The term subacromial impingement is used to describe the compression or 'pinching' of structures within in the subacromial space of the shoulder. This is not a 'diagnosis' as such but a symptom or clinical sign that can be caused by different conditions in the shoulder. The term 'subacromial pain syndrome' is a more recent diagnostic label which encompasses the different conditions that can result in subacromial impingement.
The shoulder joint is made up of three bones:
1) The upper arm bone (humerus)
2) The shoulder blade (scapula)
3) The collarbone (clavicle).
The Subacromial Space
The subacromial space lies between the humeral head, or ‘ball’, and the acromion which is part of the shoulder blade. This is indicated by the arrow on the above diagram. The sub-acromial bursa and tendons of the rotator cuff sit within the sub-acromial space and when impingement occurs they can become compressed. When this happens a ‘painful arc sign’ is often seen.
As we lift our arm, the sub-acromial space begins to narrow. This narrowing is most marked when the arm is at shoulder height. Above this, the space starts to increase again. This can often give rise to the classic ‘painful arc’ sign where pain is worst when the arm is lifted at shoulder height and decreases the further the arm is raised.
For a many years it has been thought that shoulder impingement is the result of the shape of the acromion and/or the formation of bony spurs on the underside of the acromion. As the acromion forms the roof of the subacromial space a 'hooked' acromion was thought to cause the compression or ‘pinching’ of the structures within it. A surgical procedure called an acromioplasty was developed where part of the acromion is removed.
New theories, however, suggest that the pinching is often not a result of a hooked acromion but a problem with the rotator cuff tendon itself (see below). Studies have shown that lots of people have a hooked acromion without having any shoulder pain suggesting this is not the primary cause of pain. For further information on this controversial topic please see our blog entry.
The Rotator Cuff
The rotator cuff is comprised of four muscles:
3) Teres Minor
These muscles surround the ball and socket of the shoulder joint forming a 'cuff' around it. The shoulder joint itself is inherently unstable due to the shallow socket. The main function of the rotator cuff is to contain the ball within the socket during movement of the arm.
During elevation of the arm, if the rotator cuff is not functioning properly, the ball can pulled upwards towards the acromion, by the bigger deltiod muscle. This can result in painful compression of the structures within the subacromial space; the subacromial bursa and rotator cuff tendon itself causing further irritation and it becomes a vicious cycle.
There are a number of conditions that can lead to dysfunction of the rotator cuff:
- rotator cuff tendinopathy
- an acute injury or strain of the rotator cuff
- calcific tendonitis
- partial or full thickness tear of the rotator cuff
Side view of the shoulder socket (gleniod) Type III shows a hooked acromion
Causes of Subacromial Impingement