Introduction

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  The most widely used classification is , which is based on severity of injury.

  
  
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  Clinical evaluation is especially important in assessing posterior dislocations and scapulothoracic dyskinesia.

  
  
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  Imaging should always include standard shoulder views, including an axillary view in addition to bilateral Zanca views.

  
  
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  Nonoperative treatment is the best approach for most acromioclavicular (AC) injuries and should include short-term immobilization and active physical therapy.

  
  
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  Type III injuries are the most controversial in the literature, but there is a recent trend toward conservative treatment as well.

  
  
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  Surgical treatment should be considered in late-presenting patients, high-level athletes, and overhead workers.

Articulating Surfaces

The AC joint consists of an articulation between the lateral end of the clavicle and the acromion of the scapula. Being a diarthrodial joint, the AC joint is surrounded by a capsule and has an intraarticular synovium and an articular cartilage interface. The distal clavicle articulates with the acromion via the medial facet, which is oriented posteriorly and laterally; the articular surface of the acromion is directed medially and anteriorly ( ). The average size of the joint surfaces in the adult AC joint has been found to be 9 mm (vertical) by 19 mm (anteroposterior [AP]) ( ). The average width of the AC joint ranges from 1 mm to 3 mm and decreases with advancing age, regardless of gender. It has two atypical features:

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  The articular surfaces of the joint are lined with fibrocartilage (as opposed to hyaline cartilage). The hyaline articular cartilage becomes fibrocartilage on the acromial side of the joint by the age of 17 years and on the clavicular side by the age of 24 years.

  
  
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  The joint cavity is partially divided by an articular disk, a wedge of fibrocartilage suspended from the upper part of the capsule. This disk varies in size and shape and is of mainly two types: a complete disk (very rare) and a meniscoid-like disk that only partially separates the articular surfaces and occupies the upper part of the articulation ( ). The thickness of this disk varies, ranging from 1.5 to 4 mm. Beginning in the second decade of life, this structure undergoes rapid degeneration; by the fourth decade, this degeneration is significant.

Ligaments

The AC joint is supported by multiple ligaments, including the joint capsular ligaments, the coracoacromial (CC) ligament, and the coracoclavicular ligament. The AC joint capsule, although thin, provides considerable ligamentous support. The capsule, especially its superior and posterior parts, has been identified as the primary restraint to posterior translation of the clavicle ( ). There are four AC ligaments: superior, inferior, anterior, and posterior.

The superior AC ligament is a quadrilateral band composed of parallel fibers that interlace with the trapezius and deltoid aponeuroses, covering the superior part of the articulation and extending between the upper part of the lateral end of the clavicle and the adjoining part of the upper surface of the acromion. This ligament is thicker (2.0–5.5 mm) than the inferior and has a more defined insertion into the distal clavicle. Inferiorly, it is in contact with the articular disk when this is present.

The inferior AC ligament is the thinner of the two and covers the under part of the articulation and is attached to the adjoining surfaces of the two bones. Below this ligament lies the tendon of the supraspinatus. The CC ligament, which runs from the coracoid process to the acromion, is a strong triangular band, extending between the coracoid process and the acromion. It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle and by its broad base to the whole length of the lateral border of the coracoid process. Superior to the ligament are the clavicle and undersurface of the deltoid and inferiorly is related to the tendon of the supraspinatus, a bursa being interposed. The ligament is sometimes described as consisting of two marginal bands and a thinner intervening portion, the two bands being attached, respectively, to the apex and the base of the coracoid process and joining together at the acromion. When the pectoralis minor is inserted into the capsule of the shoulder joint instead of into the coracoid process, it passes between these two bands, and the intervening portion of the ligament is then deficient.

Two other major ligaments supporting the AC joint are:

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  Conoid: runs vertically from the coracoid process of the scapula to the conoid tubercle of the clavicle

  
  
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  Trapezoid: runs from the coracoid process of the scapula to the trapezoid line of the clavicle

Collectively, the conoid and trapezoid ligaments are known as the coracoclavicular ligaments. They are a very strong structure, effectively suspending the weight of the upper limb from the clavicle ( Fig. 11.1 ).

The ligaments of the acromioclavicular joint.

Modified from TeachMeAnatomy. http://teachmeanatomy.info/upper-limb/joints/acromioclavicular . Copyright Jay F. Cox, 2006.

The conoid tubercle is located at the most posterior aspect of the clavicle, at the point where the middle third of the shaft curves into the lateral third. The trapezoid ridge extends anteriorly and laterally across the inferior surface of the lateral third of the clavicle. These landmarks represent the insertions of the corresponding ligaments ( ). The conoid ligament is the posteromedial portion and the trapezoid ligament is the anterolateral portion of the CC ligament complex. Bursae can exist between these ligaments. The clavicular insertion of the conoid ligament is approximately twice as wide (medial to lateral) and thick (anterior to posterior) as its coracoid insertion, giving rise to its inverted cone shape. The trapezoid ligament is three times thicker at its clavicular end than at its coracoid end, but it shows less narrowing of its width compared with the conoid ligament. The coracoid origin of the trapezoid covered the posterior half of the coracoid dorsum; the conoid origin is more posterior on the base of the coracoid, limited anteriorly by the trapezoid insertion. The trapezoid ligament varies from 0.8 cm to 2.5 cm both in length and in width, and the conoid ligament varies from 0.7 cm to 2.5 cm in length and from 0.4 cm to 0.95 cm in width. Several studies have shown the center of the trapezoid and the conoid ligament insertion to be located 2.5 cm and 4.6 cm from the lateral edge of the clavicle, respectively ( ).

Neurovascular Supply

Movements and Function

The AC joint allows a degree of axial rotation and AP movement. Because no muscles act directly on the joint, all movement is passive and is initiated by movement at other joints. The motions of the AC joint are described as scapular movement with respect to the clavicle, including ( ):

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  Upward and downward rotation about an axis directed perpendicular to the scapular plane facing anteriorly and medially

  
  
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  Internal and external rotation about an approximately vertical axis

  
  
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  Anterior and posterior tipping or tilting about an axis directed laterally and anteriorly

The primary functions of the AC joint are:

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  To allow additional range of rotation for the scapula on the thorax

  
  
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  To allow adjustments of the scapula (tipping and internal and external rotation) outside the initial plane of the scapula to follow the changing shape of the thorax as arm movement occurs

Applied Anatomy

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  The AC joint is extremely susceptible to both trauma and degenerative change because of its small and incongruent surfaces that result in large forces per unit area. Degenerative change is common from the second decade of life on, with the joint space itself commonly narrowed by the sixth decade.

  
  
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  The various procedures used for the operative management of AC joint dislocations are centered on the principle of reconstructing the strong conoid and trapezoid coracoclavicular ligaments in as physiological position as possible.

Etiology

An AC joint injury often occurs as a result of a direct blow to the tip of the shoulder from, for example, an awkward fall onto the dome of the shoulder, or impact with another player. This forces the acromion process downward, beneath the clavicle. The clavicle rests against the first rib, which blocks further downward displacement of the clavicle. If the clavicle is not fractured, the AC and coracoclavicular ligaments rupture. Other structures injured could include deltoid and trapezius muscles ( Fig. 11.2, A ).

The mechanism of injury.

Modified from Oster DM. AC Joint Sprains: Shoulder Separation. http://www.davidostermd.com/David_M._Oster/AC_Joint_Sprain.html .

Alternately, an AC joint injury may result from an upward force to the long axis of the humerus such as a fall that directly impacts on the wrist of a straightened arm. Most typically the shoulder is in an adducted and flexed position ( Fig. 11.2, B ).

Epidemiology

Dislocation of the AC joint is not infrequent, representing about 9% of all shoulder injuries, and should be considered whenever a young adult comes to clinical observation for a direct impact to the shoulder girdle. However, information about the basic epidemiologic features of this condition is scarce ( ). It is the third most common injury seen in college hockey players and accounts for 41% of all shoulder injuries seen in elite college rugby players ( ). In the study by Chillemi et al, the incidence of pure AC dislocations, in a retrospective database search performed to identify all urban patients with an AC dislocation over a 5-year period, was 1.8 per 10,000 per year ( ). Men between the age of 20 and 39 years were identified as significant demographic risk factors. AC joint injury has a significantly higher incidence rate for men compared with women; this is probably related to differences in lifestyle and hobbies, the men being more inclined to engage in high-risk activities. The other study that assessed the incidence of AC dislocation in a city-like population was by , who reported an incidence of 1.5 per 10,000 for males and of 0.2 for females.

In most cases, AC dislocation is the result of a direct and high-energy impact to the shoulder, which is a frequent occurrence in many sports as well as in road accidents. This explains why injuries to the AC joint are more common in the active population, highly exposed to forceful contacts. Sport activity, mainly cycling, represents the most common cause of AC dislocation, followed by road accidents. The principal mechanism of lesion regarding cycling is a direct impact to the joint when the arm is adducted or outstretched (putting the AC and CC ligaments in a position more susceptible to tears and strains) ( ). Other sporting activities, namely soccer, rugby, and basketball, are associated with a higher risk of AC injuries because of tackling or wrong landing after a jump ( ). The second most common cause of AC dislocation is road accidents. In these cases, the tightening effect from safety belts may play a key role in the genesis of the injury, with or without an additional direct trauma to the shoulder.

, in their epidemiologic study of 162 new AC joint injuries to determine the incidence and characteristics of AC joint injury at the United States Military Academy between 2005 and 2009, concluded that the majority of the AC joint injuries were low grade (145 sprains, 89%), and the injury occurred most commonly during athletics (91%). They also concluded that AC injuries resulted in at least 1359 total days lost to injury and an average of 18.4 days lost per athlete. The average time lost to injury for low-grade sprains was 10.4 days compared with high-grade injuries at 63.7 days. Of their patients with high-grade injuries, 71% elected to undergo coracoclavicular or AC reconstructions and the rate of surgical intervention in their study was 19 times higher for high-grade AC joint injuries than for low-grade injuries.

Classification

Acromioclavicular joint injuries were first described by Tossy et al in 1963. The injuries were divided into three categories depending on severity and were meant to guide treatment. Grade I was considered a sprain of the AC joint. Grade II was identified if the clavicle was displaced about 50% superiorly with regards to the acromion, and grade III included all complete separations of the AC joint. In 1967, described a similar classification for a American Academy of Orthopaedic Surgeons instructional course lecture, the main difference being a special mention of a posteriorly dislocated clavicle for grade IIIs.

In 1989, Rockwood’s group developed another classification based on Tossy’s earlier work ( ). To date, it has been the most widely used classification in the literature and involves six types ( Fig. 11.3 ).

Rockwood acromioclavicular joint injury classification.

Adapted from Rockwood CA, Young DC. Disorders of the acromioclavicular joint. In: Rockwood CA, Matsen F III, eds. The shoulder . Philadelphia: WB Saunders, 1990: 433, with permission.

Assessment

Completing a thorough history and physical examination with a patient suspected of having an AC injury is imperative. It is especially important to determine past upper extremity injuries or problems, occupation (overhead labor), and level of sport participation because these may influence the choice of treatment. In terms of the physical examination, the clinician needs to be able to see both AC joints as well as the scapulae and shoulder joints ( ). The initial assessment is performed with the arm hanging freely at the side ( ). Asymmetry is noted in terms of swelling, deformity, and bruising. The AC joint will be tender to palpation. The examiner may try to reduce the joint by translating the shoulder joint superiorly with a hand under the elbow. If the AC joint injury is not obvious, the crossed-arm adduction and active compression test are used to identify an injury ( ). To differentiate between a type III and V injury, the evaluator can ask the patient to shrug his or her shoulders. If a type V injury is present, the AC joint may not reduce itself. A type IV injury might also be assessed by grasping the acromion in one hand and the clavicle in the other and translating it anteriorly and posteriorly. All these maneuvers can be very painful for the patient and be impossible to perform at the initial evaluation. Also, the sternoclavicular joint should be examined because biclavicular dislocations have been reported ( ). The scapulothoracic joint should also be assessed with regards to resting position, especially asymmetry of the medial scapular border, and dyskinesia with motion. Unless there is a clear indication for surgery, it is quite acceptable to start with conservative treatment and reassess the patient 3 to 6 weeks after the initial injury ( ).

In terms of a chronic injury, the cross-arm adduction, active compression test, and palpation can still be used to elicit AC symptoms. However, concomitant pathology can make the diagnosis more difficult, and a diagnostic injection with xylocaine in the AC joint can become a very useful assessment tool.