Fig. 3

Comparison of operating spaces. The ultrasound image shows the L2-L3 interlaminar space in the parasagittal oblique (PSO) view. The posterior complex (PC) and anterior complex (AC) are shown at the same time. The modified preprocedural ultrasound-guided technique placed the upper edge of the inferior laminar at the center of the ultrasound screen and suggested a lower needle insertion point, which provided a larger cephalad angle (∠α), and a wider operation space (S2) than the previous technique which placed the PC and AC at the center of the screen (∠α’ and S1)