Acromioclavicular joint

The acromioclavicular (AC) joint is a small but essential joint in the shoulder complex, connecting the clavicle to the acromion of the scapula. It plays a crucial role in shoulder motion and load transfer from the upper limb to the axial skeleton. Dysfunction or injury of this joint can significantly impact upper extremity function.

Anatomy of the AC Joint

Bone Structures

The AC joint is formed by the articulation between the distal end of the clavicle and the acromion process of the scapula. The distal clavicle provides the superior surface for the joint, while the acromion forms the inferior articulating surface. Both surfaces are covered with articular cartilage that facilitates smooth movement and distributes load.

Joint Capsule and Ligaments

• Acromioclavicular Ligament: Reinforces the superior and inferior aspects of the joint capsule, providing horizontal stability.
• Coracoclavicular Ligaments: Composed of the conoid and trapezoid ligaments, these structures anchor the clavicle to the coracoid process of the scapula and maintain vertical stability of the joint.
• Coracoacromial Ligament: Extends between the coracoid and acromion, forming an arch over the humeral head and contributing to shoulder stability.

Synovial Features

The AC joint is a synovial plane joint enclosed by a thin joint capsule. The articular surfaces are covered with hyaline cartilage, and a synovial membrane lines the inner surface of the capsule. In some individuals, a fibrocartilaginous disc may be present, enhancing joint congruency and absorbing mechanical stress.

Surrounding Muscles

• Deltoid: Attaches near the AC joint and contributes to shoulder abduction and flexion.
• Trapezius: Provides dynamic stabilization of the clavicle and scapula, supporting AC joint function.
• Other Supporting Musculature: Small stabilizing muscles around the scapula assist in maintaining joint alignment during shoulder motion.

Physiology and Function

Range of Motion

The AC joint allows gliding movements that accommodate scapular rotation, elevation, and protraction. Although motion at the AC joint is minimal compared to the glenohumeral joint, it is critical for full shoulder elevation and smooth scapulothoracic rhythm. Movements include slight anterior-posterior and superior-inferior gliding of the acromion relative to the clavicle.

Load Transmission

The AC joint functions as a key link in transmitting forces from the upper limb to the axial skeleton. During weight-bearing or pushing activities, forces are transferred through the humerus to the scapula, across the AC joint, and ultimately to the clavicle and thorax. Proper ligamentous and muscular support ensures that these loads are distributed efficiently, preventing joint instability and injury.

Embryology

The acromioclavicular joint develops during fetal life as part of the formation of the shoulder girdle. The clavicle is one of the first bones to ossify in the human embryo, beginning in the fifth to sixth week of gestation, while the acromion ossifies later from multiple centers.

Development of the AC Joint

• The joint space forms as the distal clavicle and acromion ossification centers approach each other, creating a synovial cavity lined with a synovial membrane.
• Fibrocartilaginous elements may develop within the joint, contributing to the articular disc in some individuals.
• Proper alignment and ossification are crucial for normal joint function, and developmental anomalies can predispose to early degenerative changes or instability.

Clinical Significance

Acromioclavicular Joint Injuries

AC joint injuries are common, particularly in athletes and individuals involved in contact sports or falls onto the shoulder. The injuries range from mild sprains to complete separations of the joint, often classified using the Rockwood system, which grades severity from type I to type VI based on ligamentous disruption and displacement of the clavicle.

Degenerative Changes

Osteoarthritis of the AC joint is a frequent cause of shoulder pain in older adults. It may occur as a primary degenerative process or secondary to prior trauma. Degeneration leads to joint space narrowing, osteophyte formation, and subchondral sclerosis, which can be visualized on imaging and correlate with clinical symptoms.

Inflammatory and Infectious Conditions

• Septic Arthritis: Infection of the AC joint is uncommon but can result from hematogenous spread or direct inoculation. Prompt recognition is essential to prevent joint destruction.
• Rheumatologic Involvement: Conditions such as rheumatoid arthritis may involve the AC joint, causing pain, swelling, and functional impairment.

Diagnostic Evaluation

Clinical Examination

Evaluation of the AC joint begins with a thorough clinical examination, focusing on pain localization, joint stability, and functional limitations. Key components include inspection for deformity or swelling, palpation for tenderness, and assessment of joint motion.

• Special Tests: The cross-body adduction test and O’Brien test are commonly used to identify AC joint pathology by reproducing pain with specific shoulder movements.
• Observation: Prominence of the distal clavicle may indicate AC joint separation or chronic instability.

Imaging

Imaging studies complement clinical evaluation and provide detailed assessment of joint structure, alignment, and degenerative changes.

• X-ray: Standard anteroposterior and stress views can detect joint widening, subluxation, or degenerative changes.
• Magnetic Resonance Imaging (MRI): Offers high-resolution visualization of ligaments, cartilage, and surrounding soft tissues, aiding in the diagnosis of tears or inflammatory conditions.
• Ultrasound: Useful for dynamic evaluation of joint movement and ligament integrity, particularly in outpatient settings.
• Computed Tomography (CT) Scan: Provides detailed bone imaging, helpful in preoperative planning or assessing complex fractures.

Management

Conservative Treatment

Non-surgical management is the first-line approach for mild AC joint injuries, degenerative changes, or inflammatory conditions. The goals are pain relief, joint stabilization, and preservation of shoulder function.

• Rest and activity modification to reduce stress on the joint
• Immobilization with slings or supportive devices in acute injuries
• Physical therapy to strengthen surrounding muscles and improve range of motion
• Analgesics and nonsteroidal anti-inflammatory drugs to manage pain and inflammation

Surgical Treatment

Surgery is indicated in cases of severe AC joint separation, chronic instability, or advanced degenerative changes unresponsive to conservative therapy. Surgical options aim to restore joint stability and function while minimizing long-term pain.

• AC joint reconstruction using ligament repair or synthetic augmentation
• Distal clavicle resection (Mumford procedure) for degenerative arthritis
• Postoperative rehabilitation including gradual mobilization and strengthening exercises

Prognosis

The prognosis of acromioclavicular joint conditions depends on the type and severity of the pathology as well as the chosen treatment approach. Most mild injuries and early degenerative changes respond well to conservative management, allowing patients to regain normal shoulder function.

• Patients with isolated ligamentous injuries often recover fully with physical therapy and activity modification.
• Post-surgical outcomes are generally favorable, with restoration of joint stability and pain relief in most cases.
• Chronic degenerative changes or severe injuries may result in persistent discomfort or limited range of motion despite appropriate treatment.

Complications

Complications of AC joint pathology and its management can affect both function and long-term outcomes. Awareness and early management of these issues are essential for optimal recovery.

• Chronic Pain: May persist after injury or surgery, often related to residual instability or degenerative changes.
• Joint Instability: Inadequate healing or ligamentous laxity can result in recurrent subluxation or dislocation.
• Infection: Rare, but may occur following surgical intervention.
• Hardware Complications: In surgical repairs involving screws, plates, or synthetic grafts, loosening or irritation may occur.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Rockwood CA, Matsen FA, Wirth MA, Lippitt SB. The Shoulder. 5th ed. Philadelphia: Elsevier; 2017.
3. Beaty JH, Kasser JR. Rockwood and Wilkins’ Fractures in Children. 9th ed. Philadelphia: Wolters Kluwer; 2020.
4. Matsen FA, Rockwood CA. Disorders of the Acromioclavicular Joint. In: Rockwood and Matsen’s The Shoulder. 4th ed. Philadelphia: Elsevier; 2009. p. 651-700.
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7. Tauber M. Surgical Techniques for Acromioclavicular Joint Reconstruction. Orthop Clin North Am. 2016;47(4):703-712.
8. Checchia SL, Doneux PS, Miyazaki AN. Acromioclavicular Joint Injuries in Athletes. Clinics Sports Med. 2016;35(2):243-258.