[Associations among non-metric features of the atlas in the human species].

Six non-metrical variants has been studied in a series of 500 human atlases. A bipartition of the superior articular facet has been observed in 20.8% of cases, a posterior ponticulus in 14.2% of cases, a retrotransverse ponticulus in 14.2% of cases, an anterior dehiscence of the foramen tranversarium in 10.2% of cases, a dehiscence of the posterior arch in 2.8% of cases, and a lateral ponticulus in 1.8% of cases. Bilateral occurrence has been noted in 42.2 to 46.2% of cases for the anterior dehiscence of the foramen transversarium, the posterior ponticulus, and the bipartition of the superior articular facet. Bilateral occurrence is less frequent for the retrotransverse ponticulus (29.6%), and particularly for the lateral ponticulus (11.1%). The incidence of a given lateral character according to the side has been found identical or similar on the left and on the right sides. In the present series, 51.6% of the atlases showed none of the six variants studied; 46.8% of the atlases showed one or two variants; and only 1.6% of the atlases showed more than two variants; any cases with more than four variants were observed. The most interesting association observed is the association of the lateral ponticulus with the posterior ponticulus.