The sternoclavicular joint is a saddle type of synovial joint but functions as a ball-and-socket joint.

The SC joint is divided into two compartments by an articular disc.

 The disc is firmly attached to the anterior and posterior sternoclavicular ligaments as well as the interclavicular ligament.

Thus, although the articular disc serves as a shock absorber of forces transmitted along the clavicle from the upper limb, dislocation of the clavicle is rare, whereas fracture of the clavicle is common.

The sternal end of the clavicle articulates with the manubrium and the 1st costal cartilage. The articular surfaces are covered with fibrocartilage.

The joint capsule surrounds the SC joint, including the epiphysis at the sternal end of the clavicle

A synovial membrane lines the internal surface of the fibrous layer of the joint capsule, extending to the edges of the articular surfaces.

The strength of the SC joint depends on ligaments and its articular disc.

Anterior and posterior sternoclavicular ligaments, interclavicular ligament strengthens the capsule superiorly

The costoclavicular ligament anchors the inferior surface of the sternal end of the clavicle to the 1st rib and its costal cartilage, limiting elevation of the pectoral girdle.

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The SC joint is supplied by internal thoracic and suprascapular arteries

Branches of the medial supraclavicular nerve and the nerve to the subclavius supply the SC joint

Ankylosis of the Sternoclavicular Joint

When ankylosis (stiffening or fixation) of the joint occurs, or is necessary surgically, a section of the center of the clavicle is removed, creating a pseudojoint to permit scapular movement.

The acromioclavicular joint (AC joint) is a plane type of synovial joint, which is located point of the shoulder formed by the lateral part of the acromion

The sleeve-like, relatively loose  is attached to the margins of the articular surfaces (Fig. 6.58A). A synovial membrane lines the fibrous layer. Although relatively weak, the joint capsule is strengthened superiorly by fibers of the trapezius.

The acromioclavicular ligament is a fibrous band extending from the acromion to the clavicle that strengthens the AC joint superiorly 

However, the integrity of the joint is maintained by extrinsic ligaments, distant from the joint itself

Thecoracoclavicular ligaments conoid and trapezoid ligaments, which are often separated by a bursa.

The vertical conoid ligament is an inverted triangle (cone), which has its apex inferiorly where it is attached to the root of the coracoid process.

Nearly horizontal trapezoid ligament is attached to the superior surface of the coracoid process and extends laterally to the trapezoid line on the inferior surface of the clavicle.

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The acromion of the scapula rotates on the acromial end of the clavicle.

The AC joint is supplied by the suprascapular and thoracoacromial arteries 

the lateral pectoral and axillary nerves supply the AC joint

Although its extrinsic coracoclavicular ligament is strong, the AC joint itself is weak and easily injured by a direct blow

An AC joint dislocation, often called a âshoulder separation, is severe when both the AC and the coracoclavicular ligaments are torn.

When the coracoclavicular ligament tears, the shoulder separates from the clavicle and falls because of the weight of the upper limb

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The glenohumeral (shoulder) joint is a ball-and-socket type of synovial joint that permits a wide range of movement; however, its mobility makes the joint relatively unstable.

The large, round humeral head articulates with the relatively shallow glenoid cavity of the scapula

which is deepened slightly but effectively by the ring-like, fibrocartilaginous glenoid labrum

The glenoid cavity accepts little more than a third of the humeral head

which is held in the cavity by the tonus of the musculotendinous rotator cuff, or SITS, muscles (supraspinatus, infraspinatus, teres minor, and subscapularis)

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This coronal MRI shows the right glenohumeral and AC joints.A, acromion; C, clavicle; G, glenoid cavity; GrH, head of humerus; , surgical neck of humerus.

The loose fibrous layer of the joint capsule surrounds the glenohumeral joint and is attached medially to the margin of the glenoid cavity and laterally to the anatomical neck of the humerus

The joint capsule has two apertures: (1) an opening between the tubercles of the humerus for passage of the tendon of the long head of the biceps brachii

(2) an opening situated anteriorly, inferior to the coracoid process that allows communication between the subscapular bursa and the synovial cavity of the joint.

The inferior part of the joint capsule, the only part not reinforced by the rotator cuff muscles, is its weakest area

The synovial membrane lines the internal surface of the fibrous layer of the joint capsule and reflects from it onto the glenoid labrum and the humerus, as far as the articular margin of the head

The glenohumeral ligaments, which strengthen the anterior aspect of the joint capsule of the joint, and the coracohumeral

ligament, which strengthens the capsule superiorly, are intrinsic ligaments

glenohumeral ligaments are three fibrous bands, evident only on the internal aspect of the capsule, that reinforce the anterior part of the joint capsule.

The coracohumeral ligament is a strong broad band that passes from the base of the coracoid process to the anterior aspect of the greater tubercle of the humerus 

Tranverse humeral ligament converts the groove into a canal, which holds the synovial sheath and tendon of the biceps brachii in place during movements of the glenohumeral joint

The coracoacromial arch is an extrinsic, protective structure formed by the smooth inferior aspect of the acromion and the coracoid process coracoacromial ligament spanning between them.

This osseo-ligamentous structure forms a protective arch that overlies the humeral head, preventing its superior displacement from the glenoid cavity.

The coracoacromial arch is so strong that a forceful superior thrust of the humerus will not fracture it; the humeral shaft or clavicle fractures first

The glenohumeral joint is supplied by the anterior and posterior circumflex humeral arteries and branches of the suprascapular artery

The suprascapular, axillary, and lateral pectoral nerves supply the glenohumeral joint

Several bursae (sac-like cavities) containing capillary films of  (secreted by the synovial membrane) are situated near the glenohumeral joint.

Inflammation and calcification of the subacromial bursa result in pain, tenderness, and limitation of movement of the glenohumeral joint. 

This condition is also known as calcific scapulohumeral bursitis

Deposition of calcium in the supraspinatus tendon is common. This causes increased local pressure that often causes excruciating pain during abduction of the arm

The calcium deposit may irritate the overlying subacromial bursa, producing an inflammatory reaction known as subacromial bursitis.painful arc syndrome)

The musculotendinous rotator cuff is commonly injured during repetitive use of the upper limb above the horizontal (e.g., during throwing and racquet sports, swimming, and weightlifting).

Recurrent inflammation of the rotator cuff, especially the relatively avascular area of the supraspinatus tendon, is a common cause of shoulder pain and results in tears of the musculotendinous rotator cuff

Because of its freedom of movement and instability, the glenohumeral joint is commonly dislocated by direct or indirect injury.

 Because the presence of the coracoacromial arch and the support of the rotator cuff are effective in preventing upward dislocation, most dislocations of the humeral head occur in the downward (inferior) direction.

Anterior dislocation of the glenohumeral joint

It is usually caused by excessive extension and lateral rotation of the humerus

The head of the humerus is driven inferoanteriorly, and the fibrous layer of the joint capsule and glenoid labrum may be stripped from the anterior aspect of the glenoid cavity in the process

This may tear the capsule and dislocate the shoulder so that the humeral head comes to lie inferior to the glenoid cavity and anterior to the infraglenoid tubercle

The strong flexor and adductor muscles of the glenohumeral joint usually subsequently pull the humeral head anterosuperiorly into a subcoracoid position.

Inferior dislocation of the glenohumeral joint often occurs after an avulsion fracture of the greater tubercle, owing to the absence of the upward and medial pull produced by the muscles attaching to the tubercle.

The axillary nerve may be injured when the glenohumeral joint dislocates because of its close relation to the inferior part of the joint capsule of this joint

The subglenoid displacement of the head of the humerus into the quadrangular space damages the axillary nerve. 

Axillary nerve injury is indicated by paralysis of the deltoid

(manifest as an inability to abduct the arm to or above the horizontal level) 

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Tearing of the fibrocartilaginous glenoid labrum commonly occurs in athletes who throw a baseball or football and in those who have shoulder instability and subluxation (partial dislocation) of the glenohumeral joint

Usually a tear occurs in the anterosuperior part of the labrum. 

The typical symptom is pain while throwing, especially during the acceleration phase, but a sense of popping or snapping may be felt in the glenohumeral joint during abduction and lateral rotation of the arm.

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The joints of the pectoral girdle are accessory to the glenohumeral joint in positioning the upper limb.

The SC joint links the appendicular skeleton to the axial skeleton.

The SC and AC joints enable the movement at the physiological scapulothoracic joint, where approximately 1° of movement occurs for every 3° of arm movement (scapulohumeral rhythm).

In turn, approximately two thirds of the movement at the scapulothoracic joint result from motion at the SC joint, and one third is from motion at the AC joint.

The strength and integrity of the joints of the shoulder complex do not depend on congruity of the articular surfaces. The integrity of the SC and

AC joints results from intrinsic and extrinsic ligaments and the SC articular disc, whereas that of the glenohumeral joint is maintained by the tonic and active contraction of the muscles acting across it, particularly the SITS (rotator cuff) muscles.