The acronym “AC” refers to the acromioclavicular joint, a small articulation located at the top of the shoulder. This joint connects the collarbone (clavicle) to the shoulder blade (scapula). It acts as a primary link between the upper limb and the rest of the skeleton. The AC joint is frequently subject to injury, particularly in physically active individuals and athletes involved in contact sports.
The Anatomy and Role of the AC Joint
The acromioclavicular joint is formed by the lateral end of the clavicle meeting the acromion, the highest projection of the scapula. The joint surfaces are lined with fibrocartilage, providing cushioning and smooth gliding.
Stability is maintained by a complex of ligaments. The acromioclavicular ligament reinforces the joint capsule, providing horizontal stability. Stronger support comes from the coracoclavicular ligament, composed of the conoid and trapezoid bands. This complex anchors the clavicle to the coracoid process of the scapula, preventing excessive vertical movement.
The primary function of this joint is to allow for the full range of shoulder motion. It facilitates the gliding and rotational movements of the scapula, which is necessary for raising the arm fully overhead and transmitting forces across the shoulder girdle.
Common Causes of AC Joint Pain and Injury
Pain originating from the AC joint falls into two main categories: acute injury and chronic wear. The most common cause of acute pain is an AC joint separation, which is a sprain or tear of the stabilizing ligaments. This injury usually results from a powerful, direct blow to the point of the shoulder, often seen in contact sports or falls.
The impact pushes the shoulder blade downward while the clavicle remains fixed, stretching or tearing the ligaments. The severity of the separation depends on which ligaments are damaged and the extent of that damage. This traumatic event immediately causes localized pain and tenderness.
The second primary cause of discomfort is chronic arthritis, specifically osteoarthritis. This involves the gradual breakdown of cartilage over time. It is a degenerative process caused by cumulative wear and tear, or it may develop years after a previous AC joint injury. The resulting friction and inflammation produce persistent pain, especially with overhead or cross-body arm movements.
Understanding AC Joint Separation Grades
The severity of an AC joint separation is classified using a standard grading system that guides treatment decisions. This system categorizes the injury based on ligament damage and the resulting displacement of the clavicle relative to the acromion. Grades I, II, and III are the most frequently encountered separations.
A Grade I separation is the mildest form, involving only a stretching or partial tearing of the acromioclavicular ligament. The coracoclavicular ligaments remain intact, and the joint is stable. There is no significant displacement visible on X-ray, though the area is tender. Recovery is swift.
A Grade II injury involves a complete tear of the acromioclavicular ligament, though the coracoclavicular ligaments are only partially torn. This results in a slight upward displacement of the clavicle, which may be felt as a small bump. The joint is considered partially dislocated and requires dedicated healing time.
The Grade III separation represents a complete dislocation, where both the acromioclavicular and coracoclavicular ligaments are completely torn. This allows the clavicle to push upward significantly, creating a noticeable deformity. This injury involves the greatest instability among the common grades.
Non-Surgical Treatment and Recovery
The majority of AC joint injuries, including Grades I, II, and many Grade III separations, are successfully managed without surgery. Initial treatment focuses on reducing pain and inflammation immediately following the trauma. This involves a short period of rest and the application of ice to the affected area to manage swelling.
Immobilization is achieved using a sling for a short duration, which supports the arm’s weight and prevents painful movement that could aggravate the injured ligaments. Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used to control both pain and inflammation until the acute pain subsides.
A structured rehabilitation program is introduced once the initial pain is controlled. Physical therapy is necessary to restore the shoulder’s full range of motion, which is often restricted after injury. The program advances to strengthening exercises for the muscles surrounding the shoulder and scapula, helping compensate for residual ligamentous laxity.
Surgical Considerations
While most AC separations heal well non-surgically, higher-grade injuries (specifically Grades IV, V, and VI) involve significant displacement and soft tissue damage, often requiring consultation for potential surgical intervention.