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Acromio-Clavicular Separation

Radiographic Summary

Acromio-clavicular (AC) separation is an injury to the ligamentous structures of the AC joint. On AP radiographs of the shoulder, the inferior aspect of the distal clavicle should align with the inferior aspect of the acromion. In type I injuries, there is a sprain or partial tear to the AC ligament and imaging will appear normal. In type II injuries, the AC ligament is torn and the space between the distal clavicle and acromion may appear wider (as compared to the other shoulder) and the distal clavicle may be slightly superiorly displaced in relation to the acromion, but less than 100%. Type III injuries involve a tear of the AC and coraco-clavicular (CC) ligaments and the distal clavicle is 100% displaced superiorly in relation to the acromion. Type IV injuries are rare and occur when the distal clavicle is displaced posteriorly. Type V injuries are severe type III injuries; in these, the distal clavicle punctures the trapezius muscle. Type VI injuries occur when the distal clavicle is inferiorly displaced below the coracoid.

Clinical Implications

AC separation injuries occur when patients fall or sustain a blow to the top of the shoulder. Physical findings include swelling and pain on palpation of the AC joint. Patients will have pain with cross arm testing (reaching from the affected arm to the opposite shoulder). Radiographs can help determine the severity and type of the injury. Type I, II, and III injuries are treated conservatively with a sling for comfort and activity as tolerated. Type IV, V, and VI injuries are rare, and treated surgically on an outpatient basis. Patients with symptomatic arthritis as a result of these injuries may ultimately be treated with a distal clavicle excision (Mumford procedure).


  1. AP chest radiographs can be helpful in providing a comparative view of the unaffected AC joint.

  2. Weighted arm views can aid in the diagnosis but do not change overall management and are not routinely recommended.

Figure 9.1 Grade 2 Acromio-Clavicular Separation.

AP view of the left shoulder demonstrates mild widening of the left acromio-clavicular joint (it measured 12 mm; the upper limit of normal is 8 mm). The distance between the coracoid process and undersurface of the clavicle remains normal (upper normal is 13 mm). The findings are consistent with grade 2 acromio-clavicular separation. Grade 1 A-C separation is radiographically occult.

Figure 9.2 Grade 3 Acromio-Clavicular Separation.

AP view of the left shoulder shows slight widening of the left A-C joint, marked widening of the left CC interval, and superior displacement of the lateral aspect of the left clavicle in relationship to the acromion.

Figure 9.3 Remote Right Acromio-Clavicular Separation.

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