The acromioclavicular (AC) joint is a small, yet functionally significant, articulation that forms part of the shoulder complex. This joint connects the upper extremity to the torso’s skeletal structure. Although it is smaller than the main ball-and-socket joint, the AC joint is frequently injured, often leading to pain and instability that affects the entire arm’s function. Understanding this joint’s structure and mechanical role is the first step in recognizing why it can become a source of shoulder problems.
Anatomy and Location
The AC joint is located at the very top of the shoulder, where two specific bones meet. It is formed by the lateral end of the clavicle (collarbone) and the acromion, which is a bony projection of the scapula (shoulder blade). This junction is technically a plane-type synovial joint, meaning the two surfaces are relatively flat and designed to glide against each other. Stability relies on a complex arrangement of ligaments that secure the bones in place.
The primary restraints are the acromioclavicular (AC) ligaments, which wrap around the joint capsule, reinforcing the connection horizontally. Beneath this joint lies the second, stronger set of stabilizers known as the coracoclavicular (CC) ligaments. These CC ligaments consist of two distinct bands, the conoid and the trapezoid, which anchor the clavicle to the coracoid process of the scapula. These CC ligaments provide vertical stability and are the ultimate support structure for the joint.
Primary Function and Role in Movement
The AC joint acts as a mechanical link that translates forces from the arm to the axial skeleton. Its design allows the scapula to rotate and tilt relative to the clavicle, a seemingly small movement that is essential for full arm elevation. This small degree of motion, typically only about 5 to 8 degrees of rotation, is necessary for coordinating movement with the larger ball-and-socket joint.
This coordinated motion is known as the scapulohumeral rhythm, a synchronized process between the arm bone and the shoulder blade that allows the arm to lift overhead without impingement. Without the AC joint’s ability to adjust the angle of the scapula, overhead activities would be limited and functionally impaired. The joint also helps absorb impact forces that travel up the arm and into the shoulder girdle.
Common Injuries and Pathology
Two distinct conditions account for the majority of issues affecting the AC joint: traumatic separation and degenerative arthritis. AC joint separation, often called a shoulder separation, is an acute injury resulting from a direct blow to the shoulder or a fall onto the outer arm. This mechanism forces the scapula downward, stressing and tearing the stabilizing ligaments.
The severity of the injury is classified using a grading system, with Grades I through III being the most common presentations. A Grade I injury involves a sprain or stretch of the AC ligaments with the CC ligaments remaining intact, resulting in minor pain and no visible deformity. A Grade II separation involves a complete tear of the AC ligaments, but the CC ligaments are only partially torn or severely sprained. A Grade III separation signifies a complete tear of both the AC and CC ligaments, leading to the clavicle visibly separating from the acromion and creating a noticeable bump on top of the shoulder. Injuries classified as Grade IV, V, or VI are much rarer and require high-energy trauma, often involving significant displacement of the clavicle.
In contrast to traumatic separation, AC joint arthritis is a progressive wear-and-tear condition that develops over time. This condition, often termed osteoarthritis, involves the gradual breakdown of the cartilage cushioning the joint surfaces. It is frequently observed in older individuals or in specific groups like weightlifters and construction workers who perform repetitive, high-stress overhead activities.
Symptoms of AC joint arthritis include localized pain directly over the joint, which is exacerbated by motions that compress the joint, such as reaching across the body or lifting the arm overhead. The degeneration can also cause a painful catching or clicking sensation with shoulder movement. Another form, distal clavicle osteolysis, is a localized bone erosion seen in younger athletes due to chronic, high-intensity loading.
Diagnosis and Conservative Treatment
Diagnosis of AC joint issues begins with a thorough physical examination where a medical professional assesses tenderness directly over the joint and tests for pain with specific movements. The cross-arm adduction test, which involves bringing the arm across the chest, is a common maneuver used to provoke pain and help isolate the AC joint as the source of discomfort. Imaging studies, specifically X-rays, are routinely used to confirm the diagnosis, visualize the degree of separation, or identify signs of arthritis, such as joint space narrowing or bone spurs.
The majority of AC joint injuries and cases of arthritis are managed non-operatively, focusing on conservative treatment. Initial care for acute separations (Grades I and II) includes rest, using a sling for temporary immobilization, and applying ice to manage pain and swelling. Nonsteroidal anti-inflammatory drugs (NSAIDs) can be used to alleviate pain during the initial recovery phase.
Physical therapy is often initiated early, focusing on restoring the shoulder’s range of motion and strengthening the surrounding musculature to compensate for ligamentous instability. For AC joint arthritis, a corticosteroid injection directly into the joint often provides effective short-term pain relief. Surgery is reserved for severe, unstable separations (Grades IV-VI) or for symptomatic Grade III separation and arthritis that fail to improve after conservative management.