Introducing Angle B: A Novel Quantitative Approach for Sagittal Alignment Reduction in Atlanto-Axial Dislocation Surgery

Abstract

Objective: This study aims to establish a quantitative relationship between cervical lordosis (CL) and the rotation angle of the axis for the surgical reduction of atlantoaxial dislocation (AAD) and to explore the potential applications of this relationship in preoperative planning and selection of surgical techniques.

Methods: To construct the correlation between horizontal gaze and location of axis, we introduced the angle B, defined as the angle between the Frankfort horizontal line and the extension line of the posterior edge of the axis, and explored its application in surgical planning. We retrospectively analyzed imaging data from computed tomography of 23 AAD patients who had undergone posterior reduction and fixation. Theoretical equations were deduced through a series of angular transformations, and linear regression analysis was used to validate our findings.

Results: Our results showed a strong linear relationship between the change in CL (ΔCL) and the change in B(ΔB) (y=-1.0402x, coefficient of determination R²=0.978, P<0.001), supporting our deduction that ΔCL=ΔB. Furthermore, we found that the rotation angle of the axis (angle D) was equal to ΔCL. By resolving the atlantoaxial interval into the vertical dimension (h) and horizontal dimension (d), we could calculate the maximum vertical and horizontal distance that the axis could move theoretically according to a patient's maximum ΔCL. This finding supports our theory that the introduction of angle B can provide more precise preoperative planning and surgical technique selection for patients with AAD.

Conclusion: By introducing angle B and deducing the equation ΔB=ΔCL=D, we have provided an innovative tool for preoperative planning and surgical technique selection for patients with AAD. This equation not only helps surgeons achieve more precise and effective surgical reductions but also emphasizes the important role of angle B in surgical planning.

Keywords: atlanto-axial dislocation surgery; quantitative method; sagittal alignment reduction.

Figure 1

Measurements of cervical sagittal angles. The slope of the Frankfort horizontal line (sFH), the angle between the Frankfort horizontal line and the extension line of the posterior edge of the axis (B), the Cobb angle between the inferior endplate of C2 and C7 vertebrae (CL), the slope of the T1 vertebra (T1S), the angle between the Frankfort horizontal line and the inferior endplate of the axial vertebra (FXA), and the angle between the inferior endplate and extension line of the posterior edge of the axis (C) are shown. The dashed lines indicate a horizontal orientation.

Figure 2

Linear regression of ΔCL and ΔB is presented.

Figure 3

The pattern diagram of the sagittal parameter of the cervical spine and demonstration of the utility of equations in pre-surgical decision-making. (A) Shows the complete view of the sagittal parameter of the cervical spine, with the arcus anterior of atlas represented by a yellow rectangle and the vertex of dens represented by a circle. The dot-dashed lines depicts the postoperative extension line of the posterior edge of the axis and the inferior margin of the axis. Additionally, the red circle with a dotted blue outline displays the ideal location of the vertex of dens after the operation. The dashed lines represent a horizontal orientation. B’ denotes the angle between the Frankfort horizontal line and the extension line of the posterior edge of the axis after the operation, while D indicates the rotation angle of the axis. In (B), a local enlarged image of the dashed black square in Figure 3A is presented with the slope of the Frankfort horizontal line (sFH) translated vertically downward. The length of the posterior edge of the axis (g), length of the line connecting the location of axis before and after the rotation (m), the angle between the posterior edge of the axis and m (E), and the angle between m and horizontal line (F) are labeled. (C) shows a further enlarged image of the dashed black square region in Figure 3B. (D and E) Display pre- and postoperative CT images demonstrating the reduction of AAD of Patient 8.