Free Health Dissertations - The Shoulder Is A Ball And Socket Joint And Is The Most Mobile Joint In The
The shoulder is a ball and socket joint and is the most mobile joint in the body. The socket on the scapular is deepened by a fibrocartilagenous rim attached to it called the glenoid labrum. The rotator cuff of muscle surrounds this capsule. Because it is so mobile this joint is one of the commonest to dislocate accounting for about 50% of all dislocations. The surrounding soft tissues give the joint stability and are disrupted upon dislocation. Different age groups are associated with different types of dislocation. In older patients there are tears of the rotator cuff and fracture of the humerus; the surrounding muscles, their strength and co-ordination, provide day to day stability. In younger patients the anterior ligament tears the Bankart lesion; the ligaments provide ultimate stability during sports and accidents. Ninety-six percent of shoulder dislocations are traumatic.
Anterior dislocation
Eighty-five percent of shoulder dislocations are anterior. Falling onto an outstretched hand is the usual cause. It is associated with rough contact sports such as rugby and also with cycling and skiing injuries. The mechanism of injury involves external rotation, extension and abduction. On examination the deltoid muscle is flattened. There is a positive Hamilton’s ruler sign; the acromion tip is in a straight line with the lateral epicondyle of the humerus since the humeral head no longer interrupts this line. In slim patients there is a swelling anteriorly. The accessory nerve supplies sensation over a small patch of skin on the lateral aspect of the shoulder; sergeant’s stripes area which should be assessed for numbness.
To aid diagnosis two X-rays are recommended: AP and lateral. The AP film will show the humeral head and glenoid fossa overlapping; in dislocation the humeral head is usually inferomedial to the socket. A Hill-Sachs lesion may be seen. This is damage to the posterior part of the humeral head caused by the glenoid at dislocation. Fracture of the anterior part of the glenoid may cause a bony Bankart lesion visible on X-Ray.
Appart from the Bankart and Hill-Sachs lesions already described the axillary artery and nerve may be damaged. The humerus may fracture and the brachial plexus is at risk. The axillary nerve supplies the deltoid and teres minor muscles. It can also be damaged by manipulative treatment.
Axillary nerve damage causes loss of sensation over the sergeant’s stripes position and the deltoid muscle looses its power. This latter aspect means the injured person cannot abduct the arm from a neutral position. Subsequent muscle wasting erodes the curve of the shoulder. This is treated by a splint in abduction.
Treatment of anterior dislocation
Immediate reduction lessens the risk of nerve damage. If this fails reduction is under general anaesthetic and is usually still closed. Since manipulation can damage the axillary nerve the nerve function should be documented beforehand.
