Illustration of the two measurement points S1 (true L6, blue) and S2 (true S1, green) in an LSTV patient with lumbarization and six lumbar vertebrae, classified as Catellvi type IIIa by the example of PI in the multiplanar reconstruction of an abdomen–pelvis CT (in this sectional view, the intersection of the yellow and red line represents the bicoxofemoral axis; schematic illustration of the femoral head).