Radiographic Clarity of the External Auditory Meatus as a Cranial Reference Point in Spinal Deformity Patients: A Pilot Interobserver Study

Abstract

Background/Objectives: The external auditory meatus (EAM) is widely used as a surrogate cranial reference point, based on its proximity to the midpoint of the nasion-inion line (MNI). However, its radiographic clarity has not been thoroughly validated. This study aimed to evaluate the radiographic clarity and interobserver reproducibility of the EAM compared with the sella turcica as a control landmark. Methods: A retrospective review was performed on patients who underwent surgical correction for sagittal spinal deformity between 2021 and 2024. Preoperative standing whole-spine radiographs were analyzed. Horizontal and vertical distances from the EAM and the posterior border of the sella turcica to the MNI were measured. Radiographic clarity was categorized into three groups. Five independent neurosurgeons conducted all measurements, and interobserver reliability was assessed using the intraclass correlation coefficient with a two-way random-effects model [ICC(2,1)]. Results: The EAM was horizontally closer to the MNI (1.1 mm vs. 13.8 mm) but exhibited poorer radiographic clarity, with only 14.1% classified as single point and clear compared with 84.5% for the sella turcica. Interobserver reproducibility was lower for the EAM (ICC: 0.84 horizontal, 0.89 vertical) than for the sella turcica (0.97, 0.95). Horizontal deviation among observers was significantly greater for the EAM (major deviation 6.3 mm vs. 2.2 mm, p < 0.001), whereas vertical deviation did not differ significantly. Conclusions: Although anatomically close to the MNI, the EAM demonstrated inferior radiographic clarity and reproducibility. These findings suggest that the EAM may have limitations as a cranial reference landmark.