Figure 2

S1 anatomic landmark analysis. The projection of the anatomical landmarks over S1 vertebral body silhouette (the notation of landmarks are the same as in the source article [17]). r, vertebral body transverse diameters; v, transverse diameter of the spinal canal at the superior aperture of the sacrum; p, distance between the S1 facet joints; o, width of the S1 facet joint; n, height of the S1 facet joint; h, distance between the first posterior sacral foramen and superior border of the sacrum (posterior pedicle height); s, height of the S1 vertebral body.