Because of its freedom of movement and instability, the glenohumeral joint is commonly dislocated by direct or indirect
injury. Because the presence ofthe coraco-acromial arch and support ofthe
rotator cuffare effective in preventing upward dislocation, most dislocations ofthe humeral head occur in the downward (inferior) direction. However, they are described clinically as anterior or
(more rarely) posterior dislocations, indicating whether the humeral head has descended anterior or posterior to the infraglenoid tubercle and long head of the triceps. The head ends up lying
anterior or posterior to the glenoid cavity.
Anterior dislocation of the glenohumeral joint occurs most often in young adults, particularly athletes. It is usually caused by excessive extension and lateral rotation of the humerus (Fig.
B6.3 3 ). The head ofthe humerus is driven infero-anteriorly, and the fibrous layer ofthe j oint capsule and glenoid labrummay be stripped fromthe anterior aspect ofthe glenoid cavity in the process. A hard blow
to the humerus when the glenohumeral j oint is fully abducted tilts the head ofthe humerus inferiorly onto the inferior weak part ofthe j oint capsule. This may tear the capsule and dislocate the shoulder so that
the humeral head comes to lie inferior to the glenoid cavity and anterior to the infraglenoid tubercle. The strong flexor and adductor muscles ofthe glenohumeral j oint usually subsequently pull the humeral head
anterosuperiorly into a subcoracoid position. Unable to use the arm, the person commonly supports it with the other hand.