The Shoulder Joint

Bones of the upper limb | Main Anatomy Index | Joints of the upper limb (1)

The Shoulder Joint

• This is a multiaxial ball and socket type of synovial joint that permits a wide range of movement. However, mobility is gained at the expense of stability.

The Articular Surfaces of the Shoulder Joint (p. 611)

• The spheroidal head of the humerus (the ball) articulates with the shallow glenoid fossa of the scapula (the socket). Both articular surfaces are covered with hyaline cartilage.
• The shallow glenoid fossa accepts little more than a third of the large humeral head, but the glenoid fossa is deepened slightly and enlarged by a fibrocartilaginous rim called the glenoid labrum (L. lip).
• The superior portion of the labrum blends with the tendon of the long head of the biceps brachii muscle.

The Articular Capsule of the Shoulder Joint (p. 611)

• The fibrous capsule enclosing the shoulder joint is thin and loose; thus it allows a wide range of movement.

• The capsule is attached medially to the glenoid fossa, beyond the glenoid labrum.
• Superiorly, it encroaches on the root of the coracoid process so that the fibrous capsule encloses the attachment of the long head of the biceps muscle within the joint.
• Laterally the fibrous capsule is attached to the anatomical neck of the humerus.
• The inferior part of the capsule is the weakest area. The capsule is lax and lies in folds when the arm is adducted, but it becomes taut when the arm is abducted.

• There are two apertures in the articular capsule. The opening between the tubercles of the humerus is for the passage for the tendon of the long head of the biceps brachii muscle.
• The other opening is situated anteriorly, inferior to the coracoid process. It allows communication between the subscapular bursa and the synovial cavity of the joint.

• The synovial membrane lines the fibrous capsule and is reflected from it onto the glenoid labrum and the neck of the humerus, as far as the articular margin of the head.
• The synovial capsule forms a tubular sheath for the tendon of the long head of the biceps brachii muscle, where it passes into the joint cavity and lines in the intertubercular groove, extending as far as the surgical neck of the humerus.

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Intrinsic Ligaments of the Capsule of the Shoulder Joint

• These ligaments are thickenings of the fibrous capsule, which strengthen the shoulder joint.

The Glenohumeral Ligaments (p. 611)

• These are thickenings of the anterior part of the fibrous capsule.
• The superior, middle, and inferior glenohumeral ligaments run from the supraglenoid tubercle of the scapula to the lesser tubercle and the anatomical neck of the humerus.

The Transverse Humeral Ligament (p. 611)

• This is a broad band of transverse fibres passing from the greater to the lesser tubercles of the humerus.
• It forms a bridge over the superior end of the intertubercular groove, converting it into a canal that holds the synovial sheath and the tendon of the long head of the biceps as they emerge from the capsule of the shoulder joint.

The Coracohumeral Ligament (p. 611)

• This is a strong, broad band that strengthens the superior part of the capsule of the shoulder joint.
• It passes from the lateral side of the base of the coracoid process of the scapula to the anatomical neck of the humerus, adjacent to the greater tubercle.

The Coaracoacromial Arch (p. 611)

• This is formed by the coracoid process, coracoacromial ligament, and acromion.
• When force is transmitted superiorly along the humerus (e.g., when standing at a desk and partly supporting the body with the outstretched limbs), the head of the humerus is pressed against this protective arch.
• The coracoacromial arch prevents displacement of the humeral head superiorly from the glenoid cavity of the scapula.

• The supraspinatus muscle passes under this arch and lies between the deltoid muscle and the capsule of the shoulder joint. The supraspinatus tendon, passing to the greater tubercle of the humerus, is separated from the arch by the subacromial bursa.

The Coracoacromial Ligament (p. 611)

• This is a strong triangular ligament, the base of which is attached to the lateral border of the coracoid process.
• Its apex is inserted into the edge of the acromion. Superiorly, the coracoacromial ligament ids covered by the deltoid muscle.

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Movements of the Shoulder Joint (p. 611)

• This joint has more freedom of movement that any other joint in the body.
• This freedom results from the laxity of the joint's articular capsule and the large size of the humeral head compared the small size of the glenoid cavity.

• The shoulder articulation is a multiaxial ball-and-socket joint that allows movements around three axes and permits flexion-extension, abduction-adduction, circumduction, and rotation.
• In circumduction, the distal end of the humerus describes the base of a cone, the apex of which is the head of the humerus.

Stability of the Shoulder Joint (p. 611)

• The free movement of this joint leads to instability.
• The shallowness of the glenoid cavity and the laxity of the fibrous capsule also result in a considerable loss of stability.

• The strength of the joint results mainly from the muscles that surround it, particularly the rotator cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis).
• These four scapular muscles, joining the scapula to the humerus, are attached near the articular areas of the articulation and are closely related to the fibrous capsule of the joint.
• Although they have separate functions, the rotator cuff muscles work as a group in holding the head of the humerus in the glenoid fossa.

• The supraspinatus muscle and the coracoacromial arch guard the shoulder joint superiorly; the infraspinatus and teres minor muscle stabilise it posteriorly; and the subscapularis muscle protects it anteriorly.
• There is no tendinous support of the shoulder joint inferiorly; consequently, this is where it usually dislocates.

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The Rotator Cuff Muscles

The Supraspinatus Muscle (p. 537)

• This is a rounded, conical muscle and it lies in the supraspinous fossa located superior to the spine of the scapula, deep to the trapezius muscle and the coracoacromial arch.
• Its tendon is covered by the deltoid muscle.

• Proximal attachments are: supraspinous fossa of scapula.
• Distal attachments are: superior facet on greater tubercle of humerus.
• Innervation: suprascapular nerve (C4, C5, and C6)

• This muscle helps to abduct the arm, and helps the other rotator cuff muscles to hold the head of the humerus in the glenoid fossa of the scapula during all movements of the joint (i.e., it helps to stabilise the joint).
• The supraspinatus acts with the deltoid during abduction of the arm and acts strongly when a heavy weight is carried with the upper limb adducted (e.g., when carrying a heavy suitcase).

• If the deltoid is paralysed, the supraspinatus can partially abduct the humerus.

Injuries involving the supraspinatus muscle

• The tendon of the supraspinatus is separated from the coracoacromial ligament, the acromion, and the deltoid muscle by the subacromial bursa.
• When this bursa is inflamed (subacromial bursitis), abduction of the arm is painful. The supraspinatus tendon is the most commonly torn part of the rotator cuff.

The Subscapularis Muscle (p. 537)

• This is a thick triangular muscle that lies on the costal surface of the scapula and forms part of the posterior wall of the axilla.
• It crosses the anterior aspect of the shoulder joint on its way to the humerus.

• Proximal attachments are: subscapular fossa.
• Distal attachments are: lesser tubercle of humerus.
• Innervation: upper and lower subscapular nn. (C5, C6, and C7)

• The subscapularis medially rotates the arm and adducts it. It also helps the other muscles of the rotator cuff to stabilise the shoulder joint.

The Teres Minor Muscle (p. 537)

• This is an elongated, tapering muscle and is often inseparable from the infraspinous muscle, which lies along its superior border.

• Proximal attachments are: superior part of lateral border of scapula.
• Distal attachments are: inferior facet on greater tubercle of humerus.
• Innervation: axillary nerve (C5 and C6)

• The teres minor laterally rotates the arm and assists in its adduction. It also helps the other muscles of the rotator cuff top stabilise the shoulder joint.

The Infraspinatus Muscle (p. 537)

• This is a triangular muscle that occupies most of the infraspinous fossa.

• Proximal attachments are: infraspinous fossa of scapula.
• Distal attachments are: middle facet on greater tubercle of humerus.
• Innervation: suprascapular nerve (C5 and C6)

• The infraspinatus muscle laterally rotates the arm and helps the other muscles of the rotator cuff to stabilise the shoulder joint.

Injuries to the rotator cuff

• The rotator cuff holds the head of the humerus in the glenoid fossa of the scapula.
• It may be damaged by injury or disease, resulting in instability of the shoulder joint.
• Trauma may tear or rupture one or more of the tendons of the rotator cuff muscles.

• The supraspinatus tendon is the most commonly torn part of the rotator cuff.
• This injury is common in baseball pitchers.

• Degenerative tendonitis of the musculotendinous rotator cuff is a common disease, especially in older people.
• Calcium deposits may be demonstrated on radiographs in the supraspinatus tendon and in other tendons of the cuff.
• Tendonitis and inflammation of the subacromial bursa (subacromial bursitis) result in the shoulder pain that is intensified by attempts to abduct the arm.
• The supraspinatus tendon does not rupture very often in young people because their tendons are usually so strong that they will tear away (avulse) the tip of the greater tubercle of the humerus rather than rupture the tendon.

• The main stability of the shoulder joint is provided by the musculotendinous rotator cuff, which is formed by the fusion of the tendons of the rotator cuff muscles with the fibrous capsule of the joint.
• It strengthens the shoulder joint everywhere, except inferiorly.
• Consequently, if a person falls when the humerus is abducted, the head of this bone may be levered out of the glenoid fossa of the scapula, producing a dislocation of the shoulder joint.

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Bursae around the Shoulder Joint

The Subscapular Bursa (p. 613)

• This bursa is located lies between the tendon of the subscapularis muscle and the neck of the scapula.

• The bursa protects this tendon where it passes inferior to the root of the coracoid process and over the neck of the scapula.
• It usually communicates with the cavity of the shoulder joint through an opening in its fibrous capsule; thus it is really an extension of the cavity of the shoulder joint.

The Subacromial Bursa (p. 613)

• This is a larger bursa that lies between the deltoid muscle, the supraspinatus tendon, and the fibrous capsule of the shoulder joint. Its size varies, but it does not normally communicate with the cavity of the shoulder joint.
• The subacromial bursa is located inferior to the acromion and the coracoacromial ligament, between them and the supraspinatus muscle.

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Blood Supply to the Shoulder Joint

• The articular arteries to the shoulder joint are branches of the anterior and posterior circumflex humeral arteries from the axillary and the suprascapular artery from the subclavian.

Nerve Supply of the Shoulder Joint

• The articular nerves are branches of the suprascapular, axillary, and lateral pectoral nerves.

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