Fig. 3

Needle insertion angle measurements and ultrasound images illustrating the optimal needle insertion angle. (a) Photo (from superior-to-inferior) showing the measurement of the upward angulation, which represents the craniocaudal angulation of the needle in the sagittal plane. An epidural needle was held along the long axis of the spine, representing the line made by the posterior border of the vertebral body. (b) The photo (from left to right) showing the measurement of the inward angulation, which represents the lateral to medial angulation of the needle in the transverse plane. An epidural needle was held horizontally, representing the horizontal plane of the line of the spinous process. (c) The paramedian sagittal oblique view illustrating a less acute angle (60°) of needle trajectory is optimal for patients with wide interlaminar space (anterior complex visible). (d) The paramedian sagittal oblique view illustrating a more acute (55°) needle trajectory is adopted for patients with closely overlapping laminae (anterior complex not visible). (e) The trigonometry formula illustrating that, as the needle trajectory becomes more acute or the skin-to-lamina depth increases, the caudal shift should be increased