Shoulder Joint

Introduction

Introduction to the Shoulder (Glenohumeral) Joint

The shoulder joint, or glenohumeral joint, is a multiaxial ball-and-socket synovial articulation between the humeral head and the glenoid fossa of the scapula. It has the greatest range of motion of any joint in the body, a feature made possible by the shallowness of its bony socket and the laxity of its capsule. The price of this mobility is reduced stability, making the shoulder the most commonly dislocated major joint. Its stability depends on a complex interplay of static stabilisers (the glenoid labrum, glenohumeral ligaments, and joint capsule) and dynamic stabilisers (the rotator cuff muscles and the long head of biceps brachii).

This module covers the anatomy of the shoulder joint, including its articular surfaces, labrum, ligaments, bursae, surrounding muscles, neurovascular supply, movements, and the common clinical conditions affecting it.

Study

1. Articular Surfaces and Glenoid Labrum

The humeral head is approximately hemispherical, facing medially and posteriorly. It articulates with the pear-shaped glenoid fossa of the scapula, which is shallow and much smaller. The glenoid labrum, a fibrocartilaginous rim attached around the glenoid margin, deepens the socket by about 50% and provides attachment for the glenohumeral ligaments and the long head of biceps brachii. The superior labrum is continuous with the biceps tendon origin, forming the biceps-labral complex.

2. Joint Capsule and Ligaments

The fibrous capsule is loose and redundant, especially inferiorly, to permit a large range of movement. It attaches medially to the glenoid labrum and laterally to the anatomical neck of the humerus, descending further on the medial side of the surgical neck. The capsule is strengthened by three glenohumeral ligaments (superior, middle, and inferior), which are thickenings on its anterior aspect. The inferior glenohumeral ligament is the most important, acting as an anterior sling to prevent anterior dislocation of the humeral head.

The coracohumeral ligament runs from the base of the coracoid process to the greater tuberosity of the humerus, strengthening the superior capsule and helping resist inferior translation. The transverse humeral ligament bridges the bicipital groove, holding the long head of biceps tendon in place.

3. Bursae and Synovial Cavity

The synovial membrane lines the inner surface of the capsule and forms a sheath around the long head of biceps brachii as it passes through the bicipital groove. Important bursae around the joint include the subacromial-subdeltoid bursa (between the rotator cuff and the acromion/deltoid), the subscapular bursa (communicating with the joint), and the subcoracoid bursa. The subacromial bursa is often involved in impingement syndrome.

4. Muscles and Rotator Cuff

Four muscles form the rotator cuff: supraspinatus, infraspinatus, teres minor, and subscapularis. Their tendons blend with the joint capsule and provide dynamic stability by compressing the humeral head into the glenoid fossa. The supraspinatus initiates abduction; infraspinatus and teres minor externally rotate; subscapularis internally rotates the arm. The long head of biceps brachii passes intracapsularly and acts as a humeral head depressor.

5. Blood Supply and Nerve Supply

The shoulder joint receives its arterial supply from the anterior and posterior circumflex humeral arteries and the suprascapular artery. Nerve supply is provided by the axillary nerve (C5, C6), suprascapular nerve (C5, C6), and lateral pectoral nerve (C5, C6). Hilton's law applies: the nerves supplying the muscles crossing the joint also innervate the joint.

6. Movements and Range of Motion

The shoulder allows flexion (0–180°), extension (0–45°), abduction (0–180° via scapulothoracic rhythm), adduction, medial and lateral rotation (0–90° each), and circumduction. The initial 0–15° of abduction is initiated by supraspinatus; deltoid takes over from 15–90°; from 90–180°, scapular rotation contributes significantly.

7. Clinical Considerations

Anterior glenohumeral dislocation is the most common joint dislocation, often caused by abduction and external rotation. It can be associated with a Bankart lesion (avulsion of the anteroinferior labrum) and a Hill-Sachs lesion (compression fracture of the posterolateral humeral head). Axillary nerve injury is common.

Rotator cuff tears, particularly of supraspinatus, cause pain and weakness in abduction. Impingement syndrome results from compression of the supraspinatus tendon and subacromial bursa between the humeral head and the acromion. Adhesive capsulitis (frozen shoulder) is characterised by progressive pain and stiffness.

Summary

Summary of the Shoulder Joint

The glenohumeral joint is a ball-and-socket synovial joint designed for mobility. It relies on the glenoid labrum, glenohumeral ligaments, rotator cuff, and biceps tendon for stability. The joint is supplied by the circumflex humeral and suprascapular arteries, and innervated by the axillary, suprascapular, and lateral pectoral nerves. Common clinical conditions include anterior dislocation, rotator cuff pathology, impingement, and frozen shoulder.