Research Article: In vitro study of accuracy of subaxial cervical pedicle screw insertion using calipers based on the gravity line

Date Published: July 20, 2017

Publisher: Public Library of Science

Author(s): Xiang Yao, Shiqing Liu, Ara Nazarian.


There is a high probability of iatrogenic perforation of the vertebral pedicle wall during the application of subaxial cervical pedicle screw (CPS). The goal of this study was to evaluate the accuracy of CPS insertion at C3-C7 in vitro using novel calipers based on the gravity line.

Nine cadaveric cervical spines underwent computed tomography scanning and preoperative design. A lateral fluoroscopic view was taken to measure the intra-operative sagittal angle by C-arm with hanging cross structured K-wires. By referring to the gravity line, caliper A was used to locate the entry point, while caliper B was employed to guide the screw insertion. Postoperative CT scans were performed to assess the accuracy of the screw placements, according to the Neo classification.

Overall, 78 (88.6%) of the 88 pedicle screw placements were classified as grade 0 (correct position), 4 (4.5%) were grade 1 (non-critical perforation), 4 (4.5%) were grade 2 (critical perforation), and 2 (2.3%) were grade 3 (critical perforation).

Using our novel calipers and referring to the gravity line was helpful for locating and guiding individual cervical pedicle screw insertions.

Partial Text

Since Abumi et al. first reported the results of cervical pedicle screw (CPS) fixation for traumatic lesion of the subaxial cervical spine, several studies have been performed indicating its superior biomechanical stability over lateral mass screws[1–3]. The ideal placement of the CPS requires an accurate entry point, with an appropriate trajectory angle and screw size, incorrect CPS placement can sometimes result in lethal perforation in the C3–7 region[4–6].

8 specimens contained intact C3–C7 vertebrae and the last specimen contained C4–C7 vertebrae only, 88 pedicles of 44 cervical vertebrae were included in the study. The important linear and angular parameters for the preoperative design (D, Am, Ag, H1, and H2) of all 88 pedicles are displayed in Table 1.

During CPS insertion with fluoroscopy, the cervical pedicle wall perforation rate was reported to range from 6.7% to 29.8%[5, 20–23].Cong et al. conducted 10% (9/90) screws breached the pedicle cortex using the uniplanar locator[24]. In the current study, 4 out of 88 screws (4.5%) achieved non-critical perforation (grade 1), and 6 out of 88 screws (6.8%) achieved critical perforation (grade 2, 3).The perforation rates were roughly similar to previous studies using freehand techniques with the assistance of fluoroscopy or home-made instrument.

By means of the gravity line, the novel caliper A and B were helpful for locating the ideal entry point and guiding the individual stereoscopic insertion of cervical pedicle screws.




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