A bifid rib, also known as a forked or bifurcated rib, is a relatively common anatomic variant where the sternal end of a rib divides into upper and lower divisions. It is thought that the lower division represents the intended development of the rib, and the upper division represents the anomalous development.
Bifid ribs are more common in males, slightly more common on the right, and occur most frequently in the third and foruth ribs, followed by the fifth, sixth, and second ribs. Both divisions have their own costal cartilage, which may fuse before joining with the sternum.
The space between the upper division and the rib above (white *) is usually narrowed, while the lower intercostal space (red *) is wider than normal.
Familiarity with the underlying muscular, vascular, and neural anatomy may be important for planning surgical and percutaneous procedures.
- The intercostal muscles traverse the space between the upper and lower divisions.
- Intercostal nerves run their normal course along the inferior margin of the parent rib and then usually along the inferior margin of the lower division. At least one case of a pair of intercostal nerves traveling along both the upper and lower margins of the parent rib has been reported.
- I assume that the intercostal arteries take the same course as the nerves, but can't find any supporting evidence.
Bifid ribs can also be seen in association with other conditions, most famously with basal cell nevus syndrome, where they may be multiple and bilateral.
References
- Osawa T, Sasaki T, Matsumoto Y, Tsukamoto A, Onodera M, Nara E, Chen JK, Fujimura A, Nozaka Y. Bifid ribs observed in the third and the fourth ribs. Kaibogaku Zasshi. 1998 Dec;73(6):633-5.
- Song WC, Kim SH, Park DK, Koh KS. Bifid rib: anatomical considerations in three cases. Yonsei Med J. 2009 Apr 30;50(2):300-3.
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