The ribs form the bony framework of the thoracic cavity (see Image. Thoracic Cage). The thoracic cage generally contains 12 pairs of ribs. Each rib articulates posteriorly with 2 thoracic vertebrae at the costovertebral joints. The first rib is an exception because it articulates only with the first thoracic vertebra.

The ribs are classified as true, false, or floating according to their sternal attachment. The first 7 ribs are true ribs and articulate directly with the sternum through their costal cartilages. The 8th, 9th, and 10th ribs are false ribs and articulate indirectly with the sternum because their costal cartilages join the 7th costal cartilage at the costochondral joints. Floating ribs are traditionally defined as ribs that lack sternal articulation and classically include the 11th and 12th ribs. However, the 10th rib frequently lacks attachment to the ninth rib and may also be classified as a floating rib. True ribs articulate with the sternum at the sternocostal joints. The first sternocostal joint is an exception because it is a synarthrosis. The first rib may also articulate with the clavicle at the costoclavicular joint.

Results from recent cadaveric studies demonstrated substantial variability in costal margin anatomy compared with traditional descriptions. Results from a 2023 study of 30 cadavers showed that the 10th rib lacked attachment to the ninth rib and was a floating rib in more than 50% of specimens. Results from the same study showed direct attachment of the eighth rib to the sternum without joining the seventh rib in 10% of cadavers. Results from another study of 40 cadavers found a floating 10th rib in 59% of specimens and mobility of the ninth rib tip in 86% of evaluations.