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.
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.
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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