The acromion differs in individuals normally (morphological variants). These were described by Bigliani as type I, type II or type III. Type I is flat, type II is curved and type III hooked. A person with a type II or type III acromion would be at a higher risk of impingement due to the narrowing of the acromiohumeral gap and bursal space.
In addition to the above, with advancing age people tend to develop a bone spur on the front and side of the acromion. This further reduces the subacromial space increasing the risk of impingement.
Therefore somebody with a rotator cuff injury who has a type III acromion and is their 50’s has a very high risk of developing significant impingement compared to a patient in their 20’s sustaining a supraspinatus strain who may have a type I acromion and no spurs.
The rotator cuff and acromion will then rub against one another, causing a painful condition known as impingement. Each time the arm is raised there is a bit of rubbing on the tendons and the bursa between the tendons and the acromion, which may cause pain and inflammation.
Impingement may become a serious problem for some people and disturb their normal activities. This is when treatment is needed.