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. 2022 Jun 13;6(6):625-630.
doi: 10.22603/ssrr.2022-0055. eCollection 2022 Nov 27.

Accuracy of Lateral Mass Screw Insertion during Cervical Spine Surgery without Fluoroscopic Guidance and Comparison of Postoperative Screw Loosening Rate among Unicortical and Bicortical Screws Using Computed Tomography

Daisuke Inoue  1 , Hideki Shigematsu  2 , Hiroaki Matsumori  1 , Yurito Ueda  1 , Yasuhito Tanaka  2
Affiliations

Accuracy of Lateral Mass Screw Insertion during Cervical Spine Surgery without Fluoroscopic Guidance and Comparison of Postoperative Screw Loosening Rate among Unicortical and Bicortical Screws Using Computed Tomography

Daisuke Inoue et al. Spine Surg Relat Res. .

Abstract

Introduction: Pedicle screws (PSs) or lateral mass screws (LMSs) are used in posterior cervical spine fixation. The former are more firmly fixed but are associated with the risk of neurovascular injury and should be inserted using intraoperative imaging or navigation, which may prolong the surgical duration and is not feasible in all hospitals. This prospective clinical study aimed to evaluate the outcomes of LMS insertions without fluoroscopic guidance and screw loosening rates at 6 months postoperatively using computed tomography (CT).

Methods: We examined 38 patients who underwent posterior cervical spine fusion using 206 LMSs in the C3-C6 range between January 2018 and July 2021. The direction of screw insertion followed the Magerl method, and we inserted screws as bicortically as possible without intraoperative imaging. The screw position was examined using CT at 1 week postoperatively. Screw insertion angles, bicortical insertion rate, facet violation, and neurovascular injury were evaluated. Screw loosening with unicortical and bicortical screws (US and BS, respectively) was investigated using CT at 6 months postoperatively.

Results: The average LMS length was 14.1 mm. The average axial and sagittal angles were 33.9° and 29.2°, respectively. Among the 206 LMSs inserted, 167 were BS; of these, 94.6% had screw length protrusion of 0-2 mm. Facet violation was observed in 3.4% of all screws but without neurovascular injury. Six months postoperatively, loosening of 25 screws (12.1%) occurred, including 17 (18.3%) USs and 8 (8.39%) BSs. The screw loosening rate was significantly higher in US than for BS (43.6% [17/39] vs. 4.8% [8/167], P<0.01).

Conclusions: Over 80% of LMSs were inserted bicortically without intraoperative imaging. By devising the screw length selection process, we inserted for screw loosening was more common in US and more likely at the fixed end.

Keywords: Bicortical screw; Cervical spine; Fluoroscopy; Lateral mass screw; Screw loosening.

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Conflict of interest statement

Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.

Figures

Figure 1.
Figure 1.
Insertion point and direction confirmation using preoperative computed tomography. Left (axial): screw direction. Center (coronal): screw insertion point. Right (sagittal): measurement of penetration distance from the caudal side.
Figure 2.
Figure 2.
Insertion point determination of the bone hole and insertion direction using a bone model. A. Mark the lateral mass and draw a diagonal to confirm the intersection. B. Create a bone hole using a 3-mm ball air drill. Create a bone hole by shifting the ball halfway to the medial and caudal sides. C. Create a bone hole using a 3-mm ball air drill. Drilling is performed along the diagonal line drawn toward the lateral mass on the outer cranial side. The spinous process should be removed, as the drill needs to be tilted beyond the midline spinous process of the next lower vertebral lamina. D. Coronal direction is diagonal to the lateral mass. E. Axial direction follows the vertebral lamina inclination. F. Sagittal side is parallel to the facet joint.
Figure 3.
Figure 3.
Postoperative X-ray imaging. The optimal position and length on anteroposterior (left) and lateral (right) images are shown.
Figure 4.
Figure 4.
Puncturing the lateral mass screw’s contralateral cortex and the relationship between the length and angle. (A, B) Even with the same insertion point and screw length, an increased insertion angle will result in a diagonal insertion; thus, the tip will protrude. (B) There are long and short protrusions at the screw’s tip because the screw is diagonally inserted. (B, C) If same-length screws, similar to those used for penetration of the contralateral cortex, are inserted, they will protrude from the opposite side. Protrusion can be prevented by inserting a screw that is one size smaller, and a part of it can also be inserted as a bicortical screw.
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