The shoulder is a complex and mobile part of the human body, allowing extensive arm movement. Several joints work together for flexibility and stability. The acromioclavicular (AC) joint plays an important role in the overall function of the shoulder complex.
Anatomy and Location of the AC Joint
The acromioclavicular (AC) joint is located at the top of the shoulder. It forms where two bones meet: the acromion (a bony projection from the shoulder blade or scapula) and the distal clavicle (collarbone). It is a plane synovial joint, allowing gliding movements between these surfaces.
Strong ligaments stabilize the AC joint. The acromioclavicular ligaments directly connect the acromion and clavicle, reinforcing the joint capsule. Additional support comes from the coracoclavicular ligaments, two bands: the conoid and trapezoid. These ligaments attach the clavicle to the coracoid process of the scapula, important for vertical stability, suspending the scapula from the clavicle.
Role and Function of the AC Joint
While the AC joint may not move as extensively as other shoulder components, its proper function is necessary for full arm movement, especially overhead. The joint acts as a pivot point, facilitating scapula rotation and movement relative to the clavicle. This is essential for coordinated arm and shoulder girdle movements.
This joint transmits forces from the arm to the skeleton and functions as a shock absorber. Its ability to allow the shoulder blade to move independently of the collarbone is fundamental for activities like reaching, throwing, and lifting. The AC joint’s integrity supports the shoulder, enabling daily arm movements.
Common Injuries Affecting the AC Joint
The AC joint is susceptible to various injuries and conditions. A common issue is an AC joint separation, often resulting from a direct blow to the shoulder or a fall onto the tip of the shoulder or an outstretched hand. Separations are graded based on ligament damage severity and displacement between the acromion and clavicle.
Grade I separations involve a mild sprain of the acromioclavicular ligaments with minimal or no bone displacement. Grade II injuries indicate a complete tear of the acromioclavicular ligaments and a sprain or partial tear of the coracoclavicular ligaments, leading to some visible clavicle displacement. Grade III separations involve complete tears of both the acromioclavicular and coracoclavicular ligaments, resulting in noticeable displacement and a prominent bump at the top of the shoulder. Symptoms include localized pain, swelling, and tenderness over the joint, with difficulty moving the arm, especially overhead or across the body.
Beyond acute injuries, the AC joint can also be affected by osteoarthritis, a degenerative condition characterized by cartilage breakdown. This can cause pain, stiffness, and reduced range of motion, particularly during overhead or cross-body activities. Osteoarthritis in the AC joint is common in middle-aged and older individuals, and can also occur in athletes or those who engage in repetitive overhead lifting. Symptoms may include a catching sensation, clicking, or grinding with shoulder movement, and pain that may radiate into the upper arm.