The acromion process is a bony protuberance extending anteriorly from the superolateral aspect of the scapular spine. It communicates with the distal clavicle to form the acromioclavicular joint and serves as the insertion site for the coracoacromial ligament. The acromion also functions as a site of muscle attachment, with the trapezius inserting along its medial border, and the deltoid originating from its lateral border.
There are three distinct morphological shapes (Bigliani classifications) for the undersurface of the acromion: type I- flat, type II- curved, and type III- hooked. A flat, type I acromion is the least common of the three shapes, however, it is commonly achieved surgically via acromioplasty to relieve shoulder impingement. A curved, type II acromion is the most prevalent morphology, with a concave undersurface that lies parallel to the curve of the humeral head. Hooked, type III acromions curve inferiorly at the anterior aspect of the acromion and are associated with an increased incidence of shoulder impingement.