Radiological comparison of the midpoint of the nasion-inion line and the external auditory canal for measuring the cranial center of the gravity-sagittal vertical axis

Abstract

Background: Spinal sagittal malalignment affects a patient’s quality of life, and the condition has gained increasing importance. The cranial center of the gravity-sagittal vertical axis (CCG-SVA) helps in assessing sagittal alignment. Two major landmarks of the CCG are used for measuring the CCG-SVA: the midpoint of the nasion-inion line (MNI) and the external auditory canal (EAC). However, the correspondence between these two points has not been proved. In this study, we radiologically examined the positional relationship between them. Methods: We obtained lateral skull radiographs from 87 patients aged between 21 and 91 years. We evaluated the following: the vertical distance between the MNI and the upper margin of the EAC (Distance A), the horizontal distance between the plumb line from the MNI and the anterior margin of the EAC (Distance B), and the horizontal distance between the plumb line from the MNI and the center of the EAC (Distance C). Results: The average values of Distances A, B, and C were 19.6 ± 5.0, 1.65 ± 5.7, and 5.45 ± 5.5 mm, respectively, indicating that the plumb lines dropped from the MNI and the EAC did not match exactly. The measured values showed no sex-specific differences (P > 0.05, Mann–Whitney U test). Conclusions: The present study demonstrated that the MNI and EAC locations had minimal horizontal differences. The CCG-SVA should be measured by considering the differences between the MNI and the EAC.

Keywords

• Cranial center of gravity
• External auditory canal
• Sagittal vertical axis
• Radiography
• Skull