My sister had one of her vertebrae (her sixth) replaced with with a piece of tibia from the bone bank after a car accident. I was not aware this was even possible. My question is, why would they not replace it with another vertebrae from the bone bank?

Anon - Has your sister asked her surgeon this question? If not, she may want to. Without knowing the specifics of her situation, it's most likely they used the most suitable bone available. Just as in the donation of organs, skin and other components of the body, bones can be in limited availability. Some patients receive bone allographs in which tissue is processed and sterilized, then used in grafting surgeries to repair damaged bones or ligaments. You can learn more about this here: http://www.bonebank.com/