The acromioclavicular joint is composed of the articulation between the anteromedial acromion of the scapula and the lateral aspect of the clavicle.^1,2 Between these bony elements, there is a fibrocartilaginous disc, which functions to reduce the wear from stress applied across the joint during repetitive overhead lifting or even activities of daily living. The acromioclavicular ligament, which is composed of anterior, posterior, superior, and inferior components, serves as the primary stabilizer of the joint and controls horizontal motion.³ Further, the coracoclavicular ligaments, the conoid and trapezoid, function to provide vertical stability of the joint. In addition to these structures, which provide static stability, the adjacent deltoid, trapezius, and serratus anterior musculature provide dynamic stability of the joint. Acromioclavicular joint disorders are a common source of shoulder pain and can result from either age-related degeneration or traumatic injury, which are discussed below.

Acromioclavicular Joint Degeneration

In some individuals, the degeneration of this disc begins as early as twenty years old and is a main determinant of the eventual osteoarthritis of this joint.⁴ It has been shown that 54 to 57 percent of elderly patients show evidence of acromioclavicular joint degeneration on radiographs.⁵ Etiologies of acromioclavicular (AC) joint degeneration include age-related degeneration of the intra-articular disc, post-traumatic changes, and inflammatory or septic arthritis. Distal clavicular osteolysis, commonly seen among athletes and weightlifters, is another overuse injury, which causes pain in the acromioclavicular joint. Distal clavicle osteolysis is the result of repetitive excess loading during adduction, internal rotation, and flexion of the shoulder, common movements in bench pressing or overhead lifting.⁶ Patients with acromioclavicular joint degeneration will typically describe an insidious onset of anterior or superior pain of the affected shoulder.^1,7 Pain will be worse with cross-body movements or overhead activity.

Acromioclavicular Joint Separation

Although age-related degenerative changes constitute most acromioclavicular joint disorders, traumatic injury to the AC joint resulting in a separation is also a common shoulder injury. These injuries are notably more common in men, young adults in their twenties or thirties, and those who participate in contact sports. Filk et al. demonstrated that acromioclavicular joint injuries were the third most common injury in collegiate men’s ice hockey players.⁸

Traumatic acromioclavicular joint injuries can occur through either a direct or an indirect mechanism.⁷ The direct mechanism is a downward force on the lateral aspect of an adducted shoulder, which pushes the acromion inferiorly and medially. Most commonly, this occurs after a fall directly on the superolateral shoulder. In contrast, the indirect mechanism is a fall on an outstretched arm, which generates a superiorly directed force.⁷ Regardless of etiology, the pain with traumatic acromioclavicular joint injuries is classically located in the anterior or superior shoulder and worsens with movement or exertion.

The Rockwood Classification is the most widely utilized classification system to describe and stratify the degree of acromioclavicular joint separations. It organizes AC joint separation—based on involvement of the acromioclavicular joint, the coracoclavicular ligaments, and the deltoid and trapezius muscles—into six degrees of injury.⁹ Type I involves only a sprain of the acromioclavicular ligament with no significant changes on x-ray. Type II injuries involve a complete tear of the acromioclavicular ligament, a disruption of the corresponding joint, and a sprain of the coracoclavicular ligament. X-rays show an increase of the coracoclavicular distance of less than 25 percent compared to the contralateral. In Type III injuries, the acromioclavicular ligament and coracoclavicular ligaments are both torn, and the coracoclavicular distance is seen increased by 25 to 100 percent on x-rays of the affected shoulder compared to the unaffected shoulder. Type IV injuries involve tears of the acromioclavicular and coracoclavicular ligaments and a posterior dislocation of the lateral clavicle. A Type V injury includes tears of the acromioclavicular and coracoclavicular ligaments, a superior dislocation of the AC joint, and an increase of the coracoclavicular distance by 100 percent or greater in the affected compared to the unaffected shoulder on x-ray. Finally, Type VI injuries, which are rare, involve an inferior dislocation of the lateral clavicle.⁹ Additionally, the acromioclavicular and coracoclavicular ligaments are torn.

Physical Examination

Physical examination of a patient with acromioclavicular joint pathology should begin with thorough observation of the affected shoulder for swelling, bruising, prominence, or asymmetry.^7,10,11 Comparison should be made to the contralateral side, and signs of obvious deformity should be noted (figure 9.1).

Next, the examiner should palpate the entire shoulder girdle. The areas palpated should include the sternoclavicular joint, the clavicle, and the acromioclavicular joint. The examiner should note any areas that elicit tenderness and be aware of any acromioclavicular joint prominence (figure 9.2; video 9.1). If there is asymmetry or superior displacement, the distal end of the clavicle should be further palpated to see if the deformity is partially or completely reducible. Furthermore, the overlying skin should be assessed for integrity and perfusion. Although rare, cases of acromioclavicular joint cysts have been reported in the literature, most commonly in elderly patients. This manifests radiologically as the Geyser sign and results secondary to a chronic rotator cuff tear that allows for the communication of synovial fluid from the glenohumeral joint into the acromioclavicular joint, which creates a palpable subcutaneous fluid collection (figure 9.3).

Next, range of motion of the ipsilateral glenohumeral joint should be assessed, specifically with regard to any limitations in forward elevation, external rotation, and internal rotation. Active and passive range of motion measurements and strength testing should be performed on the affected shoulder and compared to the contralateral side. Patients with increasing severity of AC joint separation will experience more difficulty with forward elevation and shoulder strength.

Special Tests

In addition to point tenderness at the AC joint, several special tests can also be performed to assess the presence of acromioclavicular joint disorder. One of these tests is the cross-body adduction test (video 9.2; figure 9.4).¹² In this test, the affected arm is brought to 90 degrees of flexion. The examiner then helps the patient bring the affected shoulder to the contralateral side via horizontal adduction. This test is considered positive if it elicits pain at the acromioclavicular joint upon adduction across the body (video 9.3).