Fig. 4

Schematic images illustrating a modified lateral decubitus position and the precise skin markings. (a-b) Geometrical representation of probe angulation and resultant inaccuracy of markings. If the ultrasound probe is angled 10° caudad and the skin-to-laminar depth is 4 cm, the location corresponding to the skin marking (Point I’) in a perpendicular direction was 0.7 cm (4 × Sin10 = 0.7) cranial to the interlaminar space (Point I). Consequently, the actual landing spot (Point II with the solid needle) was cranial to the intended spot (Point II’ with the dotted needle) and too close to the interlaminar space. If the probe had a 20° caudad angulation and the skin-to-laminar depth is 4 cm, the corresponding skin marking (Point I’) was 1.4 cm (4 × Sin20 = 1.4) cranial to the interlaminar space (Point I), rendering the landing spot crossover the target interlaminar space. (c) Schematic images illustrating that skin marking only works well when the probe angulation is the same as the initial angulation of needle advancement. (d-e) Schematic image showing the relationship between needle trajectory and positioning. The conventional paramedian approach under the standard lateral decubitus position is to introduce the needle at a lateral-to-medial angulation of approximately 15°. The medial angulation can be reduced with a 10° anterior oblique position