Study Design. Retrospective Analysis Objective. To describe normative cervical sagittal alignment and to propose a classification system to guide clinical assessment and surgical planning. Background Context. Optimizing alignment is a key goal of adult cervical deformity (ACD) surgery. The purpose of this study was to understand normative alignment utilizing an asymptomatic adult cohort and to formulate a classification system that would help identify the spinal regions contributing to the cervical deformity. Methods. 468 asymptomatic adults (18-80 y) from 5 countries (USA, France, Japan, Singapore, Tunisia) formed the Multi-Ethnic Alignment Normative Study (MEANS). The C2-C7 sagittal vertical axis (cSVA), T1 slope (T1S), and C2–C7 cervical sagittal angle (CSA; positive=kyphosis, negative=lordosis), and other sagittal parameters were measured. Linear regression was utilized to correlate the C2-C7 CSA to the T1 slope. Thresholds for the C2-C7 sagittal vertical axis and T1 slope were defined as mean+2 standard deviations. Groups were compared using ANOVA with Tukey post‑hoc test. Chi square analysis was used for categorical comparisons. Results. Mean values for C2-C7 cervical sagittal angle was -0.4° (12.7°), T1 slope was 23.0° (7.9°), C2-C7 sagittal vertical axis was 19.1 (9.8). The highest mean segmental Cobb angle was 3.2° (4.8), which was at the C4-C5 segment. The T1S-CSA mismatch was 22.6 (9.4) with an interquartile range of 9.5 - 35.7. Linear regression yielded a formula CSA=-1.1(T1S) + 24.5 (R 2 =0.45, P <.0001) which was simplified to CSA=25 - T1S. Four alignment types (1A/1B/2/4) were observed in the MEANS cohort. Type 3 alignment was absent. Types 1A, 1B, 2, and 4 alignment showed significant differences across cervical, thoracic, and global sagittal parameters. Conclusion. We define normative cervical alignment utilizing the MEANS cohort and propose a classification system to identify the spinal region driving the cervical deformity. Types 1A-B represent well-compensated alignment. Type 2 and 3 were considered to have deformities in the cervical spine and thoracolumbar spine, respectively. Type 4 spines have a combined deformity. This can help guide the surgeon to determine the appropriate region that should be addressed with surgery.
Sardar et al. (Mon,) studied this question.