Relevance of Cobb method in progressing sagittal plane spinal deformity

Non-traumatic, progressing sagittal plane deformities are uncommon, but can lead to neurological deficit if untreated. The currently used Cobb method in assessing sagittal spinal curves is based on measuring the tilt of the end vertebrae. This study describes a method which quantifies the apex of the sagittal curve based on the apical quality as measured by the radius of curvature. Both this and the Cobb methods are compared to determine which has relevance in determining neurological deficit. Radiographs of 36 consecutive patients diagnosed with congenital kyphosis were reviewed. Twenty-four had normal neurology and 12 had neurological deficit as a result of sagittal curve progression. Both groups of patients had their weight bearing lateral radiographs analysed to measure the sagittal curve by the usual Cobb method and the Radius of Curvature method. There was no difference for the Cobb values for negative neurology and patients with positive neurological deficit (p = 0.3). There was a difference in these two groups when the radius of curvature method was used (p < 0.0005). The Radius of Curvature method has more relevance than Cobb method in quantifying sagittal plane deformity in congenital kyphosis when assessing neurological deficit.