Background
Reduced lumbar lordosis may make the process of identifying the intervertebral distance easier. The primary aim of this study was to measure the L3 L4 intervertebral space in the same patients undergoing spinal anesthesia in three different sitting positions, including the classic sitting position (CSP), hamstring stretch position (HSP) and rider sitting position (RSP). The secondary aim was to compare ultrasonographic measurements of the depth of the ligamentum flavum and intrathecal space in these three defined positions.

Methods
This study is a single-blinded, prospective, randomized study. Ninety patients were included in final analysis. the patients were positioned on the operating table in three different positions to perform ultrasonographic measurements of the spinal canal. The intervertebral distance (IVD), the distance between the skin and the ligamentum flavum (DBSLF) and the intrathecal space (IS) were measured in the L3 L4 intervertebral space in three different positions.

Results
The RSP produced the largest mean distance between the spinous processes. The RSP yielded a significantly larger IVD than did the CSP (p < 0.001) and HSP (p < 0.001). The DBSP was larger in the CSP than in the HSP (p = 0.001). The DBSLF was significantly larger in the RSP than in the HSP (p = 0.009).

Conclusions
Positioning the patient in the RSP significantly increased the intervertebral distance between L3 L4 vertebrae compared to the CSP and HSP, suggesting easier performance of lumbar neuraxial block.