Anatomy of the shoulder joint

The shoulder joint is formed by the head of the humerus and the glenoid cavity of the scapula. The shoulder is formed and fixed by four muscles and their tendons: the supraspinatus, infraspinatus, subscapularis, and teres minor. The tendons of these muscles form the rotator cuff. The supraspinatus muscle is located above the other muscles that form the rotator cuff. It begins in the supraspinatus fossa of the scapula, then passes under the acromial process and is attached to the anterior edge of the greater tuberosity of the humerus. The function of this muscle is to abduct the shoulder anteriorly and outward. The infraspinatus muscle begins in the infraspinatus fossa of the scapula, passes more laterally and also attaches to the greater tuberosity of the humerus, posteriorly and inferiorly to the supraspinatus muscle.

The teres minor muscle originates from the lateral border of the scapula and inserts onto the greater tuberosity of the humerus behind and below the infraspinatus muscle. The infraspinatus and teres minor muscles rotate the humerus outward. These three muscles, which form the rotator cuff, unite their fibers into a single tendon, which is then attached to the greater tuberosity.

The subscapularis is the most anterior of the four muscles. It originates from the medial aspect of the scapula, then crosses the anterior aspect of the scapulohumeral joint and inserts on the lesser tuberosity of the humerus. An extremely important object in the examination of the shoulder is the tendon of the long head of the biceps. This tendon originates from the superior articular tubercle and the posterosuperior aspect of the glenoid labrum. It passes anteriorly over the head of the humerus between the supraspinatus and subscapularis muscles to insert into the corresponding groove for the biceps tendon. The biceps tendon is thus medially bordered by the lesser tuberosity and laterally by the greater tuberosity of the humerus. The synovial sheath of the biceps tendon extends up to 3 cm inferiorly from the biceps groove. The biceps tendon and rotator cuff are separated from the deltoid muscle by the subdeltoid-acromial bursa.

Normally, there is no communication between the subdeltoid bursa and the scapulohumeral joint. This bursa is located anteriorly and superiorly above the biceps tendon, between the deltoid and infraspinatus muscles.

The radial nerve is the largest branch of the brachial plexus. It is formed from various combinations of C5-T1 roots and innervates the motor bundles of the triceps, brachioradialis, brachialis, and extensor carpi radialis. The radial nerve supplies sensation to the posterior surface of the skin in the distal 2/3 of the arm, the posterior surface of the forearm, and the posterior surface of the 1st, 2nd, 3rd fingers and, partially, the 4th from the lateral side. The radial nerve emerges from the posterior part of the brachial plexus and follows the triceps. Initially, it follows between the coracobrachialis and teres muscles and then between the medial and lateral bellies of the triceps. The deep brachial artery accompanies the radial nerve along its course. In the middle third of the arm, the nerve follows around the posterior surface of the humerus along the prominent line of the deltoid muscle. The nerve is anchored to the humerus where it penetrates the muscular septum in the distal humerus, causing the most common nerve injury in below-mid and distal humeral fractures.

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