Pain in the middle of the chest between the breasts can be caused by various conditions, including joint strains, muscle injury, acid reflux, and gastrointestinal problems. Some of the most common causes of sternum and substernal pain are costochondritis, sternum fracture, sternoclavicular joint injury, collarbone injuries, muscular strain or bruise, hernia, acid reflux, pleurisy, bronchitis, and pneumonia. Costochondritis is inflammation of the cartilage that links the ribs to the sternum and is the most common cause of chest bone pain and rib pain. It can mimic the symptoms of a heart attack or other heart conditions, but it is usually associated with sharp pain when pushing on the sides of the sternum. Pain caused by costochondritis might also be accompanied by swelling (Tietze syndrome) . If the pain is severe, worsening, or accompanied by other symptoms such as shortness of breath, nausea, or sweating, it is important to seek medical attention immediately to rule out life-threatening causes such as a heart attack.