Anatomical identification of ischial spines applicable to intrapartum transperineal ultrasound based on magnetic resonance imaging of pregnant women

Abstract

Objective

Intrapartum transperineal ultrasound is considered useful in judging fetal head descent; however, the inability to detect ischial spines on ultrasound images has been a drawback to its legitimacy. The current study aimed to determine the anatomical location of ischial spines, which can be directly applied to intrapartum transperineal ultrasound images.

Method

Based on magnetic resonance imaging (MRI) of 67 pregnant women at 33⁺² [31⁺⁶-34⁺⁰] weeks gestation (median [interquartile range: IQR]), we calculated the angle between the pubic symphysis and the midpoint of ischial spines (midline symphysis-ischial spine angle; mSIA), which is theoretically equivalent to the angle of progression at fetal head station 0 on ITU, by determining spatial coordinates of pelvic landmarks and utilizing spatial vector analysis. Furthermore, we measured symphysis-ischial spine distance (SID), defined as the distance between the vertical plane passing the lower edge of the pubic symphysis and the plane that passes the ischial spines.

Results

As a result, mSIA was 109.6° [105.1–114.0] and SID 26.4 mm [19.8–30.7] (median, [IQR]). There was no correlation between mSIA or SID and maternal characteristics, including physique.

Conclusions

We established a novel method to measure the components of the pelvic anatomy by analyzing the three-dimensional coordinates of MRI data and identified the anatomical location of ischial spines which can be applied to ultrasound images. Our results provide valuable evidence to enhance the reliability of intrapartum transperineal ultrasound in assessing fetal head descent by considering the location of ischial spines.

Keywords:

• Intrapartum transperineal ultrasound
• angle of progression
• ischial spines
• station 0
• magnetic resonance imaging

Author contributions

E.Y. and T.I. designed the research. E.Y. and S.H. analyzed MRI images and acquired the data. E.Y., T.I., and S.S. wrote the manuscript. M.I, M.T., T.S., K.S., K.K., T.N., and K.K. have made substantial contribution to study design, interpretation of data, and made critical comments on the manuscript. T.F. and Y.O. organized this research as project managers.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data generated or analyzed during this study are included in this published article.