Atlas Morphology in Symptomatic Chiari I Malformation: A Statistical Shape Analysis

Introduction: Chiari I Malformation (CMI) is traditionally diagnosed radiologically as caudal descent of the cerebellar tonsils past McRae’s line. Although patients may satisfy this one-dimensional criterion, they may be asymptomatic, potentially never becoming symptomatic. This same uncertainty clouds the etiology as current research focuses on occipital hypoplasia; however, recent investigations have linked CMI with craniocervical instability. This study aims to describe the shape of the superior articular facets of the atlas differentiating between CMI and matched controls.

Methods: Atlas bones from 30 symptomatic CMI patients and 32 age/sex-matched controls were manually segmented from computer tomography scans. The markups module in 3DSlicer was used to place 20 equidistant semilandmarks around the perimeter of the superior articular surface of the lateral mass bilaterally. The slicermorph module was used to apply a generalized Procrustes superimposition and principal component analysis (PCA) to assess shape variation in the sample. An independent-sample Mann-Whitney U test was applied to the principal component (PC) scores to test for group differences.

Results: PCA demonstrated 62 PCs with the first PC accounting for 24.7% of the variability in the sample. The PC1 shape axis was characterized by changes in facet depth and angle, with more negative scores associated with deeper, elongated, and obliquely angled surfaces. Positive scores along PC1 were more shallow, circular, and horizontally angled. Scores were significantly different between groups (U = 319, p = 0.023), with CMI patients clustered toward the negative end of the axis.

Conclusion : Results suggest that the superior articular facet of the atlas in CMI is distinctly different. Their horizontal orientation and shallower facets may contribute to instability at the atlanto-occipital joint, potentially exacerbating symptoms by introducing turbulence to cerebrospinal fluid dynamics around the cisterna magna.