The costal margin is the cartilage along the edge of the eighth through tenth ribs in the middle of the chest. It marks the lower portion of the chest and the location of the diaphragm. This cartilage allows these ribs, which do not come into contact with the sternum, to articulate rather than float in the chest cavity, protecting the heart and lungs from physical impact. People can feel the costal margin by locating the base of the sternum and moving down and to the side to find the costal cartilage.
Locating the costal margin can be important for some surgical procedures, and it is one of the points of orientation people can use to locate the positions of the bones in the torso. The cartilage marks two sides of the triangular space at the bottom of the rib cage, with the base of the triangle indicating the divide between the upper and lower chest. A related medical term some people may find useful is the costal angle, the term describing the angle between the two costal margins.
This is part of the body's medial anatomy, the structures falling roughly in the middle of the body. Doctors may need to access the costal margin for some surgical procedures, and it can provide an anchoring point for the use of surgical retractors to expose areas of interest in the chest and abdomen during open surgeries.
The ribs attached to the costal margin are known as the false ribs because they do not connect directly with the sternum; they form part of the protective network of bone known as the rib cage. The costal cartilage connects with the true ribs to reinforce the false ribs and make sure they remain in a stable position. Just below them lie the 11th and 12th ribs, the floating ribs. These ribs do not articulate at the front of the body, attaching only to the spinal column.
Medical problems with the costal margin and attached ribs can include rib fractures, deformities, and displacement. These are usually evident on an x-ray of the chest, and the approach to treatment can vary depending on the nature of the problem. Setting and casting fractures of the ribs is challenging because the ribcage needs a high degree of flexibility. Immobilizing the ribs to allow fractures to heal is not an option, as the patient would not be able to breathe.