What is the anatomy of the shoulder?

Shoulder Anatomy

The shoulder is a complex multi-joint system consisting of four articulations (glenohumeral, acromioclavicular, sternoclavicular joints, and the scapulothoracic gliding mechanism) that work synergistically to provide the greatest range of motion of any joint in the human body 1, 2.

Bony Components

The shoulder girdle consists of three primary bones: the humerus, scapula, and clavicle 3, 4.

• The proximal humerus ossifies completely by 17-18 years of age 5
• The glenoid (part of the scapula) ossifies by 16-18 years 5
• The clavicle is the last to ossify, completing by 18-20 years 5
• The sternal manubrium articulates with the clavicle at the sternoclavicular joint 3

Glenohumeral Joint

The glenohumeral joint is the primary articulation of the shoulder, where the humeral head articulates with the glenoid fossa of the scapula 4, 1.

Static Stabilizers

• Glenohumeral ligaments (superior, middle, and inferior) provide passive stability 4
• The inferior glenohumeral ligament complex serves as the main passive stabilizer of the shoulder 6
• The glenoid labrum is a fibrocartilaginous structure that deepens the glenoid socket and provides attachment for ligaments 4
• The joint capsule surrounds the glenohumeral joint and is stronger than the epiphyseal plates in adolescents 5

Dynamic Stabilizers: The Rotator Cuff

The rotator cuff consists of four muscles and their tendons: supraspinatus, infraspinatus, teres minor, and subscapularis 3, 4, 6.

• The supraspinatus compresses the humeral head into the glenoid and assists with abduction, working most optimally between 0-60 degrees of abduction 6
• The infraspinatus and teres minor provide external rotation and posterior stability 3, 4
• The subscapularis provides internal rotation and anterior stability 3, 4
• The rotator cuff functions primarily as an active stabilizer, resisting translational forces at the glenohumeral joint 5, 6

Additional Muscular Components

• The deltoid muscle works synergistically with the supraspinatus for abduction and can nearly completely compensate for supraspinatus failure 6
• The biceps brachii (long head) originates from the supraglenoid tubercle and contributes to shoulder stability 3
• The pectoralis major provides internal rotation and adduction 3

Acromioclavicular and Sternoclavicular Joints

The clavicular joints permit scapular movement against the chest wall during arm movements, allowing the glenoid fossa to follow the humeral head 2.

• The acromioclavicular joint connects the acromion process of the scapula to the lateral clavicle 5, 3
• The sternoclavicular joint connects the medial clavicle to the sternal manubrium 3, 2
• These joints contribute significantly to total arm movement, and pathology in either disrupts upper limb function 2

Scapulothoracic Mechanism

The scapulothoracic articulation is not a true joint but a gliding mechanism between the scapula and thoracic cage that is critical for shoulder function 1, 2.

Subacromial Space

The subacromial-subdeltoid bursa is the most clinically significant bursa, located between the acromion/deltoid and the rotator cuff, reducing friction during shoulder movement 3.

Vascular Supply

The humeral head receives blood supply primarily from the anterior circumflex humeral artery and its ascending branches, with contributions from the posterior circumflex artery and intraosseous anastomoses 6.

Age-Related Anatomical Considerations

Adolescent Shoulder Anatomy

In skeletally immature individuals, the epiphyseal plates are weaker than the surrounding joint capsule and ligaments, making them vulnerable to injury 5.

• Adolescents produce significantly more type III collagen (the major protein of ligaments and tendons) compared to adults 5
• This increased collagen production affects the mechanical properties of supporting structures 5

Clinical Imaging Considerations

For traumatic shoulder evaluation, standard radiographs should include anteroposterior views in internal and external rotation plus an axillary or scapula-Y view, as glenohumeral and acromioclavicular dislocations can be misclassified on AP views alone 5, 7.