The SC joint, an inherently unstable diarthrodial synovial joint, requires stabilization by intrinsic and extrinsic structures to maintain congruency.1–3 In the event of traumatic injury, thorough evaluation of the joint capsule and surrounding structures must be completed to determine joint stability and injury extent.

The articulating surface of the SC joint is oriented in an anterior lateral to posterior medial sloping direction with a relatively flat articular surface in the axial plane (Figure 77.1)¹ Intrinsic and extrinsic stabilizers as well as the surrounding musculature provide added stability.^1,4 The posterior ligamentous complex is crucial for joint stability regardless of dislocation direction.^1,5 When the posterior capsule is disrupted there is an estimated 41% increase in anterior translation and 106% increase in posterior translation of the joint.⁵ Only 25% anterior and 0.7% posterior translation of the SC joint occurs with anterior capsule disruption.⁵ When all but the capsular ligaments were disrupted, no significant instability was reported.⁶ Subclavius adds additional protection by reducing the upward displacement of the clavicle under compressive loads.²

Traumatic posterior SC joint dislocations occur due to a direct anteromedial force over the medial clavicle or an indirect force onto the posterior lateral shoulder. The amount of initial displacement reflects the magnitude of the applied force and degree of soft tissue injury.⁴ Of note, the medial clavicular physis is the last to fuse.¹ Therefore, a physeal fracture dislocation is more common in individuals under 25 years of age.¹

Posterior SC joint dislocations are associated with a 3–4% mortality rate secondary to life‐threatening neurovascular, tracheal, and esophageal injuries.⁷ The brachiocephalic veins lie immediately posterior to the SC joints bilaterally increasing the risk for massive hemorrhage after dislocation or during reduction. Literature review of 60 cases of SC dislocations report a 26% incidence of mediastinal complications.⁸ Common compressive symptoms include dysphagia, dyspnea, or vascular/neurologic compromise.^1,7 Urgent reduction of posterior dislocations are recommended.

CT scans should be obtained for any patient with concern of posterior SC dislocation. CT scans provide valuable information on displacement and the integrity of surrounding structures.⁴ CT angiograms provide excellent visualization of vascular structures and 3D reconstructions prove beneficial in determining the degree of vertical displacement (Figure 77.2).⁴ Chest X‐rays or dedicate clavicular films may provide additional information about associated intrathoracic injuries or increase the suspicion for an SC joint dislocation; however, CT scans provide better diagnostic images.^2,9