Computed Tomography Imaging Study of Basilar Invagination and Atlantoaxial Dislocation

OBJECTIVE: We sought to analyze axis deformities and the biomechanics related to atlantoaxial dislocation (AAD) in patients with basilar invagination (BI). METHODS: Seventy-six patients were retrospectively analyzed including 21 patients who had BI (group B), 32 patients with BI and AAD (group C), and 23 nondeformity control subjects (group A). Using 3-dimensional computed tomography imaging the distance from the tip of the dens above the Chamberlain line in the sagittal plane, atlantodental interval, sagittal inclination, coronal inclination, and craniocervical tilt, the height of the odontoid and ratio of the height to the odontoid basal width were measured in each patient. We statistically analyzed the resulting data for correlations among physiologic measurements and disease state. RESULTS: The height of the odontoid process in groups A, B, and C was 13.38 +/- 1.50 mm, 10.87 +/- 1.48 mm, and 8.49 +/- 2.49 mm, respectively. The ratio of height-to-basal width of the odontoid in groups A, B, and C was 1.32 +/- 0.21, 0.91 +/- 0.21, and 0.65 +/- 0.17, respectively. The sagittal inclination in groups A, B, and C was 85.85 +/- 4.55 degrees, 105.76 +/- 10.72 degrees, and 123.48 +/- 12.43 degrees, and the coronal inclination was 108.95 +/- 24.09 degrees, 105.40 +/- 25.16 degrees, and 108.82 +/- 21.41 degrees, respectively. The craniocervical tilt in groups A, B, and C was 60.31 +/- 6.98 degrees, 84.53 +/- 18.94 degrees, and 71.79 +/- 11.69 degrees, respectively. The height of the odontoid, height-to-basal width, and sagittal inclination were significantly correlated with both BI and AAD (P < 0.001). Odontoid height and height-to-basal width ratio were significantly correlated with BI, AAD, and sagittal inclination (P < 0.001). Finally, craniocervical tilt was correlated only with the BI severity (P < 0.001). CONCLUSION: This study confirms that deformities of the odontoid process and the lateral joint correlate with severity of BI, while deformity of the odontoid process may be a primary factor in AAD for patients with BI.