Sub-acromial impingement is a generic diagnosis that encompasses a range of more specific diagnoses to the shoulder. The term is often used instead of the more specific diagnoses as a clinical examination and imaging findings may not be conclusive and there is often considerable overlap of these specific diagnoses.
The sub-acromial space is the term given to the space at the top of the shoulder complex (see diagram). It is bordered above by the end of the collar bone (clavicle), the acromion bone of the shoulder blade (scapular) and the joint they form (acromio-clavicular joint). Below the sub-acromial space lies the top of the upper arm bone (humeral head) and the shoulder joint (gleno-humeral joint).
The sub-acromial space itself contains a fluid sac termed the sub-acromial bursa as well as the tendon of the supraspinatus muscle. Under normal circumstances, the sub-acromial bursa adds a little protection to the shoulder complex and the supraspinatus tendon and the tendon itself is able to move freely within this space to aid lifting and load bearing through the upper arm.
As the name suggests, sub-acromial bursitis is the term given for inflammation to the sub-acromial bursa.
The rotator cuff muscles and tendons are a group of muscles which attach from the shoulder blade (scapular) and span the front, back and top of the shoulder joint to insert into the upper arm bone (humeral head).
Some cases of shoulder pain may be caused by an acute bout of inflammation to part of the rotator cuff tendon. This is often referred to as rotator cuff tendinitis or a supraspinatus tendinitis (if this is the tendon affected).
This condition may cause significant shoulder pain and is most often caused by a sudden unaccustomed use of the shoulder involving significant load being placed through the rotator cuff tendon. Examples may be repetitive lifting of weight or overhead activities.
The supraspinatus forms the top part of the rotator cuff tendon and runs directly through the sub acromial space. It also bears the most force during many of the common shoulder movements we perform and as such is the most common tendon to cause shoulder pain.
A tendinopathy refers to pain which is related to changes within the tendon structure rather than inflammatory changes to the tendon (tendinitis).
The exact cause of why individuals begin to suffer with sub-acromial impingement is not known. It was traditionally thought that the sub-acromial bursa and supraspinatus tendon became painful as a result of being squashed within the sub-acromial space during overhead shoulder movements. However, several studies have demonstrated that these structures are regularly compressed within the space without causing any pain in individuals without a history of shoulder problems.
Contemporary research also indicates that tendon disorders may be caused by repeated load through tendons which are not able to regenerate themselves between load. This has been termed a failed healing response.
Most clinicians take a pragmatic view in that for different individuals there may be a combination of these factors taking place to cause sub-acromial pain.
Ultrasound and MRI scans often demonstrate several anatomical faults within the shoulder in individuals suffering from sub-acromial impingement. These include inflammation, thickening or calcific deposits to the tendons of the rotator cuff. Swelling and enlargement to the sub-acromial bursa is also a common finding.
Some individuals may have additional thickening and bony spurs known as osteophytes to the underside of the acromio-clavicular joint. The shape of the acromion bone may also be curved or hooked downwards. Both of these changes may narrow the sub acromial space from above.
Of note is that many of these findings are seen on the opposite side to the pain and are commonly found among individuals with no history of shoulder pain.
Several factors may increase the forces being placed through the rotator cuff tendons and may also lead to an increased narrowing of the sub acromial space during overhead shoulder movements. Poor flexibility through the middle and upper spine or the pectoral muscles reduce the amount of movement that can be created by the shoulder blade (scapular). Weakness of the muscles which rotate the shoulder blade upwards may also limit the range of motion.
These factors may mean that when lifting the arm above the head, the shoulder joint (gleno-humeral joint) has to contribute more range to the movement, thereby, increasing forces through the rotator cuff tendons and increasing the likelihood of compressing the structures within the sub-acromial space.
Weakness and poor condition to the rotator cuff tendons themselves may also contribute to an inability to maintain the sub-acromial space during overhead movements as the upper arm bone may slide upwards in the shoulder socket where weakness exists.