Shoulder Joint-Movements-Stability of the Shoulder Joint-Dislocations of the Shoulder Joint-Anterior Inferior Dislocation--Posterior Dislocations-Shoulder Pain

Shoulder Joint

■■ Articulation: This occurs between the rounded head of the humerus and the shallow, pear-shaped glenoid cavity of the scapula. The articular surfaces are covered by hyaline articular cartilage, and the glenoid cavity is deepened by the presence of a fibrocartilaginous rim called the glenoid labrum.

■■ Type: Synovial ball-and-socket joint

■■ Capsule: This surrounds the joint and is attached medially to the margin of the glenoid cavity outside the labrum; laterally, it is attached to the anatomic neck of the humerus. The capsule is thin and lax, allowing a wide range of movement. It is strengthened by fibrous slips from the tendons of the subscapularis, supraspinatus, infraspinatus, and teres minor muscles (the rotator cuff muscles).

■■ Ligaments: The glenohumeral ligaments are three weak bands of fibrous tissue that strengthen the front of the capsule. The transverse humeral ligament strengthens the capsule and bridges the gap between the two tuberosities . The coracohumeral ligament strengthens the capsule above and stretches from the root of the coracoid process to the greater tuberosity of the humerus.

■■ Accessory ligaments: The coracoacromial ligament extends between the coracoid process and the acromion. Its function is to protect the superior aspect of the joint

■■ Synovial membrane: This lines the capsule and is attached to the margins of the cartilage covering the articular surfaces. It forms a tubular sheath around the tendon of the long head of the biceps brachii. It extends through the anterior wall of the capsule to form the subscapularis bursa beneath the subscapularis muscle .

■■ Nerve supply: The axillary and suprascapular nerves

Movements

The shoulder joint has a wide range of movement, and the stability of the joint has been sacrificed to permit this.

(Compare with the hip joint, which is stable but limited in its movements.) The strength of the joint depends on the tone of the short rotator cuff muscles that cross in front, above, and behind the joint—namely, the subscapularis, supraspinatus, infraspinatus, and teres minor. When the joint is abducted, the lower surface of the head of the humerus is supported by the long head of the triceps, which bows downward because of its length and gives little actual support to the humerus. In addition, the inferior part of the capsule is the weakest area.

Stability of the Shoulder Joint

The shallowness of the glenoid fossa of the scapula and the lack of support provided by weak ligaments make this joint an unstable structure. Its strength almost entirely depends on the tone of the short muscles that bind the upper end of the humerus to the scapula—namely, the subscapularis in front, the supraspinatus above, and the infraspinatus and teres minor behind. The tendons of these muscles are fused to the underlying capsule of the shoulder joint. Together, these tendons form the rotator cuff.

The least supported part of the joint lies in the inferior location, where it is unprotected by muscles.

Dislocations of the Shoulder Joint

The shoulder joint is the most commonly dislocated large joint.

Anterior Inferior Dislocation

Sudden violence applied to the humerus with the joint fully abducted tilts the humeral head downward onto the inferior weak part of the capsule, which tears, and the humeral head comes to lie inferior to the glenoid fossa. During this movement, the acromion has acted as a fulcrum. The strong flexors and adductors of the shoulder joint now usually pull the humeral head forward and upward into the subcoracoid position.

Posterior Dislocations

Posterior dislocations are rare and are usually caused by direct violence to the front of the joint. On inspection of the patient with shoulder dislocation, the rounded appearance of the shoulder is seen to be lost because the greater tuberosity of the humerus is no longer bulging laterally beneath the deltoid muscle. A subglenoid displacement of the head of the humerus into the quadrangular space can cause damage to the axillary nerve, as indicated by paralysis of the deltoid muscle and loss of skin sensation over the lower half of the deltoid. Downward displacement of the humerus can also stretch and damage the radial nerve.

 

Shoulder Pain

The synovial membrane, capsule, and ligaments of the shoulder joint are innervated by the axillary nerve and the suprascapular nerve. The joint is sensitive to pain, pressure, excessive traction, and distention. The muscles surrounding the joint undergo reflex spasm in response to pain originating in the joint, which in turn serves to immobilize the joint and thus reduce the pain.

Injury to the shoulder joint is followed by pain, limitation of movement, and muscle atrophy owing to disuse. It is important to appreciate that pain in the shoulder region can be caused by disease elsewhere and that the shoulder joint may be normal; for example, diseases of the spinal cord and vertebral column and the pressure of a cervical rib (see page XXX) can cause shoulder pain. Irritation of the diaphragmatic pleura or peritoneum can produce referred pain via the phrenic and supraclavicular nerves.

Scapula-Fractures of the Scapula-Dropped Shoulder and Winged Scapula-Clavicle-Fractures of the Clavicle-Compression of the Brachial Plexus, Subclavian Artery, and Subclavian Vein by the Clavicle-

Scapula

The scapula is a flat triangular bone that lies on the posterior chest wall between the 2nd and 7th ribs. On its posterior surface, the spine of the scapula projects backward.

The lateral end of the spine is free and forms the acromion, which articulates with the clavicle. The superolateral angle of the scapula forms the pear-shaped glenoid cavity, or fossa, which articulates with the head of the humerus at the shoulder joint. The coracoid process projects upward and forward above the glenoid cavity and provides attachment for muscles and ligaments. Medial to the base of the coracoid process is the suprascapular notch.

The anterior surface of the scapula is concave and forms the shallow subscapular fossa. The posterior surface of the scapula is divided by the spine into the supraspinous fossa above and an infraspinous fossa below. The inferior angle of the scapula can be palpated easily in the living subject and marks the level of the 7th rib and the spine of the 7th thoracic vertebra.

Fractures of the Scapula

Fractures of the scapula are usually the result of severe trauma, such as occurs in run-over accident victims or in occupants of automobiles involved in crashes. Injuries are usually associated with fractured ribs. Most fractures of the scapula require little treatment because the muscles on the anterior and posterior surfaces adequately splint the fragments.

Dropped Shoulder and Winged Scapula

The position of the scapula on the posterior wall of the thorax is maintained by the tone and balance of the muscles attached to it. If one of these muscles is paralyzed, the balance is upset, as in dropped shoulder, which occurs with paralysis of the trapezius, or winged scapula, caused by paralysis of the serratus anterior. Such imbalance can be detected by careful physical examination.

Clavicle

The clavicle is a long, slender bone ,acts as a strut that holds the arm away from the trunk that lies horizontally across the root of the neck just beneath the skin. It articulates with the sternum and 1st costal cartilage medially and with the acromion process of the scapula laterally.. It also transmits forces from the upper limb to the axial skeleton and provides attachment for muscles. The medial two thirds of the clavicle is convex forward and its lateral third is concave forward. The important muscles and ligaments attached to the clavicle.

Fractures of the Clavicle

The clavicle is a strut that holds the arm laterally so that it can move freely on the trunk. Unfortunately, because of its position, it is exposed to trauma and transmits forces from the upper limb to the trunk. It is the most commonly fractured bone in the body.

The fracture usually occurs as a result of a fall on the shoulder or outstretched hand. The force is transmitted along the clavicle, which breaks at its weakest point, the junction of the middle and outer thirds. After the fracture, the lateral fragment is depressed by the weight of the arm, and it is pulled medially and forward by the strong adductor muscles of the shoulder joint, especially the pectoralis major. The medial end is tilted upward by the sternocleidomastoid muscle.

The close relationship of the supraclavicular nerves to the clavicle may result in their involvement in callus formation after fracture of the bone. This may be the cause of persistent pain over the side of the neck.

Compression of the Brachial Plexus, Subclavian Artery, and Subclavian Vein by the Clavicle

The interval between the clavicle and the first rib in some patients may become narrowed and thus is responsible for compression of nerves and blood vessels.