Left sternal protrusion may be due to congenital development, calcium deficiency, inappropriate posture, etc. It may also be due to left pleural effusion, costochondritis, scoliosis, old fracture and so on. 1. Physiological factors: Normal human congenital development will have an imbalance in the development of the sternum, and there may be a protrusion of the left side of the sternum. In addition, the lack of calcium in infants and young children may lead to rib ectopia, and improper posture may also lead to the imbalance of the force on both sides of the sternum, causing the left side of the sternum to protrude. 2. Left pleural effusion: when pleurisy is caused by inflammation or tumor, it may lead to left pleural effusion, which is manifested as left sternal protrusion. 3. costochondritis: due to exertion and cold and other adverse factors, the periosteum is stimulated, which causes local aseptic inflammation and local swelling and pain. 4. Scoliosis: especially the scoliosis of thoracic vertebrae, which side the thoracic vertebrae protrudes, the ribs on this side will protrude with it. 5. Old fracture: after the rib fracture is dislocated, deformity healing may occur, which is manifested as protruding left sternum. There are other causes of left sternal protrusion, and it is recommended that patients seek timely medical treatment to avoid delaying the condition.