Subacromial impingement is a condition that causes pain in the shoulder and upper arm. It happens when the tendons of the shoulder joint are strained or injured, or when a spur of bone develops under a flat bone found at the top of your shoulder blade, called the acromion. Either of these conditions can result in the inflammation, pinching or rubbing of any of four tendons (known as the rotator cuff) against the acromion. When this happens it is known as impingement.
Why does it happen?
The exact cause of subacromial impingement is not known. Some people may be
more susceptible to a wear-and-tear process in the tendon, where it can start to
fray or split. It can occur spontaneously with no apparent cause or due to the
result of minor injury, although it is often triggered by subjecting the shoulder to
repetitive overhead activities.
Age may be a factor in the onset of subacromial impingement, since bone spurs
can develop as you get older. The rotator cuff group of tendons is also more
prone to strains and tears with increasing age, and is most common in people
over the age of 40. In younger people impingement can often be caused by
shoulder instability (such as dislocation).
The shape of the acromion can also make some people more susceptible to the development of subacromial impingement.
How common is it?
Extremely common. Subacromial impingement is the most common problem of the shoulder. It is thought that as many as 20% of all people will have symptoms at some time.