Anatomy of the Ribs, Sternum, and Costal Margin.

The ribs, sternum, and costal margin provide a rigid, but flexible chest wall that functions to provide protection to the vital cardiothoracic organs, while also allowing for varying levels of respiration based on physiologic need. The latter function is accomplished through various muscular attachments and rib articulations with both the axial spine posteriorly and the sternum anteriorly. The accessory muscles of inspiration rely on the downward slope and outward curve of each rib, which when contracted move the ribs upward and outward, in turn forcing the sternum anterior and increasing the thoracic volume. Ribs are also classified as true, false, or floating based on their attachment anteriorly to the costal cartilage and sternum. Ribs 1-7 are considered true ribs as their costal cartilage directly articulates with the manubrium and sternum. Ribs 8-10 are considered false ribs because their costal cartilage connects to the cartilage of the immediately superior rib instead of the sternum itself. This complex cartilaginous structure is referred to as the costal margin, which provides attachments for the diaphragm and various abdominal muscles. Although historically thought to be a consistent structure, its variable nature has recently been defined in cadaveric studies. Ribs 11 and 12 are considered floating ribs because they have rudimentary cartilage caps and are not attached to the sternum at all. The sternum comprises the manubrium, body, and xiphoid process. Each component has differing mobility, muscle attachment, and function.