J Korean Neurosurg Soc.  2021 Nov;64(6):913-921. 10.3340/jkns.2021.0011.

Analyzing the Significance of T1 Slope minus Cervical Lordosis in Patients with Anterior Cervical Discectomy and Fusion Surgery

• Lee HJ
• You ST
• Sung JH
• Kim IS
• Hong JT

• KMID: 2521981
• DOI: http://doi.org/10.3340/jkns.2021.0011

Abstract

Objective
: Accurate measurement of T1 slope (a component of T1s minus cervical lordosis [CL]) is often constrained by anatomical limitations. In this situation, efforts should be made to find the exact meaning of T1s-CL and whether there are any alternatives to it.
Methods
: We enrolled 117 patients who received two-level anterior cervical discectomy and fusion (ACDF). Occipital slope, C2 slope (C2s), C7 slope (C7s), T1, O-C2 angle (O-C2A), C2-7 angle (C2-7A), O-C7 angle (O-C7A), T1s-CL, C7-T1 angle (C7-T1A), and C2-7 sagittal vertical axis were measured. We determined 16° (T1s-CL) as the reference point for dividing subjects into the mismatch group and the balance group, and a comparative analysis was performed.
Results
: The mean value of C7-T1A was constantly maintained within 2.6° peri-operatively. In addition, C2s and T1s-CL showed the same absolute change (Δ|0.8|°). The mean values of T1s-CL of the mismatch and balance groups were 23.0° and 7.6°, respectively. The five factors with the largest differences between the two groups were as follows : C2s (Δ13.3°), T1s-CL (Δ15.4°), O-C2A (Δ8.7°), C2-7A (Δ14.7°), and segmental angle (Δ7.9°) before surgery. Only four factors showed statistically significant change between the two groups after ACDF : T1s-CL (Δ4.0° vs. Δ0.2°), C2s (Δ3.2° vs. Δ0.7°), O-C2A (Δ2.6° vs. Δ1.3°), C2-7A (Δ6.3° vs. Δ1.3°). A very strong correlation between T1s-CL and C2s was also found (r=|0.88–0.96|).
Conclusion
: C2s itself may be the essential key to represent T1s-CL. The amounts and directions of change of these two factors (T1s-CL and C2s) were also almost identical. The above phenomenon was re-confirmed once again through the correlation analysis.

Keyword

T1 slope; Cervical lordosis; Cervical sagittal balance; Anterior cervical discectomy and fusion; C2 slope

Figure

• 

  Fig. 1. Parameters measured before (A) and after surgery (B). The slope angle was measured with a horizontal line and a specific reference line at each cervical level : McGregor line (occipital slope), lower endplate of C2 body (C2 slope), lower endplate of C7 body (C7 slope), upper endplate of T1 body (T1 slope), upper segmental slope (USS), lower segmental slope (LSS). C2-7SVA (distance between the C2 plumb line and the superior posterior corner of C7 body), cranio-vertebral angle (CVA; the angle between the connecting line from the EAC to the distal tip of the spinous process of C7 and the horizontal line.

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  Fig. 2. Example of sagittal alignment changes after ACDF (A and B) in mismatch group patients (T1s-CL, 20). Two levels ACDF induced improvement of cervical lordosis (C2-7A) and T1s-CL (20→8). Because, C7-T1A remained constant with a small difference (3°), the difference value between T1s-CL and C2 slope was also maintained constantly. ACDF : anterior cervical discectomy and fusion, T1s-CL : T1 slope minus cervical lordosis, C2-7A : C2-7 angle, C7- T1A : C7-T1 angle.

• 

  Fig. 3. This figure shows that the degree and trend of correlation among variable cervical radiologic parameters (occipital slope [Os], C2 slope [C2s], C7 slope [C7s], T1 slope [T1s], T1 slope minus cervical lordosis [T1s-CL], C2-7 angle [C2-7A], cranio-vertebral angle [CVA], C2-7 sagittal vertical axis [C2-7SVA]) in Table 4 (pre-operation). T1s-CL and C2s have very similar degrees and trends of correlation with other parameters and C7s and T1s also show very similar patterns of correlation.

Reference

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