Validating the Reliability of External Auditory Meatus as a Reference Point for Cranial Center in Spinal Deformity Patients: A Preliminary Report

Study Design: A retrospective comparative study.Purpose: This study aimed to evaluate the reliability of the external auditory meatus (EAM) as a reference point for determining the cranial center in patients with sagittal spinal deformity. Specifically, we assessed the radiographic visibility and spatial relationship of the EAM relative to the midpoint of the nasion-inion line (MNI) and compared it with the sella turcica.Overview of Literature: The cranial center is commonly defined as the midpoint of the nasion-inion line (MNI). The EAM has been widely used as an approximate surrogate of the MNI; however, its radiographic clarity and consistency are often limited. In contrast, the sella turcica is a singular anatomical structure that is more distinctly visualized on radiographic imaging.Methods: A total of 71 patients who underwent corrective surgery for spinal deformity between 2021 and 2024 were included. Preoperative full-spine radiographs were analyzed to evaluate the spatial relationship between both the EAM and the sella turcica relative to the MNI. Radiographic visibility was classified into three categories: single-point and clear, duplex but distinguishable, and unclear.Results: The EAM was located, on average, 1.1 mm posterior and 17.1 mm inferior to the MNI, whereas the sella turcica was located 13.8 mm anterior and 2.0 mm superior to the MNI. Relative to the femoral head axis, angular analysis revealed that the EAM exhibited a mean posterior tilt of 0.2°, while the sella turcica showed a mean anterior tilt of 1.3°. The sella turcica was clearly identifiable in 84.5% of cases, whereas the EAM was clearly visualized in only 14.1%, with the majority being either duplicated or indistinct (p<0.001).Conclusions: Although the EAM is anatomically closer to the MNI, its radiographic visibility and consistency are inferior to those of the sella turcica, thereby reducing its reliability as a cranial reference point. Further studies are needed to determine whether the sella turcica can serve as a more reliable alternative reference.