Device Prototype for Vaginal Delivery of Extremely Preterm Fetuses in the Breech Presentation

Abstract

Vaginal delivery is typically avoided in the extremely preterm breech population due to the concern of entrapment by the cervix of the aftercoming head. A mechanical device concept is presented to enable vaginal delivery by preventing retraction of the cervix against the fetus during delivery. The two-part device was designed to dilate the cervix, prevent prolapse of small fetal parts and maintain sufficient dilation during delivery. The two-part device was designed and manufactured with the following modules: an inflatable saline-filled cervical balloon for dilation and a cervical retractor composed of semirigid beams to stabilize the cervix and maintain adequate dilation. The device was tested using a cervical phantom designed to simulate the compressive force the cervix exerts. The cervical balloon reached a maximum dilation of 8.5 cm, after which there was leakage of saline from the balloon. While this dilation was less than the target goal of 10 cm, the leaking was attributed to prototype manufacturing defects, which could be resolved with further development. The cervical retractor was able to withstand between 1-3 kPa. Although estimates of cervical pressure values can be upward of 30 kPa, there are no in vivo measurements to formally identify the pressure values for patients in preterm labor. This device serves as a viable proof-of-concept for utilizing an inflatable balloon device to prevent cervical retraction in the setting of extremely preterm vaginal breech delivery. Further manufacturing improvements and design changes could improve the device for continued development and testing.

Fig. 1

Schematic of the cephalic position, breech position, and head entrapment with anatomy labeled

Fig. 2

Schematic of the two part device to dilate cervix and hold birth canal open. A cervical balloon dilates the cervix while a cervical retractor device with a ring balloon (uterine ring) in the uterus and a ring balloon (perineal ring) outside the vagina connected together with beams holds the birth canal open.

Fig. 3

Cervix phantom comprising of a wooden frame, eyelet screws, nylon webbing, and high extension spring. Close up of cervical balloon being tested in the phantom on bottom right.

Fig. 4

(Left) Diagram of cervix in phantom for Eq. (5). (Right) Free body diagram of the phantom strap around the cervix balloon.

Fig. 5

Redesign of cervical retractor beams to follow the curvature of the pelvis

Fig. 6

abaqus model of the cervical retractor beams with the mesh shown

Fig. 7

abaqus model of cervical retractor with locations of the pressure experienced while holding open the cervix and birth canal

Fig. 8

Von Mises stress and deformations in the finite element model. Stress is displayed in N/mm².

Fig. 9

abaqus simulation showing von Mises stress and radial expansion of the balloon when axial expansion is constrained with a central column. The simulation used an internal pressure of 30 kPa. Stress is displayed in N/mm². The balloon stretches radially because it experiences the most stress on the outer surface.

Fig. 10

Cervical balloon pre-inflation, left, and postinflation, right, showing preferential inflation in the radial direction. Initial height is 3 cm and initial diameter is 4 cm. Final height is 5 cm and final diameter is 8.5 cm.

Fig. 11

Schematic of syringe pump system. The three headed arrows represent three-way stop cocks. The single headed arrow represents a one-way valve.

Fig. 12

Testing results and analysis for cervical balloon inflation