Acromioclavicular Joint Separations 

The acromioclavicular (AC) joint is a diarthrodial articulation with an interposed fibrocartilaginous meniscal disk that links the hyaline cartilage articular surfaces of the acromial process and the clavicle. The joint is stabilized by a combination of dynamic muscular and static ligamentous structures, which allow a normal anatomic range of motion. Because of the transverse orientation of the articulation, direct downward forces may result in shear stresses that cause disruption of these stabilizing structures and create displacement beyond the normal limits. This is evidenced by abnormal positioning of the clavicle relative to the acromion, usually in the superior direction.

The severity of an AC separation is dependent upon the degree of ligamentous injury. The capsular AC ligaments and the extracapsular coracoclavicular (CC) ligament are the primary static stabilizers of the AC joint, as seen in the image below. The anterior and posterior AC ligaments are predominantly responsible for maintaining stability in the anteroposterior (AP) plane. The 2 components of the CC ligament, the trapezoid and conoid ligaments, provide restraint against compression and superior-inferior translation, respectively.^[1] The deltoid and trapezius muscles are especially important in providing dynamic stabilization when these ligamentous structures are damaged.

Recent studies

Stine and Vangsness analyzed the capsular and ligamentous insertions around the AC joint in 28 cadaveric shoulders to determine the amount of bone that can be removed without destabilizing the joint. Their recommendations for AC joint resection (5-7 mm) is that 2-3 mm of the medial acromion and 3-4 mm of the distal clavicle can be resected without removing the AC capsular insertions. They note that medial resections greater that 15 mm will begin to take down the trapezoid ligament and suggest that arthroscopic bone resection be directed into the AC joint at approximately 50º in the axial plane and 12º in the coronal plane for safe symmetric resection.^[2]

Leidel et al studied the operative outcome of acute grade III AC joint separations after temporary K-wire transfixation in 70 patients divided into 3 groups: 1-2 years after procedure; 3-5 years after procedure; and 6-10 years after procedure. The study showed no significant differences among the 3 groups, with good functional results in all treatment groups and consistent outcome over the long term.^[3]

Treatment of acromioclavicular (AC) separations has been a subject of debate. Type I and type II injuries are generally accepted to be treated nonoperatively in the acute setting. However, reaching a consensus regarding the optimal management of acute type III injuries has been difficult.

The earliest writings on the subject were by Hippocrates, Galen, and Paul of Aegina, who recommended conservative management with compressive bandages to keep the clavicle in a normal position.^[4] However, subsequent medical history shows that nonoperative and operative treatments for acute type III AC separations have gained popularity in cyclical fashion.

Surgical management gained popularity with the advent of improved anesthesia and sterile surgical techniques. The first surgical repair of an acute AC dislocation is credited to Samuel Cooper, who used a loop of silver wire to approximate the clavicle and acromion process.^[5] Subsequently, numerous other techniques were reported, including suture repair of the AC ligaments and CC ligament,^[5] tendon graft for reconstruction,^{[5, 6]} and fixation with nails, screws, or wires.^{[5, 7, 8, 9, 10]}

To understand contemporary opinion regarding nonoperative versus operative repair of acute type III AC separations, approved orthopedic residency program chairpersons have been polled twice. In 1974, Powers and Bach found that most chairpersons advocated open surgical repair of type III injuries and concluded that surgical repair was the most popular method of treatment.^[11] In 1992, Cox reported the results of a similar poll, which found that 72.2% of chairpersons who responded preferred nonoperative, symptomatic management.^[12] He concluded that this represented a shift toward conservative management of such injuries.

The management of acute type III AC separations is still somewhat controversial. In most patients with such injuries, nonoperative care achieves an excellent functional outcome. In younger, more active patients, surgical correction should at least be considered.^{[13, 3, 14]}

The degree of damage to the acromioclavicular (AC) ligaments and coracoclavicular (CC) ligament with resultant displacement of the clavicle relative to the acromion is the primary criterion for the classification of AC separations. Originally, Cadenat differentiated incomplete injuries, in which only the capsular AC ligaments were torn, from injuries that involved disruption of both the AC and CC ligamentous structures.^[5] Allman and Tossy later recognized 3 different types of AC separation based on similar criteria. Type I injuries were characterized by sprained but intact CC and AC ligaments. Type II injuries involved a complete disruption of the AC ligaments with a sprained but intact CC ligament. In the more severe type III injury, both the CC and AC structures were completely disrupted.^{[15, 16, 17]}

In more recent years, this system has been expanded to include 3 additional variants.^[18] Type IV injuries are defined by posterior displacement of the clavicle relative to the acromion with buttonholing through the trapezius muscle. In type V injuries, the clavicle is widely displaced superiorly relative to the acromion as a result of disruption of muscle attachments. The rare type VI injuries are characterized by inferior displacement of the distal clavicle below the acromial process or the coracoid process. See image below for classification.

Coracoid process fractures and CC ligament bony avulsions associated with AC joint separations represent variations on this theme. Recognition of these unique injury patterns helps guide treatment.

Frequency

Acromioclavicular joint injuries are seen especially in competitive athletes, such as rugby or hockey players, and occur most frequently in the second decade of life. Males are more commonly affected than females, with a male-to-female ratio of approximately 5:1.^[13]

The most common mechanism of injury is a direct force applied to the superior aspect of the acromion, usually from a fall with the arm in an adducted position. This impact drives the acromion inferiorly, spraining the intra-articular acromioclavicular ligaments. If the force is great enough, the extra-articular coracoclavicular ligament may also be damaged.

Less commonly, an indirect force may be transmitted up the arm as a result of a fall on an outstretched hand. The force continues through the humeral head to the acromial process, displacing it superiorly and stressing the AC ligaments. The coracoacromial (CA) ligaments are not injured with this type of mechanism.

Patients typically present with pain and restricted shoulder motion after a fall. Specifying the acuteness of the pain is important, as patients may experience similar pain and limitation of movement with chronic conditions affecting the shoulder. Age is also a helpful diagnostic clue. Incidence of acromioclavicular (AC) separations is highest in patients in the second decade of life; patients in this age group almost never present with degenerative rotator cuff tears or impingement.

Visual inspection of the patient may also provide a significant key to diagnosis. A prominent clavicle with loss of the normal contour of the shoulder caused by sagging of the acromion is highly suggestive of a ligamentous disruption of the AC joint. These findings may be clearer when the patient is asked to hold a 10- to 15-pound weight in the hand of the affected arm. Patients should undergo a standard shoulder examination, including evaluation of range of motion and assessment of neurovascular status. Pain with palpation over the AC joint and inhibited movement due to discomfort are generally noted.

Treatment for acute type III acromioclavicular (AC) injuries remains controversial despite randomized trials that indicate success with nonoperative treatment in most cases.^{[19, 20, 21]} Many surgeons believe that this degree of displacement leads to muscle-fatigue discomfort and difficulty manipulating heavy loads. As a result, operative intervention should be considered in heavy laborers and younger patients who are athletic or unsure of their future careers.

Type IV, V, and VI injuries generally require surgical repair. Because the clavicle is so far displaced from the acromial process in the posterior, superior, or inferior direction, respectively, conservative management is inadequate. The patient continues to experience pain and dysfunction if the articulation is not reduced and stabilized.

Type I, II, and III injuries that are managed conservatively may result in persistent shoulder pain, dysfunction, or both. Type I and II separations may progress to develop symptomatic degenerative disease. Type III separations may result in impingement symptoms, muscle-fatigue discomfort, and/or neurovascular symptomatology. Late surgical management may be required.

The acromioclavicular (AC) joint capsule is strengthened by the anterior and posterior AC ligaments. These ligaments are intracapsular and provide stability in the anterior and posterior directions. The fibers of the trapezius and the deltoid muscles provide additional stability.

The trapezoid and conoid components of the coracoclavicular (CC) ligament add vertical stability. They attach to the coracoid process and the inferior surface of the distal clavicle (see image below).^[22]

Although not an AC joint–stabilizing structure, the coracoacromial (CA) ligament is also relevant to the discussion of surgical treatment. This structure runs from the superior surface of the coracoid process to the inferior surface of the acromial process in a nearly horizontal direction. During operative repair of type III injuries, the CA ligament may be resected from its acromial insertion and used to reconstruct the torn CC ligament.

Type III, IV, V, and VI AC separations are, in fact, double disruptions of the superior shoulder suspensory complex (SSSC). The SSSC is a bony and soft-tissue ring composed of the glenoid process, the coracoid process, the CC ligament, the distal clavicle, the AC joint, and the acromial process at the end of a superior bony strut (the midshaft clavicle) and an inferior bony strut (the junction of the lateral scapular body and the medial glenoid neck). Type III through type VI separations are characterized by disruptions of both the CC and AC ligaments; therefore, they constitute unstable injuries that must be accounted for or require surgical reduction and stabilization.^[23]

The presence of infection is a contraindication to repair of an acromioclavicular separation. Additionally, an open wound with compromised soft tissues may necessitate delay of the surgery until the risk of infection is minimized by irrigation and antibiotic prophylaxis. Other considerations include the patient's overall health and ability to undergo anesthesia.