Table 3 Definition and description of sacrum indexes adopted from Kanika et al. (2011) and Shreekrishna et al. (2013)

I1S

Sacral index

Ratio of sacral basal width to sacral height (sacral mid-ventral straight length) (%)

\(\frac{{\text{P1}}}{{\text{P5}}}\)

I2S

Corporobasal index

Ratio of transverse diameter of S1 body (corpus width) to sacral basal width of S1 (%)

\(\frac{{\text{P2}}}{{\text{P1}}}\)

I3S

S1 body index

Ratio of anteroposterior (A-P) diameter to transverse diameter of S1 body (%)

\(\frac{{\text{P3}}}{{\text{P2}}}\)

I4S

Alar base-wing index

Ratio of average bilateral ala width to transverse diameter of S1 body (%)

\(\frac{{\text{P4}}}{{\text{P2}}}\)

I5S

Auricular index

Ratio of average bilateral auricular surface length to sacral basal width (%)

\(\frac{{\text{P6}}}{{\text{P1}}}\)

I6S

Alar diagonal index

Ratio of average bilateral ala width to average bilateral ala anteroposterior (A-P) length (%)

\(\frac{{\text{P4}}}{{\text{P7}}}\)