gms | German Medical Science

Objectives: Fractures of the atlas (C1) occur typically bimodal with peaks at both early adulthood (20-30s) and later adulthood (60s-up). For surgical treatment, Harms technique using lateral mass screws provides rigid fixation and good clinical outcome. The use of the posterior arch of C1 as "pedicle" would allow for better screw anchorage that could yield beneficial biomechanical stability. Therefore, the aim of the study was to introduce a bilateral safe zone of c1 pedicle screws regarding the angulation and the pedicle height.

Methods: We retrospectively reviewed the CT scans of 500 patients. Cervical fractures, previous spinal trauma or previous surgery as well as anatomical variations and CT scans showing artefacts were exclusion criteria. Three-dimensional reformats were generated for detailed measurements. Idealized screw entry point (EP), length of lateral mass as screw trajectory (LML), lateral mass width (LMW), maximal divergence (DI) and maximal convergence (CON) from EP without perforation and pedicle height (PH) of posterior arch were measured.

Results and Conclusion: Of the 500 subjects, 335 males and 165 females were included. Mean age of the patients was 49.48 years (18-92). All measurements did not demonstrate significant side-related differences. The idealized mean screw entry point was 22.81 mm from midline-axis (left 22,6; right 23 mm). From this entry-point, a safe zone between 11.6° of divergence and 19.6° of convergence was detected. Mean LMW was 9.5 mm (range 5.2-14) and mean length of screw trajectory (LML) was 29.6 mm. Measurements of female patients were generally smaller with statistically significant differences compared to male patients (p<0.05). Regarding pedicle height (PH), 158 subjects (31.6%) had <4mm height of posterior arch which makes pedicle screw placement very challenging.

Due to the variable anatomy of the axis, the screw placement in C1 is crucial to avoid perforation of either, the vertebral foramen including the vertebral artery and the foramen magnum. Compared to lateral mass screws, which are generally possible for most patients, pedicle screws might be feasible in a majority of patients. Therefore, detailed knowledge of C1 anatomy and detailed measurement is essential in the operative planning. The safe zone regarding screw angulation provides further safety and avoidance of potential perforation. Furthermore, the pedicle height <4 mm might be a valuable cut off to use lateral mass screws as valid alternative.