acromion

acromion, bony projection on the scapula (shoulder blade) that forms the highest point of the shoulder. It projects laterally from the spine of the scapula and curves over the shoulder joint, where it connects with the clavicle (collarbone) at the acromioclavicular (AC) joint. The acromion forms a protective arch over the shoulder joint and serves as a major attachment site for the deltoid and trapezius muscles; it therefore functions in both protection and movement of the shoulder.

Structurally, the acromion is broad and triangular, with a superior surface that attaches the trapezius muscle and an inferior surface that forms the roof of the subacromial space. Within this space lie the supraspinatus tendon, which forms part of the rotator cuff and is responsible for lifting the arm, and the subacromial bursa, a fluid-filled sac that acts as a cushion to help prevent friction between the rotator cuff and the acromion. The lateral edge of the acromion articulates with the clavicle at the AC joint, a synovial joint that contributes to shoulder girdle mobility, and its anterior margin provides attachment for fibers of the deltoid muscle, which covers the shoulder and plays a central role in arm abduction.

The acromion exhibits several anatomical variations. Traditionally, its morphology has been classified into three main types: flat (type I), curved (type II), and hooked (type III). A flat acromion generally provides more space within the subacromial region than the curved and hooked forms. Some studies also describe a convex (type IV) variant, though it is less common. Other classifications for acromion morphology also exist, including acromial angle classification, which is based on the measured angle formed by lines connecting specific points on the acromial undersurface, and modified Epstein classification, which is based on a perpendicular height relative to acromion length and the location of the highest point along this length. In addition to shape, variations occur in the degree of acromial tilt, thickness, and the presence of an os acromiale, a condition in which the acromion fails to fuse during development.

The differences in acromion morphology influence shoulder biomechanics and susceptibility to injury. This is mainly because of alterations in the space above the rotator cuff tendons: Curved or hooked shapes narrow the subacromial space and increase the risk of impingement and rotator cuff tears, whereas flatter shapes generally reduce this risk.