitulum anteriorly and posteriorly to include the coronoid, radial and olecranon fossae. The capsule is thin anteriorly and posteriorly but thicker medially and laterally where reinforced by the collateral ligaments. It attaches distally to the articular margins of the trochlear notch, then passes onto the superior border of the annular ligament, with which it becomes continuous. The collateral ligaments of the elbow differ in shape: the radial is fan-shaped, radiating onto the annular ligament from the lateral humeral epicondyle, while the ulnar has three bands constituting a triangle between the medial epicondyle and the lateral sides of the coronoid and olecranon processes (Fig. 12.15). The synovium of the elbow, lining the capsule, is continuous with that of the superior radio-ulnar joint, and extends distally a little way below the annular ligament. The synovium is separated from the capsule anteriorly and posteriorly by fat pads, whose displacement can be used in the radiological diagnosis of small effusions or haemarthroses of the elbow. Muscles crossing, and thus acting upon, the joint are summarised below. The nerves to the elbow derive, by Hilton’s Law, from all three major nerves of the limb. The arterial anastomosis around the elbow is made up of branches from the brachial, radial and ulnar vessels. The elbow is maximally stable in full extension, when the anterior capsule is tense. A swollen elbow is best aspirated from the lateral side, where no important neurovascular structures cross the joint. A posterior approach, lateral to the main body of the triceps tendon, may also be used. Open surgical approaches are mainly lateral or posterior, depending upon the size of exposure required.

Fig. 12.15 The right elbow joint. lateral view. medial view.

Source: Rogers op. cit.

Movements and muscles

• flexion: brachialis, biceps brachii, brachioradialis, pronator teres; and

• extension: triceps brachii, anconeus.

Major anatomical relations

• anterior: median nerve; brachial artery;

• anterolateral: radial nerve; and

• posteromedial: ulnar nerve.

Radio-ulnar joints

The shafts of radius and ulna are strongly linked in all positions of the forearm by the interosseous membrane, whose fibres pass distally and medially, transmitting force from radius to ulna. The proximal ends of the bones articulate at the superior radio-ulnar joint, described above with the elbow, with which it is structurally continuous. The distal ends articulate at the inferior radio-ulnar joint, between the cylindrical ulna and the concave ulnar notch of the radius. The capsule of this joint is weak, but the joint is strengthened by a triangular intra-articular fibrocartilage passing between the ulnar styloid and the distal margin of the ulnar notch of the radius. If intact, this fibrocartilage separates the synovial lining of the inferior radio-ulnar joint from that of the wrist joint. The radio-ulnar joints move together during forearm rotation into pronation and supination, the axis passing from the centre of the radial head to theulnar styloid.

Movements and muscles

• supination: biceps brachii (especially when elbow flexed), supinator; and

• pronation: pronator teres (especially when elbow flexed), pronator quadratus.

Major anatomical relations

• superior radio-ulnar joint: the posterior interosseous nerve runs in supinator close to the distal capsular attachment; and

• inferior radio-ulnar joint: the dorsal branch of the ulnar nerve passes close to the joint posteriorly.

Wrist joint

In strict terms the ‘wrist’ joint is the radiocarpal joint, that between the distal radius (and the distal surface of the triangular fibrocartilage) and the proximal row of carpal bones. There are also the intercarpal joints, between the individual carpal bones, and the midcarpal joint, between the two rows of carpal bones. The capsule of the radiocarpal joint is attached distal to the (distal) growth plates of radius and ulna, and is stronger in its palmar than in its dorsal component. There are two collateral ligaments, radial and ulnar, attaching to the respective styloid processes and fusing with the capsule. The synovium of the radiocarpal joint is usually separate from the continuous synovial lining of the intercarpal, midcarpal and carpometacarpal joints. The nerve supply to the wrist is from the interosseous nerves (branches of median and radial) and there is an arterial anastomosis from the radial, ulnar and interosseous vessels. The wrist is maximally stable in extension. It is usually approached, both surgically and for aspiration, from the dorsal aspect. Note that, on clinical examination, most of the carpus lies distal to the distal wrist crease, i.e. ‘in the hand’.

Movements and muscles

The wrist complex can be flexed, extended, adducted (ulnar deviated) and abducted (radially deviated). These movements rarely occur in isolation. A combination of all four produces circumduction. In the working wrist, extension usually occurs with radial deviation, against gravity, while flexion and ulnar deviation occur together as gravity-assisted movements. Flexion and radial deviation both take place mainly at the midcarpal joint, extension and ulnar deviation at the radiocarpal. Translational and rotational movements also occur in the carpus. The movements are produced by all the named flexors and extensors of wrist and digits. Radial and ulnar deviation are produced by the respective flexors and extensors of the wrist working together.

Joints of the hand

The carpometacarpal joints move little, with the exception of the carpometacarpal joint of the thumb. This is a very mobile saddle-shaped joint, giving the thumb much of its versatility and precision of movement. The metacarpophalangeal (MCP) and IP joints each have a pair of obliquely running collateral ligaments reinforcing the capsule. The IP joints are hinge joints, while the MCP joints also allow abduction, adduction and a little rotation. Note that the palmar skin crease at the base of each finger does not overlie the MCP joint: the joint is situated much more proximally in the palm. Make a fist and check this.

Between the joints: fasciae and compartments; nerves

The muscles of the limb are ensheathed in a continuous ‘tube’ of deep fascia: this is not as conspicuous or as thick as the deep fascia of the lower limb, and is more easily defined in the forearm than in the upper arm. It is locally thickened to form subcutaneous extensor retinacula at the wrist; the corresponding flexor retinaculum is more deeply placed. The intermuscular septa attach to the inner surface of this fascial sleeve, limiting the osteofascial compartments. In the proximal segment of the limb, the anatomical ‘arm’, there are two such compartments, separated by the humerus and the medial and lateral intermuscular septa:

• the flexor compartment contains the elbow flexors, innervated by the musculocutaneous nerve (Fig. 12.16) and with a functional root value of C5,6; and

• the extensor compartment contains triceps (and anconeus), innervated by the radial (Fig. 12.17) root value C7,8. Its main artery is the profunda brachii.

Fig. 12.16 The course and distribution of the median and musculocutaneous nerves.

Source: Rogers op. cit.

Fig. 12.17 The course and distribution of the axillary and radial nerves.

Source: Rogers op. cit.

The median nerve (Fig. 12.16) and the brachial artery remain in the flexor compartment. The ulnar nerve (Fig. 12.18) lies in the flexor compartment proximally but passes into the extensor compartment before crossing the elbow. The radial nerve does exactly the reverse, lying anterolaterally at the elbow.

Fig. 12.18 The course and distribution of the ulnar nerve.

Source: Rogers op. cit.

In the distal segment, the forearm, there are two flexor osteofascial compartments and one extensor, separated and defined by the bones and the interosseous membrane:

• the superficial flexor compartment contains flexores carpi radialis and ulnaris, flexor digitorum superficialis, palmaris longus (if present) and, proximally, pronator teres;

• the deep compartment contains flexor digitorum profundus, flexor pollicis longus and, distally, pronator quadratus; and

• the extensor compartment contains all the wrist and finger extensors, with, at a deeper level, supinator proximally and the group of muscles passing to the extensor aspect of thumb and index distally. This group includes extensors pollicis longus and brevis, abductor pollicis longus and extensor indicis (proprius). The brachioradialis muscle is unusual in that, although most of it lies in the forearm, it does not cross the wrist. It lies along the radial side of the forearm, and, like extensor carpi radialis longus, is supplied by the radial nerve before it divides. It is thus part of the extensor group, though its position allows it to function as an elbow flexor and as a rotator of the forearm.

The nerves of the flexor compartments are median and ulnar superficially and anterior interosseous (of median) deeply. The nerve of the extensor compartment is the posterior interosseous branch of the radial, and its artery the posterior interosseous branch of the ulnar (usually via the common interosseous). The sensory cutaneous branch of the radial lies superficially.

Transitional zones

The axilla has been dealt with above. The other transitional zones could be considered as the antecubital fossa, the carpal tunnel and the anatomical snuff box.

Carpal tunnel

The flexor retinaculum forms the roof of a tunnel, the floor and walls of which are made up by the concavity of the carpal bones (Fig. 12.19). Within this tunnel are the tendons of flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus and flexor carpi radialis (the latter tendon is in its own separate osseofascial compartment. The most important structure to pass through the tunnel is the median nerve. Any lesion diminishing the size of the tunnel may result in compression of the median nerve (carpal tunnel syndrome). The superficial palmar branch of the nerve is given off proximal to the flexor retinaculum and, therefore, there is no sensory impairment on the lateral side of the palm if the nerve is compressed in the carpal tunnel.