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In an answer to How important is it that a person be able to touch their toes?, someone said that they found out they had an extra vertebra from their chiropractor.

I'm skeptical about their specific claim if it's based on an evidence-challenged medicine practitioner, but is it possible for a human to have extra vertebrae without noticing it?

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Fun trivia: giraffes apparently have the same number of vertebrae in their neck as other mammals, including humans: en.wikipedia.org/wiki/Giraffe#Neck –  Andrew Grimm May 25 '11 at 3:52
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up vote 17 down vote accepted

Yes, and this is fairly common.

It's not an "extra" vertebra as such, but is actually nonfusion of a vertebra that is supposed to fuse. From wikianswers (I know, bad source, but it's a good summary):

The sacrum bone in the posterior pelvis comes from the fusing of 5 sacral vertebrae, which are numbered from superior (S1) to anterior (S5). The sacral bones fully fuse typically by around age 23, but sometimes the S1 vertebra does not fuse with the others. When this happens, it's called lumbarization.

enter image description here

Image credit, the disc where the white arrow is pointing is more commonly not present.

There is no evidence to suggest this would make a person taller or affect their flexibility, and is probably not even associated with increased lower back pain. Pubmed (paywall) has a study from the Australian Spinal Association that studied around 500 spinal x-rays and found:

The prevalence of... lumbarization [was] 6.0%... There was no greater prevalence in patients suffering from low back pain when compared against those who did not... Contrary to previous claims that lumbarization is more common in men, we found a moderate predilection for this finding among women.

An Indian anatomical department did a study where they directly examined 332 dried sacra, and found:

A total of 3.9% of sacra presented lumbarization, seven (2.1%) with partial and six (1.8%) with complete separation of the S1.

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Very cool. I'd like to know if the chiropractor told the client which extra vertebrae was present. Otherwise: hearsay. I teach circus, work with contortionists, now I wonder.. –  Nthaoe May 25 '11 at 0:56
    
while interesting, lumbarization doesn't really answer the question. A person with this still has 33 vertebrae, it's just that they have one extra lumbar vertebra at the expense of one less sacral vertebra due to improper fusion. Although, there is some debate in certain circle regarding the total number after fusion. The general numbers are 7cervical 12thoracic 5lumbar 5sacral 4coccyx. Some can have slightly more, some can have slightly less. The number a person has is easily verifiable with a simple X-ray. –  Monkey Tuesday May 25 '11 at 1:10
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@Monkey I've most commonly heard 24 separate vertebrae plus the sacrum and the coccyx, lumbarization would mean one extra separate vertebra. If 32-34 is normal, then what is "extra"? 34? 35? Where must these be located to count as extra, as if they're in the coccyx or sacrum, does it really matter? –  jozzas May 25 '11 at 1:45
    
Clearly,our problem is that we are using two different systems for counting vertebrae.Going with your method(that after fusion,the coccyx and sacrum are one bone each)clearly causes one to be considered "extra".By that logic your answer is appropriate.It may also be the rationale used when someone says they have an extra vertebra.Medically,I would count anyone with 34 vertebrae on Xray to have one extra,because 33 bones is considered normal, how they fuse is a slightly different matter. –  Monkey Tuesday May 25 '11 at 3:04
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