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.
Keywords: Atlas; Congenital non-union; Geometric morphometrics; Landmark; Posterior arch; Semilandmark; Shape change.
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