What is the anatomy of the shoulder?

Anatomy of the Shoulder

Structural Components

The shoulder is a complex joint system comprising five interconnected articulations that work together to provide the upper extremity with the greatest range of motion of any joint in the human body. 1, 2

The Five Joints

• Glenohumeral joint – the primary ball-and-socket articulation between the humeral head and glenoid fossa of the scapula 1
• Acromioclavicular joint – connects the acromion process of the scapula to the lateral clavicle 1
• Sternoclavicular joint – articulation between the medial clavicle and sternum 1
• Scapulothoracic joint – a gliding mechanism between the scapula and posterior chest wall (not a true synovial joint) 1
• Subacromial joint – the space beneath the acromion containing the subacromial bursa 1

Rotator Cuff Muscles

The rotator cuff functions primarily as the active stabilizer of the glenohumeral joint and consists of four muscles with their associated tendons. 1, 3

Individual Muscles and Functions

• Supraspinatus – initiates and assists arm abduction, working optimally between 0-60 degrees of abduction; compresses the humeral head into the glenoid fossa 1
• Infraspinatus – primary external rotator of the shoulder 3
• Teres minor – assists with external rotation 3
• Subscapularis – primary internal rotator, located on the anterior scapula 4, 3

The supraspinatus and deltoid work synergistically during abduction, and if the supraspinatus fails, the deltoid can nearly completely compensate for its function 1

Ligamentous Stabilizers

• Inferior glenohumeral ligament complex – serves as the main passive stabilizer of the shoulder joint 1
• Superior glenohumeral ligament complex – contributes to anterosuperior stability 3
• Capsulolabral structures – the glenoid labrum deepens the socket and enhances concavity-compression stability 3, 5

Vascular Supply

• Anterior circumflex humeral artery and its ascending branches provide the primary blood supply to the humeral head 1
• Posterior circumflex humeral artery contributes several small branches 1
• Arcuate artery (from the lateral ascending branch of the anterior circumflex artery) is the most important intraosseous vessel supplying the humeral head 1

Scapulohumeral Rhythm

The coordinated movement of the clavicle, scapula, and humerus produces scapulohumeral rhythm, which is essential for full shoulder function. 5

• The clavicular joints permit scapular movement against the chest wall during arm motion, allowing the glenoid fossa to follow the humeral head 2
• This functional interrelationship is critical for achieving full range of motion 2
• Disruption of any single component will impair overall upper limb function 2

Additional Structures

• Long head of biceps tendon – travels through the bicipital groove and can be a source of anterior shoulder pain 4, 6
• Deltoid muscle – the primary abductor of the shoulder, covering the rotator cuff superficially 6
• Subacromial-subdeltoid bursa – the most clinically significant bursa, located between the rotator cuff and acromion 6
• Subcoracoid bursa – situated beneath the coracoid process 6

Clinical Relevance

Pathology in any component of this complex system can significantly impair shoulder function, with age-specific patterns of injury. 4

• Patients under 35-40 years predominantly experience instability and labral pathology 4
• Patients over 35-40 years primarily develop rotator cuff disease, degenerative changes, and impingement syndrome 4