Morphometric determinants of the sagittal dimensions of the cervical spinal canal in achondroplasia: an analysis of the reliability of the Torg ratio.

This study was undertaken to evaluate the accuracy of the Torg ratio for predicting the spinal canal stenosis in achondroplasia, by trying to establish its correlation with the corresponding effective sagittal canal diameter (ESCD) and dural sac ratio (DSR) on the MRI of the cervical spine. Lateral radiographs and the sagittal and axial MRI study of the cervical spine from C3 to C7 level were carried out in 18 asymptomatic achondroplasia subjects. A total of 90 levels were evaluated on the lateral radiographs and on the mid sagittal T2 MR images. On statistic analysis, the highest correlation coefficient for the Torg ratio compared with the ESCD was 0.74 at C6 level and the least was 0.45 at the C5 level. The highest correlation coefficient between the Torg ratio and the DSR was 0.79 at C3 level and the least was 0.42 at the C5 level. The highest coefficient of determination for the ESCD was 54% at C6 which translates into a poor correlation. The highest coefficient of determination for the DSR was 62 at C3 level, indicating a variable correlation. These results clearly show that the Torg ratio is of limited value in the assessment of the true sagittal spinal canal diameter in achondroplasia and its reliability as an indicator of severe cervical canal stenosis is questionable.