Shoulder Complex (Glenohumeral + AC + SC)

complexus articulationis humeri

The shoulder complex is the most mobile joint complex in the body, achieving the greatest arc of motion of any joint through the coordinated action of four joints: the glenohumeral ball-and-socket (primary motion), the acromioclavicular joint (scapular rotation on clavicle), the sternoclavicular joint (clavicular elevation and rotation), and the scapulothoracic pseudoarticulation (scapular rotation on the thoracic wall). Loss of any component produces compensatory overloading of the others.

Anatomical Data

Structure

Bones	Humeral head Glenoid Acromion + lateral clavicle (AC) Medial clavicle + sternum + first rib (SC)
Ligaments	Glenohumeral ligaments (superior, middle, inferior) Coracohumeral ligament Coracoclavicular ligaments (conoid, trapezoid) AC ligament SC ligaments

Biomechanics

Function & Actions

Movements	Total arm elevation 180 degrees (120 glenohumeral + 60 scapulothoracic rotation) External rotation 90 degrees Internal rotation to T7 behind the back Combined motion requires all four joints to contribute normally

Clinical Relevance

Clinical Notes

The 2:1 glenohumeral-to-scapulothoracic rhythm describes the coordinated movement ratio, for every 2 degrees of glenohumeral elevation there is 1 degree of scapular upward rotation. Disruption of this rhythm (scapular dyskinesis) from rotator cuff pathology, AC joint dysfunction, or thoracic kyphosis alters shoulder mechanics and contributes to impingement.

Pathology

Common Injuries & Conditions

Frozen Shoulder (Adhesive Capsulitis)

Glenohumeral capsular contracture producing the classic pattern of equal active and passive restriction in external rotation, abduction, and internal rotation, managed with physiotherapy, injection, and hydrodilatation.

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