The needle penetrates the gluteus maximus and the piriformis in order to reach the target sacral plexus in the greater sciatic foramen between the sacral bone and the ischial bone.
Sacral plexus (yellow shadow), piriformis (triangular brown shadow), gluteus maximus (cyan asterix), surface of sacral bone (green arrows), surface of iliac bone (red arrows), needle (red), M = medial, L = lateral.
