The left edge of the sternum is often caused by the following reasons: First, it may be caused by an injury here, where the sternum meets the rib cartilage, the bone in this area is still relatively weak, once this area suffers a serious violent blow, it may cause contusions or even fractures here, which may lead to pain. Second, this situation may be caused by excessive fatigue, such as the patient using the left upper limb to do excessive activities, or excessive exercise for the chest, may cause fatigue of the pectoralis major muscle in the stop here, which leads to pain. Third, this condition may be caused by costochondritis, which is often the most common site of costochondritis. Once a sterile inflammation of the rib cartilage occurs, it can also lead to pain in this area. Fourth, this condition may also be related to a series of internal organ lesions in the chest cavity, such as the possibility of severe bronchitis, pneumonia, lung tumors or the presence of aortic coarctation, acute myocardial infarction, angina pectoris and a series of other diseases in this area.