Impingement syndrome refers to a condition where elevation and rotation of the arm brings the rotator cuff into close contact with the corner of the shoulder blade called the acromion. The acromion creates a roof of sorts above the rotator cuff and the space beneath the acromion, referred to as the subacromial space, is quite limited. Anything that closes down this space, be it extra bone formation on the underside of the acromion or poor shoulder mechanics that allow the entire shoulder blade to pitch forward, a condition termed scapular dyskinesis, can lead to impingement of the acromion on the supraspinatus and infraspinatus.
Symptoms of subacromial impingement mirror those of other rotator cuff disorders, generally leading to pain with forward elevation or rotation of the arm. The pain is typically experienced in the upper arm region often leading patients to believe that they have an arm problem as opposed to a true shoulder problem.
A less common type of impingement called subcoracoid impingement has been described where a separate segment of the shoulder blade called the coracoid comes into contact with the anterior most portion of the rotator cuff called the subscapularis. The symptoms here may be more vague and difficult to characterize but are felt more anteriorly in the shoulder often with crossing over the front of the body motions with the shoulder.