The acromioclavicular (AC) joint is a small but important component of the shoulder, located at its uppermost part. This joint forms a connection integral to the shoulder’s overall function and range of motion, enabling various arm movements.
Anatomy and Location
The “AC” in acromioclavicular refers to the two bones that form this joint: the acromion and the clavicle. The acromion is a bony projection extending from the scapula, commonly known as the shoulder blade, forming the highest point of the shoulder. The clavicle, or collarbone, is a long, slender bone that runs horizontally across the top of the chest. The AC joint is precisely where the lateral, or outer, end of the clavicle meets the acromion of the scapula.
This articulation is classified as a synovial plane joint, meaning its flat surfaces allow for gliding movements. Surrounding this joint is a fibrous capsule, and within it, an articular disc may sometimes be present. Stability for the AC joint primarily comes from a network of strong ligaments. The acromioclavicular ligaments, located both superiorly and inferiorly, directly connect the acromion and clavicle. Additionally, the coracoclavicular ligaments, which consist of the conoid and trapezoid ligaments, provide further support by connecting the clavicle to the coracoid process of the scapula, a separate bony projection. These ligaments work together to maintain the joint’s alignment and integrity.
Function of the AC Joint
The AC joint is fundamental to the comprehensive mobility and stability of the shoulder complex. It enables the scapula to rotate, which is necessary for a full range of arm motion, allowing actions like raising the arm overhead. The joint also facilitates the transmission of forces from the upper limb to the rest of the skeleton.
Acting as a pivot point, the AC joint contributes significantly to the shoulder’s ability to perform complex movements like throwing or lifting. While the AC joint itself permits only gliding movements, its interaction with other shoulder structures allows for greater degrees of arm rotation. The AC joint effectively links the arm to the axial skeleton, supporting the entire upper extremity.
Common Issues Affecting the AC Joint
The AC joint is frequently susceptible to injuries, often due to its exposed location and the forces it withstands. Acromioclavicular joint sprains, commonly known as shoulder separations, are among the most prevalent issues. These injuries typically result from direct trauma to the shoulder, such as a fall directly onto the shoulder or an impact during sports activities. The force can cause the ligaments stabilizing the joint to stretch or tear.
Symptoms of an AC joint sprain can include pain at the top of the shoulder, swelling, and tenderness to the touch. In more severe cases, a visible bump may appear above the shoulder, indicating that the clavicle has become displaced due to torn ligaments. Movement of the arm, especially overhead or across the body, often exacerbates the pain and can be limited.