The gleno-humeral joint is commonly accepted to be the most complex joint in the human body. Possibly due to the difficulty of arriving at an accurate diagnosis, the term "shoulder impingement syndrome" (SIS) has become a standard term in shoulder diagnosis. Any condition that narrows the space between the anterior/inferior aspect of the acromion and coracoacromial ligament can result in SIS (1). The most common causative factors are a thickened subacromial bursa and rotator cuff tendonopathy.
Jablonski's Dictionary of Syndromes and Eponymic Diseases (2nd edition) defines shoulder impingement syndrome as "Compression of the rotator cuff tendons and subacromial bursa between the humeral head and structures that make up the coracoacromial arch and the humeral tuberosities. This condition is associated with subacromial bursitis and rotator cuff (largely supraspinatus) and bicipital tendon inflammation, with or without degenerative changes in the tendon. Pain that is most severe when the arm is abducted in an arc between 40 and 120 degrees, sometimes associated with tears in the rotator cuff, is the chief symptom."
(1) Calis M, Akgun K, Birtane M, Karacan I, Calis H, Tuzun F. Diagnostic values of clinical diagnostic tests in subacromial impingement syndrome. Ann Rheum Dis 2000; 59(1):44-47.;(2) Jablonski's Dictionary of Syndromes and Eponymic Diseases, 2d ed