The acromion is a prominent bony projection located at the top of the shoulder. It forms the most lateral part of the scapula, also known as the shoulder blade. This structure serves as a crucial point of connection for several important muscles and ligaments. Its position makes it integral to the overall architecture of the shoulder girdle, providing a stable base for various soft tissue attachments.
Muscles Connected to the Acromion
The deltoid muscle, a large, triangular muscle that gives the shoulder its rounded contour, has a significant origin point on the acromion. Its anterior fibers originate from the anterior border of the lateral third, while the middle fibers arise from the lateral margin. The posterior fibers originate from the spine of the scapula, which is continuous with the acromion.
The lateral border of the acromion features small tubercles where the deltoid muscle’s tendons attach. This broad origin allows the deltoid to cover the shoulder joint from three aspects and contributes to its fan-like shape. The deltoid then crosses the shoulder joint to insert onto the humerus, the upper arm bone.
The trapezius muscle, a large, flat, triangular muscle extending across the upper back and neck, also connects to the acromion. Its superior, or descending, fibers insert onto the medial aspect of the acromion and the lateral third of the clavicle. These fibers form a broad, flat sheet as they converge towards the shoulder girdle. The connection of both the deltoid and trapezius muscles to the acromion highlights its importance as a muscular anchor.
Ligaments Linked to the Acromion
The coracoacromial ligament is a strong, triangular band of fibrous tissue extending between the coracoid process and the acromion. It originates from the summit of the acromion, just in front of the clavicular articulation, and inserts broadly along the lateral border of the coracoid process. This ligament does not directly stabilize a joint but forms a protective structure.
Along with the coracoid process and the acromion, it creates the coracoacromial arch. This arch acts as a protective vault above the humeral head, preventing the humerus from dislocating upwards and ensuring the integrity of the shoulder joint.
The acromioclavicular (AC) ligament directly connects the acromion to the clavicle, or collarbone. This ligament is an integral part of the joint capsule surrounding the acromioclavicular joint, which is a plane synovial joint. It reinforces the joint from above and below, with superior and inferior components.
The superior acromioclavicular ligament, generally stronger, connects the superior surfaces of the acromion and the acromial end of the clavicle. This ligament provides crucial horizontal stability to the AC joint, ensuring the clavicle remains properly articulated with the acromion.
Acromion Attachments and Shoulder Movement
The muscles attached to the acromion play a significant role in enabling the diverse movements of the shoulder joint. The deltoid muscle, with its extensive origin on the acromion, is the primary muscle responsible for shoulder abduction, lifting the arm away from the body. All three sections of the deltoid work in concert to achieve this motion.
The anterior fibers of the deltoid contribute to shoulder flexion, moving the arm forward and upward, and also assist in internal rotation of the arm. The middle fibers are most effective at pure shoulder abduction. The posterior fibers, conversely, facilitate shoulder extension by moving the arm backward, and also contribute to external rotation.
The trapezius muscle, connected to the acromion, influences shoulder movement primarily through its actions on the scapula. The upper fibers of the trapezius elevate the scapula, as seen in shrugging motions, and contribute to its upward rotation. This upward rotation of the scapula, especially in conjunction with the serratus anterior muscle, is essential for lifting the arm above the head beyond 90 degrees.
The middle fibers of the trapezius retract the scapula, pulling it closer to the spine, while the lower fibers depress the scapula. These coordinated movements of the scapula, driven by the trapezius, allow the glenohumeral joint to achieve its full range of motion. The combined actions of the deltoid and trapezius are crucial for complex movements like throwing.
Acromion Attachments and Shoulder Stability
The attachments to the acromion significantly contribute to the overall stability of the shoulder complex. The acromioclavicular ligament, connecting the acromion and clavicle, provides horizontal stability to the acromioclavicular joint, preventing excessive movement between these bones. This ligament helps maintain the proper alignment of the clavicle with the acromion.
The coracoacromial ligament forms part of the coracoacromial arch, which acts as a protective roof over the glenohumeral joint. This arch helps prevent superior displacement of the humeral head, especially during arm movements. It also shields underlying structures like the rotator cuff tendons from direct pressure.
The strong muscular attachments to the acromion, particularly the deltoid and trapezius, provide dynamic stability to the shoulder joint. The deltoid muscle helps stabilize the humeral head against the glenoid cavity, especially when carrying loads. This muscular support works in conjunction with the ligaments to ensure the shoulder joint remains secure during movement.