Morphometric analysis of the seventh cervical vertebra for pedicle screw insertion

Anatomy of the pedicles of the seventh cervical vertebra (C7) at the cervicothoracic junction is different from other cervical vertebrae. Fixation of C7 is required during cervical vertebra and upper thoracic injuries in clinical practice. However, the typical pedicle screw insertion methods may have problems in clinical practice based on the anatomical features of C7. This study is to explore a new pedicle screw insertion technique for C7 and to provide anatomical and radiographic basis for clinical application.

C7 vertebral specimens from six human cadavers were observed for the relative position between the posterior bony landmark and the pedicle projection. Computed tomography (CT) was performed for 30 patients with cervical spondylosis (26-61 years old, mean age was 42.3 years old). The CT scan data were processed by Mimics 8.1 software for associated parameter measurement. Appropriate screw entry points (Eps) and insertion angles were selected. A total of 12 pedicle screws were inserted and then observed. The six specimens were observed after inserting the screw using this method. The junction site of the middle 1/3 and outer 1/3 segment of line G [The junction between point A (the intersection point of the superior margin of the lamina of C7 and the medial margin of the superior articular process) and point B (the intersection point of the lateral margin of the inferior articular process and the transverse process)] was taken as the Ep. The screw insertion direction parallel horizontally to the upper terminal lamina of C7 and the sagittal angle was between 35° and 45°.

Gross and imaging observations revealed that pedicle projection was on the line (line G) between point A (the intersection point of the superior margin of the lamina of C7 and the medial margin of the superior articular process) and point B (the intersection point of the lateral margin of the inferior articular process and the transverse process) and located at the middle 1/3 and outer 1/3 segments of the line (point L[also it is the screw entry points (Eps)]. No significant difference in the measurements on the left and right sides were observed (P > 0.05). No penetration of the 12 screws through pedicle was observed.

The junction site of the middle 1/3 and outer 1/3 segments of line G are the projection points of C7 pedicles on the lateral mass. The junction site anatomical position was simply and easy to be controlled during surgery, simultaneously avoided uncertainty of other methods. This study provides a new method for determining an Ep for C7 pedicle screw insertion.