|MadSci Network: Medicine|
An extra rib on the bottom of the rib cage. First, I’d like to know why you think you have this extra rib. The 12th rib is typically short and stubby, and might seem like an “extra” rib since it doesn’t do much. Assuming that this is not the case, what you are describing is most likely a lumbar rib. These extra ribs are fairly common and typically have no clinical significance. Since you indicate that your doctors are not concerned about this, there is probably no reason that you should be either. However, if this “extra rib” is giving you problems, bring those problems to the attention of your health care provider. It is possible that something else is going on – let the professionals make the diagnosis. Ok, enough with the dire warnings. Why would you have a lumbar rib? It’s really all a matter of the basic body plan and growth regulatory genes. The vertebrate body (mammals, reptiles, birds, etc.) is built on a segmental design – sort of like a stack of building blocks. Each segment contains exactly the same parts. Regional distinctions among those segments come about through growth regulatory genes that make some of the parts bigger and other parts smaller. Thus, if we look at the spinal column as the basis of the segmental blocks, each block contains the same parts – a vertebral body, a transverse element, and a costal (or rib) element. In the thorax, the costal element enlarges to become the bone we call a rib. In the cervical (neck) region the costal element fuses with the transverse element to produce the unique cervical vertebral pattern (all of these vertebrae have three holes in them, the large one for the spinal cord and two smaller ones for blood vessels). In the lumbar (lower back) region the transverse element stays small (a mere bump known as the accessory tubercle) and the costal elements become projections that stay attached to the vertebrae, but shoot out about two inches on either side of the bone (the transverse processes). Sometimes the regulatory genes that govern bone growth get interrupted and cause the costal element to grow into a true rib. In the neck this is known as a cervical rib (the costa cervicalis that you mentioned) and this can cause clinical problems. In the lower back this is known as the lumbar rib. A lumbar rib is basically a transverse process that is not attached to its vertebra. Under normal circumstances, the lumbar rib has no clinical significance at all. If you experienced some type of injury to your lower back that broke this rib, you might increase the risk of damage to your kidney, but this type of risk would also be associated with a broken 12th or 11th rib. In any case, a broken rib here is not a “normal circumstance.” The lumbar rib has no known association with spina bifida or scoliosis, although I should mention that both of these conditions are fairly common by themselves. So, why do you have a lumbar rib? There is really no way to know. Although genetics are important, there is such a strong interaction between genetic traits and the developmental environment that I would say it is extremely unlikely that you can link this trait to a specific gene. I should also point out to you that first cousin marriages were fairly common until very recently (in some segments of society it was actually the preferred marriage!). Most people around you are probably descended from a first cousin marriage, they just don’t know it. So in this regard you are unusual only in the fact that you can name your ancestors from four generations ago – most people can’t. References: Sadler, TW (2000) Langman’s Medial Embryology. Lippincott Williams & Wilkins: Philadelphia Romer, AS & Parsons, TS (1986) The Vertebrate Body. Saunders Publishing Co.: Philadelphia Agur, AMR & Lee, MJ (1999) Grant’s Atlas of Anatomy. Lippincott Williams & Wilkins: Philadelphia. Especially see pages 240 & 275
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