Until now, placement of pedicle screw in subaxial cervical spine is still one of the most challenging procedures in spine surgery due to both the small size of cervical pedicle and the proximity of screw trajectory to the vertebral artery or spinal cord [8–9, 15]. Pedicle screw misplacement in C3 to C6 might lead to catastrophic consequences, which limits the wide application of pedicle screw in these levels [16–17]. Although navigation assistance and robotics have been developed to increase the accuracy of cervical pedicle screw placements, these techniques are only available in a few hospitals in developed countries or regions. In most hospitals, LMS is still the first choice for cervical fixation in C3 to C6. However, the C7 had thicker lateral mass when compared with C3 to C6 and most of C7s had no vertebral artery passing through the transverse foramens [12–13]. Such unique anatomic characteristics made free-hand technique of pedicle screw safe and feasible for C7 fixation.
Several entry points and trajectories for C7 pedicle screw insertion had been recommended in the literature. Abumi et al. firstly reported the technique for C3-C7 pedicle screws in 1994 [18]. The entry point was located lateral to the center of the articular mass and adjacent to the posterior edge of the superior articular surface with convergent angle of 30–40°. In 1997, Ebraheim et al. introduced a horizontal line between left and right inferior articular processes of upper cervical vertebrae and an ordinate between the outer edge of the lateral mass of the adjacent vertebrae [19]. The entry points were located 1.6–2.6 mm below the horizontal line and 4.5–6.4 mm inward from the ordinate at C3 to C7 levels. In theory, both entry point and convergent angle of C7 should be different from those at other levels since C7 has unique anatomic characteristics as a transitional vertebra. Unfortunately, both Abumi et al. and Ebraheim et al. treated C3–C7 as a whole and did not differentiate the entry points for these levels.
Nowadays, there were an increasing number of studies specifically focus on the techniques for C7 pedicle screw insertion. Karaikovic et al. reported different entry points at different cervical levels. [20]. He suggested that the entry point was located at the lateral vertebral notch at C3 and C4, but gradually moved medially at C5–C7. However, such description of entry point was too vague for other surgeons to reproduce. Li et al. chose the entry point for C7 pedicle screw as the intersection of the horizontal line through the midpoint of the transverse process root and the vertical line through the intersection of the posterolateral and posterior planes of the isthmus [21]. The screw direction should incline inward by about 60° using this point. Lee et al. recommended a starting point for the C7 pedicle screw to be 2 mm lateral and 2 mm superior to the center of lateral mass with average transverse angles of 28° at C7. [22]. Liao et al. introduced a line connecting 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) [23]. The junction site of the middle 1/3 and outer 1/3 segment of this line was selected as the entry point for C7 pedicle screw. The average inclination angle of the screw trajectory was 41.1°.
It is noticeable that most of these recommended entry points for C7 screw were located laterally to the middle of lateral mass, which was often used as the entry points for LMS. In our study, the projection of C7 pedicle axis on lateral mass (Ep A) was also located lateral to the middle of posterior part of C7 with average horizontal offset of 3.7 mm from Ep B no the left side and 3.6 mm on the right side. Selecting these entry points for C7 pedicle screw placement often made rod insertion difficult when the LMS was used at C3 to C6 levels. In addition, a more lateral entry point led to a larger transverse angle for C7 pedicle screw, which increased the difficulty of accurate screw insertion. The average transverse angle of trajectory A was 33.6° on the left side and 32.0° on the right side, which was similar to the results of previous studies. In the current study, we introduced a easily identified entry point (Ep B) for C7 pedicle screw. This point was kept in line with the entry points for LMS in C3 to C6, which facilitated the procedure of rod insertion (Fig. 5). After exposure of the posterior elements of C7, the base of superior facet is clearly visible and the middle of the base can be quickly identified. The average vertical distance between Ep A and Ep B was only − 0.7 mm on the left side and − 0.8 mm on the right side, which means these 2 entry points were approximately located at the same height in the sagittal plane. However, the, Ep B was located medially to Ep A. The transverse angle should be smaller for pedicle screw trajectory with a more medial entry point. The average transverse angle for trajectory B was only 15.3° on the left side and 14.4° on the right side, which was significantly smaller than that for trajectory A. Due to the small transverse angle, trajectory B can be easily explored by free-hand technique. Although the average screw length for trajectory B was shorter than that for trajectory A, no screw length was smaller than 22 mm for trajectory B.
As we know, intraoperative X-ray film of cervicothoracic junction can not be clearly visualized due to the overlap of the shoulder joint. Therefore, fluoroscopic guidance is not a reliable method for C7 pedicle screw insertion. A safe free hand technique for C7 pedicle screw insertion is necessary for CSM patients underwent posterior cervical fixation. To validate the feasibility of our free hand technique, 53 CSM patients underwent C7 pedicle screw fixation by our technique and the overall accuracy of screw insertion was as high as 92.5% (98/106) with only 8 screws mildly perforated (grade 1). Among these 8 screws, 4 was medially displaced and 4 was downwards displaced. The accuracy of screw placement was similar between left side and right side (L:94.3% vs R:90.6%, P > 0.05). No perforation was larger than 2 mm and no screw-related complication occurred. Therefore, the midpoint of the base of C7 superior facet is a reliable landmark for C7 pedicle screw placement. Using this landmark, high accuracy of screw placement was obtained in CSM patients. Our method is is easy to learn for beginners and is worthy of being widely popularized.