2) Secondary outcome

The endpoints of the secondary outcomes were JOACMEQ, neck muscle strength, and cervical spine alignment. JOACMEQ and neck muscle strength were evaluated before, 2 weeks after, and 3 months after surgery, and cervical spine alignment was evaluated before and 3 months after surgery. JOACMEQ was used to determine the treatment effect before and after surgery. Patients with JOACMEQ scores ≥90 points before surgery and after treatment were excluded from the analysis.

Neck muscle strength was evaluated by measuring neck extension muscle strength and flexion muscle strength using a handheld dynamometer (HHD, Mobie MT-100, Sakai Medical Co.). Each measurement was performed twice in a sitting posture, and the average value was calculated. The measured values were divided by body weight for normalization.

To evaluate cervical spine alignment (Fig. 2), the 1: C2-7 lordosis angle, 2: C7 slope, and 3: C2-7 sagittal vertical axis were measured using plain radiography of the cervical spine from a standing side view and functional imaging (forward/backward bending)²²⁾. Measurements were performed by the same examiner. Each measurement was performed three times, and the average value was calculated. Cervical forward/backward ROM (forward/backward bending ROM) was calculated from the difference in C2-C7 lordosis angles at forward and backward bending. Left and right cervical bending and rotation ROM were measured in the sitting posture twice using a goniometer, and the average value was adopted.

Radiographic evaluation.

a.1: C2–C7 lordosis [lordosis (degree)]; defined as the angle tangential to the lower endplate of the C2 vertebra and upper endplate of the C7 vertebra. b.2: C7 sagittal slope [C7 slope (degree)], defined as the angle tangential to the lower endplate of the C7 vertebra and horizontal line; range of motion of the cervical spine. 3: C2 sagittal vertical axis [C2 SVA (mm)], defined as the distance between the C2 plumb line and the superior-posterior aspect of the C7 vertebra.

･Range of motion of the cervical spine [ROM (degree)], defined as the residual C2–C7 lordosis angle during flexion and extension.