The acromioclavicular (AC) joint is a small but functionally significant articulation located in the shoulder girdle. This joint links the upper limb to the main body skeleton and provides a platform for the expansive movement of the arm. It contributes substantially to the stability of the entire shoulder complex. Though its range of motion is relatively small, the AC joint is necessary for achieving the full, unrestricted overhead motion that the shoulder is known for.
Anatomy and Location
The acromioclavicular joint is located at the top of the shoulder, articulating the lateral end of the clavicle (collarbone) and the acromion, the highest part of the scapula (shoulder blade). The joint surfaces are lined with fibrocartilage, an atypical feature for a synovial joint, which helps manage the forces transmitted across this small connection point.
Stability relies on a complex arrangement of supporting structures, including a joint capsule and two distinct groups of ligaments. The acromioclavicular ligament runs horizontally, covering the joint capsule to bind the two bones directly together.
The primary static restraint against vertical separation comes from the coracoclavicular ligaments, a strong bundle located slightly below the joint. This coracoclavicular complex is composed of two separate bands: the conoid and the trapezoid ligaments. These ligaments anchor the clavicle to the coracoid process, a hook-like projection of the scapula, effectively suspending the weight of the upper limb from the collarbone.
Role in Shoulder Movement
The acromioclavicular joint functions as a gliding or plane joint, allowing the scapula to move relative to the clavicle in three dimensions. While no muscles directly initiate movement at this joint, it is a passive structure that reacts to the larger motions of the arm. This subtle movement is crucial for maintaining contact between the shoulder blade and the rib cage throughout the shoulder’s full range of motion.
As the arm is raised overhead, the AC joint acts as a pivot point, permitting the necessary upward rotation of the scapula. This rotation accounts for a portion of the total arc of movement, ensuring the bony socket remains correctly positioned to receive the head of the upper arm bone. Without this small, synchronized movement, the arm’s ability to elevate above shoulder height would be significantly restricted.
Understanding AC Joint Separation
The most common injury to this area is an AC joint separation, often mistakenly called a shoulder separation. This injury occurs from a direct impact, such as falling onto the point of the shoulder or sustaining a blow in contact sports. The force drives the acromion downward, tearing the stabilizing ligaments and causing the collarbone to separate from the shoulder blade.
Symptoms include immediate, localized pain at the joint, swelling, and tenderness to the touch. A visible step-off deformity or “bump” may appear on the top of the shoulder, which is the elevated end of the clavicle. The degree of ligament damage dictates the severity of the injury, which is commonly described using a three-grade system.
A Type I separation involves only a sprain or partial stretching of the acromioclavicular ligament, with the coracoclavicular ligaments remaining intact. A patient with a Type I injury experiences pain but little to no noticeable deformity.
A Type II separation progresses to a complete tear of the acromioclavicular ligament and a sprain of the coracoclavicular ligaments, resulting in a slight upward displacement of the clavicle.
The more severe Type III separation involves a complete tear of both the acromioclavicular and coracoclavicular ligaments. This total disruption allows for a noticeable elevation of the clavicle and a prominent bump on the shoulder.