Akromion

The acromion is a bony process on the scapula that forms the lateral, superior aspect of the shoulder blade. It extends laterally from the spine of the scapula and ends near the shoulder joint, where it articulates with the clavicle at the acromioclavicular joint. Together with the coracoid process and the coracoacromial ligament, the acromion contributes to the coracoacromial arch, which forms a protective roof over the glenohumeral joint and helps define the subacromial space.

The acromion varies in shape. A commonly used classification (Bigliani) lists three main forms: Type I, flat; Type II, curved; and Type III, hooked. The hooked (Type III) form is associated with greater risk of subacromial impingement and rotator cuff pathology. Ossification centers in the acromion fuse with age, but in some individuals a secondary center can fail to fuse, producing an accessory fragment called os acromiale, which may cause pain or dysfunction.

Attachment sites include the origin of parts of the deltoid muscle, and the trapezius also lies in proximity to the acromion. The shape and position of the acromion influence the anatomy of the subacromial space and the mechanics of shoulder movement, including the passage of rotator cuff tendons beneath the acromion.

Clinical significance includes impingement syndromes related to acromial shape or osteophytes, and potential pain from os acromiale or acromial fractures, though fractures are relatively uncommon. Surgical procedures such as acromioplasty reshape the undersurface of the acromion to relieve impingement in selected cases.