The Aetiology of Subacromial Impingement Syndrome

Introduction Subacromial impingement syndrome (SIS) is a general term used to describe a variety of conditions that may act independently or in combination, and manifest as anterior or anterior-lateralsuperior shoulder pain. SIS occurs as a result of pathology of one or more of the structures located within the subacromial space. The pain is associated with a loss of shoulder function, especially during overhead activities, occurring during vocational, sporting or the normal activities associated with daily living. The subacromial space has also been termed the acromio-humeral joint or the bursal joint (Wiles, 1955), the supraspinatus outlet (Neer and Poppen, 1987), and the suprahumeral space (Calliet, 1991). These names reflect the borders and contents of this region. The superior border is the coracoacromial arch, comprising the inferior surface of the acromion, the coracoacromial ligament and the coracoid process. The inferior border comprises the greater tuberosity and superior aspect of the head of the humerus. Petersson and RedlundJohnell (1984) in a series of 175 radiographs reported that the mean distance between the inferior border of the acromion and the superior border of the humerus in an anteroposterior projection was between 9 to 10 millimetres (mm). Located within the subacromial space are the tendons of the rotator cuff and the long head of biceps, the subacromial/subdeltoid bursa and the superior capsule of the glenohumeral joint. During elevation of the arm the greater tuberosity moves closer to the acromion, narrowing the space. SIS results from pathology of any of these structures. The inferior aspect of the acromioclavicular joint has also been implicated in the aetiology and pathogenesis of SIS (Petersson and Gentz, 1983).