Comparison of the skin-to-epidural space distance at the thoracic and lumbar levels in children using magnetic resonance imaging

BACKGROUND: Several studies have attempted to estimate the approximate distance from the skin-to-epidural space using different imaging modalities (computed tomography [CT], ultrasound, and magnetic resonance imaging [MRI]) and direct needle measurements. The objective of our study was to compare the distance from the skin to the epidural space (SED) at multiple levels, focusing on T(6-7), T(9-10), and L(2-3) using MRI. METHODS: After institutional review board (IRB) approval, sagittal T2-weighted MRI images of the spine of 108 children in the age group ranging from 3 months to 8 years undergoing radiological evaluation in the supine position at our institution were analyzed. The SED at T(6-7) and T(9-10) levels (straight and inclined) and SED at L(2-3) (straight) were determined and compared using repeated-measures ANOVA and paired t-tests with a Bonferroni correction for 10 pairwise comparisons (P < 0.005 was considered statistically significant). RESULTS: The average SED (measured straight and inclined) was 18.2 mm and 21.6 mm at T(6-7); 18.3 mm and 20.5 mm at T(9-10); and 21.8 mm (straight) at L(2-3). The repeated-measures ANOVA F-test indicated significant variability in SED (P < 0.001) among the 5 measurements obtained. At the P < 0.005 significance level, corrected for multiple comparisons, the SED (straight) at T(9-10) straight was shorter than the other measured distances. CONCLUSION: The distance from the skin to the epidural space is not constant at various vertebral levels. At the levels measured, it was greatest at the lumbar level and at least at the thoracic level of T(9-10). A single predictive formula was not applicable for calculating the approximate SED at all vertebral levels.