Shoulder joint

The large humeral head articulates with a glenoid cavity only about one-third its area, deepened by a fibrocartilaginous glenoid labrum. The capsule is loose, attached around the glenoid beyond the labrum and to the anatomical neck of the humerus (extending down the medial shaft); it has a redundant inferior axillary recess that permits abduction and is the weakest part. The long head of biceps passes through the joint from the supraglenoid tubercle, enveloped in synovium, and exits through the intertubercular sulcus.

Because bony and ligamentous stability is limited, the joint relies on dynamic stabilisers — chiefly the rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis), whose tendons blend with the capsule and compress the humeral head into the glenoid. Static stabilisers include the glenohumeral ligaments (superior, middle, inferior — the inferior glenohumeral ligament is most important in abduction and external rotation), the coracohumeral ligament, the transverse humeral ligament and negative intra-articular pressure. Above, the coracoacromial arch (acromion, coracoid and coracoacromial ligament) prevents superior displacement, with the subacromial bursa cushioning the supraspinatus beneath.

Movements: flexion–extension, abduction–adduction, medial–lateral rotation and circumduction. Full elevation (≈180°) combines ≈120° of glenohumeral movement with ≈60° of scapular rotation (scapulohumeral rhythm 2:1).