infrasternal angle infrasternal angle the lower opening of the thorax is formed by the twelfth thoracic vertebra behind, by the eleventh and twelfth ribs at the sides, and in front by the cartilages of the tenth, ninth, eighth, and seventh ribs, which ascend on either side and form an angle, the infrasternal angle or subcostal angle, into the apex of which the xiphoid process projects. infrasternal anglethe human rib cage.the thorax from the right.detailsidentifierslatinangulus infrasternalista98a02.3.04.008ta21103fma7573anatomical terminology pregnancy causes the angle to increase from 68° to 103°.