C7 sagittal vertical axis is the determinant of the C5–C7 angle in cervical sagittal alignment

Abstract Background Context Previous studies have indicated that the T1 slope correlates with cervical lordosis. In contrast, the specific impact of the C7 sagittal vertical axis (C7SVA) on cervical lordosis remains unknown. Purpose This study aimed to investigate the specific role of C7SVA in cervical lordosis. Study Design/Setting This was a retrospective radiographic study. Patient Sample Forty-eight consecutive patients who underwent lateral standing radiography of the entire spine were retrospectively reviewed. Outcome Measures Radiographic parameters included occipito (Oc)-C7, Oc–C2, C2–C7, C2–C4, and C5–C7 angles; T1 slope; C7SVA; T1 pelvic angle (TPA); pelvic incidence; pelvic tilt; and sacral slope. Methods The radiographs of 96 consecutive patients who underwent lateral standing radiography of the entire spine in June 2015 in our hospital were retrospectively reviewed. Patients having cervical deformities, having undergone cervical fusion, and under 18 years of age were excluded. A total of 48 Asian patients (14 men and 34 women; mean age, 54.6 years) were eligible. Pathologies included scoliosis, myelopathy, thoracolumbar deformity, and spondylosis. Spearman rank correlation coefficients were used to examine correlations between the parameters. The relationship between C5–C7 lordosis and the radiographic parameters was calculated using the forward stepwise multivariate regression analysis. The authors do not have financial associations relevant to this article. Results C7SVA correlated with the Oc–C7 (r=0.42) and C2–C7 (r=0.50) angles. However, the correlation coefficient was smaller than that between the T1 slope and Oc–C7 (r=0.83) or C2–C7 (r=0.76) angles. When the C2–C7 angle was divided into C2–C4 and C5–C7 angles, C7SVA correlated with the C5–C7 (r=0.63) angle but not with the C2–C4 angle. The correlation coefficient between the C5–C7 angle and C7SVA was higher than that between the C5–C7 angle and T1 slope (r=0.53) or the C5–C7 angle and TPA (r=0.60). Using radiographic parameters and age, multiple regression analysis revealed that only C7SVA affected the C5–C7 angle. Conclusions C7SVA was the only radiographic parameter that affected the C5–C7 angle. Both T1 slope and C7SVA are key to the shape of the cervical sagittal alignment. The results of this study can be a starting point to improve our understanding of cervical sagittal alignment.