Magnetic Resonance Imaging of the Cervical Spine Under-Represents Sagittal Plane Deformity in Degenerative Myelopathy Patients

Curvature angle measured using the Cobb technique on MR, on the same individual seen in Figure 1. A line was placed along the inferior aspect of C2 (solid line), and a perpendicular line (yellow dashed) was drawn. A line was placed along the inferior aspect of C7 (red line), and a perpendicular line (yellow dashed) was then drawn. The acute angle subtended between the two crossing lines is the curvature angle. In this example the measurement was 1.6 degrees of kyphosis, notably different than the measurement performed on plain radiograph.

Curvature index (Ishihara) measured on plain radiograph. A line is connected between the most posterior-inferior portion of C2 (A) and the most posterior-inferior portion of C7 (B). The distance of line (AB) was recorded. The distances between the posterior-inferior aspects of C3, C4, C5, and C6 to the orthogonal intersection of the line from C2 to C7 was calculated (d_1-4). The curvature index was calculated:

In this example the measurement was 31.1.

Curvature index (Ishihara) measured on MRI, on the same individual seen in Figure 3. A line is connected between the most posterior-inferior portion of C2 (A) and the most posterior-inferior portion of C7 (B). The distance of line (AB) was recorded. The distances between the posterior-inferior aspects of C3, C4, C5, and C6 to the orthogonal intersection of the line from C2 to C7 was calculated (d_1-4). The curvature index was calculated:

In this example the measurement was 30.9.

Bland-Altman plots of comparisons of curvature angle in lordotic (a) and kyphotic (b) patients. The average between pairs of measurements are plotted against their difference. The solid line indicates mean bias (average difference), and the dotted lines are limits of agreement (95% confidence limit lines).