Background From a biomechanical point of view, pedicle screws (PS) are better than other kinds of screws for implantation in the seventh cervical vertebra (C7). However, the application of PS is limited because of the high risk of severe complications. It is essential to define the optimal entry point and trajectory. The aim of this study was to comprehensively analyze the starting point and trajectory for C7 PS insertion using three dimensional (3D) models. Methods Overall, 60 subjects aged 18 to 67 years old were included. All CT images were used to construct 3D computer models of the C7 vertebrae. A new coordinate system was established for the next evaluation. The pedicle axis was calculated with respect to the entire pedicle; then, the ideal entry point, screw diameter and length, sagittal angle and lateral angle were assessed. Results All the ideal entry points were located at the medial superior to lateral notch (LN), and the mean distance between the entry point and LN was 5.86 ± 1.67 mm in the horizontal direction and 3.47 ± 1.57 mm in the vertical direction. The mean distance between the entry point and the middle point of the inferior edge of the C6 articular process (MP) was 0.74 ± 1.83 mm in the horizontal direction. The mean sagittal angle of the pedicle axis was 90.42°, and the mean pedicle transverse angle was 30.70°. The average diameter and length of the PS were 6.51 ± 0.76 mm and 31.58 ± 4.40 mm, respectively. Conclusions This study provided a novel method to calculate the ideal starting point and trajectory for C7 PS insertion. These measurements may be helpful for preoperative planning. It is recommended that 3D CT imaging is used preoperatively to carefully evaluate the anatomy of each individual.

Background: Butterfly vertebrae are a rare congenital vertebral anomaly. An overlap of this spinal anomaly with other diseases has been reported. However, to the authors' knowledge, the coexistence of butterfly vertebrae and spinal cord injury has not been reported in the literature. Case presentation: A 42-year-old male was admitted to our emergency department after a motor vehicle accident. His complaint was back pain, and he was unable to move both lower limbs. Upon physical examination, the patient was not ambulatory. Sensory examination revealed the absence of sensation below the T12 level. The strength of the bilateral lower limbs was grade 0. The patient received a radiographic evaluation. The initial diagnosis was T11 fracture with complete paraplegia of the lower limbs. Magnetic resonance imaging (MRI) was then performed. Sagittal MRI demonstrated an isointense lesion on T1-weighted imaging and a high-signal spindle-like lesion on T2-weighted imaging of the spinal cord adjacent to the T11 vertebra. The fat-suppressed sequence also revealed hyperintensities of the cord. There was no evidence of acute injury of the T11 vertebral body except for cuneiform anterior wedging. The patient was ultimately diagnosed with complete paraplegia with a T11 butterfly vertebra. He underwent urgent posterior decompressive and fixation surgery from T10 to T12. His postoperative recovery was uneventful. Conclusions: The coexistence of a butterfly vertebra with spinal cord injury was reported for the first time. Although butterfly vertebrae may be incidentally detected, it is important to be familiar with their radiographic features to distinguish them from fractures.