The acromion is a segment of bone on the scapula, or shoulder blade, that forms the highest point of the shoulder. This structure acts as a protective cover, often described as the “roof” of the shoulder joint, sitting directly over the complex structures beneath it.
Anatomy of the Acromion
The acromion is a continuation of the scapular spine, the prominent ridge on the back of the shoulder blade. This process extends laterally and then curves forward and upward, creating an arch that overhangs the head of the humerus (the upper arm bone). This positioning forms the summit of the shoulder. Its top surface is rough for muscle attachment, while its undersurface is smooth and concave.
This bony process connects with the clavicle (collarbone) at the acromioclavicular (AC) joint, a joint that allows for gliding movements and links the scapula to the rest of the skeleton. Directly beneath the acromion lies the subacromial space, a narrow channel that contains the rotator cuff tendons and a bursa, a fluid-filled sac that reduces friction. The shape of the acromion varies among individuals and is classified as Type 1 (flat), Type 2 (curved), or Type 3 (hooked), which influences the space available for the tendons.
Function in Shoulder Movement
The acromion serves as a stable anchor point for muscles responsible for the range of motion in the arm. It is an attachment site for both the deltoid and trapezius muscles. The deltoid muscle, which gives the shoulder its rounded contour, originates from the acromion and is the primary muscle for lifting the arm out to the side.
The trapezius muscle inserts onto the acromion and spine of the scapula. This muscle is responsible for stabilizing and moving the shoulder blade, including rotation and retraction. The leverage provided by these muscle attachments facilitates powerful overhead and reaching motions. The acromion also functions as a protective shield, covering the rotator cuff tendons as they pass underneath it during arm movement.
Common Acromial Injuries and Pain
Shoulder impingement syndrome, a frequent source of shoulder pain, occurs when the rotator cuff tendons become pinched or compressed within the subacromial space. This compression often happens during overhead activities, as the space naturally narrows when the arm is lifted. The anatomical shape of the acromion can play a part in this condition.
An individual with a Type 3, or hooked, acromion has a bone structure that curves downward, reducing the volume of the subacromial space and increasing the likelihood of the tendons being pinched. This can lead to tendinitis (tendon inflammation) or bursitis (bursa inflammation). Over time, this persistent rubbing can contribute to fraying or tearing of the rotator cuff tendons.
Another common issue is an injury to the acromioclavicular joint. An AC joint sprain or separation typically results from a direct fall onto the shoulder, forcing the joint apart and damaging the ligaments that hold it together. A direct and significant trauma, such as a hard fall or impact, can also cause a fracture of the acromion bone itself.