The acromion is a bony structure that serves as the highest point of the shoulder. This projection is part of the shoulder girdle, providing a structural roof and a complex junction point for movement. Understanding the acromion’s anatomy and function is fundamental to grasping how the shoulder works and facilitating the arm’s extensive range of motion. This bony process often becomes a source of pain and limitation when common issues, such as pinching or fracture, arise.
Defining the Acromion’s Structure and Location
The acromion is an extension of the spine of the scapula, or shoulder blade, projecting outward and curving forward over the main shoulder joint. It forms the roof of the shoulder, creating a protective arch over the joint’s deeper structures. Its flattened, somewhat triangular shape originates from the back of the shoulder blade and extends laterally toward the side of the body.
The front end of the acromion forms a joint with the collarbone, or clavicle, known as the acromioclavicular (AC) joint. This connection provides a secondary point of stability for the shoulder complex and allows for subtle gliding movements that assist in arm elevation. Beneath the acromion is a narrow channel called the subacromial space, which contains the rotator cuff tendons and a fluid-filled sac called the bursa.
Scientists have classified the shape of the acromion into several types, most commonly using the Bigliani classification system. The three primary shapes are Type I (flat), Type II (curved undersurface), and Type III (distinct hooked shape). These variations are normal anatomical differences present from birth, but they can influence the amount of space available beneath the bony arch.
Role in Shoulder Movement and Stability
The acromion’s primary functional role is to serve as a bony shield, arching over the rotator cuff tendons and the top of the humerus, or upper arm bone. This arch helps prevent the tendons from being compressed or damaged during overhead movements. The subacromial space created by the arch allows for smooth, friction-free movement of the underlying soft tissues.
The acromion also acts as a major anchor point for some of the largest muscles of the upper body. Its rough, outer surface provides a significant attachment area for the deltoid muscle, the primary muscle responsible for lifting the arm away from the body. The trapezius muscle, which helps stabilize the scapula and facilitates movement of the neck and upper back, also attaches to the acromion’s medial border. The integration of these muscles allows for the shoulder’s mobility and strength.
Common Issues Affecting the Acromion
One of the most frequent problems involving this structure is shoulder impingement syndrome, which occurs when the subacromial space narrows and pinches the soft tissues beneath the acromion. This compression often involves the supraspinatus tendon or the subacromial bursa, causing pain, especially when lifting the arm overhead. Anatomical variations, particularly the curved (Type II) or hooked (Type III) acromion shapes, are associated with a higher risk of chronic impingement because they naturally reduce the available space.
Acromion fractures represent a less common but more severe issue, typically resulting from significant trauma to the shoulder blade. These injuries are often seen after high-energy events, such as a direct blow to the shoulder or a motor vehicle accident. A fracture can displace the bone fragment, and this displaced piece may narrow the subacromial space, leading to secondary impingement. Fractures that significantly narrow this space are generally classified as Type III and may require surgical intervention to restore the proper shoulder mechanics.