The shoulder joint

The coracoacromial arch prevents injury to the joint from above, and prevents upward dislocation of the humeral head. The arch is formed by the acromion, the coracoid process and the coracoacromial ligament. Below the arch lie parts of the subacromial bursa, the supraspinatus muscle and its tendon, the superior portion of the joint capsule, the origin of the biceps tendon, and loose connective tissue. As the humerus moves, the arch and these components may interfere, presenting an obstacle to abduction of the humerus as the greater tuberosity attempts to pass under the coracoacromial ligament.

The glenoid fossa is located on the anterior superior angle of the scapula. It is a shallow ovoid socket that faces anteriorly, laterally and upward. The glenoidal labrum, a fibrous lip, runs around the edge of the glenoid fossa increasing its depth. The humeral head is more strongly curved than the glenoid fossa, so that only a small area of the surface of the head is in contact with the glenoid fossa at any one time. Since this is not a true ball and socket arrangement,(the socket is flat and not congruent with the rounded head), muscle action must be synchronized to maintain stability of the joint during movement.

The joint capsule is a loose bag attached around the rim of the glenoid fossa. The capsule inserts around the anatomical neck of the humerus. The long head of biceps arises from the supraglenoid tubercle. The tendon invaginates the capsule as it runs across the joint superiorly to lie in the intertubercular groove. The tendon does not lie within the joint, but rather lies in a fold of the capsule.The capsule is sufficiently lax that when the arm is adducted the superior capsule is tight, but inferiorly the axillary fold of the capsule is loose and pleated. When the arm is abducted the opposite is the case. In a similar way medial and lateral rotation of the arm alternately loosens and tightens the capsule anteriorly and posteriorly. The capsule is strengthened by superior middle and inferior glenohumeral ligaments. These horizontal pleats in the capsule run from the medial side of the intertubercular groove to beyond the edge of the glenoid fossa. Anteriorly the capsule is loose enough to form a small pouch. Anterior strengthening is aided by the coracohumeral ligamentThe ligament originates at the coracoid process and attaches to the humerus at the bicipital groove.

Movement at the shoulder joint is usually accompanied by movement of the scapula. Two groups of muscles are involved, the short muscles of the rotator cuff and the larger muscles attached to the axial skeleton.

The rotator cuff is formed by the supraspinatus, infraspinatus, teres minor and subscapularis muscles. The rotator action of the cuff muscles relates to rotation around a point located in the centre of the humeral head. The supraspinatus muscle originates from the supraspinous fossa above the spine of the scapula. It passes laterally below the coracoacromial ligament and attaches to the greater tuberosity. The infraspinatus muscle arises below the scapular spine in the infraspinous fossa and travels laterally to insert just below the insertion of the supraspinatus muscle on the greater tuberosity. The teres minor muscle arises from the middle third of the lateral border of the scapula and passes upward and laterally to insert into the greater tuberosity below the insertion of the infraspinatus. The insertion of these three muscles forms a conjoined tendon on the greater tuberosity. The subscapularis muscle arises from the subscapular fossa and passes across the front of the shoulder joint to insert into the lesser tubercle. A subscapular bursa usually lies between the muscle and the neck of the scapula. As the tendons of the rotator cuff muscles cross the shoulder joint they are closely applied to the joint capsule.

The large muscles involved in shoulder movement are trapezius, deltoid, serratus anterior, pectoralis major and minor, assisted by the smaller muscles teres major, the rhomboids and levator scapulae. The origin and insertion of each of these muscles is shown on the accompanying diagrams, together with their major action in shoulder movement.

Movement at the shoulder joint is accompanied by coordinated movement of the scapula. The scapula is suspended in muscles and glides on the body wall. The two major muscles involved are trapezius and serratus anterior.

Trapezius is a complex muscle with three components. The upper fibres arise from the ligamentum nuchae in the cervical region, the posterior spinous processes of the cervical and upper thoracic vertebrae. The fibres converge from this broad origin to insert on the superior surface of the lateral and intermediate scapular spine. The middle fibres take origin from the spinous processes of the upper thoracic vertebrae and travel laterally to insert into the medial border of the scapular spine. The lower fibres of trapezius arise from the spinous processes of the lower thoracic vertebrae and attach to the medial portion of the spine of the scapula. The serratus anterior muscle arises on the anterolateral thoracic wall by slips from the upper eight ribs, and runs posteriorly, closely applied to the chest wall. The curving fibres insert into the anterior surface of the medial border of the scapula. Many fibres converge on the inferior angle, with some having to run upwards while others travel downwards from their origin.

The trapezius and serratus anterior muscles act in concert to move the scapula. Upward rotation of the scapula is necessary during abduction at the shoulder. The superior fibres of trapezius pull upward on the clavicle while the inferior fibres pull downward on the medial scapular spine. The inferior fibres of serratus anterior pull the medial border of the scapula around the body wall. The tendency of the inferior fibres of serratus anterior is to pull the scapula down. This is counteracted by the upper fibres of trapezius. The end result is rotation and elevation of the scapula. This scapular movement allows a greater level of abduction and a higher reach.