A novel method to judge the safe position of C2 lamina screw with double holes

To evaluate the feasibility of C2 laminar screw (C2LS) fixation assisted by double holes and to explore a novel method for judging its safe positioning.

The Digital Imaging in Communications format data of 25 C2 vertebrae specimens were obtained by computed tomography thin-slice scanning, and the data were imported into Mimics software for three-dimensional reconstruction. The bone cortex was removed at the starting and terminal midpoints of the lamina as the observation holes. Points A, M, and P represented the three different entry points of the C2LS. Point M was designated as the central entry point, point A was 1 mm from the extension surface of point M to the ventral side, and point P was 1 mm from the extension surface of point M to the dorsal side. Further classified into groups A, M, and P based on the screw path correlation parameters at each entry point. The distance between holes 1 and 2 (D1 and D2) was measured at the maximum safe inclination, parallel, and maximum safe extroversion positions when the screw-insertion points of C2LS were at the center points A, M, and P, respectively. The data of D1 and D2 of the three groups (A, M, P) in different screw directions were analyzed and compared.

Twenty-five specimens were measured in the parallel, maximum safe inclination, and maximum safe extroversion groups. In the parallel relationship, there were significant differences in the D1 and D2 data between the M and P groups. In the introversion relationship, there were significant differences in the D1 and D2 data between the M and P groups. In the extroversion relationship, there were significant differences in the D1 and D2 data among the A, M, and P groups, respectively.

Use of the C2LS assisted by double holes was able to determine the safe position of the screw in operation. When D1 is greater than D2, the screw position can be considered relatively safe, with the intraoperative visual manifestation of a deep screw in hole 1 and a shallow screw in hole 2. If D1 is close to D2, it is necessary to ensure that the screws in both holes 1 and 2 are shallow, then the screw position is safe. If the screw in hole 1 is observed to be shallower than in hole 2 during surgery, regardless of the entry point position, there is a significant risk of ventral cortical perforation. Especially when the entry point is more anterior, even when inserting the screw parallel to the lamina, caution must be taken against the risk of ventral cortical perforation.