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Shoulder Anatomy

by dr kalman piper

The Shoulder Joint

The shoulder joint is a “ball and socket” joint, between the humerus (upper arm bone) and the scapula (“shoulder blade”). The ball is called the humeral head. Its rounded end has an articulating surface covered in smooth cartilage to reduce friction when moving, and some boney prominences, called the tuberosities, to which are attached the tendons that move the arm. The glenoid is the socket part of the scapula, and it slightly “pear” shaped”, rather than perfectly round. It also has articular cartilage lining its surface. The humeral head is very large compared to the smaller, shallow glenoid – visualise a golf ball sitting on a tee. This allows the arm greater freedom of movement, but at the expense of stability – shoulder dislocations are very common. The hip joint is another ball and socket joint, however the ball is set deeply in its socket. The hip joint cannot move through as greater range as the shoulder, but dislocation is very rare.

The Labrum and Biceps Tendon

Surrounding the glenoid is a tough fibrous wedge-shaped ring of tissue called the labrum. It helps increase the stability of the shoulder joint (preventing dislocation) while still allowing freedom of movement. It is attached to the rim of the glenoid and acts like a chock preventing the humeral head slipping off the glenoid. It is prone to injury, particularly when the shoulder dislocates. Damage to the front (anterior) part of the labrum is called a Bankart injury and is associated with anterior shoulder dislocation, whereas damage to the top (superior) part of the labrum is called a SLAP tear. SLAP tears are usually associated with a fall on the outstretched arm or repetitive overhead activities and throwing sports. At the top of the ring, one of the tendons of the biceps (the long head) is attached to the labrum. This area is called the biceps anchor and it is where SLAP tears occur. The biceps muscle has two tendons (hence the name) called the short head and the long head. The short head is the strongest and performs most of the work during biceps muscle use. The long head of biceps courses over the humeral head, travels through the shoulder joint and then attaches to the superior labrum. Its function is limited and cutting the tendon (tenotomy) doesn’t cause any loss of strength of the biceps muscle in most people. Occasionally, a rupture or tenotomy of the long head of biceps can cause a change in the appearance of the biceps muscle belly (called the “Popeye” sign), however there is no change in the function of the muscle, only the cosmetic appearance.

The Rotator Cuff

The rotator cuff is a group of four muscles and their tendons that attach to the tuberosities on the humeral head. By pulling on the tendons, the muscles rotate the round head on the glenoid and consequently move the arm, hence the name rotator cuff. One tendon attaches to the front of the humeral head, one tendon attaches to the top of the head and two tendons attach to the back of the humeral head.

The tendon at the top of the humeral head is called the supraspinatus tendon. Its action rotates the humeral head and causes elevation of the arm above the head. Similarly, the tendon at the front of the shoulder (subscapularis) internally rotates the humeral head and moves the arm behind the body, for tasks such as tucking in a shirt at the back or doing up a bra strap. There are two tendons (infraspinatus and Teres minor) that attach to the back of the humeral head and externally rotate the humeral head. This action is particularly important flor activities such as drinking from a cup, holding a telephone against your ear or brushing your hair.

Rotator cuff tears are very common. The supraspinatus tendon is the most commonly torn tendon. Isolated tears of the other tendons are uncommon, but commonly occur in association with tears of supraspinatus. The long head of biceps tendon is also frequently involved in rotator cuff tears.

The Acromion and Deltoid Muscle

The boney tip of the shoulder that can be felt beneath the skin is part of the scapula called the acromion. The deltoid muscle (which forms the bulk of the muscle felt around the shoulder) attaches to the acromion and aides the rotator cuff in movements of the arm. The rotator cuff tendons move back and forth underneath the acromion. The space between the rotator cuff and the under surface of the acromion is called the subacromial bursa. The space available for movement of the rotator cuff can be diminished by a spur of bone that commonly occurs underneath the acromion. This spur of bone can rub on the rotator cuff (impingement) and cause inflammation in the subacromial bursa (subacromial bursitis) and subsequent rotator cuff tear.

The Clavicle and Acromioclavicular (AC) Joint

The clavicle (collar bone) runs between the sternum and the acromion and acts as a strut between the body and the shoulder joint. It takes load when the arm is lifted above head height, particularly during activities such as stacking high shelves or lifting weights. Fractures of the clavicle are very common and usually occur from an impact directly on side the shoulder during a fall or rugby tackle. In the past, most clavicle fractures were treated without surgery; however, for displaced fractures recent research shows that long term results are better with surgery. The joint between the acromion and clavicle is the AC joint and it can be felt underneath the skin over the shoulder. Arthritis of the AC joint is very common and nearly all people have arthritis of this joint in their later years, but for most it doesn’t cause a probable. Very occasionally arthritis of this joint causes pain, which is usually felt directly over the joint. Very severe arthritis of the joint can form spurs underneath the joint which causes shoulder impingement and rotator cuff tear, similar to a subacromial spur.