A case of an atlas vertebra bilaterally without costal elements of transverse processes

An unusual atlas vertebra whose costal elements were not formed bilaterally was detected in an anatomy laboratory. Because the vertebral arteries have bony protections anteriorly by the costal elements of the transverse foramina of a usual atlas, surgical operations of the atlas’ without transverse foramina need much more attention while approaching to that region. As well as the vertebral artery, the nerve plexus around vertebral artery may also be responsible of some complications due to failure of this bony protection.