Application of a correlation between the lumbar Torg ratio and the area of the spinal canal to predict lumbar stenosis: a study of 420 postmortem subjects

BACKGROUND: A cervical Torg ratio of 0.8 has been used as a screening tool to determine the presence of cervical spinal stenosis. However, there have been no studies done to define the Torg ratio in the lumbar spine for predicting lumbar spinal stenosis (LSS). Torg ratios have never been correlated with the actual calculated canal area as derived from anatomic specimens. The aim of this study was to provide an analysis of the utility of the lumbar Torg ratio for predicting LSS based on objective measurements of skeletal specimens. MATERIALS AND METHODS: 420 adult skeletal specimens from the Hamann Todd Collection in the Cleveland Museum of Natural History were selected. Digital calipers were used to measure the sagittal diameter (SCD), interpedicular distance, pedicle length, and vertebral body diameter. The canal area at each level was calculated using a geometric formula. A standard distribution curve for canal area and Torg ratio was created, and values that were that is less than the mean minus two standard deviations (SD) below the mean were considered stenotic. Regression analysis was performed to determine if the Torg ratio was correlated with canal area, and if a “below normal” Torg ratio was predictive of LSS. RESULTS: The Torg ratio for 2SD below the mean was defined as 0.43 at L1, 0.43 at L2, 0.41 at L3, 0.38 at L4, 0.37 at L5. Regression analysis revealed a significant association of the Torg ratio with canal area (p < 0.01). A Torg ratio that was less than the mean − 2SD predicted canal stenosis at L2, L3, L4, and L5 (p < 0.01). Using a Torg ratio of <0.5 predicted stenosis with a sensitivity of 86 % and specificity of 52 % at all lumbar levels. CONCLUSIONS: Based on the results of our study, we have defined the lower limit of the normal Torg ratio at each level. A Torg ratio of <0.5 predicts LSS and could be a useful radiological tool for LSS screening.