Odontoid Process Retroflexion in an Adult Population with Basilar Invagination and in Normal Adults

Purpose or Learning Objective: Literature has shown that the retroflexion angle of the odontoid process was higher in patients with Chiari malformation. Basilar invagination (BI) often accompanies Chiari malformation. This study tested the hypothesis that the odontoid retroflexion angle (ORA) in patients with BI is lower than in the normal population.

Methods or Background: This retrospective study included patients diagnosed with type B BI and a matched adult control group. All participants had undergone magnetic resonance imaging (MRI). Two observers measured (1) distance from odontoid apex to Chamberlain's line (OA-CL); (2) ORA; and (3) posterior basion to C-2 line measured to the dural margin (pBC2 line). Statistical analysis was made with the intraclass correlation coefficient (ICC), t test, and receiver operating characteristic (ROC) curve.

Results or Findings: The study included 33 (19 female and 14 male) BI patients and 34 (20 female and 14 male) controls. The mean ages in patient and control groups were 52.30 years (standard deviation [SD]: 16.59 years) and 48.18 years (SD: 17.69 years), respectively (p = 0.33). In the interrater evaluation, ICC was 0.98 (95% confidence interval [CI], 0.97–0.99) for OA-CL, 0.69 (95% CI, 0.54–0.80) for ORA, and 0.82 (95% CI, 0.50–0.92) for the pBC2 line (p < 0.001 for each). In the t test, patient and control group measurements were 10.08 mm (SD: 5.25 mm) and − 0.64 mm (SD: 2.83 mm) for OA-CL, 73.60 degrees (SD: 3.74 degrees) and 78.06 degrees (SD: 4.06 degrees) for ORA, and 8.55 mm (SD: 2.70 mm) and 6.57 mm (SD: 1.20 mm) for the pBC2 line (p < 0.001 for each). In ROC analysis, OA-CL was classified as accurate in all participants. Area under the curve was calculated as 0.79 for ORA (95% CI, 0.69–0.90, cutoff value: < 76.45 degrees) and 0.77 for the pBC2 line (95% CI, 0.66 –0.89, cutoff value: > 7.78 mm).

Conclusion: In patients with BI, the ORA is narrowed and the pBC2 line is widened. These two anatomical changes may be candidates to explain the increased frequency of Chiari malformation in patients with BI. However, we do not recommend their use as a diagnostic imaging marker due to the relatively low interrater reliability and accuracy compared with OA-CL.

Publication History

Article published online:
02 June 2022