Congenital failure of midline fusion of the posterior atlas arch with an associated unilateral cleft

Shape change in the atlas with congenital midline non-union of its posterior arch: a morphometric geometric study

BACKGROUND CONTEXT

The congenital midline non-union of the posterior arch of the atlas is a developmental variant present at a frequency ranging from 0.7% to 3.9%. Most of the reported cases correspond to incidental findings during routine medical examination. In cases of posterior non-union, hypertrophy of the anterior arch and cortical bone thickening of the posterior arches have been observed and interpreted as adaptive responses of the atlas to increased mechanical stress.

PURPOSE

We sought to determine if the congenital non-union of the posterior arch results in a change in the shape of the atlas.

STUDY DESIGN/SETTING

This study is an analysis of the first cervical vertebrae from osteological collections through morphometric geometric techniques.

METHODS

A total of 21 vertebrae were scanned with a high-resolution three-dimensional scanner (Artec Space Spider, Artec Group, Luxembourg). To capture vertebral shape, 19 landmarks and 100 semilandmarks were placed on the vertebrae. Procrustes superimposition was applied to obtain size and shape data (MorphoJ 1.02; Klingenberg, 2011), which were analyzed through principal component analysis (PCA) and mean shape comparisons.

RESULTS

The PCA resulted in two components explaining 22.32% and 18.8% of the total shape variance. The graphic plotting of both components indicates a clear shape difference between the control atlas and the atlas with posterior non-union. This observation was supported by statistically significant differences in mean shape comparisons between both types of vertebra (p<.0001). Changes in shape were observed in the superior and inferior articular facets, the transverse processes, and the neural canal between the control and non-union vertebrae.

CONCLUSIONS

Non-union of the posterior arch of the atlas is associated with significant changes in the shape of the vertebra.

Possible Further Evidence of Low Genetic Diversity in the El Sidrón (Asturias, Spain) Neandertal Group: Congenital Clefts of the Atlas

We present here the first cases in Neandertals of congenital clefts of the arch of the atlas. Two atlases from El Sidrón, northern Spain, present respectively a defect of the posterior (frequency in extant modern human populations ranging from 0.73% to 3.84%), and anterior (frequency in extant modern human populations ranging from 0.087% to 0.1%) arch, a condition in most cases not associated with any clinical manifestation. The fact that two out of three observable atlases present a low frequency congenital condition, together with previously reported evidence of retained deciduous mandibular canine in two out of ten dentitions from El Sidrón, supports the previous observation based on genetic evidence that these Neandertals constituted a group with close genetic relations. Some have proposed for humans and other species that the presence of skeletal congenital conditions, although without clinical significance, could be used as a signal of endogamy or inbreeding. In the present case this interpretation would fit the general scenario of high incidence of rare conditions among Pleistocene humans and the specific scenariothat emerges from Neandertal paleogenetics, which points to long-term small and decreasing population size with reduced and isolated groups. Adverse environmental factors affecting early pregnancies would constitute an alternative, non-exclusive, explanation for a high incidence of congenital conditions. Further support or rejection of these interpretations will come from new genetic and skeletal evidence from Neandertal remains.