Background:During craniotomies using the transpetrosal-presigmoid approach,exposure of the sigmoid sinus remains an essential but hazardous step.In such procedures,accurate localization of the anterosuperior point of the transverse-sigrmoid sinus junction (ASTS) is very important for reducing surgical morbidity.This study aimed to create an accurate and practical method for identifying the ASTS.Methods:On the lateral surfaces of 40 adult skulls (19 male skulls and 21 female skulls),a rectangular coordinate system was defined to measure the x and y coordinates of two points:the ASTS and the squamosal-parietomastoid suture junction (SP).With the coordinate system,the distribution characteristics of the ASTS were statistically analyzed and the differences between the ASTS and SP were investigated.Results:For ASTS-x,significant differences were found in different sides (P =0.020);the ASTS-x in male skulls was significantly higher on the right side (P =0.017);there was no significant difference between the sides in female skulls.There were no significant differences in gender or interaction of gender and side for ASTS-x,and for ASTS-y,there were no significant differences in side,gender,or interaction of gender and side.For both sides combined,the mean ASTS-x was significantly higher than the mean SP-x (P =0.003) and the mean ASTS-y was significantly higher than the mean SP-y (P =0.011).Conclusions:This reference coordinate system may be an accurate and practical method for identifying the ASTS during presigmoid craniotomy.The SP might be difficult to find during presigmoid craniotomy and,therefore,it is not always a reliable landmark for defining the ASTS.