Background and Objectives This study was performed to devise a method for predicting epidural depth more accurately with a variety of physical measurements not previously studied.

Methods Computed tomography was used to accurately measure the L3-4 epidural depth. The inclusion criteria were restricted to healthy men, 20-25 years of age, in order to rule out the influences of age and sex.

Results Significant correlations with depth from skin to the center of the “triangular” posterior epidural space (Sk-Ep) were found for waist circumference, waist circumference/height ratio, body mass index (BMI), weight/height ratio, weight/neck circumference ratio, and weight. The depth from the supraspinous ligament to the center of the posterior epidural space (Sl-Ep) did not correlate with any physical measurements. However, Sk-Sl, which equals Sk-Ep minus Sl-Ep (ie, the depth from the skin to the supraspinous ligament) correlated with waist circumference/height ratio, waist circumference, BMI, and weight/height ratio.

Conclusion Addition of the physical parameters such as waist circumference/neck circumference ratio or BMI results in a higher predictive value for epidural depth than use of more traditional physical parameters such as weight/height ratio and/or weight only. The value of Sl-Ep is independent of any physical parameters. Thus, the significant correlation between the physical measurements and the epidural depth seems to be due only to obesity-related factors.