Principal components analysis of the atlas vertebra

Objective: This project explored morphological asymmetry of the atlas by comparing multiple structural variables bilaterally using multivariate statistics. Methods: Two hundred thirty-seven atlases were obtained from the Hamann-Todd Osteological Collection (Cleveland Museum of Natural History, Cleveland, Ohio). The following dimensions were bilaterally measured: anteroposterior length of the superior articular facet, width of the superior articular facet, length of the transverse process, width of the vertebral canal, anteroposterior diameter of the inferior facet, height of the lateral mass, transverse diameter of the inferior articular facet, width of the inferior lateral mass, slope of the inferior facet, convergence angle of the superior facet, circumference of the inferior articular facet, and circumference of the superior articular facet. Principal components analysis (varimax rotation) was used to determine sources of variation within the dataset. Multivariate analysis of variance and paired t tests were used to identify statistical differences between right and left sides for each variable. Results: More than 99% of variance was accounted for across 11 eigenvectors, with most eigenvectors dominated by a single variable. Bilateral comparison of variables showed a significant difference between sides for the anteroposterior length of the superior facet, the width of the superior facet, the width of the vertebral canal, anteroposterior diameter of the inferior facet, and convergence angle of the superior facet (all significant at P < .05). Conclusion: Based on statistical analysis of the atlantal variables, vertebral canal width and the morphology of the superior and inferior articular facets showed significant asymmetry. The role of these asymmetries related to the biornechanics of the C0 through C1 and encroachment on the spinal cord warrant further investigation.