Acromioclavicular JointEdit

The acromioclavicular joint (AC joint) is a small but clinically important articulation that sits at the top of the shoulder. It marks the meeting point of the distal end of the clavicle with the acromion of the scapula. Although it is a minor joint in terms of daily movement, its integrity is essential for the comfort and function of the shoulder, especially in athletes and laborers who rely on overhead or push-pull shoulder actions. The joint is encased in a capsule and reinforced by several ligaments, including the acromioclavicular ligaments that directly cross the joint and the coracoclavicular ligaments (the conoid and trapezoid ligaments) that suspend the clavicle from the coracoid process. Its stability is the product of both these ligaments and the surrounding muscular and soft-tissue envelope.

In everyday function, the AC joint allows the scapula to rotate and tilt with arm movements, helping to position the glenohumeral joint for a wide range of motions. When the joint is injured or degenerates, pain is often felt at the top of the shoulder, sometimes with a visible bump or “step-off” where the clavicle sits relative to the acromion. Because of its superficial location, the AC joint is particularly susceptible to direct impacts—such as falls onto the shoulder or collisions in contact sports—and it is one of the joints clinicians assess first when shoulder trauma occurs.

Anatomy

  • The distal clavicle articulates with the acromion to form the AC joint, a plane-type synovial joint with limited intrinsic motion but a wide range of functional contributions to shoulder mechanics. For the bones involved, consider clavicle and acromion as the primary partners.
  • The joint capsule is relatively thin, but reinforced by the acromioclavicular ligaments that cross directly over the joint and by the coracoclavicular ligaments that connect the clavicle to the coracoid process of the scapula. The coracoclavicular ligaments consist of two components, the conoid ligament and the trapezoid ligament, which provide vertical stability and help suspend the clavicle.
  • The articular surfaces are covered with hyaline cartilage and are shaped to permit small translational movements that accompany shoulder motion while maintaining rigid linking of the clavicle and acromion during lifting and pushing.
  • Nearby structures include the scapula and the glenohumeral joint, with the AC joint functioning as part of the broader shoulder girdle.

Biomechanics and stability

Stability at the AC joint arises from a combination of the joint capsule, the AC ligaments, the coracoclavicular ligaments, and the surrounding musculature. The coracoclavicular ligaments are particularly important for preventing excessive clavicular elevation and maintaining the vertical relationship between the clavicle and the scapula. The AC ligaments contribute to horizontal stability, resisting anterior and posterior displacement. Together with dynamic stabilizers such as the deltoid and trapezius muscles, these structures ensure coordinated movement of the shoulder during overhead and pushing activities.

Injury classification and clinical presentation

AC joint injuries are a common consequence of direct trauma to the shoulder. Clinicians often reference a grading system that describes the degree of ligamentous disruption and displacement, with the Rockwood classification being the most widely used in practice. Broadly, injuries range from minor sprains with preserved alignment to complete disruption with visible deformity and marked displacement. Grade I and II injuries generally involve sprains or partial tears with little or no step-off, while grade III indicates a complete disruption of the AC and CC ligaments with noticeable elevation of the distal clavicle. Higher grades (IV–VI) describe more severe displacements, including posterior displacement into the trapezius region or significant misalignment that may require more complex management.

Key clinical features include: - Pain and tenderness over the AC joint - Pain with cross-body adduction or horizontal adduction - Swelling and sometimes a palpable bump or deformity in higher-grade injuries - Limited range of motion or weakness, particularly with overhead activities

Imaging helps confirm the diagnosis and assess associated injuries. Standard radiographs include views that emphasize the AC joint alignment, and specialized views (such as a dedicated AC joint view) can improve detection of subtle displacements. In more complex or ambiguous cases, magnetic resonance imaging (MRI) or ultrasound may be employed to evaluate soft-tissue injury and adjacent structures.

Diagnosis

  • History and physical examination focus on mechanism of injury, local tenderness, and deformity.
  • Radiographs: upright AP views of the shoulder, dedicated AC joint views, and sometimes weighted views to reveal subtle instability.
  • Advanced imaging: MRI or CT may be used to evaluate concomitant shoulder injuries, chronic changes, or to plan surgical reconstruction when indicated.
  • Special considerations: injuries are often assessed in the context of the entire shoulder girdle, since associated injuries to the glenohumeral joint or rotator cuff tend to influence management and prognosis.

Management

Treatment decisions balance the severity of the ligamentous injury, the degree of displacement, patient activity demands, and the likelihood of restoring function with acceptable risk. A pragmatic, evidence-based approach emphasizes nonoperative management when appropriate, reserving surgical intervention for more severe injuries or for patients whose activities demand rapid return to high-level overhead work or contact sports.

  • Nonoperative management (typical for grades I–II and some grade III injuries): Rest, ice, analgesia, and a structured rehabilitation program focusing on gradual restoration of range of motion, followed by progressive strengthening of the periscapular muscles and rotator cuff. A period in a sling or brace may be used to allow ligament healing, with a plan to reintroduce activity as symptoms permit.
  • Surgical management (considered for higher-grade injuries, persistent instability, or high-demand athletes): Reconstruction or stabilization of the coracoclavicular ligaments, direct repair of the AC ligaments, or more complex procedures to restore the clavicular–acromial relationship. Common surgical options include anatomical reconstruction using grafts to recreate the CC ligaments and, in some cases, fixation devices or plates to maintain alignment during healing. The choice of procedure depends on the specific pattern of injury, patient goals, and surgeon experience. See coracoclavicular ligament reconstruction and Weaver-Dunn procedure as related concepts.

Contemporary debates in this area often center on the management of Rockwood grade III injuries. Proponents of nonoperative care argue that many patients achieve excellent functional outcomes without the risks and costs of surgery, while proponents of early surgical stabilization contend that certain patients (e.g., high-level overhead athletes or laborers with persistent instability) benefit from faster, more predictable restoration of joint alignment and function. The evidence shows that for many patients, outcomes are similar across approaches, but individual priorities—such as cosmetic appearance, speed of return to sport, or willingness to accept surgical risk—shape decisions. In all cases, the goal is to maximize functional recovery while minimizing unnecessary intervention and expense.

Rehabilitation after either approach emphasizes: - Early protection and gradual restoration of motion - Strengthening of the shoulder girdle and scapular stabilizers - Return-to-sport or work timelines tailored to the individual and the injury grade

Prognosis and complications

Most AC joint injuries heal without long-term consequences when managed appropriately, but some patients experience residual pain, persistent instability, or post-traumatic arthritis, especially after high-energy injuries or inadequate rehabilitation. Complications from surgery can include infection, hardware irritation, loosening or failure of grafts, and stiffness, underscoring the importance of careful patient selection and meticulous technique.

See also