As with any acute injury, a complete history and physical exam should be performed for any patient suspected of having an injured AC joint. The history will help determine treatment options for the patient and should focus on the injury mechanism, treatment timing, and the potential for postinjury morbidity. Age, desired sport, and occupational demands warrant special consideration should operative intervention be contemplated. On physical exam, concomitant traction injuries should be noted and a careful examination of the affected shoulder be completed.

Incomplete injuries to the AC joint will likely result in pain localized to the upper shoulder. At times, this pain can be poorly localized. The AC joint has dual innervation from the suprascapular nerve as well as the lateral pectoral nerve, and pain can be referred to their respective dermatomal distributions. Isolation of the AC joint as the location of injury can be accomplished by eliciting pain with direct palpation or provocative maneuvers such as the cross-chest adduction test. Relief of pain with injection of a local anesthetic confirms the diagnosis. With complete injuries, there is almost always pain, swelling, and deformity of the AC joint. Subtle deformities can be confirmed with radiographs.

Radiographs are usually sufficient for evaluating potential AC joint injuries. Specific views for optimal assessment have been described. In general, three orthogonal views are adequate to initially evaluate the shoulder for a traumatic injury. These views include an AP, a scapular-Y, and an axillary. If an AC joint injury is further suspected, additional views may be indicated.

Although the AP and scapular-Y views of the shoulder can indicate an AC joint injury, more specific views can reveal additional, more subtle findings. The Zanca view angles the X-ray beam 10° to 15° cephalad in a true AP orientation to eliminate the overlap of the scapular spine (1). The X-ray power should be reduced to 30% to 50% of normal to avoid overpenetration of the less dense AC joint. The Zanca view will reveal any relative displacement of the clavicle. Bilateral Zanca views on the same X-ray cassette allow for comparison with the contralateral side (Fig. 4.1). Bearden et al. (2) demonstrated in their study
that the mean distance between the superior border of the coracoid and the inferior border of the clavicle was 1.1 to 1.3 cm. As this distance will vary between patients and radiographic techniques, it is necessary to compare side-to-side differences. Distances of >25% to 50% compared with the unaffected side have been shown to be diagnostic for a complete CC ligament disruption (3).

The axillary view, although not specific, can be helpful in type IV injuries in which the clavicle is posteriorly displaced. In addition, the cross-arm view can illuminate the degree of injury by accentuating the displacement of the clavicle (4) (Fig. 4.2). Although classically described and often cited in the evaluation of AC joint injuries, weighted stress radiographs are no longer thought to be the gold standard and have been largely superseded by bilateral Zanca views.

Most AC joint injuries are the consequence of a force directed inferiorly at the acromion with the arm adducted. This motion forces the entire shoulder girdle down. During the initial motion, the AC joint remains congruent. However, the clavicle eventually impacts the first rib, inhibiting further inferior translation. The clavicle will either fracture, with the first rib acting as a fulcrum, or the AC ligament complex will sequentially fail.

Although first recognized by Hippocrates (5), it was not until 1917 when the first description of the sequential failure of the AC joint complex was published by Cadenat (6). Tossy (7) later proposed a classification scheme including three degrees of injury ranging from type I injuries, which represent a sprain of the AC ligaments, to type III with complete disruption and separation of the clavicle and scapula. Rockwood et al. (3) expanded Tossy’s classification, adding three additional injury grades for a total of six.

Rockwood’s classification (Fig. 4.3) begins with a minimal injury to the AC joint proper. This injury pattern, type I, represents a sprain of the joint capsule and surrounding ligaments without displacement. In a type II injury, the AC joint capsule and surrounding ligaments are disrupted but without significant elevation of the clavicle, usually <50%. The coracoclavicular (CC) ligaments are intact preventing superior displacement but increased anterior-posterior translation will often be apparent. With further significant force a type III injury occurs. The AC ligament complex as well as the CC ligament complex rupture resulting in up to 100% displacement of the clavicle relative to the scapula. Type IV injuries, best visualized on axillary radiographs, occur when the clavicle is forced posteriorly through the trapezius and are generally irreducible
on physical exam due to soft-tissue interposition. Type V injuries are secondary to violent force and represent complete soft-tissue disruption of the clavicle from the scapula with up to 300% displacement of the clavicle. This displacement occurs because of the disruption of the deltotrapezial fascia. Finally, the extremely rare type VI injury occurs when the clavicle becomes displaced and entrapped beneath the coracoid or acromion.