Injury to the acromioclavicular (AC) joint is a common injury among athletes and young individuals.
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 providing horizontal stability across the joint. Supporting structures include two coracoclavicular ligaments (trapezoid and conoid ligaments), which provide vertical stability, as well as the coracoacromial ligament. These ligaments can be injured, and if the injury is severe, one or more of these ligaments can be torn. The acromioclavicular joint can also be disrupted clinically and radiographically. 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.
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 account for more than 40% of all shoulder injuries and nearly 10% of all injuries in collision sports such as football, lacrosse, and ice hockey.
The most common mechanism of injury is direct trauma to the lateral shoulder or acromion process with the arm in adduction. Falling on an outstretched hand or elbow may also lead to AC joint separation.
Patients with an AC joint injury typically present with shoulder pain, usually superior-anterior in location, and will describe a mechanism of injury suggesting 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 active and passive range of motion secondary to pain. The examiner can assess the stability of the AC joint with anterior-posterior mobility (acromioclavicular ligament) and vertical mobility (coracoclavicular ligaments). It is important 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 used 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.
Acromioclavicular joint injuries follow a Rockwood classification system of type I to type VI. Type I is referred to as a sprain 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 but less than a 25 mm 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 20 mm; the patient is a laborer, elite athlete, or concerned about cosmesis; or is not improving with conservative management, then surgical intervention should be considered. Posterior displacement of the distal clavicle into the trapezius defines type IV injuries. Superior displacement of the distal clavicle by more than 25 mm defines type V injuries. Type VI is rare and is 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 the case should be discussed with an orthopedic surgeon.
Rockwood Classification of AC Joint Injuries (acromioclavicular [AC], coracoclavicular [CC]).
The prognosis for AC joint injuries is generally favorable. Most injuries are non-operative, and individuals typically regain functional motion by 6 weeks and return to normal activity by 12 weeks. Surgically managed injuries have a longer recovery time including immobilization for 6 weeks and a gradual return to full activity around 6 months.
The most common 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.
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 be evaluated by an orthopedic surgeon or sports medicine physician.
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 important to consult the orthopedic surgeon when the diagnosis is made 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. Pharmacist 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. 
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