Pubic Arch Angle Postural Mobility Evaluation in Different Patient's Positions for Clinical Research

Abstract

Childbirth is a dynamic process involving mutual adaptation between the maternal pelvis and the presenting fetal part. The ability of the pelvis to maintain optimal mobility during labor plays a crucial role in achieving favorable obstetric outcomes. The pubic arch angle (PAA) increases amplitude during pregnancy, showing pelvic tissue adjustment. The PAA evaluated with ultrasound in a single position predicts the risk of dystocia in labor and, consequently, anal sphincter trauma and incontinence after delivery. The hip flexion degree was found to reduce lumbar lordosis, shift the sacral promontory, affect the pubic arch angle, and increase pelvic diameter, creating more space for the fetus to descend during labor. Studies with magnetic resonance have demonstrated the modification of pelvic diameters and the PAA with maternal position change in the degree of hip joint flexion. The present technical report intends to describe the technique for evaluating the PAA amplitude change in supine, kneeling, and standing patients' different leg positions. The procedure is designed for clinical research in labor biomechanics. The supine leg positions for pubic angle measurement can vary from hyperextension, as in Walcher's position, to neutral supine position, mild hip flexion, and hyperflexion, which is the position of the McRoberts maneuver. The kneeling and standing positions mimic labor and delivery in the flexible sacrum maternal positions. The 2D ultrasound technique can assess the PAA in the clinical research setting during the obstetrical examination. The transducer transversely positioned on the perineum shows the pubic symphysis and the two symmetrical ischiopubic branches, as described in the literature. Evidence from ultrasound, magnetic resonance imaging, and computational modeling highlights the adaptability of pelvic structures influenced by hip flexion and soft tissue elasticity. Preliminary studies confirm significant positional differences in pubic arch angle and pelvic measurements, supporting the clinical relevance of assessing pelvic mobility. The proposed ultrasound-based approach for evaluating PAA measurements in various maternal positions offers a practical tool for research in labor management and predicting vaginal birth outcomes. Ongoing research aims to elucidate further the relationship between pelvic dimensions in different maternal positions, fetal progression, and obstetric outcomes, contributing to safer, more effective childbirth practices.

Keywords: childbirth; mobility limitation; obstetrics; pelvimetry; posture; pubic arch angle; ultrasound.

Figure 1. Supine leg positions for PAA measurements.

The pelvic joints are mobilized by varying the degree of hip flexion-extension, influencing the pelvic space. A: Walcher position. B: Supine position (p1). C: 45° bent leg position (p2). D: 90° bent leg position. E: Hyperflexed leg position (p3). PAA: Pubic arch angle Image credit: Marco Siccardi.

Figure 2. Standing and kneeling postures for pelvimetry evaluation.

A, B: Standing positions. C: Kneeling positions. sp1: Standing erect position; sp2: standing 90° flexed leg position; sp3: hyperflexed leg position. kp1: kneeling erect position; sp2: "all-four" position; kp3: kneeling squat position. Image was modified from [17]. The authors granted permission to use and modify the image, published under the Creative Commons Attribution License CC-BY 4.0.

Figure 3. Perineal probe position and ultrasound PAA measurement.

The optimal position of the probe on the perineum to visualize the ischio-pubic branches of the pubic arch angle is on the lower edge of the pubic symphysis at 45° tilt. The ultrasound PAA measurements of a multiparous non-pregnant supine patient demonstrate an amplitude difference of 18.2° between the hip extended and 90° hip flexed positions. A: The correct probe position and orientation on the perineum. B: Ultrasound image in the supine position (sup), PAA= 90.7°. C: Ultrasound image in the 90° flexed leg position (90), PAA= 108.9°. PAA: Pubic arch angle Image credit: Marco Siccardi.