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Acromioclavicular Joint Injury

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Last Update: May 13, 2022.

Continuing Education Activity

Acromioclavicular joint injuries account for more than forty percent of all shoulder injuries. Mild injuries are not associated with any significant morbidity, but severe injuries can lead to significant loss of strength and function of the shoulder. Acromioclavicular injuries may be associated with a fractured clavicle, impingement syndromes, and more rarely neurovascular insults. This activity reviews the evaluation and management of this condition and highlights the role of the interprofessional team in caring for patients effected by this acromioclavicular joint injuries.


  • Describe the most common mechanism of injury associated with acromioclavicular joint injury.
  • Review the evaluation of a patient with a suspected acromioclavicular injury.
  • Identify the difference in management of patients with Rockwood Classification type I-II injuries from patients with Rockwood Classification type III-VI.
  • Outline strategies for the interprofessional care team to provide optimal management and counseling to improve outcomes for patients with acromioclavicular joint injuries.
Access free multiple choice questions on this topic.


Injury to the acromioclavicular (AC) joint is a common injury among athletes and young individuals. Acromioclavicular joint injuries account for more than forty percent of all shoulder injuries. Mild injuries are not associated with any significant morbidity, but severe injuries can lead to significant loss of strength and function of the shoulder. Acromioclavicular injuries may be associated with a fractured clavicle, impingement syndromes, and more rarely neurovascular insults.


The AC joint is a diarthrodial joint defined by the lateral process of the clavicle articulating with the acromion process as it projects anteriorly off the scapula. The joint is primarily stabilized by the acromioclavicular ligament, which is composed of an anterior, posterior, inferior, and superior component. Of note, the superior portion of the AC ligament is the most important component for the stability of the AC joint. Supporting structures include two coracoclavicular ligaments (trapezoid and conoid ligaments), which provide vertical stability, as well as the coracoacromial ligament. Mild injuries are not associated with any significant morbidity, but severe injuries can lead to considerable loss of strength and function of the shoulder. Acromioclavicular injuries may be associated with a fractured clavicle, impingement syndromes, and more rarely neurovascular insults.[1][2]


AC injuries are frequently seen after sporting events, car accidents, falls from a bicycle, and other sports-related activities (e.g., skiing). AC joint injuries may account for as much as 40% of all shoulder injuries and nearly 10% of all injuries in collision sports such as football, lacrosse, and ice hockey.[3][4]


The most common mechanism of injury is direct trauma to the lateral aspect of the shoulder or acromion process with the arm in adduction. Falling on an outstretched hand or elbow may also lead to AC joint separation.

History and Physical

Patients with an AC joint injury typically present with anterosuperior shoulder pain and will describe a mechanism of injury of blunt trauma to the abducted shoulder or landing on an outstretched arm, suggestive of this type of injury. They may describe pain radiating to the neck or shoulder, which is often worse with movement or when they try to sleep on the affected shoulder. On examination, the clinician may observe swelling, bruising, or a deformity of the AC joint, depending on the degree of injury. The patient will be tender at that location. They may have a restriction in the active and passive range of motion secondary to pain. "Piano key sign" may be seen, with an elevation of the clavicle that rebounds after inferior compression. Finally, It is essential to evaluate the entire clavicle for possible fracture or sternoclavicular injury as well as perform a full neurovascular exam on the affected extremity.


Standard X-rays are adequate to make a diagnosis of acromioclavicular joint injury and should be used to evaluate for other causes of traumatic shoulder pain. AC joint injuries may not always be evident on regular radiographic views (anteroposterior [AP], lateral). Additional views include the Zanca view; an AP view performed by tilting the beam 10 to 15 degrees cranial, as well as bilateral AP views to compare displacement to the contralateral shoulder. Weighted stress views may be useful to evaluate the displacement of the joint when the diagnosis is uncertain on standard AP views. If there is continued uncertainty in diagnosis, the provider may also consider ultrasound or MRI for further diagnostic evaluation.

Consider the Rockwood Staging System outlined below on the evaluation of radiographs, especially when compared to the contralateral shoulder, which will be important for guiding treatment. Identification of the coracoclavicular interspace distance compared to the contralateral view will help to guide treatment options in the majority of cases. An example of the appropriate method for measuring coracoclavicular interspace distance appears below.

Treatment / Management

Acromioclavicular joint injuries follow the Rockwood classification system of type I to type VI. Type I is referred to as a sprain of the acromioclavicular ligaments only and demonstrates no radiographic displacement. Type II involves tearing of the acromioclavicular ligament and sprain of the coracoclavicular ligament with less than 25% increase in the coracoclavicular interspace or with the clavicle elevated but not superior to the border of the acromion. Type I and II sprains are managed non-operatively with a sling, analgesia, ice, and physical therapy. Type III AC joint separation involves tearing of both the acromioclavicular ligament and coracoclavicular ligaments resulting in clavicle elevation above the border of the acromion with a 25 to 100% increased coracoclavicular distance on x-ray compared to the contralateral side. Type III injuries are frequently managed non-operatively similar to type I and II; however, if the displacement is greater than 75%; the patient is a laborer, elite athlete, or concerned about cosmesis; or is not improving with conservative management, then surgical intervention may be considered. Posterior displacement of the distal clavicle into the trapezius defines type IV injuries. Superior displacement of the distal clavicle by more than 100% compared to contralateral defines type V injuries. Type VI is rare and is defined as an inferolateral displacement in a subacromial or subcoracoid displacement behind the coracobrachialis or biceps tendon. Type IV through VI injuries are typically managed surgically, and warrants referral to an orthopedic surgeon.[1][3][5]

In the setting of Rockwood type Types III and V injuries, a distal clavicle hook plate may be employed to restore the alignment of the acromioclavicular joint. The "hook" portion of the hook plate is placed inferior to the acromion, with superior plating on the clavicle. While this does provide excellent reduction of the joint, the plate must be removed in a subsequent surgery to prevent subacromial irritation and iatrogenic damage to the rotator cuff.[6]

Differential Diagnosis

  • Acromioclavicular distal clavicle osteolysis
  • Acromioclavicular joint arthritis
  • Acromion fracture
  • Adhesive capsulitis
  • Anterior humerus subluxation
  • Complex pain syndrome
  • Erb-Duchenne injury
  • Glenoid labrum tear
  • Os acromiale
  • Rotator-cuff injury
  • Superior labral tear
  • Septic arthritis
  • Shoulder dislocation


Rockwood Classification of AC Joint Injuries (acromioclavicular [AC], coracoclavicular [CC]).[7]

  • I: AC ligament sprain; CC ligament intact; no radiographic abnormalities
  • II: AC ligament is torn; CC ligament sprain; the clavicle has elevated but is not superior to the border of the acromion, or exhibits a less than 25% increase in the CC interspace compared to the contralateral
  • III: AC and CC ligaments are torn; clavicle has elevated above the border of the acromion, or there is an increase of 25% to 100% in the CC interspace compared to the contralateral
  • IV: AC and CC ligaments are torn; posterior displacement of the distal clavicle into the trapezius
  • V: AC and CC ligaments are torn; superior displacement of the distal clavicle by more than 100% in the CC interspace compared to the contralateral
  • VI: AC and CC ligaments are torn; inferolateral displacement in a subacromial or subcoracoid displacement behind the coracobrachialis or biceps tendon

The Rockwood classification remains the gold standard for driving treatment decisions and is an excellent tool in the management of acromioclavicular joint injuries.


The prognosis for AC joint injuries is generally favorable. Most injuries receive non-operative management, and individuals typically regain functional motion by six weeks and return to normal activity by 12 weeks. Surgically managed injuries have a more extended recovery timeframe, including immobilization for six weeks and a gradual return to full activity around six months.[8][9][10]


The most frequently encountered complication of AC joint separation is residual joint pain affecting anywhere from 30% to 50% of individuals. AC joint arthritis is also a known complication and is more common with surgical management.

Postoperative and Rehabilitation Care

Rehabilitation of AC joint separation primarily includes rest, a brief period of immobilization in a sling (typically 3 to 7 days), ice, NSAIDs, and physical therapy. Return to physical activity and sport is guided by both the physician and physical therapist.


Type I and II AC joint separations do not necessarily require an evaluation by a specialist; however, type III through VI should have an evaluation by an orthopedic surgeon or sports medicine physician.

Deterrence and Patient Education

Injuries of the acromioclavicular joint are common causes of shoulder pain and frequently occur in athletes and the setting of traumatic injury. Patients should seek an evaluation from a trained orthopedic or sports management physician. Early management with sling immobilization, ice, rest, and anti-inflammatory medications is generally an appropriate treatment for these injuries. Patients can generally return to normal activities after they have been evaluated and cleared by their treating physician.

Pearls and Other Issues

  • The position of the AC joint makes it vulnerable to direct trauma to the superior or lateral aspect of the shoulder.
  • In an isolated injury, patients typically have pain and swelling over the joint with or without a deformity depending on the degree of injury.
  • All patients should receive a radiographic evaluation of the affected shoulder, and if the diagnosis is in question, obtain radiographs of the contralateral AC joint as well.
  • Type I and II injuries occur twice as frequently as type III to VI injuries.
  • Most injuries can have management non-operatively. If uncertainty exists or it is a Rockwell grade III or greater, an orthopedic physician should be urgently consulted.
  • Non-operative management includes rest, protection with a sling, ice, NSAIDs, and physical therapy.

Enhancing Healthcare Team Outcomes

AC joint injuries commonly present to the emergency department. The mild injuries are usually managed conservatively, but the severe injuries may require surgery; hence it is crucial to consult an orthopedic surgeon when arriving at the diagnosis, to grade the injury. For mild injuries, the nurse practitioner, emergency department physician, and primary caregiver should recommend physical therapy, once the acute pain has resolved. Pharmacists and nurses should work with the team to educate the patient and family and assist in pain control.  Those wishing to return to sports should first consult with the orthopedic surgeon.[11]

Review Questions

Radiograph Shouder AC Joint Separation/Injury Type III/IV


Radiograph Shouder AC Joint Separation/Injury Type III/IV. Contributed by Scott Dulebohn, MD


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