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

Object: 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. Methods: 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. Results: 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). Conclusions: Using our novel calipers and referring to the gravity line was helpful for locating and guiding individual cervical pedicle screw insertions.