(A) Causes Subcutaneous emphysema of the chest wall is usually secondary to sternal and/or rib fractures with pneumothorax, especially in patients with multiple multiple rib fractures with tension pneumothorax, and can also be complicated by tracheal, bronchial, pulmonary and esophageal injuries. Occasionally, it is seen secondary to endoscopic injury. (II) Pathogenesis Closed chest injury and open injury are often accompanied by subcutaneous air accumulation, and there are usually three ways for air to enter the subcutaneous tissue through the damaged area: ① when the pneumothorax is accompanied by wall pleura damage, the air in the chest cavity can enter the subcutaneous tissue of the chest wall through the damaged area; ② when the trachea, bronchus or esophagus is ruptured, the air can enter the mediastinum directly through the rupture, and then diffuse to the neck, face and chest through the superior sternal recess subcutaneous tissues; ③ air enters subcutaneous tissues directly through the body surface wound of chest wall.