Relationship between cervical curvature index (Ishihara) and cervical spine angle (C2--7)

The purpose of this study was to determine the relationship between the cervical curvature index (Ishihara) and the cervical spine angle (C2--7) from both cervical lateral radiographs and a geometrical model. The cervical lateral radiographs of 295 outpatients with no cervical symptoms were reviewed to measure the cervical curvature index (Ishihara) and the cervical spine angle (C2--7). The average cervical curvature index (Ishihara) was 10.9 (SD, 15.3) and the average cervical spine angle (C2--7) was 20.3 (SD, 14.3). A highly significant correlation (0.95; 95% confidence interval [CI], 0.94--0.96) was found between the cervical curvature index (Ishihara) and the cervical spine angle (C2--7). Their linear correlation was also validated in the geometrical model. Their correlation diminished, however, in subjects with an S-shaped cervical spine. The cervical curvature index (Ishihara) and the cervical spine angle (C2--7) were interchangeable, but this interchangeability was lost in the S-shaped cervical spines.