## 4 Discussion
Therefore, using stainless steel as the material of vacuum extractor does not cause permanent deformation of the vacuum extractor, while using silicone rubber as the material is prone to deformation. In addition, by observing the stress on the simulated fetal head skull structure, when silicone rubber was used as the vacuum extractor material, the stress on the fetal head skull was subjected to the attraction by the vacuum extractor. This would cause the fetal head skull structure to be greatly deformed at the center of the sphere on the vacuum extractor, so there would be relatively high stress, and the stress value would be greater with an increase in vacuum pressure. In addition, when stainless steel was used as the vacuum extractor material, because metals have relatively high strength, there was relatively high stress in the outer area of the vacuum extractor when pulling out the infant during delivery. In addition, when stainless steel was used as the material, the value of von Mises stress on the fetal head skull was larger, compared with using silicone rubber. The main reason is that for the load conditions, when the same displacement was applied and stainless steel was used as the vacuum extractor material, the structural deformation of the fetal head skull was relatively large, owing to the high strength and small deformation of stainless steel, so the stress value on the fetal head skull was relatively high when using stainless steel. Therefore, the previous literature indicates that using non-metallic vacuum extractors can reduce fetal skull injury [21]. The reason may be that using non-metallic vacuum extractors have relatively low stress on the head skull. There are some limitations in the finite element analysis. In this study, to evaluate the data on reaction force, among the boundary conditions, the fetal head neck was set as a fixed area, so such a setting would possibly cause the numerical results of reaction force in this study to be even larger than the actual situation. The model structures of vacuum extractors fabricated from two different materials evaluated herein were simulated with the same model structure shape, mainly owing to the need to evaluate the influences of different materials. To avoid the influences of different shapes, we herein used the same model for vacuum extractors fabricated from silicone rubber and stainless steel. In addition, the structure of fetal head model was simplified, and only two structures--the scalp and the skull--were established. This simplification could simply evaluate the model we want to analyze so that the research results could focus on identifying the differences in influences of the factors we want to observe and investigate. In addition, this study simplified many structures and organs within the pelvic cavity during the pregnant status, compared with other, previous computer models [22]. This simplification can not only reduce uncertain and overly complicated results caused by too many mutation parameters, but also reduce the resistance of pregnant women to the fetal head during delivery. However, the data of this study are slightly different from the actual data; such a simplification does not affect the theme trend of this study. With the help of the method of the finite element analysis, the results adopted in this study revealed that the stainless-steel vacuum extractor with a higher Young's modulus has a greater reaction force and stress on the fetal skull than the materials with a lower Young's modulus; the attractive pressure of vacuum extractor on the fetal head still has a slight upward trend as the attractiveness is raised.