SPINE Volume 34, Number 12, pp E422–E426 ©2009, Lippincott Williams & Wilkins Transverse Foramen of the Atlas (C1) Anteriorly Unclosed A Misknown Human Variant and Its Evolutionary Significance Franck Billmann, MD, PhD, and Jean-Marie Le Minor, MD, PhD Study Design. Morphologic study of 500 human adult atlases, 256 atlases of nonhuman adult primates repre- senting 37 genera, and 25 human atlases of newborns and young individuals from birth to 3 years of age. Objective. 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 evo- lutionary and functional significance of this disposition. Summary of Background Data. Anteriorly unclosed transverse foramen is one of the classic variants of the human atlas, however, rare quantitative data are avail- able in the literature and this disposition remains little understood. Methods. Macroscopical study (dried bones). Results. In humans, an anteriorly unclosed trans- verse 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 ob- served in 45.1% of the anterior unclosures and unilat- eral 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%). Conclusion. The anteriorly unclosed transverse fora- men 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 ju- venile pattern (i.e., paedomorphosis). It must not be in- terpreted as a pathologic erosion or destruction but as being a simple normal variant. Key words: cervical spine, anatomy, variants, physical anthropology, primates. Spine 2009;34:E422–E426 Anatomic variants of the first cervical vertebra or atlas in humans have already been noted in early works. 1–5 More recently, the renewal of interest in the nonmetric or quantitative traits of the skeleton 6,7 has instigated an increase in the number of studies devoted to these vari- ants. Transverse foramen anteriorly unclosed is one of the classic variants of the human atlas but was not re- tained in the classic list of nonmetric variants of the in- fracranial skeleton published by Finnegan. 7 Because no specific term exists in the international anatomic nomen- clature (Nomina Anatomica) to designate this variant, several synonyms have been used in the literature, lead- ing to possible confusion: (1) anterior aperture (or dehis- cence) of the transverse foramen, 4,8,9 (2) absence (or aplasia) of the anterior root of the transverse process, 5 and (3) absence of the transverse foramen, in extreme forms. 10 Because only rare quantitative data are available in the literature on this nonmetric variant and this disposi- tion remains little understood with explanations pro- posed being essentially theoretical, the aim of the present study, based on extensive human and comparative ver- tebral samples, was 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 re- garding the evolutionary and functional significance of this disposition. Materials and Methods The material used in this study consisted of 500 human atlases (Homo sapiens) (Figure 1) and of 393 atlases of nonhuman primates representing 41 genera. These vertebrae (dried bones) came from adult subjects without pathologic signs of the cer- vical spine. The precise age and sex were unknown. The 393 atlases of nonhuman primates were distributed as follows: Prosimii (102 individuals from 15 genera): Lemuridae: 3 Hapalemur, 31 Eulemur;6 Lemur,1 Lepilemur; Indriidae: 2 Avahi,6 Indri, 12 Propithecus; Daubentoniidae: 4 Daubento- nia; Cheirogaleidae: 6 Cheirogaleus, 10 Microcebus; Galagi- dae: 5 Galago; Lorisidae: 2 Loris,5 Nycticebus,5 Perodicticus; Tarsiidae: 4 Tarsius; Platyrrhinii (60 individuals from 13 gen- era): Callitrichidae: 10 Callithrix,3 Leontopithecus,1 Sagui- nus; Cebidae: 3 Aotus,1 Callicebus, 14 Cebus,5 Saimiri; Ateli- dae: 2 Alouatta, 10 Ateles,1 Brachyteles,2 Cacajao,5 Lagothrix,3 Pithecia; Catarrhinii (161 individuals from 9 gen- era): Cercopithecinae: 13 Cercocebus, 28 Cercopithecus,7 Erythrocebus, 43 Macaca, 42 Papio,8 Theropithecus; Colobi- nae: 4 Colobus,4 Nasalis, 12 Presbytis; Hominoidea (70 indi- viduals from 4 genera): 18 Hylobates (gibbons), 19 Gorilla gorilla (gorillas), 21 Pan troglodytes (chimpanzees), 12 Pongo pygmaeus (orangutans). Moreover, 25 human atlases of new- borns and young individuals from birth to 3 years of age (dried bones) were also studied. These vertebrae belong to the following collections: (1) Laboratoire d’Anatomie and Muse ´e Delmas-Orfila-Rouvie `re (UFR des Saints-Pe `res, Paris, France), (2) Laboratoire d’Anatomie Compare ´e (Muse ´um National d’Histoire Na- turelle, Paris, France), (3) Laboratoire d’Anatomie (Faculte ´ de From the Institute of Normal Anatomy, ULP, Strasbourg, France. Acknowledgment date: September 16, 2008. Revision date: December 15, 2008. Acceptance date: December 19, 2008. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Jean-Marie Le Minor, MD, PhD, Institut d’Anatomie Normale (EA 3428), Faculte ´ de Me ´- decine (ULP), F-67085 Strasbourg, France; E-mail: jean-marie. leminor@medecine.u-strasbg.fr E422