Despite being a common radiologic diagnosis, many patients with CMI diagnosis never develop symptoms and the mechanisms driving symptomatology are poorly understood. While most research attention has been focused on occipital hypoplasia, recent investigations have linked CMI and craniocervical instability. 

Axis bones from 22 symptomatic CMI patients and 25 age/sex-matched controls were manually segmented from computed tomography scans. The markups module in 3DSlicer was used to place 20 bilateral equidistant semilandmarks around the perimeter of the superior articular surface of the lateral mass. 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 samples t-test was applied to resulting principal component (PC) scores to test for differences between groups.

The PCA resulted in 47 PCs with the first PC accounting for 26.4% of the variability in the sample. The PC1 shape axis was characterized by changes in facet depth and angle, with more negative scores being associated with deeper, elongated, and obliquely angled surfaces. Positive scores along PC1 were associated with more shallow, circular, and horizontally angled surfaces. Scores were significantly different between groups (t (45) = 2.211, p = 0.016), with CMI patients clustered toward the negative end of the axis.

These results suggest that there are distinct bony differences in the CMI atlas. Horizontal orientation and shallower facets may contribute to instability at the atlantooccipital joint, exacerbating symptoms.