Background. The purpose of this study was to analyze the position of the conus medullaris and Tuffier's line in the same patient population, to correlate this position with age and sex, and to determine an objective guide for the selection of a safe intervertebral space during spinal block. Methods. Magnetic resonance imaging studies of the lumbar spine were reviewed in 690 consecutive patients. The study population consisted of patients older than 20 yr who had been referred for imaging to assess possible causes of low back pain. The position of the corms medullaris was defined as the most distal point of the cord that could be visualized on the sagittal sequence. A line perpendicular to the long axis of the cord was extended to the adjacent vertebra. in the lumbar plain films, the Tuffier's line was defined by drawing a horizontal fine across the highest points of the iliac crests. Each vertebral body and intervertebral space was divided into four segments: upper, middle, and lower thirds of a vertebral body, and the intervertebral space. Each segment of a vertebral body or intervertebral space that the lines crossed was identified and recorded. The positions, stratified by decade of age, were compared using analysis of variance. The Tukey test was used for post hoc comparisons. Comparisons between sex were performed with the unpaired t test. Results. The conus medullaris and Tuffter's line (median [range]) were positioned at L1-lower (T12-upper-L3-upper) and L4L5 (L3L4-L5S1), respectively. The distance between the corms medullaris and Tuffier's line (mean +/- SD [range]) was 12.6 +/- 1.9 [7-18] segments, which corresponded to the height of approximately three vertebral bodies and intervertebral spaces. In no case did Tuffier's line overlap with the conus medullaris. The distance in segments between the conus medullaris and Tuffier's line was shorter with increased age (P < 0.001). The position of the conus medullaris and Tuffier's line was lower in female patients than in male patients (P < 0.001) and higher in patients with sacralization than in those with lumbarization or without transitional vertebra (P < 0.001). The in-between distances were not significantly different regardless of sex or presence of transitional vertebra. Conclusions: During spinal block, there seems to be a safety margin of 2-4 vertebral bodies and intervertebral spaces between the conus medullaris and Tuffier's line, which is consistent regardless of sex or presence of transitional vertebra. However, because the corms medullaris and Tuffier's line become closer with age and the clinical use of Tuffier's line requires palpation through subcutaneous fat, caution must be exercised regarding selection of the intervertebral space, especially in the aged and obese population.
