The acromion process is a prominent bony feature of the shoulder blade, or scapula, that significantly shapes the upper shoulder contour. This structure is a large, flattened extension projecting outward from the scapula’s posterior surface. The acromion is an integral component of the shoulder girdle, providing stability and the framework necessary for the arm’s extensive mobility.
Where the Acromion Process Is Located
The acromion process is a direct continuation of the spine of the scapula, the noticeable horizontal ridge of bone on the back of the shoulder blade. As the spine extends laterally toward the arm, it widens and turns forward, forming the broad, somewhat triangular acromion. This bony projection is easily felt just beneath the skin at the highest point of the shoulder.
The structure serves as a point of articulation with the collarbone (clavicle), creating the acromioclavicular (AC) joint. This small, gliding joint enables the scapula to move against the clavicle, increasing the overall range of motion for the shoulder complex. The acromion also forms a protective, bony arch that overhangs the main ball-and-socket joint, known as the glenohumeral joint.
This position above the main joint establishes the acromion as the “roof” of the shoulder, defining the space beneath it called the subacromial space. Within this confined area reside soft tissues, including the tendons of the rotator cuff muscles and the fluid-filled subacromial bursa. The arch formed by the acromion helps shield these structures from direct impact and trauma.
The Role of the Acromion in Shoulder Movement
The acromion is directly linked to the mechanics of arm movement, particularly the ability to lift the arm away from the body (abduction). Its broad surface provides a point of origin and attachment for two muscle groups that power the shoulder and upper back. The superior surface and medial border of the acromion serve as an anchor for portions of the trapezius muscle, which assists in stabilizing and rotating the scapula.
The lateral border of the acromion is a primary origin point for the fibers of the deltoid muscle, the muscle that gives the shoulder its rounded contour. This attachment generates the force needed to raise the arm, as the deltoid pulls on the upper arm bone (humerus) during lifting. The acromion, together with the coracoacromial ligament, creates the coracoacromial arch, which acts as a static shield for the underlying rotator cuff.
During overhead movements, the rotator cuff tendons, especially the supraspinatus tendon, must glide smoothly through the subacromial space, directly beneath the acromion. The integrity of this space is maintained by the cushioning effect of the subacromial bursa, which reduces friction between the bone and the moving tendons. The mechanical relationship between the acromion and the soft tissues beneath it dictates the efficiency and health of shoulder function.
Injuries and Conditions Affecting the Acromion
The shape of the acromion influences the likelihood of developing common shoulder pain conditions, most notably Subacromial Impingement Syndrome. This syndrome occurs when the space beneath the acromion narrows, causing the bony projection to repeatedly rub against or pinch the rotator cuff tendons and the bursa during arm movement. The acromion shape is classified into three main types, which can predispose an individual to this condition.
Type I is a flat shape that provides the most generous space, making it the least likely to contribute to impingement. Type II, a curved acromion, narrows the subacromial area and increases the chance of friction and irritation of the underlying tendons. Type III, known as a hooked acromion, drastically reduces the space and is strongly associated with chronic impingement and rotator cuff tears.
Beyond impingement, the acromion is susceptible to other issues, including the development of bone spurs with age, which can further narrow the subacromial space. The acromioclavicular (AC) joint, where the acromion meets the clavicle, can develop arthritis, leading to pain and joint enlargement. Although less common, a fracture of the acromion can occur, typically due to a direct blow or a severe injury, which can sometimes displace the bone fragment.