The acromioclavicular (AC) joint is a small but important connection point in the shoulder complex. While the shoulder is often associated with the large ball-and-socket joint, the AC joint provides a necessary link within the shoulder girdle. This joint helps coordinate movement between the arm and the torso, ensuring smooth and extensive reach.
Precise Location and Anatomy
The acromioclavicular joint is located at the top of the shoulder, easily felt just above the arm. It is formed where two bones meet: the clavicle (collarbone) and the acromion, a bony projection of the scapula (shoulder blade). The clavicle extends horizontally from the chest, and the acromion forms the highest point of the shoulder blade, creating the roof of the shoulder.
This junction is classified as a synovial joint, meaning it is enclosed by a capsule and contains fluid to allow for movement. The joint surfaces are lined with fibrocartilage, which provides cushioning. Stability is provided by a set of ligaments, including the acromioclavicular ligaments that cross directly over the joint. Stronger support comes from the coracoclavicular ligaments, which connect the clavicle to a different part of the shoulder blade and act as the main stabilizer against displacement.
The Joint’s Role in Shoulder Function
The AC joint is a gliding joint, allowing only slight movement on its own. Its importance lies in how it works with the other shoulder components, acting as a pivot point for the scapula. This joint enables the shoulder blade to rotate and tilt, which is necessary for the arm’s full range of motion.
When the arm is raised overhead, the scapula must rotate upward to maintain a stable socket for the upper arm bone, and the AC joint permits this motion. This ensures the arm achieves full elevation and rotation without bony impingement. The AC joint also helps transmit forces from the arm to the rest of the skeleton. Although no muscles directly move the AC joint, its movements are passive, driven by larger shoulder muscles that move the scapula and clavicle.
Why the AC Joint Causes Trouble
Since the acromioclavicular joint is positioned at the outer, exposed part of the shoulder, it is susceptible to injury from direct trauma. A common issue is an AC separation, which occurs when a person falls directly onto the shoulder or onto an outstretched arm. This impact can stretch or tear the stabilizing acromioclavicular and coracoclavicular ligaments, causing the clavicle to become misaligned or displaced from the acromion.
Degenerative issues, primarily osteoarthritis, are another source of problems. Over time, the joint’s fibrocartilage can wear down due to age-related changes or repetitive microtrauma, such as from heavy overhead activity. This wear causes inflammation and pain, particularly when moving the arm across the body or lifting objects. Repetitive stress, especially among weight lifters, can lead to distal clavicular osteolysis, where the end of the collarbone begins to break down.