6. Analysis of radiological findings

Two spinal surgeons independently performed the radiological evaluations using a digital imaging and communications in medicine viewer (Synapse ver. 4.1.0; FUJIFILM Medical, Tokyo, Japan). For postoperative image evaluation, images at the final follow-up were used. Standing lateral view plain radiographs were obtained with the neck in a neutral posture, flexion, and extension, preoperatively, and at the final follow-up. Neutral posture lateral radiographs were obtained with the patient standing in a comfortable position with the head facing forward and gazing in the horizontal direction. The C2–C7 angle was measured as the Cobb angle between the C2 and C7 vertebral bodies. A positive value in the C2–C7 angle indicates lordosis in this study. The range of motion (ROM) was calculated as the difference between the C2–C7 angle during flexion and extension. We defined the C2–C7 sagittal vertical axis (SVA) as the distance between the C2 plumb line and the superior posterior corner of the C7 vertebral body. The C7 slope was determined as the angle between the superior endplate of C7 and a horizontal line. The angles associated with C2–C5 and C5–C7 angle were measured in the same way as the angle for C2–C7 [¹³].

The lower disk angle (LDA) was defined as the disk angle at the decompression site. LDA was measured as the C4–C5 Cobb angle for C3–C4 laminectomies, C5–C6 Cobb angle for C4–C5 laminectomies, C6–C7 angle for C5–C6 laminectomies, and C7–Th1 angle for C6–C7 laminectomies (Fig. 2). A positive LDA value means lordosis at the targeted disk in this study.

Lower disc angle. Lower disc angle (LDA) was defined as a LDA at the decompression site. LDA was measured as C4–C5 Cobb angle for C3–4 laminectomy.

We evaluated the intervertebral disc height according to the Matsumoto method [^15,16]. We defined disk height grade (DHG) as the percentage of height reduction compared with the most adjacent normal disk on sagittal magnetic resonance images, graded from 0 to 2: grade 0, no narrowing of the disk space, grade 1, less than 50% loss of height, and grade 2 more than 50% loss of height. The total disk height is the sum of the grades of all intervertebral discs. The lower adjacent DHG was the DHG of the intervertebral space measured at the LDA.