An examination of atlanto-occipital curvature in adult Chiari malformation type 1 and control groups

Objective: To compare computed tomography (CT) scan-based morphometric measures associated with atlanto-occipital joint (AOJ) instability between Chiari malformation type I (CMI) patients and controls and to examine associations between these measures, clinical symptoms, and established craniocervical/posterior fossa morphometrics in CMI. Materials and Methods: Four AOJ-related morphometric measures were evaluated on CT: condyle–C1 interval (CCI), C1 socket depth-to-length ratio (DL ratio), condyle depth-to-depth ratio (CD ratio), and atlas tilt angle. Measurements were compared between CMI ( n = 45) and controls ( n = 55). Within CMI, associations were tested between AOJ measures and symptom variables, as well as 13 established craniocervical/posterior fossa morphometrics. Results: Mean CCI was slightly larger in CMI than in controls (1.00 vs. 0.91 mm, P < 0.05). DL ratio, CD ratio, and atlas tilt angle did not differ significantly between groups. Within CMI, CCI showed a modest association with hypermobility ( r = 0.34, P < 0.05). Moderate correlations were observed between AOJ measures and selected morphometrics, suggesting relationships between AOJ geometry and posterior fossa/craniocervical anatomy. Conclusion: Among the AOJ measures examined, only CCI demonstrated a small but statistically significant group difference and a modest association with hypermobility in CMI. The shape of the AOJ curvature was not different between CMI and control. However, further investigation of AOJ-related morphometrics as potential contributors to symptom heterogeneity among CMI patients is necessary.