Transpedicular screwing of the seventh cervical vertebra: anatomical considerations and surgical technique

The purpose of this study was first to assess the feasibility of C7 transpedicular screwing with a morphological study and secondly to evaluate the safety of such a surgical technique when guided only by posterior landmarks. Eighteen C7 vertebrae, harvested from fresh human cadavers, were included in this study. First the morphometry of C7 pedicle was performed on computed tomography with multiplanar reconstructions. Results of this quantitative anatomy were compared with the literature data. Secondly 30 pedicle screws, whose placement was guided only by anatomical features on the posterior face of the dorsal arch, were inserted in 15 C7 vertebrae. A second computed tomographic examination was done after the surgical procedure to check the screw placement in both planes. The average pedicular width was 6+/-1.2 mm and the average height was 5.8+/-1.1 mm. The pedicle angulation in the transverse plane was 33.3 degrees +/-6.6 degrees, the pedicle angulation in the sagittal plane was 4.3 degrees +/-4.5 degrees downward with reference to the lower endplate of C7. The average distance from the entry point of transpedicular screwing to the anterior cortex of the vertebral body was 29+/-3 mm. Concerning the safety of transpedicular screwing, 63% of screws were found entirely inside the pedicle without any violation of the pedicle cortex. Most of pedicle violations were observed in the transverse plane. No grade II violation of the pedicle was observed. Dimensions of the C7 pedicle are amply compatible with transpedicular fixation using 3.5 mm screws. Such a surgical technique seems to be an interesting option when posterior fixation of C7 is required. Nevertheless morphological guidelines appeared not to be sufficient to ensure safe transpedicular screwing. Laminoforaminotomy is strongly recommended, although it has not been evaluated in this study.