Movement of the pectoral girdle as a whole

Movements of the pectoral girdle serve to increase the range of movement of the shoulder joint, principally by changing the relative position of the glenoid fossa with respect to the chest wall. In all of these movements the clavicle, acting as a strut, holds the shoulder away from the trunk thereby securing greater freedom of movement of the upper limb. It should be remembered that movements of the pectoral girdle accompany virtually all movements of the shoulder joint.

In movements of the pectoral girdle, the glenoid fossa travels in an arc of a circle whose radius is the clavicle, while the medial border of the scapula, held against the chest wall, travels in a curve of shorter radius. Consequently, the relative positions of clavicle and shoulder blade must be capable of changing. This occurs at the acromioclavicular joint. It now becomes obvious that a rigid union between clavicle and shoulder blade would severely limit the mobility of the upper limb.

Lateral movement of the shoulder blade around the chest wall brings it to lie more in a sagittal plane so that the glenoid fossa faces more directly forwards(a,b). Medial movement towards the vertebral column brings the shoulder blade to lie more in the frontal plane, with the glenoid fossa facing more directly laterally. These two extreme positions of the scapula form a solid angle of 40° to 45°(a). Furthermore, the angle between the clavicle and shoulder blade decreases to approximately 60° on full lateral movement of the shoulder blade, and increases to approximately 70° on full medial movement(a). The total range of linear translation of the shoulder blade around the chest wall is about 15cm.

Elevation and depression of the shoulder blade has a linear range of some 10 to 12cm(c). However, it is usually accompanied by some degree of rotation, so that in elevation the glenoid fossa comes to face increasingly  upwards. In depression, the fossa points increasingly downwards. The rotation of the shoulder blade that occurs with respect to the chest wall does so about an axis perpendicular to the plane of the scapula, and is situated a little below the spine close to the superolateral angle. The total range of angular rotation of the scapula amounts to 60°(d). This involves a displacement of the inferior angle of the scapula of 10 to 12cm, and that of the superolateral angle of 5 to 6cm.

During abduction or flexion of the arm, the clavicle rotates about its long axis so that its anterior surface is increasingly directed upwards. Towards the end of the range of rotation of the scapula against the clavicle, the coracoclavicular ligament becomes taut and transmits the rotating force to the clavicle, whose rotation then accounts for the scapular rotation on the chest wall. Any impairment of the clavicle to rotate at either the sternoclavicular or the acromioclavicular joints will interfere with the free movement of the shoulder blade and upper limb as a whole.

Although movements of medial and lateral translation of the scapula, elevation and depression, and its rotation with respect to the chest wall have been discussed, it is important to remember that they do not occur as pure movements. All movements of the pectoral girdle will involve some degree of each of the above pure movements.

Biomechanics

Stresses on the clavicle

The clavicle is subjected to both compression and tension stresses, which under normal conditions are absorbed within it. They only become apparent when the integrity of the pectoral girdle is compromised, as by fracture, dislocation or muscular imbalance. Unlike the pelvic girdle, because the pectoral girdle is not a complete bony ring, the intrinsic stresses require the cooperation of muscles attached to it for there to be equilibrium.

A compressive stress along the length of the clavicle  directed towards the sternoclavicular joint is produced by the action of trapezius and pectoralisminor as they pull the clavicle towards the sternum. Forces transmitted medially from the upper limb to the glenoid fossa are transmitted from the scapula to the clavicle by trapezoid ligament, and from the clavicle to the first rib by the costoclavicular ligament. Consequently, falling on an outstretched hand or elbow puts no strain on either end of the clavicle at the joints. If the clavicle fractures as a result, it does so between these two ligaments. In such fractures, the two fragments tend to over-ride one another. These compressive stresses are increased by lying on one shoulder.

Tension stresses within the clavicle are produced, under the action of deltoid, when the upper limb is abducted. Hanging and swinging forwards by the arms increases these tension stresses. If sufficiently large they may lead to some discontinuity of the pectoral girdle; however,  a joint dislocation is much more likely to occur than a fracture.

A rotational force transmitted through the scapula to the clavicle tends to damage the acromioclavicular and/or sternoclavicular joints and their associated ligaments rather than causing a fracture of the bone. Downward forces applied to the lateral end of the clavicle create bending stresses within it, which, if sufficiently large or if the clavicle comes into contact with the coracoid process, may lead to fracture of the bone.

The acromioclavicular joint

The synovial acromioclavicular joint connects the clavicle with the shoulder blade. The role that this joint plays in the movement of the pectoral girdle is considered by some to be greater than that of the sternoclavicular joint, particularly for movements in or close to the sagittal plane.

Articular surfaces

The articulation is between an oval flat or slightly convex facet on the lateral end of the clavicle, and a similarly shaped flat or slightly concave facet on the anteromedial border of the acromion process. Both joint surfaces are covered with fibrocartilage. The major axis of both facets runs from anterolateral to posteromedial, so that the clavicular facet faces laterally and posteriorly, and that on the acromion faces medially and anteriorly. Consequently, the lateral end of the clavicle tends to over-ride the acromion, which together with the slope of their articulating surfaces favours displacement of the acromion downwards and under the clavicle in dislocations.

Joint capsule and synovial membrane

A relatively loose fibrous capsule surrounds the joint attaching to the articular margins. Its strong coarse fibres run in parallel fasciculi from one bone to the other. The capsule is thickest and strongest above where it is reinforced by the fibres of trapezius. Some authorities contend that the joint capsule is reinforced by two strong ligaments, the superior and inferior acromioclavicular ligaments, passing between the adjacent surfaces of the two bones. In reality these are no more than capsular thickenings, which will show varying degrees of thickening in different individuals.

Synovial membrane lines the inner surface of the capsule attaching to the margins of the articular surfaces. 

Intra-articular surfaces

A wedge-shaped, fibrocartilaginous articular disc partially divides the cavity in most joints. When present, the disc is attached to the upper inner part of the capsule and dips down between the two articulating surfaces. Only rarely does the disc form a complete partition within the joint. The presence of the articular disc partially compensates for the small degree of incongruity between the two joint surfaces.

Ligaments

Apart from the capsular thickening alluded to above, the strength of the acromioclavicular joint is provided by an extracapsular accessory ligament, the coracoclavicular ligament.

Coracoclavicular ligament

The coracoclavicular ligament is extremely powerful, anchoring the lateral end of the clavicle to the coracoid process. The ligament, which is medial to the acromioclavicular joint, stabilizes the clavicle with respect to the acromion. It is in two parts which are named according to their shapes, these being the posteromedial conoid and anterolateral trapezoid ligaments. The two parts tend to be continuous with each other posteriorly but are separated anteriorly by a small gap in which is found a synovial bursa.

The apex of the fan-shaped conoid ligament is attached posteromedially to the “elbow” of the angulated coracoid process. From here the ligament broadens as it passes upwards, more or less in the frontal plane, to attach to the conoid tubercle on the under surface of the clavicle.

The stronger and more powerful trapezoid ligament is a flat quadrilateral band. It is attached inferiorly to a roughened ridge on the upper surface of the coracoid process. The wider superior surface of the ligament is attached to the trapezoid line on the under surface of the clavicle, which runs anterolaterally from the conoid tubercle. Although the two surfaces of the trapezoid ligament are set obliquely, the ligament lies more or less in the sagittal plane, being more nearly horizontal than vertical.

Because the conoid and trapezoid ligaments lie in different planes, which are more or less at right angles to each other, and because the posterior edge of the trapezoid ligament is usually in contact with the lateral edge of the conoid ligament, a solid angle facing anteromedially is formed between them.

The two parts of the coracoclavicular ligaments are set so as to restrain opposite movements of the scapula with respect to the clavicle. The conoid ligament limits forward movement of the scapula, while the trapezoid limits backward movements. The importance of these limiting movements will be discussed more fully in the section on movements of the pectoral girdle. Both ligaments, but especially the trapezoid ligament, prevent the acromion being carried medially under the lateral end of the clavicle when laterally directed forces are applied to the shoulder. 

Blood and nerve supply

The arterial supply to the joint is by branches from the suprascapular branch of the subclavian and acromial branch of the thoracoacromial trunk. Venous drainage is to the external jugular and axillary veins. Lymphatic drainage will be to the apical group of axillary nodes.

The nerve supply to the joint is by twigs from the lateral supraclavicular, lateral pectoral, suprascapular and axillary nerves, from roots C4, 5 and 6.

Relations

The attachments of trapezius and deltoid cover the posterosuperior and anterosuperior aspects of the joint respectively. Medial to the coracoclavicular ligament the transverse superior scapular ligament converts the scapula notch into a foramen. The suprascapular vessels pass above the ligament, while beneath it and through the foramen runs the suprascapular nerve. The lateral supraclavicular nerve crosses the clavicle medial to the acromioclavicular joint.

Although not directly associated with the joint, the coracoacromial ligament, as its name suggests, is attached to both the coracoid and acromion process.

Stability

The stability of the joint is essentially provided by the coracoclavicular ligament. Trapezius and deltoid by virtue of their crossing the joint will also provide a certain amount of stability during movement of the joint.

Movement

The movements of the joint are entirely passive as there are no muscles connecting the bones which could cause one to move with respect to the other. Muscles which move the shoulder blade cause it to move on the clavicle. Indeed, all movements of the shoulder blade involve movement at both the acromioclavicular and sternoclavicular joints. All movements at the acromioclavicular joint, except that of axial rotation, are gliding movements with the coracoclavicular ligament acting so as to limit these movements.

The acromioclavicular joint has three degrees of freedom of motion about three axes. These movements are probably best described in terms of their relation to the shoulder blade rather than with respect to the cardinal axes of the body, since the joint constantly changes its relation to the trunk. The most important function of the joint is that it provides an additional range of movement for the pectoral girdle after the range of movement at the sternoclavicular joint has been exhausted.

Movement about a vertical axis(a)

This movement is associated with protraction and retraction of the shoulder blade. The axis of movement passes vertically through the lateral end of the clavicle midway between the joint and the coracoclavicular ligament. As the acromion glides backwards with respect to the clavicle, the angle between the clavicle and shoulder blade increases: similarly as the acromion glides forwards this angle decreases. Backward movement of the acromion is checked by the anterior joint capsule and actively limited by the trapezoid ligament as it becomes stretched. Forward movement will be checked by the posterior joint capsule and limited by the stretching of the conoid ligament. Towards the end of forward movement of the acromion, the trapezoid ligament may also be put under tension and therefore help to limit the movement. Compensatory movements of the clavicle at the sternoclavicular joint accompany these actions at the acromioclavicular joint.

Movement about a sagital axis(b)

Movement about this axis occurs when the shoulder blade is elevated or depressed. It has been estimated that the total range of movement about this axis is no more than 15°. Elevation is limited by tension developed in both parts of the coracoclavicular ligament, with the conoid ligament becoming stretched first; depression is checked by the coracoid process coming into contact with the under surface of the clavicle.

Axial rotation(c)

Axial rotation at the acromioclavicular joint is associated with medial and lateral rotation of the shoulder blade, that is when the glenoid fossa faces inferiorly or superiorly respectively. The range of rotation of the shoulder blade with respect to the clavicle is of the order of 30°, and occurs about an axis that passes through the conoid ligament and the acromioclavicular joint. The movement allows the flexed arm to be fully elevated. Restraints to rotation are provided by both parts of the coracoclavicular ligament.

Accessory movements

With the subject lying supine, downward pressure applied with the thumb on the lateral end of the clavicle causes it to glide backward against the acromion process.

Palpation

The line of the acromioclavicular joint can be palpated from above by applying a downward pressure to the lateral end of the clavicle.

The sternoclavicular joint

The synovial sternoclavicular joint provides the only point of bony connection between the pectoral girdle and upper limb, and the trunk. Although the joint is functionally a ball and socket joint, it does not have the form of such a joint.

Articular surfaces

The medial end of the clavicle articulates with the clavicular notch at the superolateral angle of the sternum and the adjacent upper medial surface of the first costal cartilage. The clavicular articular surface tends to be larger than that on the sternum. Consequently, the medial end of the clavicle projects above the upper margin of the manubrium sterni.

The articular surfaces are reciprocally concavoconvex, although they do not usually have similar radii of curvature. The joint, therefore, is not particularly congruent. Congruence is partly provided by an intraarticular fibrocartilaginous disc. The articular surface on the manubrium sterni is set at approximately 45° to the perpendicular. It is markedly concave from above downwards, and convex from behind forwards, being covered with hyaline cartilage. The clavicular articular surface is convex vertically and flattened or slightly concave horizontally, with the concavity being continued over the inferior surface of the shaft for articulation with the first rib costal cartilage. The greater horizontal articular surface of the clavicle overlaps the sternocostal surface anteriorly and particularly posteriorly, the whole being covered with fibrocartilage rather than hyaline cartilage. 

Joint capsule and synovial membrane

A fibrous capsule surrounds the whole joint like a sleeve attaching to the articular margins of both the clavicle and the sternum, with its inferior part passing between the clavicle and the upper surface of the first costal cartilage. Except for this inferior part, which is weak, the joint capsule is relatively strong, being strengthened anteriorly, posteriorly and superiorly by capsular thickenings known as the anterior and posterior sternoclavicular ligaments and the interclavicular ligament respectively.

Because there are two separate cavities associated with the joint(see below), there are two synovial membranes. A relatively loose lateral membrane lines the capsule, being reflected from the articular margin of the medial end of the clavicle to the margins of the articular disc. Similarly, the medial membrane attaches to the articular margins on the sternum and to the margins of the disc.

Intra-articular structures

A complete, intra-articular, fibrocartilaginous disc separates the joint into two synovial cavities. The disc is flat and round, being thinner centrally, where it may occasionally be perforated and permit communication between the two cavities, than around the periphery. It is attached at its circumference to the joint capsule, particularly in front and behind. More importantly, however, the disc is firmly attached superiorly and posteriorly to the upper border of the medial end of the clavicle, and inferiorly to the first costal cartilage near its sternal end. Consequently, as well as providing some cushioning between the articular surfaces, from forces transmitted from the upper limb, and compensating for incongruity of the joint surfaces, the disc also has an important ligamentous action. Although mainly fibrocartilaginous, it is fibrous or ligamentous at its circumference, and holds the medial end of the clavicle against the sternum. It prevents the clavicle moving upwards and medially along the sloping sternochondral surface under the influence of strong, thrusting forces transmitted from the limb, or when the clavicle is depressed as by a heavy weight carried in the hand.

Ligaments

The joint capsule is strengthened anteriorly, posteriorly and superiorly by the anterior and posterior sternoclavicular ligaments and the interclavicular ligament respectively. In addition, an accessory ligament, the costoclavicular ligament,  binds the clavicle to the first costal cartilage just lateral to the joint.

Anterior sternoclavicular ligament

The anterior sternoclavicular ligament is a strong, broad band of fibres attaching above to the superior and anterior parts of the medial end of the clavicle, passing obliquely downwards and medially to the front of the upper part of the manubrium sterni. It is reinforced by the tendinous origin of sternomastoid.

Posterior sternoclavicular ligament

The posterior sternoclavicular ligament, although not as strong as the anterior ligament, is also a broad band running obliquely downwards and medially. Laterally it attaches to the superior and posterior parts of the medial end  of the clavicle, while medially it is attached to the back of the upper part of the manubrium sterni. The sternal attachment of sternohyoid extends across, and reinforces part of, the posterior ligament.

Interclavicular ligament

The interclavicular ligament strengthens the capsule superiorly, and is formed by fibres attaching to the upper aspect of the sternal end of one clavicle passing across the jugular notch to join similar fibres from the opposite side. Some of these fibres attach to the floor of the jugular notch.

Costoclavicular ligament

The extracapsular costoclavicular ligament is an extremely strong, short, dense band of fibres. It is attached to the upper surface of the first costal cartilage near its lateral end, and to a roughened area on the posterior aspect of the inferior surface of the medial end of the clavicle. The ligament is in two laminae, usually separated by a bursa, which are attached to the anterior and posterior lips of the clavicular rhomboid impression. The anterior fibres run upwards and laterally, while those of the posterior lamina run upwards and medially; thus the fibres have a cruciate arrangement. The direction of fibres in the two laminae is the same as those in the external and internal intercostal muscles respectively.

The costoclavicular ligament essentially limits elevation of the clavicle; however, it is also active in preventing excessive anterior or posterior movements of the medial end of the clavicle. Its position and strength compensate for the weakness of the adjacent inferior part of the joint capsule.

Blood and nerve supply

The arterial supply of the sternoclavicular joint is from branches of the internal thoracic artery, the superior thoracic branch of the axillary artery, the clavicular branch of the thoracoacromial trunk, and the suprascapular artery. Venous drainage is to the axillary and external jugular veins. Lymphatics from the joint pass to the lower deep cervical group of nodes, sometimes called the supraclavicular nodes, and thence to the jugular trunk. A few lymphatics may pass to the apical group of axillary nodes.

The nerve supply of the joint is by twigs from the medial supraclavicular nerve(C3, 4) and the nerve to subclavius(C5 and 6).

Relations

Overlying the joint anteriorly is the tendinous attachment of the sternal head of sternomastoid. Posteriorly, the sternoclavicular joint is separated from the brachiocephalic vein and common carotid artery on the left and the brachiocephalic trunk, sternohyoid and sternothyroid muscles on the right. The superior vena cava is formed on the right hand side, by the union of the two brachiocephalic veins, just below the joint at the lower border of the first costal cartilage.

On the right hand side, both the phrenic and vagus nerves lie lateral to the sternoclavicular joint as they enter the thorax from the neck. However, on the left hand side, the vagus may pass behind the joint as it descends between the common carotid and subclavian arteries.

Stability

The shape of the articular surfaces and the surrounding musculature provide only a limited amount of security for the joint. The stability of the sternoclavicular joint is primarily dependent on the strength and integrity of its ligaments, particularly the costoclavicular ligament. Unfortunately, when dislocation of the joint takes place it is liable to occur.

Movements

Although the articular surfaces do not conform at those of a ball and socket joint, the sternoclavicular joint nevertheless has three degrees of freedom of movement, that is elevation and depression, protraction and retraction, and axial rotation. The fulcrum of these movements, except axial rotation, is not at the joint centre but through the costoclavicular ligament. Consequently, the movements of elevation and depression, and protraction and retraction involve a gliding between the clavicle and the intra-articular disc, and between the disc and sternum.

Elevation and depression

The axis of rotation for elevation and depression runs horizontally and slightly obliquely, anterolaterally through the costoclavicular ligament. Some authorities have suggested that two axes of rotation can be identified for elevation and depression, one for the gliding of the clavicle with respect to the disc, and the other for gliding of the disc against the sternum. Nevertheless, functionally the combined axis of movement runs through the costoclavicular ligament.

Because the axis of movement is somewhat removed from the joint centre, as the lateral end of the clavicle moves, so its medial end moves in the opposite direction. Consequently, elevation of the lateral end of the clavicle causes the medial end to move downwards and laterally. The range of movement of the lateral end of the clavicle is approximately 10cm of elevation and 3cm of depression, giving a total angular range of movement of some 60°. Elevation is limited by tension in the costoclavicular ligament and by tone in the subclavius muscle. Depression of the clavicle, in which the medial end moves upwards and medially, is limited by tension in the interclavicular ligament and by the intra-articular disc. If these two mechanisms fail, then movement is eventually limited by contact between the clavicle and upper surface of the first rib.

Protraction and retraction

The axis of movement for protraction and retraction lies in a vertical plane running obliquely, inferolaterally through the middle part of the costoclavicular ligament. Again the two ends of the clavicle move in opposite directions because of the position of the fulcrum about which movement takes place, so that in protraction of the lateral end, the medial end moves back and vice versa. In these movements, the medial end of the clavicle and the intra-articular disc tend to move as one unit against the sternum. The range of movement of the lateral end of the clavicle is approximately 5cm of protraction(anterior movement) and 2cm of retraction(posterior movement), giving a total angular range of movement of about 35°. Movement anteriorly is limited by tension in the anterior sternoclavicular and costoclavicular ligaments, while posterior movement is limited by the posterior sternoclavicular and costoclavicular ligaments.

Axial rotation

Whereas elevation, depression, protraction and retraction of the clavicle are active movements brought about by direct muscle action, axial rotation is entirely passive, being produced by rotation of the scapula and transmitted to the clavicle by the coracoclavicular ligament. Pure axial rotation of the clavicle is not possible in the living subject; it always accompanies movements in other planes. The axis about which rotation occurs passes through the centre of the articular surfaces.

The range of movement is small when the clavicle is in the frontal plane, but increases considerably when the lateral end of the clavicle is carried backwards. The degree of axial rotation possible is between 20° and 40° depending on the position of the clavicle.

That there should be any axial rotation possible at the sternoclavicular joint is due to the relative incongruity of the articular surfaces, the presence of an intra-articular disc and the relative laxness of the capsular thickenings.

Accessory movements

With the subject lying supine, a downward pressure by the thumb on the medial end of the clavicle produces a posterior gliding of the clavicle against the sternum.

Palpation

The line of the sternoclavicular joint can be easily identified through the skin and subcutaneous tissues at the medial end of the clavicle. The projection of the medial end of the clavicle above the sternum can also be palpated.

The pectoral girdle

The upper limb has become highly specialized in its functions of prehension and manipulation. Evolutionary forces have produced a limb which is extremely mobile yet without losing the stability required to give the acquired mobility, force and precision. The culmination of this evolutionary development is an upper limb which has no locomotor function, except in infants and in those who, of necessity, use walking aids.

All vertebrates possess four limbs of one form to another, these limbs being connected to the axial skeleton by the pectoral and pelvic girdles. The pelvic girdle is firmly anchored to the vertebral column, it articulates only with thoracic cage, which while providing a mechanism whereby forces generated in the upper limb can be partially transferred to the axial skeleton, it does not unduly restrict movement of the girdle as a whole.

The shoulder blade(scapula) and clavicle are the bones of the pectoral girdle. The shoulder blade is usually referred to as the scapula in descriptive anatomy; morphologically the term has a slightly more restricted meaning. Although movements of the shoulder joint accompany nearly all movements of the joints of the pectoral girdle, it is not part of the pectoral girdle. The shoulder blade is slung in muscle from the thoracic cage, while the clavicle interposes itself between shoulder blade and thorax providing a strut which steadies and braces the pectoral girdle during adduction movements. The clavicle articulates with the thorax by the sternoclavicular joint, and with the shoulder blade by the acromioclavicular joint. The humerus of the upper limb joins the scapula at the shoulder(glenohumeral) joint. It is the summation of the mobility of these three individual, yet mutually interdependent, joints which gives the upper limb its freedom of movement, consequently, the ultimate range of movement of the shoulder complex is much greater than that of the hip. The clavicle moves with respect to the sternum, the scapula with respect to the clavicle and the humerus with respect to the scapula. In addition, the scapula moves with respect to the thoracic wall. This arrangement favours mobility of the shoulder-arm complex; however, it makes stabilization of the upper limb against the axial skeleton much more difficult. Stability is achieved by the powerful musculature which attaches the pectoral girdle to the thorax, vertebral column, head and neck. Furthermore, this musculature acts as a shock absorber when body-weight is received by the upper limbs.

Just as the innominate bone consists of three parts which meet and fuse at the acetabulum, so there are three parts of the shoulder blade. The broad, flat, dorsal part of the shoulder blade is the scapula; the ventral part is the coracoid process, the two joining together in the upper part of the glenoid fossa.( These are counterparts of the ilium and ischium respectively.) A small piece of bone, the precoracoid, ossifying separately at the tip of the coracoid process, is the counterpart of the pubis.

The clavicle has no counterpart in the pelvic girdle. Forces from the upper limb are transmitted by the trapezius to the cervical spine and by the clavicle to the axial skeleton by the coracoclavicular and costoclavicular ligaments, so that normally neither end of the clavicle transmits much force. 

Fasciae of the upper limb

The superficial fascia

The superficial fascia of the upper limb shows regional differences between, for example, the shoulder region and the hand. In the shoulder region and arm, it contains a variable amount of fat. In the female there is a deposition of fat in this region – a secondary sexual characteristic, the amount of which tends to increase after middle age. At the elbow, a subcutaneous bursa is present between the skin and the olecranon process. This may become enlarged in people who often tend to lean on their elbows, giving rise to a condition known as “student’s elbow”.

There is nothing particularly noteworthy about the superficial fascia in the forearm. However, in the hand there are several specializations, most of which enhance the hand’s tactile or prehensile capabilities. On the dorsum of the hand the fascia is loose and thin, and can be readily lifted away from the underlying tissue. It is in the palm of the hand, as well as the palmar surface of the digits where specializations of the fascia can be seen. In the centre of the palm, strong bands of connective tissue connect the skin to the palmar aponeurosis, which is a thickening of the deep fascia. Overlying the thenar and hypothenar regions the fixation of the skin to the deep fascia is less well marked, but here the superficial fascia is thicker and less fibrous to facilitate the gripping action of the hand. This is because it can adapt to the contours of the object being held. Palmaris brevis lies in the superficial fascia over the hypothenar eminence. By wrinkling the skin, it improves the grip. Similar less fibrous pads of tissue are also found opposite the metacarpophalangeal joints, where the superficial transverse metacarpal ligament(a band of transverse fibres) connects to the palmar surfaces of the fibrous flexor sheaths of the fingers.

The pads on the palmar surfaces of the distal phalanges are highly specialized regions which house numerous tactile nerve endings. Here the skin is firmly attached to the latter two-thirds of the distal phalanx. However, the blood supply to the distal phalanx itself runs through this highly specialized pad. If the pad becomes infected there may be compression of the artery with death of this part of the bone. On the dorsum of the distal phalanx is the nail and there is no superficial fascia deep to it.

The deep fascia

The deep fascia of the upper limb is continuous with that of the upper back, and consequently can be traced superiorly to the superior nuchal line on the occipital bone, to the ligamentum nuchae centrally in the cervical region, and to the supraspinous and interspinous ligaments in the thoracic region. In the shoulder region, the deep fascia is extremely strong over infraspinatus and teres minor, being firmly attached to the medial and lateral borders of the scapula. Superiorly, a sheath is formed for deltoid, which attaches to the clavicle, and the acromion process and spine of the scapula.

The deep fascia covering pectoralis major attaches above to the clavicle, and may be traced, via the clavicle, to the neck. Inferiorly, it is continuous with the fascia of the anterior abdominal wall. Medially, the fascia is firmly attached to the sternum, whereas laterally it becomes thickened as the axillary fascia, which forms the floor of the axilla. Further laterally it becomes continuous with the deep fascia of the arm.

Deep to pectoralis major is the clavipectoral fascia. Medially, this is attached to the first costal cartilage and passes to the coracoid process and coracoclavicular ligament laterally. The clavipectoral fascia splits to surround subclavius superiorly, and thus attaches to the undersurface of the clavicle. It also splits to enclose pectoralis minor inferiorly. An extension of the fascia from the lateral border of pectoralis major passes into the axilla and attaches to the axillary floor. This is often referred to as the suspensory ligament of the axilla. The deep surface of the clavipectoral fascia is connected to the axillary sheath surrounding the axillary vessels and brachial plexus.

In the arm, the deep fascia forms an investing layer around the muscles. It attaches at the elbow to the medial and lateral epicondyles of the humerus and the olecranon process, becoming continuous with the deep fascia of the forearm. Two intermuscular septa arise from the deep surface of this investing layer and pass to attach to the supracondylar ridges of the humerus. Both the medial and lateral intermuscular septa are found only in the lower half of the arm. Besides separating the arm into flexor and extensor compartments they also give attachment to muscles in each compartment. Of the two, the medial intermuscular septum is said to be the stronger.

In the forearm, the deep fascia of the elbow is very strong because many of the muscles arising from either the common flexor or extensor tendons also arise from the overlying fascia. The bicipital aponeurosis helps to strengthen the fascia anteriorly, while the triceps insertion does so posteriorly. The deep fascia is also strong and thick where it attaches to the posterior border of the ulna, because it gives attachment to flexor digitorum profundus, and flexor and extensor carpi ulnaris. However, the fascia becomes thinner as it approaches the wrist, although at the wrist there are two thickenings of the transverse fibres forming the flexor and extensor retinaculae. These serve to hold the tendons entering the hand in place and prevent “bowstringing”.

The flexor retinaculum

The flexor retinaculum is found anterior to the carpus where it acts as a strong band for retention of the long flexor tendons, converting the carpal sulcus into a tunnel. It attaches laterally to the tubercle of the scaphoid and to both lips of the groove on the trapezium, and medially to the pisiform and the hook of the hamate. Flexor carpi radialis passes below the flexor retinaculum in its own lateral compartment surrounded by its synovial sheath. Medial to it, the single tendon of flexor pollicis longus is present, lying within its synovial sheath. All eight tendons of flexor digitorum superficialis and profundus run below the retinaculum in a common synovial sheath. The median nerve also enters the hand by passing below the flexor retinaculum, where it lies in front of the superficial tendons. It is here that it may become compressed if the synovial sheaths become inflamed, thus giving rise to the “carpal tunnel syndrome”.

In the palm of the hand there are two layers of fascia. The deeper layer covers the interossei and encloses adductor pollicis. The more superficial layer is strong in its central part forming the palmar aponeurosis. On each side of this, the fascia thins out to cover the thenar and hypothenar muscles. The palmar aponeurosis strengthens the hand for gripping, yet also protects the underlying vessels and nerves. It is a dense, thick triangular structure bound to the overlying superficial fascia. The apex is at the wrist and its base at the webs of the fingers. From the base four slips pass into the fingers to become continuous with the digital sheaths of the flexor tendons. Each slip further divides and has attachments to the deep transverse metacarpal ligament, the capsule of the metacarpophalangeal joint and the sides of the proximal phalanx. The slips cross in front of the lumbricals, and the digital vessels and nerves. 

Fibro-osseous canals, retinaculae and synovial sheaths of the flexors of the wrist and fingers

The tendons of the digital flexors are held in close proximity to the phalanges by fibrous sheaths. These act to prevent “bowstringing” of the tendons and ensure that their pull produces immediate movement at the interphalangeal joints. The canals are formed by a shallow groove on the anterior surface of the phalanges and by a fibrous sheath which attaches to the raised lateral and medial edges of the palmar surfaces of the proximal and middle phalanges and the palmar surface of the distal phalanx. The canal is closed distally by attaching to the distal phalanx, but is open proximally deep to the palmar aponeurosis. Most of the fibres of this sheath are arranged transversely, but at the interphalangeal joints they have a criss-cross arrangement to allow flexion to occur. All five of the fibroosseous canals are lined with a synovial sheath which surrounds the enclosed tendons. In the fingers, the synovial sheath surrounds the tendons of superficialis and profundus and is connected to them by the vinculae. The sheath of the thumb contains only the tendon of flexor pollicis longus within a synovial covering.

Muscles abducting / adducting / opposing the fingers

Palmar interossei

Dorsal interossei

Abductor digiti minimi

Opponens digiti minimi

Abductor digiti minimi

Abductor digiti minimi is the most superficial of the hypothenar muscles, lying in series with the dorsal interossei. It arises from the pisiform, the pisohamate and pisometacarpal ligaments, and the tendon of flexor carpi ulnaris, to insert by a tendon into the ulnar side of the proximal phalanx of the little finger and its dorsal digital expansion.

Nerve supply

Abductor digiti minimi is supplied by the deep branch of the ulnar nerve, root value C1.

Action

Abductor digiti minimi pulls the little finger away from the ring finger into a position of abduction. It also helps to flex the metacarpophalangeal joint.  By its attachment to the dorsal digital expansion, the muscle may help in extending the interphalangeal joints. Abductor digiti minimi is a powerful muscle playing an important role in grasping a large object with outspread fingers.

Opponens digiti minimi

Opponens digiti minimi lies deep to abductor digiti minimi, arising from the hook of the hamate and adjacent flexor retinaculum, to insert into the medial half of the palmar surface of the fifth metacarpal.

Nerve supply

Opponens digiti minimi is supplied by the deep branch of the ulnar nerve, root value T1.

Action

Opponens digiti minimi pulls the little finger forwards towards the palm and rotates it laterally at the carpometacarpal joint. This movement deepens the hollow of the hand, and is a necessary part of opposition of the little finger.

Palpation

The hypothenar muscles are closely related to one another; palpation of the individual muscles is therefore difficult. Resistance to abduction of the little finger enables abductor digiti minimi to be palpated on the ulnar border of the hand. For the movements of flexion and opposition of the little finger it is difficult to localize the action of the remaining hypothenar muscles, so accurate palpation is hard to achieve.