Surface anatomy of Shoulder .
The scapula is a thin triangular plate of bone which lies at a tangent to the posterolateral surface of the thorax. It has a long, slightly thickened medial border which meets the thin, concave superior border at the superior angle and the markedly thickened lateral border (margin) at the inferior angle. The lateral angle is the thickest part of the bone. It is truncated (the apex of the angle is cut), so that the end forms the shallow, pear-shaped glenoid cavity.
The glenoid cavity is continuous with the rest of the scapula through the neck of the scapula and faces anterolaterally to articulate with the hemispherical head of the humerus. The coracoid is a thick buttress which extends anterosuperiorly from the neck of the scapula and the upper part of the glenoid cavity to end in the coracoid process. The scapular notch separates the coracoid laterally from the remainder of the superior border. The dorsal surface of the scapula is divided into supraspinous and infraspinous fossae by the spine of the scapula.
The spine runs from the medial border of the scapula to its neck and increases in height laterally. The posterior surface of the spine (crest of the spine) widens laterally to become the flattened acromion which projects forwards at the palpable acromial angle. The acromion and coracoid form two parts of an incomplete bony arch above the glenoid cavity and the head of the humerus which articulates with it. This coracoacromial arch is completed by the coracoacromial ligament. Above the coracoacromial ligament, the lateral end of the clavicle articulates with the acromion.
The clavicle is attached to the coracoid process through the powerful coracoclavicular ligaments. The acromion and the coracoid process arch forwards, each leaving a space beneath. The subscapularis passes from the subscapular fossa of the scapula beneath the coracoid process. The supraspinatus passes from the supraspinous fossa beneath the acromion. The thick lateral border of the scapula gives attachment to the two teres muscles which also pass to the humerus. The expanded proximal end of the humerus consists of the hemispherical articular head, which faces medially, upwards, and backwards.
The remainder of the expanded end is formed by two tubercles—the greater tubercle faces laterally, the lesser tubercle anteriorly. The tubercles are separated from each other by the intertubercular sulcus. This sulcus transmits the tendon of the long head of the biceps muscle which arises from the supraglenoid tubercle. The intertubercular sulcus continues inferiorly on the surgical neck and upper part of the body of the humerus as a shallow groove bounded by the medial and lateral lips of the intertubercular sulcus .
The lateral lip of the intertubercular sulcus is continuous inferiorly with the anterior limb of the V-shaped deltoid tuberosity. The lesser tubercle is the site of insertion of the subscapularis. The greater tubercle has three facets (superior, posterosuperior, and posterior) for the insertion of the supraspinatus, infraspinatus, and teres minor, respectively. The greater and lesser tubercles are separated from the head by a shallow sulcus, the anatomical neck. The upper end of the humerus is separated from the body by the surgical neck.
Muscles attaching the humerus to the scapula .
Muscles attaching the humerus to the scapula are :-
(a) Those which have considerable mechanical advantage over the shoulder joint by being attached at some distance from it—deltoid, teres major, coracobrachialis, and biceps brachii (short head).
(b) Those which lie close to the shoulder joint and have a smaller mechanical advantage over it. They help to stabilize the joint in any position and act as the main ligaments of the joint—subscapularis, supraspinatus, infraspinatus, teres minor, and the long heads of the biceps and triceps brachii.
1. Deltoid Muscle .
This large muscle forms the bulge of the shoulder. It takes origin from the lateral third of the clavicle, acromion, and spine of the scapula and is inserted into the deltoid tuberosity of the humerus . Nerve supply: axillary nerve. Actions: the anterior fibres flex and medially rotate the shoulder; the middle fibres abduct the shoulder and the posterior fibres extend and laterally rotate the shoulder.
2. Subscapularis Muscle .
The subscapularis originates from a wide area on the costal surface of the scapula . It is inserted on the lesser tubercle of the humerus . The fibres fuse with the capsule of the shoulder joint and are separated from it and the lateral part of the costal surface of the scapula by the subscapular bursa . The subscapular bursa frequently communicates with the cavity of the shoulder joint. Nerve supply: upper and lower subscapular nerves. Actions: medial rotation of the shoulder. Plays an important role in stabilizing the shoulder joint.
3. Supraspinatus Muscle .
The supraspinatus arises from the supraspinous fossa of the scapula and is inserted on the superior facet on the greater tubercle of the humerus . Its fibres pass under the coracoacromial ligament. Nerve supply: suprascapular nerve. Action: abduction of the shoulder joint.
4. Infraspinatus Muscle .
The infraspinatus arises from the infraspinous fossa of the scapula and is inserted on the middle facet on the greater tubercle of the humerus. Nerve supply: suprascapular nerve. Action: lateral rotation of the shoulder joint.
5. Teres major Muscle .
The teres major takes origin from the lateral one-third of the lateral margin of the scapula. Its fibres pass to the anterior surface of the humerus and are inserted into the medial lip of the intertubercular sulcus . The lower margin is closely related to the latissimus dorsi and forms the posterior axillary fold. Nerve supply: lower subscapular nerve. Actions: adduction and medial rotation of the shoulder joint.
6. Teres minor Muscle .
The teres minor takes origin from the superior two-thirds of the inferior margin of the scapula and is inserted into the posterior facet on the greater tubercle of the humerus . Nerve supply: axillary nerve. Actions: lateral rotation and stabilization of the shoulder joint.
Axillary nerve .
The axillary nerve is a terminal branch of the posterior cord of the brachial plexus and is formed near the lower border of the subscapularis. It curves back on the lower border of the subscapularis and passes through the quadrangular space with the posterior circumflex humeral artery. It lies medial to the surgical neck of the humerus, immediately inferior to the capsule of the shoulder joint. The nerve gives a branch to the shoulder joint and then divides into anterior and posterior branches.
The posterior branch supplies the teres minor and the posterior part of the deltoid. It then descends over the posterior border of the deltoid and supplies the skin over the lower half of that muscle as the upper lateral cutaneous nerve of the arm . The anterior branch continues horizontally between the deltoid muscle and the surgical neck of the humerus. It supplies the deltoid and sends a few branches through it to the overlying skin.
The axillary nerve is at risk in downward dislocation of the head of the humerus and in fractures of the surgical neck of the humerus because of its close relation to the joint and the bone . When it is damaged, the deltoid and teres minor muscles are paralysed.
Circumflex humeral arteries .
The anterior and posterior circumflex humeral arteries are branches of the third part of the axillary artery. Together they form a circular anastomosis at the surgical neck of the humerus . They supply the surrounding muscles, the shoulder joint and the upper end of the humerus. They also anastomose with the profunda brachii artery by a descending branch.
Suprascapular nerve .
The suprascapular nerve arises from the upper trunk of the brachial plexus (C. 5, 6) . It passes downwards and backwards, superior to the plexus to join the suprascapular vessels. It enters the supraspinous fossa through the scapular notch. Here it supplies the supraspinatus and gives branches to the acromioclavicular and shoulder joints. The nerve then descends immediately lateral to the root of the spine of the scapula to enter the infraspinous fossa and supply the infraspinatus and shoulder joint.
Suprascapular artery .
The suprascapular artery arises from the first branch of the subclavian artery. It enters the supraspinous fossa of the scapula above the scapular ligament, and runs deep to and supplies, the supraspinatus. It then passes through the spinoglenoid notch and supplies the infraspinatus. It anastomoses with the circumflex scapular artery and with branches of the transverse cervical artery.
Coracoacromial ligament .
The coracoacromial ligament is a strong triangular band. Its base is attached to the lateral border of the coracoid process, and its apex to the tip of the acromion. It lies between the subacromial bursa inferiorly and the deltoid muscle superiorly .
Coracoclavicular ligament .
The powerful coracoclavicular ligament passes between the upper surface of the coracoid process and the clavicle. It has two parts. The posteromedial part is the conoid ligament. It is shaped like an inverted cone and is attached above to the conoid tubercle of the clavicle . The anterolateral triangular part is the trapezoid ligament.
It passes superolaterally to the trapezoid line on the clavicle. The coracoclavicular ligament is the main structure suspending the scapula and hence the upper limb from the clavicle. It supports the acromioclavicular joint and helps to transmit forces from the upper limb to the trunk .
Shoulder joint .
The disproportionately large head of the humerus, the small, shallow glenoid cavity and the loose articular capsule gives the shoulder joint a wide range of movements but make the joint inherently unstable . This instability is overcome by the powerful muscles which closely surround the joint. These muscles support the joint in any position without restricting movement which ligaments would do.
However, there is an increased risk of displacement of the head of the humerus from the glenoid cavity (dislocation) when the joint is suddenly pulled upon. This displacement frequently occurs through the lower part of the joint capsule which is inadequately supported by the long head of the triceps. This inferior dislocation of the humeral head can result in damage to the adjacent axillary nerve.
Articular capsule of Shoulder .
The outer fibrous membrane of the articular capsule is a thin, but relatively strong, tubular structure. It is attached to the margin of the glenoid cavity and to the anatomical neck of the humerus, except inferiorly where it extends downwards 1.5–2.0 cm on the surgical neck of the bone . With the arm by the side, this inferior part of the membrane hangs down in a redundant fold between the teres major and minor muscles. When the arm is abducted to a right angle, this fold is tensed.
In the latter position, the lower part of the articular surface of the humeral head lies on this part of the articular capsule, with the long head of the triceps and the teres major muscles supporting it below. Anteriorly, the attachment of the fibrous membrane extends inferiorly (from the anatomical neck) between the tubercles of the humerus bridging over the upper part of the intertubercular sulcus. Deep to it is a synovial-lined tunnel, through which the tendon of the long head of the biceps muscle leaves the joint. This is one of three apertures in the fibrous membrane; the other two are extensions of the synovial membrane through the fibrous membrane to form the subscapular and infraspinatus bursae.
The subscapular bursa is large and more constant. It lies close to the root of the coracoid process and occasionally allows dislocation of the head of the humerus at this point. There are four slight thickenings of the articular capsule: (1) the coracohumeral ligament lies obliquely across the upper surface of the joint from the base of the coracoid process to the superior surface of the greater tubercle of the humerus; and (2) two or three glenohumeral ligaments may be visible as thickenings of the anterior part of the membrane, when viewed from the interior of the joint.
The synovial membrane lines the fibrous capsule and covers the intracapsular part of the surgical neck of the humerus. The synovial membrane also forms a sheath around the tendon of the long head of the biceps and is continuous with the lining of the bursae which communicate with the joint. The nerve supply of the shoulder joint is by the axillary, suprascapular, and lateral pectoral nerves.
Movements of the Limb at the shoulder joint .
The wide range of movements which is possible at the shoulder joint is the result of: (1) the nature of the articular surfaces (the large hemispherical head of the humerus fitted to the small, shallow glenoid cavity); (2) a loose-fitting articular capsule and (3) the replacement of ligaments by a group of muscles.
The glenoid cavity faces anterolaterally at rest, and its plane is parallel to the axis around which the scapula is rotated in movements of the shoulder girdle. Movement of the shoulder joint can take place independently but is usually accompanied by movements of the shoulder girdle. Even when the scapula is not moved, the muscles would be in tension to maintain a stable scapula on which the limb may be moved.
1. Flexion Movement .
Flexion is carried out by muscles which pass anterior to the shoulder joint (the short head of the biceps, the coracobrachialis and the clavicular parts of the deltoid and pectoralis major), but the arm can be flexed to the horizontal position only if the inferior angle of the scapula is also pulled forwards on the chest wall (lateral rotation) by the serratus anterior, thus turning the glenoid cavity upwards.
2. Extension Movement .
Two sets of muscles extend the shoulder. Extension of the shoulder from the anatomical position is restricted and produced by the posterior fibres of the deltoid, assisted initially by the latissimus dorsi. Further extension is completed by elevation of the scapula on the convex thoracic wall by the trapezius and levator scapulae.
Extension of the flexed shoulder joint back to the anatomical position against resistance is produced by the latissimus dorsi, the teres major, and the sternocostal part of the pectoralis major, assisted by the rhomboid major and pectoralis minor, both of which rotate the scapula medially.
3. Abduction Movement .
Abduction is produced by the middle fibres of the deltoid and by the supraspinatus, both of which pass superior to the joint. The supraspinatus is responsible for initiating the movement. While the deltoid is contracting, simultaneous contraction of the teres minor and the lower fibres of the subscapularis prevent the humerus from being pulled up against the coracoacromial arch .
The deltoid can abduct the humerus on the scapula to the horizontal, but this movement is associated from the beginning with the lateral rotation of the scapula . Lateral rotation of the scapula (produced by the serratus anterior and trapezius) permits the humerus to be carried upwards to the vertical position, by turning the glenoid cavity to face superiorly. To confirm this, note the elevation of the shoulder and the lateral projection of the inferior angle of the scapula in full abduction in the living.
4. Adduction Movement .
Adduction against resistance of the arm abducted above the head is first produced by the latissimus dorsi and the lowest sternocostal fibres of the pectoralis major, and is assisted by the teres major and the medial rotators of the scapula . Once the horizontal position has been passed, progressively higher fibres of the pectoralis major are involved.
When this movement is not resisted, the muscles which are active in abduction act eccentrically to control the pull of gravity on the limb. This situation is common to every movement where gravity is the driving force. It can be demonstrated in the shoulder by the continuing firmness (contraction) of the deltoid as the arm is lowered to the side and its immediate flaccidity when the movement encounters resistance.
5. Medial and lateral rotation of the humerus .
Medial and lateral rotation of the shoulder may occur in any position but are best demonstrated with the arm by the side and the elbow flexed at right angle. The hand can then be swung laterally (lateral rotation of the humerus) or medially (medial rotation of the humerus).
In this position, medial rotation is produced by muscles passing to the front of the humerus from the trunk (pectoralis major, latissimus dorsi, subscapularis, teres major, anterior fibres of the deltoid). Lateral rotation results from the contraction of muscles passing to the back of the humerus from the trunk (infraspinatus, teres minor, and posterior fibres of the deltoid).
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