Learning Objectives

By the end of this article, you should be able to:

Describe the anatomy of the glenohumeral joint, including articulating surfaces, capsule, ligaments, and bursae.
List and explain the movements permitted at the shoulder joint.
Understand the balance between mobility and stability in the joint.
Identify the blood supply and innervation of the shoulder joint.
Recognize common clinical conditions affecting the shoulder joint, including dislocations and rotator cuff injuries.

The shoulder joint, also known as the glenohumeral joint, is a ball-and-socket synovial joint formed between the head of the humerus and the glenoid cavity of the scapula. It is one of the most mobile joints in the human body, allowing the upper limb a wide range of movements, but this mobility comes at the cost of relative instability.

Anatomical Structure

Articulating Surfaces

The joint is formed by the humeral head and the glenoid fossa.
The humeral head is much larger than the glenoid fossa, giving the joint a wide range of motion.
To enhance stability, the glenoid cavity is deepened by a fibrocartilaginous glenoid labrum.
Both articulating surfaces are lined with hyaline cartilage.

Joint Capsule

The fibrous joint capsule extends from the anatomical neck of the humerus to the rim of the glenoid cavity.
The capsule is lax, particularly inferiorly, permitting extensive movement.
The synovial membrane lines the inner surface and produces synovial fluid to reduce friction.

Ligaments

Key ligaments stabilizing the shoulder joint include:

Glenohumeral ligaments (superior, middle, inferior) – reinforce the anterior capsule.
Coracohumeral ligament – supports the superior capsule from the coracoid process to the greater tubercle.
Transverse humeral ligament – holds the long head of the biceps tendon in the intertubercular groove.
Coracoacromial ligament – forms the coracoacromial arch, resisting superior displacement of the humeral head.

Bursae

Bursae reduce friction between tendons and bone:

Subacromial bursa – beneath the deltoid and acromion; reduces friction for the supraspinatus tendon.
Subscapular bursa – between the subscapularis tendon and the scapula; facilitates tendon movement.
Minor bursae exist between other tendons and structures around the joint.

Movements

The shoulder joint allows:

Flexion – pectoralis major, anterior deltoid, coracobrachialis; biceps brachii assists.
Extension – posterior deltoid, latissimus dorsi, teres major.
Abduction – supraspinatus (0–15°), deltoid (15–90°), scapular rotation beyond 90° (trapezius, serratus anterior).
Adduction – pectoralis major, latissimus dorsi, teres major.
Internal rotation – subscapularis, pectoralis major, latissimus dorsi, teres major, anterior deltoid.
External rotation – infraspinatus, teres minor.
Circumduction – a combination of all the above movements.

Mobility and Stability

The shoulder joint’s high mobility is balanced by several stabilizing structures:

Factors for mobility:

Ball-and-socket design.
Large humeral head and shallow glenoid cavity (approx. 4:1 ratio).
Lax joint capsule.

Factors for stability:

Rotator cuff muscles – compress the humeral head into the glenoid cavity.
Glenoid labrum – deepens fossa and enhances congruency.
Ligaments – reinforce the capsule; the coracoacromial arch resists superior displacement.
Biceps tendon – minor humeral head depressor, contributing to stability.

Blood Supply

Anterior and posterior circumflex humeral arteries (branches of the axillary artery).
Suprascapular artery (branch of thyrocervical trunk).

Innervation

Axillary nerve
Suprascapular nerve

Clinical Relevance

Shoulder Dislocation

Anterior dislocation is most common (~95%) due to excessive extension and lateral rotation.
Posterior and inferior dislocations are rare.
Hill-Sachs lesions – posterolateral humeral head impaction fractures.
Bankart lesions – detachment of the antero-inferior labrum.
Axillary nerve injury may occur, leading to deltoid paralysis and loss of sensation over the regimental badge area.

Rotator Cuff Tendonitis

Overuse or strain can cause inflammation of the tendons (especially the supraspinatus).
May progress to bursitis or shoulder impingement.
Painful arc (60–120° of abduction) is a key clinical sign.