Risk Stratification of Lamina and Pedicle Morphometry for Screw Fixation in the Axis Vertebra

Screw fixation of the second cervical vertebra (C2), also known as the axis, is a mainstay in the management of upper cervical trauma and instability. The safety of C2 fixation via translaminar screw (TLS) and transpedicular screw (TPS) techniques is fundamentally influenced by individual anatomical variability. This study addresses the need to move beyond reliance on mean values by establishing quantifiable, integrated risk thresholds for both fixation pathways within the study population.

A total of 50 dry human C2 vertebrae were evaluated. The length, height, and thickness of the lamina were measured, and pedicle dimensions were recorded. Safety thresholds were defined according to commonly accepted minimum screw corridor requirements. Lamina and pedicle categories were cross-tabulated to determine the proportion of vertebrae that could safely accommodate laminar fixation, pedicular fixation, or both.

The mean C2 laminar length and height demonstrated no statistically significant side-to-side differences (p>0.05), with a mean middle third laminar thickness of 5.8 mm bilaterally. Despite favorable mean values, laminar thickness decreased to 3.3 mm and pedicle width to 2.1 mm in some specimens, falling below accepted safety thresholds. Risk stratification of laminar thickness revealed that 36 vertebrae (72%) demonstrated bilateral laminar thickness ≥4.0 mm, indicating suitability for TLS fixation, while two specimens (4%) showed bilateral laminar thickness less than 4.0 mm, rendering them unsuitable for TLS fixation. This variability underscores the importance of individualized morphometric assessment prior to C2 screw fixation.

The quantified bilateral laminar risk identified in this study supports a paradigm shift in preoperative planning for C2 fixation. Translating morphometric measurements into a quantified safety model provides an objective framework to guide selection of the most appropriate fixation technique, thereby enhancing operative planning, reducing neurovascular risk, and ensuring selection of the safest available anatomical corridor for each patient.