Subacromial impingement is the most common complaint concerning the shoulder joint. It is defined as an irritation of the tendons which compose the rotator cuff as they pass through the subacromial space.
Factors involved in injuries to rotator cuff tendons are as follows:
- Vascular factors: The tendons which make up the rotator cuff have sufficient vascularisation except the supraspinatus muscle tendon. Supraspinatus vascularisation relies on the contribution of the blood circulation vessels: the anterior circumflex humeral artery, from the sub and suprascapularis muscle artery and from the tendinous vessels resulting from the prior anastomosis of the anterior ones. The critical area is a small region located medially at 1cm from the insertion of the rotator cuff, a relatively avascular area. It is from this area that fluids flow through the tendon. This capacity diminishes with age.
- Mechanical factors: Neer proved that in the shoulder adduction movement, the rotator cuff chafes against the lower section of the acromion. The mechanical aetiology is aggravated by the presence of an “aggressive” acromion due to its shape or due to an osteophyte (spur) on its lower side. According to Bigliari there are three acromion shapes: type I or flat which is “benign” and types II (curved) and type III (pointed) which are aggressive.
- Another cause of subacromial impingement is the presence of an unfused acromionor “Os Acromiale” which is a variation on the normal situation whereby the acromion is not fused or in one single piece but is unfused or in two parts probably due to the fact that fusion did not occur during childhood. The “Abnormal” movement of this acromion also causes this chafing against the tendons of the rotator cuff.
- Thickening of the subacromial bursais also a factor causing chafing of the structure passing through subacromial space.
- Degenerative factors: Degenerative changes to the rotator cuff are produced in the over-50 age group and this is aggravated by friction against the acromion.
Subacromial impingement is known to be insidious, progressive and mechanical. It does not occur all of a sudden but starts with a slight pain which can gradually lead to a functional lack of power and acute pain. It is a dull pain with a sensation of tiredness which appears at night. When the tissue is at rest the inflammation does not drain properly and for this reason the pain increases. The pain also increases as a result of excessive movement. The pain may also affect the elbow.
The pain may become more intense when raising one’s arms above one’s head or when putting one’s arms behind one’s back, for example in the movement to adjust a bra.