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Emergency and Acute Medicine – Acromioclavicular (AC) Joint Injury
Anatomy and Injury Overview
The acromioclavicular (AC) joint is formed by the distal clavicle articulating with the acromion of the scapula. Stability is provided by the acromioclavicular (AC) ligament, the coracoclavicular (CC) ligament, and dynamic support from the deltoid and trapezius muscles. The AC ligament primarily maintains horizontal stability, while the CC ligament provides vertical stability. Injury occurs along a predictable spectrum, classically described by the Rockwood classification, which reflects progressive disruption of these stabilizing structures.
Rockwood Classification
Type I injuries involve a sprain of the AC ligament without rupture. The CC ligament, deltoid, and trapezius remain intact, and radiographs are normal; diagnosis is clinical, with localized AC joint tenderness.
Type II injuries feature rupture of the AC ligament with horizontal instability of the distal clavicle and a sprained but intact CC ligament. Mild deltoid and trapezius injury may be present. Imaging shows slight widening of the AC joint (normally <5 mm) with a normal cc distance (11–13 mm).< />pan>
Type III injuries include rupture of both the AC and CC ligaments, producing horizontal and vertical instability. The deltoid and trapezius are detached, and radiographs demonstrate widening of the AC joint with superior displacement of the distal clavicle, typically about 100%.
Types IV, V, and VI represent severe injuries and are best appreciated on axillary or lateral views. All require operative management and carry a higher risk of prolonged disability. Type IV involves posterior displacement of the clavicle into the trapezius; Type V shows marked superior displacement with a 100–300% increase in CC distance and dramatic shoulder droop; Type VI, usually associated with high-energy trauma, features inferior displacement of the clavicle into a subacromial or subcoracoid position and is often accompanied by neurovascular injury.
Mechanism and Epidemiology
AC joint injuries are most common in young, active males, particularly during contact sports. The typical mechanism is a direct blow to the superior or lateral shoulder with the arm adducted, such as during a fall. Less commonly, injury occurs indirectly through a fall on an outstretched hand or elbow, transmitting force to the AC joint.
Clinical Presentation
Patients usually report pain over the superior or anterior shoulder following trauma, worsened by cross-body movement, reaching behind the back, or overhead activity. The severity of symptoms correlates with the degree of ligament disruption. Associated neurovascular complaints or cervical spine symptoms should heighten concern for severe injury.
Physical Examination
Examination is best performed with the patient seated or standing, as gravity accentuates instability. Inspection may reveal swelling, ecchymosis, deformity, asymmetry, or skin tenting. A prominent distal clavicle with a sagging acromion suggests at least a Type II injury.
Palpation should proceed systematically from the sternoclavicular joint along the clavicle to the AC joint, CC ligament, coracoid process, scapula, and proximal humerus. AC joint tenderness suggests AC ligament injury, while horizontal instability indicates rupture. Tenderness or vertical instability implicates CC ligament disruption.
Provocative maneuvers include the cross-body adduction test, which compresses the AC joint and reproduces pain, and the O’Brien test, which may elicit superior shoulder pain in AC pathology. A complete distal neurovascular examination, including assessment of the brachial plexus, and a careful cervical spine evaluation are essential.
Diagnostic Evaluation
Diagnosis relies on history, examination, and targeted imaging. Dedicated AC joint radiographs with bilateral comparison are recommended when injury is suspected, as standard shoulder views may obscure subtle findings. The Zanca view (10–15° cephalic tilt) improves visualization of the AC joint, while axillary views are critical for identifying posterior or inferior clavicular displacement. Stress views are no longer routinely recommended. Advanced imaging with CT or MRI is reserved for surgical planning or complex injuries, and angiography may be indicated if vascular compromise is suspected.
Differential Diagnosis
Conditions that may mimic AC joint injury include shoulder dislocation, clavicle or acromion fractures, rotator cuff pathology, tendinitis, adhesive capsulitis, cervical radiculopathy, osteoarthritis, osteomyelitis, and—in children—distal clavicle physeal fractures.
Management in the Emergency Setting
Initial care includes ice application, sling immobilization, analgesia, and cervical spine precautions when indicated.
Type I and II injuries are managed conservatively with rest, ice, NSAIDs, brief sling use (typically 3–7 days), and early range-of-motion exercises. Most patients return to normal activity within 2–4 weeks once painless strength and motion are restored.
Type III injuries require sling immobilization, analgesia, and early orthopedic referral (within 72 hours). Management is controversial, with treatment individualized based on patient age, activity level, occupation, hand dominance, and reinjury risk.
Types IV, V, and VI injuries necessitate immediate orthopedic consultation and operative repair.
A recognized long-term complication of AC joint injury is post-traumatic arthritis.
Pediatric Considerations
In children, the clavicle is encased in a periosteal sleeve, with the CC ligament contained within and the AC ligament external and more vulnerable. True AC joint dislocations are uncommon in pediatrics and are usually Type I or II. Distal clavicle physeal fractures are more frequent. Younger children with Type III injuries are typically managed conservatively, while adolescents may require more aggressive treatment. Types IV–VI generally require surgery.
Disposition and Follow-Up
Patients with Type I or II injuries may be discharged with orthopedic follow-up in 2–4 weeks. Type III injuries require urgent orthopedic evaluation, and Types IV–VI typically warrant admission for operative management. Open injuries and pediatric cases should receive prompt specialist assessment.
Clinical Pearls and Pitfalls
Normal CC spacing distinguishes Type I and II injuries and supports conservative management. Type III injuries demonstrate complete superior displacement and require early orthopedic input. Types IV–VI share ligamentous disruption with Type III injuries but differ by clavicular position and uniformly require surgical repair.
Anatomy and Injury Overview
The acromioclavicular (AC) joint is formed by the distal clavicle articulating with the acromion of the scapula. Stability is provided by the acromioclavicular (AC) ligament, the coracoclavicular (CC) ligament, and dynamic support from the deltoid and trapezius muscles. The AC ligament primarily maintains horizontal stability, while the CC ligament provides vertical stability. Injury occurs along a predictable spectrum, classically described by the Rockwood classification, which reflects progressive disruption of these stabilizing structures.
Rockwood Classification
Type I injuries involve a sprain of the AC ligament without rupture. The CC ligament, deltoid, and trapezius remain intact, and radiographs are normal; diagnosis is clinical, with localized AC joint tenderness.
Type II injuries feature rupture of the AC ligament with horizontal instability of the distal clavicle and a sprained but intact CC ligament. Mild deltoid and trapezius injury may be present. Imaging shows slight widening of the AC joint (normally <5 mm) with a normal cc distance (11–13 mm).< />pan>
Type III injuries include rupture of both the AC and CC ligaments, producing horizontal and vertical instability. The deltoid and trapezius are detached, and radiographs demonstrate widening of the AC joint with superior displacement of the distal clavicle, typically about 100%.
Types IV, V, and VI represent severe injuries and are best appreciated on axillary or lateral views. All require operative management and carry a higher risk of prolonged disability. Type IV involves posterior displacement of the clavicle into the trapezius; Type V shows marked superior displacement with a 100–300% increase in CC distance and dramatic shoulder droop; Type VI, usually associated with high-energy trauma, features inferior displacement of the clavicle into a subacromial or subcoracoid position and is often accompanied by neurovascular injury.
Mechanism and Epidemiology
AC joint injuries are most common in young, active males, particularly during contact sports. The typical mechanism is a direct blow to the superior or lateral shoulder with the arm adducted, such as during a fall. Less commonly, injury occurs indirectly through a fall on an outstretched hand or elbow, transmitting force to the AC joint.
Clinical Presentation
Patients usually report pain over the superior or anterior shoulder following trauma, worsened by cross-body movement, reaching behind the back, or overhead activity. The severity of symptoms correlates with the degree of ligament disruption. Associated neurovascular complaints or cervical spine symptoms should heighten concern for severe injury.
Physical Examination
Examination is best performed with the patient seated or standing, as gravity accentuates instability. Inspection may reveal swelling, ecchymosis, deformity, asymmetry, or skin tenting. A prominent distal clavicle with a sagging acromion suggests at least a Type II injury.
Palpation should proceed systematically from the sternoclavicular joint along the clavicle to the AC joint, CC ligament, coracoid process, scapula, and proximal humerus. AC joint tenderness suggests AC ligament injury, while horizontal instability indicates rupture. Tenderness or vertical instability implicates CC ligament disruption.
Provocative maneuvers include the cross-body adduction test, which compresses the AC joint and reproduces pain, and the O’Brien test, which may elicit superior shoulder pain in AC pathology. A complete distal neurovascular examination, including assessment of the brachial plexus, and a careful cervical spine evaluation are essential.
Diagnostic Evaluation
Diagnosis relies on history, examination, and targeted imaging. Dedicated AC joint radiographs with bilateral comparison are recommended when injury is suspected, as standard shoulder views may obscure subtle findings. The Zanca view (10–15° cephalic tilt) improves visualization of the AC joint, while axillary views are critical for identifying posterior or inferior clavicular displacement. Stress views are no longer routinely recommended. Advanced imaging with CT or MRI is reserved for surgical planning or complex injuries, and angiography may be indicated if vascular compromise is suspected.
Differential Diagnosis
Conditions that may mimic AC joint injury include shoulder dislocation, clavicle or acromion fractures, rotator cuff pathology, tendinitis, adhesive capsulitis, cervical radiculopathy, osteoarthritis, osteomyelitis, and—in children—distal clavicle physeal fractures.
Management in the Emergency Setting
Initial care includes ice application, sling immobilization, analgesia, and cervical spine precautions when indicated.
Type I and II injuries are managed conservatively with rest, ice, NSAIDs, brief sling use (typically 3–7 days), and early range-of-motion exercises. Most patients return to normal activity within 2–4 weeks once painless strength and motion are restored.
Type III injuries require sling immobilization, analgesia, and early orthopedic referral (within 72 hours). Management is controversial, with treatment individualized based on patient age, activity level, occupation, hand dominance, and reinjury risk.
Types IV, V, and VI injuries necessitate immediate orthopedic consultation and operative repair.
A recognized long-term complication of AC joint injury is post-traumatic arthritis.
Pediatric Considerations
In children, the clavicle is encased in a periosteal sleeve, with the CC ligament contained within and the AC ligament external and more vulnerable. True AC joint dislocations are uncommon in pediatrics and are usually Type I or II. Distal clavicle physeal fractures are more frequent. Younger children with Type III injuries are typically managed conservatively, while adolescents may require more aggressive treatment. Types IV–VI generally require surgery.
Disposition and Follow-Up
Patients with Type I or II injuries may be discharged with orthopedic follow-up in 2–4 weeks. Type III injuries require urgent orthopedic evaluation, and Types IV–VI typically warrant admission for operative management. Open injuries and pediatric cases should receive prompt specialist assessment.
Clinical Pearls and Pitfalls
Normal CC spacing distinguishes Type I and II injuries and supports conservative management. Type III injuries demonstrate complete superior displacement and require early orthopedic input. Types IV–VI share ligamentous disruption with Type III injuries but differ by clavicular position and uniformly require surgical repair.
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