The acromion is a bony structure forming a significant part of the shoulder. An extension of the scapula (shoulder blade), it is a key landmark that contributes to the shoulder’s shape and mechanics, enabling a wide range of arm movements.
Understanding the Acromion’s Structure
The acromion is a flat, broad projection of the scapula, extending laterally over the shoulder joint. It continues from the scapular spine, a bony ridge on the shoulder blade’s posterior surface. Often described as forming a “roof” over the shoulder joint, its superior surface is convex and rough for muscle attachment, while its inferior surface is smooth and concave, facing the joint.
The acromion articulates with the lateral end of the clavicle (collarbone), forming the acromioclavicular (AC) joint. This plane synovial joint allows gliding movements, connecting the scapula to the clavicle. The acromion also overhangs the glenoid cavity, the shallow socket where the humerus (upper arm bone) articulates to form the main shoulder joint. This relationship creates a crucial space beneath it for shoulder function.
The Acromion’s Role in Shoulder Movement
The acromion’s position and structure are integral to the shoulder’s extensive range of motion. It protects underlying structures like the rotator cuff tendons and the subacromial bursa. Rotator cuff tendons, which rotate and lift the arm, pass through the narrow subacromial space beneath the acromion. The subacromial bursa, a fluid-filled sac in this space, reduces friction between the acromion and moving tendons, ensuring smooth movement.
The acromion serves as an attachment site for several muscles contributing to shoulder movement. The deltoid muscle, responsible for arm abduction (lifting away from the body), originates partly from the acromion. The trapezius muscle also attaches here, influencing scapular rotation and stability. This facilitates the complex mechanics of the shoulder, enabling actions like reaching, lifting, and throwing.
Common Acromion-Related Conditions
The acromion’s anatomy can predispose individuals to certain shoulder conditions. A common issue is shoulder impingement syndrome, where rotator cuff tendons or the subacromial bursa pinch between the acromion and humeral head during arm elevation. A hooked or curved acromion can narrow the subacromial space, increasing impingement likelihood. Symptoms include pain, weakness, and limited movement, especially with overhead activities.
Another condition is an acromioclavicular (AC) joint separation, an injury to the ligaments connecting the acromion and clavicle. These injuries typically result from a direct fall or blow to the shoulder, common in contact sports. Severity varies from a mild sprain to a complete ligament tear, potentially causing a noticeable bump on the shoulder due to clavicle displacement.
Acromial fractures are less common but can occur from high-energy trauma like motor vehicle accidents or direct shoulder impacts. While often from significant force, they can also be stress fractures or related to bone quality issues. Symptoms include severe pain, swelling, and reduced range of motion in the affected shoulder.