Morphometric variation of the C1 posterior arch

Surgical stabilization of the atlantoaxial joint (C1–C2) is required in various conditions, including type II odontoid fractures, rheumatoid arthritis, congenital malformations, tumors, and infections. However, the unique morphology of the C1 vertebra presents challenges for implant fixation due to the proximity of critical neurovascular structures and limited bony surface. While dimensions related to lateral mass screw placement have been well studied, less is known about the morphometric variation of the C1 posterior arch for alternative fixation methods such as hooks and clamps. This study aimed to characterize posterior arch morphology using both anatomic measurements and statistical shape modeling (SSM). A retrospective review of cervical spine CT scans from patients aged 65 or older was conducted to evaluate 77 intact C1 vertebrae. Using 3D Slicer software, fiducial landmarks were placed on five sagittal slices (midline, ±8 mm, ±15 mm) to obtain measurements of posterior arch height, width, and slope. Anatomical symmetry was analyzed, and principal component analysis was used in SSM to assess 3D shape variation. The analysis revealed substantial variability in posterior arch morphology, with no clear correlation between height and width across sagittal slices. The average coronal slope was approximately 15°, but ranged widely from 2° to over 30°. Lateral symmetry in height, width, and slope was generally preserved. SSM showed the greatest variation occurred in the transverse plane, with larger sagittal cross-sections associated with increased posterior curvature. These findings underscore the wide anatomical variation in the size, shape, and curvature of the C1 posterior arch—critical considerations for implant design and surgical planning. This study is the first to quantify variation in coronal slope and provides important guidance for developing alternative fixation techniques. Hooks and clamps must be designed to accommodate the full spectrum of posterior arch anatomy to ensure safe and effective stabilization.