home Anatomy, Education, Emergency Medicine Shoulder Dislocation

Shoulder Dislocation

Anterior view of the left shoulder. © Medicine CF

The shoulder joint, or glenohumeral joint is a synovial ball and socket articulation. This allows the joint a wide range of movement including: flexion, extension, abduction, adduction, medial rotation, lateral rotation and circumduction. The increased movement comes at a cost of skeletal stability, which is instead provided by the rotator cuff muscles, the long head of the biceps brachii muscle, bony processes of the scapula, and extracapsular ligaments.

The shoulder joint or glenohumeral joint is a synovial ball and socket articulation. This allows the joint a wide range of movement including flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction. The increased movement comes at a cost of skeletal stability, which is instead provided by the rotator cuff muscles, the long head of the biceps brachii muscle, bony processes of the scapula, and extracapsular ligaments.

The surrounding muscle tendons and a skeletal arch formed superiorly by the coracoid process and acromion and the coracoacromial ligament provide joint stability. Tendons of the rotator cuff muscles are the: supraspinatus, infraspinatus, teres minor, and subscapularis muscles (mnemonic = “SITS”). They form a musculotendinous collar that surrounds the posterior, superior and anterior aspects of the glenohumeral joint. This stabilizes and holds the head of the humerus in the glenoid cavity of the scapula without compromising the arms flexibility and range of motion. The long head of the biceps brachii muscles restricts upward movement of the humeral head on the glenoid cavity.

Anterior-Posterior X-Ray demonstrating an anteroinferior dislocation of the humoral head. © Medicine CF

Shoulder dislocation is a common problem and may occur repetitively. Anterior dislocation occurs most frequently with the head of the humerus appearing anteroinferior. To get there, the humoral head must be displaced inferiorly, then anteriorly, and finally superiorly. Once a joint capsule and cartilage are disrupted, the joint is susceptible to recurrent dislocations. An anteroinferior dislocation may damage the axillary nerve, which originates from the brachial plexus at the level of the axilla, by direct compression of the humeral head on the nerve inferiorly as it passes through the quadrangular space. The axillary nerve supplies 3 muscles: the deltoid, the teres minor, and the triceps brachii. Injury to the axillary nerve leads to axillary nerve palsy characterized by paralysis of the deltoid and teres minor resulting in loss of abduction of the arm (from 15-90 degrees), weak flexion, extension, and rotation of the shoulder.

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The lengthening effect of the humerus may also stretch the radial nerve, which is tightly bound within the radial groove, and produce a radial nerve paralysis. The radial nerve and its branches provide motor innervation to the dorsal arm muscles (the triceps brachii and the anconeus) and the extrinsic extensors of the wrists and hands; it also provides cutaneous sensory innervation to most of the back of the hand.

Posterior shoulder dislocation is rare. when seen, the clinician should focus on its cause, the most common being extremely vigorous muscle contractions, which may be associated with an epileptic seizure.

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Juan Antonio Aguilar Garcia, M.D.

Having the knowledge to help people live healthier and longer lives is a privilege. Sharing that knowledge with the world is an obligation.