Sagittal balance of the cervical spine: an analysis of occipitocervical and spinopelvic interdependence, with C-7 slope as a marker of cervical and spinopelvic alignment.

OBJECT: Sagittal malalignment of the cervical spine has been associated with worsened postsurgical outcomes. For better operative planning of fusion and alignment restoration, improved knowledge of ideal fusion angles and interdependences between upper and lower cervical spine alignment is needed. Because spinal and spinopelvic parameters might play a role in cervical sagittal alignment, their associations should be studied in depth.
METHODS: The authors retrospectively analyzed digital lateral standing cervical radiographs of 145 patients (34 asymptomatic, 74 symptomatic; 37 surgically treated), including full-standing radiographs obtained in 45 of these patients. Sagittal measurements were as follows: C2-7, occiput (Oc)-C2, C1-2 Cobb angles, and C-7 slope (the angle between the horizontal line and the superior endplate of C-7), as well as T4-12 and L1-S1 Cobb angles, sacral slope, pelvic incidence, and C-7 sagittal vertical axis (SVA). A correlation analysis was performed, and linear regression models were developed.
RESULTS: Statistical analyses revealed significant correlations between C2-7 and Oc-C2 (r = -0.4, p < 0.01), Oc-C2 (r = -0.3, p < 0.01), and C1-2 angle (r = -0.3, p < 0.01). C-7 slope was significantly correlated with C2-7 (r = -0.5, p < 0.01) and with Oc-C2 angle (r = 0.2, p = 0.02). Total cervical (Oc-C7) lordosis was 30.2° and did not differ significantly among asymptomatic, symptomatic, and surgically treated patients. Correlations between C2-7 and Oc-C2 alignment were stronger in asymptomatic patients (r = -0.5, p < 0.01) and surgically treated patients (r = -0.5, p < 0.01) than in symptomatic patients (r = -0.3, p = 0.01), but the between-group difference was not significant (p > 0.1). Comparing cervical and spinopelvic alignment revealed a significant correlation between sacral slope and C-7 slope (r = -0.3, p = 0.04) and C2-7 (r = 0.4, p < 0.01). The C-7 SVA correlated significantly with the C-7 slope (r = -0.4, p < 0.01). The interdependences were stronger within the occipitocervical parameters than between the cervical and remaining spinal parameters.
CONCLUSIONS: Significant correlations between the upper and lower cervical spine exist, confirming the existence of inherent compensatory mechanisms to maintain overall balance; no significant differences were found among asymptomatic, symptomatic, and surgically treated patients. The C-7 slope is a useful marker of overall sagittal alignment, acting as a link between the occipitocervical and thoracolumbar spine.