Experimental Assessment of Traction Force and Associated Fetal Brain Deformation in Vacuum-Assisted Delivery

Abstract

Vacuum-assisted delivery (VAD) uses a vacuum cup on the fetal scalp to apply traction during uterine contractions, assisting complicated vaginal deliveries. Despite its widespread use, VAD presents a higher risk of neonatal morbidity compared to natural vaginal delivery and biomechanical evidence for safe VAD traction forces is still limited. The aim of this study is to develop and assess the feasibility of an experimental VAD testing setup, and investigate the impact of traction forces on fetal brain deformation. A patient-specific fetal head phantom was developed and subjected to experimental VAD in two testing setups: one with manual and one with automatic force application. The skull phantom was 3D printed using multi-material Polyjet technology. The brain phantom was cast in a 3D-printed mold using a composite hydrogel, and sonomicrometry crystals were used to estimate the brain deformation in three brain regions. The experimental VADs on the fetal head phantom allowed for quantifying brain strain with traction forces up to 112 N. Consistent brain crystal movements aligned with the traction force demonstrated the feasibility of the setup. The estimated brain deformations reached up to 4% and correlated significantly with traction force (p < 0.05) in regions close to the suction cup. Despite limitations such as the absence of scalp modeling and a simplified strain computation, this study provides a baseline for numerical studies and supports further research to optimize the safety of VAD procedures and develop VAD training platforms.

Keywords: Brain deformation; Experimental setup; Fetal head phantom; Traction force; Vacuum-assisted delivery.

Fig. 1

Skull phantom. a Lower part (blue) and upper part (red) of the CAD skull model, b lower and upper part of the 3D-printed skull phantom mounted by a screwing system, and c lower part of the skull phantom with the three fixed piezoelectric crystals (F1, F2, F3)

Fig. 2

Brain phantom construction. a The two parts of the brain mold, b brain mold with internally positioned brain crystals, c brain phantom, and d schematic of the brain phantom with initial crystal positions in the X–Z plane of the initial reference frame, i.e. the frontal view

Fig. 3

Experimental VAD testing setups, a manual force setup: 1 VAD handle, 2 force gage, 3 sonomicrometry system, 4 VAD cup, 5 fetal head phantom, and b automatic force setup: 1 Instron actuator, 2 Instron software, 3 sonomicrometry system, 4 VAD cup, 5 fetal head phantom, 6 vacuum pump

Fig. 4

Post-processing workflow of brain phantom deformation with gray boxes referring to the four outputs of the method presented in the results section. In Trilateration: the three spheres (red, blue, and green) are defined by their centers (yellow dots) being the positions of the fixed crystals (F1, F2, F3) and the inter-crystal distances between each fixed crystal and a brain crystal (B1, blue dot). Initial reference frame/Traction force reference frame: initial position of the brain crystals in the cortical region under the cup (blue), the ventricular region (red), and the sensorimotor cortical region (green). Traction force reference frame: the black arrow designates the manual traction force direction

Fig. 5

Schematic of the fetal head phantom, brain phantom with initial crystal positions and trajectories and directions of traction force in the initial reference frame. a Traction force direction in the manual and automatic force setup, b initial positions of the brain crystals (B1, B2, B3) for each crystal set (cortical region under the cup, ventricular region and sensorimotor cortical region) in the brain phantom (realistic scale of the markers with respect to the piezoelectric crystals), c trajectories of the selected crystal pairs at maximum manually applied traction force (cortical region under the cup: B2–B3, 106 N, ventricular region: B1–B3, 111 N, sensorimotor cortical region: B1–B3, 108 N) (realistic scale of the amplitude of the crystal trajectories with respect to the brain phantom), and d zoom on the trajectory (blue line) and initial position (yellow dot) of the brain crystal B2 in the cortical region under the cup at minimum (31 N) and maximum (106 N) manually applied traction force with traction force direction marked with the black line

Fig. 6

Traction force, crystal displacement, and corresponding strain over time in the direction of the traction force, over four repeated experiments, for a crystal pair B2–B3 of the cortical region under the cup in the manual force setup reaching a peak around 70 N and 100 N, b crystal pair B1–B3 of the ventricular region in the manual force setup reaching a peak around 70 N and 100 N, and c crystal pair B2–B3 of the cortical region under the cup in the automatic force setup reaching a peak around 70 N with the peaks of the traction forces and the strain amplitudes aligned in post-processing for each crystal set and experimental VAD testing setup. The colored straight line represents the mean and the colored area the standard deviation

Fig. 7

Sample traction force (a), strain (b), and crystal displacement curves (c) for crystal pair B2–B3 in the cortical region under the cup in the manual force setup, crystal pair B1–B3 in the ventricular region in the manual force setup, crystal pair B1–B3 in the sensorimotor cortical region in the manual force setup, and crystal pair B2–B3 in the cortical region under the cup in the automatic force setup

Fig. 8

Correlation between the force and strain peaks of each crystal set in both experimental VAD testing setups where the dots represent estimated strain and force peaks, the full line is the linear regression with the associated r and p values in the manual force setup (blue: cortical region under the cup, red: ventricular region, green: sensorimotor cortical region), the squared markers represent the strain and force peaks, and the dashed line is the linear regression in the automatic force setup with the associated r and p values (blue: cortical region under the cup)