The sternoclavicular joint, or SC joint, is the connection of the sternum (breastbone) to the clavicle (collarbone). This SC joint is uncommonly injured, but it can be problematic when an injury is sustained.
The sternoclavicular joint can dislocate from its normal position. The clavicle will either dislocate in front (an anterior SC dislocation) or behind (a posterior SC dislocation) the sternum. The posterior SC dislocations are more worrisome because of the important structures that are located just behind the sternum. These structures include major blood vessels and the trachea (windpipe). Damage to these can cause life-threatening problems with blood flow and breathing.
Dislocations of the SC joint are usually the result of an injury to the shoulder. Most commonly, the outside/front of the shoulder is struck forcefully, and an anterior SC dislocation occurs. In some cases, a blunt force directly to the front of the chest can cause a posterior SC dislocation.
Source: Verywell Health