Fig. 1

Steps of modified lateral decubitus position and skin markings for the needle entry point. (a) Standard left-lateral decubitus position with the shoulder and the hip being placed at the edge of the bed. To obtain an optimal paramedian sagittal oblique (PMSO) view with basically horizontal laminae, the probe was placed with a cephalad-to-caudal orientation, as indicated by the blue sector. The white dotted line represents the edge of the bed. (b) The hip was moved forward (illustrated by the white arrow) to achieve a perpendicularly (to the edge of the bed) placed probe with an optimal PMSO view, as indicated by the blue rectangle. The caudal edge of the T6/7 interlaminar space was marked on the overlying skin. (c) The back was pushed forward with a 10° anterior oblique, verified via an inclinometer tool on a smartphone. In the transverse median (TM) interlaminar view, the midline of spinous processes was marked on the skin. (d) Determination of the needle entry site, approximately 0.5–1 cm lateral to the midline and 1–1.5 cm caudal to the interlaminar space. The blue dot (C1) represents the needle entry point for patients stratified as Category I, 0.5 cm lateral to the midline and 1 cm caudal to the interlaminar space. The yellow dot (C2) represents the needle entry point for patients stratified as Category III, 1 cm lateral to the midline and 1.5 cm caudal to the interlaminar space