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Background: Traditional rib anatomy and costal margin teaching contends that the costal margin consists of a combined costal cartilage made up of ribs 7 to 10. Variations in 9th and 10th rib anatomy have been observed. We sought to evaluate the variability of interchondral joints and the make-up of the costal margin., Methods: Cadaveric dissections were performed to evaluate the anatomy of the anterior ribs and the composition of the costal margin. Experienced chest wall surgeons evaluated this anatomy through a standardized dissection and assessment. Dissection videos were performed to allow for further review/assessment., Results: Bilateral chest wall anatomy of 30 cadavers was evaluated (15 male, 15 female). The average age was 78 ± 12 years, and all patients were Caucasian. In all patients, the first rib attached to the manubrium, the second rib attached to the manubriosternal junction, and ribs 3 to 6 attached directly to the sternum. Interchondral joints were present between ribs 4/5-3%, 5/6-68%, 6/7-83%, 7/8-72%. Ribs combining to form a common costal cartilage via cartilaginous unions were observed between 6/7-3%, 7/8-45%, 8/9-30%, and 9/10-20%.The 8th rib attached directly to the sternum without joining the 7th rib in 10% of cadavers. The 8th and 9th ribs had free tips in 45% and 60% of evaluations, respectively. The 10th rib was found to have a hooked tip in 25% of cases and was a floating rib without attachment to the 9th rib 52% of the time. Rib tip mobility was noted in ribs 8, 9, and 10 in 52%, 70%, and 90%, respectively., Conclusion: Interchondral joints are common between ribs 5 and 8. Significant variability exists in the chest wall and costal margin compared with traditional teaching. It is important for chest wall surgeons treating diseases of the costal margin to appreciate this anatomic variability., (Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.)

Background: Classical teaching of rib anatomy contends that the false ribs (7th to 10th) fuse anteriorly to form the costal margin of the chest. Slipped rib syndrome consists of false rib subluxation into the thorax with symptomatic impingement of the intercostal nerve. We sought to determine the incidence of this anatomic finding through anatomic study of the costal margin., Methods: Cadavers were evaluated for mobility and attachment of the ninth and tenth ribs. Experienced anatomists and chest wall surgeons conducted a standardized dissection and assessed rib tip mobility using predefined criteria. Videos of dissections were submitted to a single investigator who reviewed the findings., Results: Costal margins of 40 cadavers (45% male) were evaluated bilaterally. The average age was 83 years ± 11 years. The ninth rib was found to be attached to the eighth rib 100% of the time by an interchondral cartilaginous attachment along the body of the eighth and ninth ribs. Internal subluxation was noted in 19% (15 of 80), and the tip of the rib was mobile in 86% (69 of 80) evaluations. The tenth rib was attached to the ninth rib in 18%(14/80). A "floating" 10th rib was noted in 59% (47 of 80) of specimens. Subluxation was noted in 33% (26 of 60). Half of the ribs that subluxed moved medially to the ninth rib and half moved externally. An upwardly hooked tip was noted in 10% (8 of 80). Ribs with a hooked tip subluxed in 63% (5 of 8), and all of these ribs (5 of 5) moved to the interior of the chest ( p = 0.020)., Conclusion: The ninth rib is commonly attached to the eighth rib, but the tenth rib is often not attached to the ninth rib. Most commonly, the tenth rib is a "floating" rib. Internal subluxation of the tenth rib as well as the presence of a hooked tip may predispose individuals to the development of "slipped rib syndrome.", Level of Evidence: Diagnostic Tests or Criteria; Level III., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)