Selection of the apposite vacuum extractor during operative delivery: A biomechanical study

Background: Operative delivery is a technique used during vaginal or cesarean birth to facilitate the patient's labor course through the assistance of a vacuum extractor. This method is increasingly used compared with forceps. This study aimed to investigate the forced effects of vacuum extractors comprising vacuum cups with different thicknesses on the fetal head and the vacuum extractor during vacuum-assisted delivery and to determine the optimal thickness for reducing the failure rate and minimizing neonatal and maternal morbidity.

Methods: A biomechanical model was developed to examine the impact of vacuum cups with varying thicknesses. This simulation three-dimensional (3D) geometry model was used to evaluate hemispherical-shaped vacuum extractors made of silicone rubber having a similar cup diameter of 70 mm with varying thicknesses (1 mm to 5 mm), which were applied to the three models (flat surfaces, hemispherical balls, and fetal head). Under one boundary condition and two different loading conditions, finite element analysis was utilized to simulate the force of vacuum extractors on the fetal head during the process of operative delivery. The main observation indicators were the reaction forces of the constructed model, and von Mises stress on both the vacuum extractors and fetal head.

Results: For the reaction forces on each axis, we found that the sum of the reaction force values on each axis was increased as the thickness of the vacuum extractor was increased, regardless of the surface type. Additionally, the reaction force of the fixed-support end was increased with the increased thickness of the vacuum extractor. The von Mises stress distributions of vacuum extractors comprising vacuum cups with different thicknesses, revealed that the thinner the cup, the greater the von Mises stress exerted on the extractor itself regardless of the surface type. The distribution of von Mises stress on the skull structure of the fetal head, showed that the thinner the cup, the greater the von Mises stress exerted on the skull structure regardless of the surface type.

Conclusion: A thinner vacuum extractor cup may result in greater injury to the fetus; hence, a thicker vacuum extractor cup is preferably utilized during vacuum-assisted operative delivery. Using a thicker vacuum extractor should yield a higher successful delivery rate and reduce fetal injury.