FIGURE 1.

(a) Drawing of the lower spine and the pelvis in anterior view illustrating the spatial relationships between the iliac crest and the fourth and fifth lumbar vertebrae, the linea terminalis, which is composed of the arcuate line and the margin of the pelvic brim on the ala of the first sacral vertebra (marked as “ala margin”). (b) Visualization of the categorial relationship between the iliac crest and the fourth (L4) and fifth (L5) lumbar vertebrae, namely (1) center of L4, (2) inferior surface of L4, (3) superior surface of L5, and (4) center of L5. (c) The trajectories from the arcuate line on the hipbones to the margin of the pelvic brim of the sacral alae. The pelvic inlet thereby forms a flat circular plane (2) but can also be elongated, bent upwards (1), or curtailed and bent downwards (3)