Fig. 2. Trigonometric procedure for needle proximity assessment. Depiction of the determination of the proximity (P^t) of any given point (AP) to the lumbar plexus from measured (black circles) and computed (white marks) coordinates to the level L^4/5, posterior superior iliac spine (PSIS), and nerve crossing points (NCP). Details on trigonometric model development can be found in Supplemental Digital Content 1,  https://links.lww.com/ALN/A536. L³–L⁵= spinous processes of the respective lumbar vertebrae.