The rotator cuff - the group of muscles and their accompanying tendons that act to stabilize the shoulder joint - occupies a very tight space between the acromion (the rigid bony arch of the shoulder blade) and the humeral head (top of the upper arm bone). The rotator cuff's limited space and its role as a dynamic stabilizer of the shoulder joint, which is itself designed to accommodate the upper arm's tremendous range of motion, leaves it susceptible to becoming pinched or trapped between the bony surfaces surrounding it, a condition referred to as impingement.
What is shoulder impingement syndrome?
Any condition that causes an excessive and painful narrowing of the already limited space under the acromion (or other locations in the shoulder) is referred to as shoulder impingement syndrome. Such conditions include existing (abnormal) anatomical structures, muscle or tendon inflammation and degenerative arthritis. The vexing problem, and perhaps the reason behind the reference to a syndrome, is that these conditions can quickly lead to a vicious cycle, whereby the triggering of inflammation (e.g. in a rotator cuff tendon or in the bursa under the acromion) leads to a further narrowing of the subacromial space, and therefore further irritation and inflammation.
A Further Explanation
The arm's tremendous range of motion and the mobility and special anatomy of the shoulder joint that allow for this range of motion make it is necessary to ensure the exact centering of the humeral head in the shoulder joint. Unlike in the case of the hip whose deep socket ensures such centering while also limiting its range of motion, this task is performed in the shoulder by the soft-tissue components of the rotator cuff. The tendons and muscles of the rotator cuff are responsible for dynamically holding the humeral head properly aligned in the dish-shaped portion of the lateral scapula. Only optimal positioning will prevent a portion of one of the tendons from becoming impinged during movement.
What are the known causes of impingement syndrome?
Various factors may play a role in the excessive narrowing (impingement) of the subacromial space (i.e. the space between the undersurface of the acromion and the top of the humeral head). One important distinction in this regard is made between functional impingement and structural impingement.
Functional impingement is involved, for instance, when the impingement is caused by a momentary malfunctioning of the otherwise entirely healthy muscles that enter the rotator cuff. In contrast, structural impingement arises as a result of abnormal joint structure or changes in the bursa, tendons, bones, or ligaments.
- Causes of Structural Impingement
- Thickened or calcified ligaments
- Subacromial spurs
- Wear-related enlargement of the acromioclavicular joint
- Calcified rotator cuff tendon
- Thickened rotator cuff tendon as a result of chronic inflammation
What are the symptoms that are associated with shoulder impingement syndrome?
The crowding of the tendon under the acromion leads to shoulder pain when you move your arm in certain ways. This is because the movement actually leads to the pinching or tearing of the tendon. This pain tends to be most intense when raising your arm up from your side or to the rear. You may also feel pain on the outside of your upper arm.
Other symptoms may appear as the disorder persists. Irritation of the bursa between the acromion and the rotator cuff tendon often leads to shoulder pain at night and can make it virtually impossible to sleep on the affected shoulder. The shoulder joint's range of motion may also decrease significantly as the impingement persists. This is referred to as secondary shoulder stiffness or frozen shoulder.
Generally speaking, impingement can lead to prolonged or chronic shoulder pain.
What forms of treatment are available for shoulder impingement syndrome?
Conservative forms of treatment, including rest and physical therapy, are usually effective. Shockwave therapy may be advisable as a complement to the full array of conservative measures. The high energy sonic waves produced can pulverize calcific deposits and inflamed or necrotic tissue. This then permits their absorption by the body.
If conservative forms of treatment prove ineffective, arthroscopic shoulder surgery may help to eliminate the pinching of the tendon. The aim of arthroscopic shoulder surgery is usually to expand the available space by shaving and smoothing the underside of the acromion and removing the bursa that is located between it and the rotator cuff.
