Acromion

Acromion (from Greek akron — extremity, summit and omos — shoulder) is a large flattened bony projection of the scapula in the shape of a triangle, which is a lateral continuation of its spine.

It hangs over the humeral head, forming a “roof” over the shoulder joint, and serves as the articulation point between the scapula and the clavicle.

Etiology and pathophysiology

The acromion plays a fundamental role in the biomechanics of the shoulder girdle. Powerful bundles of deltoid and trapezius muscles are attached to its rough upper surface and edges. On the anterior-medial edge of the process is a small articular facet, which together with the clavicle forms the slightly mobile acromioclavicular joint.

A crucial anatomical feature is the shape of the underside of the acromion, which faces the rotator cuff. According to Bigliani’s morphological classification, there are three types of structure: flat, curved, and hooked.

The hooked shape of the process, as well as the formation of age-related osteophytes (bone spurs) on it, critically narrows the subacromial space. This leads to mechanical conflict: when the arm is raised, the supraspinatus tendon is pinched between the head of the humerus and the edge of the acromion.

Clinical significance

The pathological anatomy of the acromion is a major cause of shoulder impingement syndrome. Chronic trauma to the rotator cuff against this bony protrusion leads to persistent bursitis and degenerative tendon tears, often requiring surgical resection of part of the process.

Isolated acromion fractures are rare and usually occur from a direct heavy blow to the shoulder area. In differential diagnosis, it is important to remember the existence of a congenital anomaly — failure to fuse the ossification centers of the acromion, where the distal part remains mobile and can be mistakenly interpreted as a fresh fracture on radiographs.