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The acromion is a bony structure on the scapula that serves as an attachment point for the deltoid and trapezius muscles, and joins with the clavicle to form the AC joint, allowing for arm movement.
The acromion, also known as the acromion process, is a bony structure on the top of the scapula, or scapula. It arises from a crest that horizontally crosses the upper portion of the scapula on the posterior side and projects at the apex of the shoulder, forming a club shape. Coupled with the coracoid process, a similar club-shaped protrusion that arises from the anterior side of the scapula and crosses laterally toward the shoulder joint, the acromion serves as the attachment point for the deltoid and trapezius muscles. Its upper surface is convex and rough, sloping up and out over the shoulder joint, while its lower surface is concave and smooth.
One function of the acromion is to join the clavicle to form the acromioclavicular (AC) joint. Here the clavicle with its flattened lateral end meets the medial or inner border of the acromion process to form a type of synovial joint known as a sliding joint. In a slip joint, adjacent bone surfaces slide past each other. The articulation surfaces of the clavicle and the acromion, sliding against each other, make possible the action of raising the arm above the head.
Another purpose of the acromion is to serve as a muscle attachment site. At the shoulder, the fibers of the middle deltoid originate at the lateral border of the acromion process, traverse the shoulder joint, and insert into the deltoid tuberosity partially along the outside of the humerus bone of the upper arm. The main function of the deltoid muscle, especially its middle fibers, is to abduct the arm or lift it laterally away from the body. This action occurs at the glenohumeral, or shoulder, joint, but the attachment of the muscle to the acromion provides the leverage that helps lift the weight of the arm.
The trapezius muscle of the upper back, especially its central fibers, also attaches to the acromion. Originating from the spinous processes of the thoracic vertebrae, the middle trapezius runs horizontally across the upper back and inserts into the medial border of the acromion process. The function of the middle fibers of the trapezius is to retract the shoulder blades, pulling them back and together. This in turn pulls the arms back to the shoulder joint, a movement that begins between the shoulder blades and ends with the back movement of the shoulders.
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