Figure 1

A. Schematic anatomy of a healthy glenohumeral joint and subacromial space. B. Schematic anatomy of a shoulder joint with the presence of several etiologic mechanisms for Subacromial Impingement Syndrome. In theory, impingement ("a disbalance between acromial space and the space needed for subacromial structures") can be caused by 1) A dynamically reduced subacromial space due to a pathologic pattern of arm-shoulder movements (e.g. scapular dyskinesia), resulting in relative cranialisation of the humerus with respect to the scapula/acromion, or 2) A more statically reduced subacromial space, due to 2a) structural anatomic variations (e.g. a hooked acromion), eventually in combination with altered arm-scapula motion patterns; 2b) A subacromial inflammatory reaction (e.g. caused by micro-trauma or overuse) causing subacromial oedema, fibrosis and tendinosis; 2c) Encroachment of subacromial tissues by an adjoining pathology or structures other than the acromion (e.g. acromioclavicular (AC)-joint osteoarthritis and subacromial osteophytes, calcifying tendinitis, and coracoid impingement).