Assessment of the relationships among the PS3L, PCL and vaginal axis
The MRI measurements are also summarized in Table S1. The length of the PS3L was longer than that of the PCL (median, 12.8 cm vs 10.1 cm; p < .001). The median angle between the PS3L and PCL (PS3L-PCL angle) was 27.0 [interquartile range (IQR), 24.0 – 30.0] degrees, ranging from 13.0 to 41.0 degrees. When stratified by age, no significant differences were observed in the lengths of both reference lines among the subgroups (p > .05), and no significant differences were observed in the PS3L-PCL angle in the subgroup analysis (p > .05) (Table S1).
The median distances from the distal, middle, and apical points of the upper two-thirds of the vagina to the PS3L were -0.5 [IQR, -0.9 - 0.0] cm, 0.0 [IQR, - 0.4 - 0.6] cm, and -0.2 [IQR, -0.9 - 0.0] cm, respectively, in the total sample (Table S1). These points showed a trend of a relatively concentrated distribution along the PS3L, and this trend was observed in all age subgroups (Fig. S1), especially in the women aged ≤ 30 years (with median distances from these three points to the PS3L of 0.0 [IQR, -0.5- 0.0] cm, 0.0 [IQR, 0.0 - 0.7] cm, and -0.2 [IQR, -0.8 - 0.0] cm) and the women aged 31 to 49 years (with distances of -0.4 [IQR, -0.7- 0.0] cm, 0.0 [IQR, -0.4 - 0.7] cm, and 0.0 [IQR, -0.8 - 0.2] cm). In the women aged ≥ 50 years, the distances from all three points to the PS3L were slightly lower than those in the two younger groups (p < .01) (Table S1, Figs. 3 and S1), with median distances of -0.8 [IQR, -1.2 - -0.5] cm, 0.0 [IQR, -0.6 - 0.3] cm, and -0.7 [IQR, -1.1 - 0.0] cm.
The median distances from the distal, middle, and apical points of the upper two-thirds of the vagina to the PCL were 0.4 [IQR, 0.0 - 0.7] cm, 2.1 [IQR, 1.7 - 2.5] cm, and 3.1 [IQR, 2.5 - 3.7] cm, respectively (Table S1). Because both the PS3L and PCL originate from the most inferior aspect of the pubic symphysis, the distal point of the upper two-thirds of the vagina was also close to the PCL, but the middle and apical points gradually shifted upward away from the PCL, and this trend was observed in all age subgroups (Fig. S1). The locations of all three points to the PCL in the women aged ≥ 50 years were also slightly lower than those in the two younger groups (p < .001), although the difference was limited to no greater than 1 cm (Table S1, Figs. 3 and S1).
The distributions of these measurements when using the PS3L as a reference line were more concentrated than those using the PCL (Fig. 3).
The median angle between the PS3L and the upper two-thirds of vaginal axis (PS3L-vaginal angle) was 0.0 [IQR, -4.0 - 7.0] degrees and ranged from -27.0 to 30.0 degrees (Table S1). The PS3L seemed to be nearly parallel to the vaginal axis, and this trend was observed in all age subgroups (Fig. S1), especially in the women aged ≤ 30 years (with a PS3L-vaginal angle of 0.0 [IQR -6.5 - 3.0] degrees) and the women aged 31 to 49 years (with an angle of 0.0 [IQR, -0.3 – 6.0] degrees). In the women aged ≥ 50 years, the PS3L-vaginal angle (mean angle, 3.2 degrees ± 9.5) was slightly larger than that in the two younger groups (p < .05), but the difference was limited (Table S1).
The median angle between the PCL and the vaginal axis (PCL-vaginal angle) was 29.0 [IQR, 23.0 - 34.0] degrees, ranged from 2.0 to 60.0 degrees and was significantly larger than the PS3L-vaginal angle (p < .001). The PCL and the vaginal axis demonstrated an acute angulation, and this trend was observed in all age subgroups (Fig. S1). In the women aged ≤ 30 years, this angle was slightly smaller than that in the two older groups (p < .001), although the difference was limited (Table S1).
The median distances of Pb’, Pc, and Pd were 2.4 [IQR, 2.1- 2.6] cm, 4.5 [IQR, 3.9- 5.3] cm, and 7.7 [IQR, 7.0- 8.5] cm and were nearly one-fifth, two-fifths, and three-fifths of the length of the PS3L, respectively (Table S1). All positions of the three marker points in the 614 patients relative to the PS3L are shown in Fig. 4.