Transverse foramen of the atlas (C1) anteriorly unclosed: a misknown human variant and its evolutionary significance.

Abstract

Morphologic study of 500 human adult atlases, 256 atlases of nonhuman adult primates representing 37 genera, and 25 human atlases of newborns and young individuals from birth to 3 years of age. To provide original observations to attempt to understand the anterior unclosure of the transverse foramen of the human atlas, to ascertain its occurrence in primates, and to offer some elements regarding the evolutionary and functional significance of this disposition. Anteriorly unclosed transverse foramen is one of the classic variants of the human atlas, however, rare quantitative data are available in the literature and this disposition remains little understood. Macroscopical study (dried bones). In humans, an anteriorly unclosed transverse foramen was observed on one or both sides in 51 individuals of the 500 in the present series (10.2%); bilateral occurrence (symmetric manifestation) was observed in 45.1% of the anterior unclosures and unilateral occurrence in 54.9%; no significant right or left predominance was observed in the frequency of lateral occurrence. In human newborns and young individuals (from birth to 3 years of age), the transverse foramen was anteriorly unclosed in all of the 25 cases studied. In nonhuman primates, some bilateral or unilateral cases of anterior unclosure were only observed in hominoids, except gibbons: orangutans (8.3%), gorillas (10.5%), and chimpanzees (14.3%). The anteriorly unclosed transverse foramen of the atlas seems to be the result of an evolutionary tendency characteristic of hominoids within primates and could be interpreted as a shared derived character (i.e., synapomorphy) and as the persistence in adults of a juvenile pattern (i.e., paedomorphosis). It must not be interpreted as a pathologic erosion or destruction but as being a simple normal variant.