In this study, we developed an evaluation scale to evaluate the correct application of the obstetric vacuum called kiwi-cup. We demonstrated that the use of simulation for the application of this instrument is pivotal in the process of learning and acquisition of technical skills. Simulation training plays an important role not only because it provides operators with the technical skills to carry out the application of the kiwi-cup correctly but also and above all because the awareness of having certain skills improves the safety of operators in doing the right things at the right time.
Much has been written in the literature in recent years about strategies to increase the accuracy and safety of OVD with specific attention to simulation  .
Although there is a general approval on the use of the simulation to improve maternal and fetal outcomes after OVD, there is not a standardized assessment scale for the application of the kiwi-cup. Beyond the successful rate, it is not possible to evaluate the technical skills of the trainee during the application of the kiwi-cup which allows the completion of the birth.
On the other hand, the successful rate does not automatically correspond to a correct application of the kiwi-cup. In clinical practice, this is important because simulation training improves maternal and fetal outcomes even if it does not increase the successful rate . The correct application of the cup limits iatrogenic damage but if the tractions are not performed correctly, it can be detached anyway. Previous papers use the successful rate as a unique and exclusive reference and even in the last large study (STROBE), the primary outcome is to evaluate failed operative vaginal birth with the first intended instrument with no consideration about the used techniques . In a recent systematic review of 30,813 articles about simulation training for operative vaginal delivery, only eight papers evaluated the effects of simulation on technical skills . Among those, only one regarded the application of kiwi-cup and it evaluated an overall "improved knowledge" .
It is, therefore, necessary to establish an assessment scale to evaluate whether the operator performs the necessary procedures and whether the improvement of the successful rate (which we also have in our study) does not depend on greater confidence of the operator with the mannequin but rather from a real acquisition of technical skills. At the same time, this evaluation scale allows us to establish exactly which technical skills are lacking and how we can improve the performance of each operator by correcting them.
For this reason, we have idealized an evaluation scale based on the indications directly exposed by Vacca .
Among these parameters, the time factor is essential. The duration of OVD is associated with adverse obstetric and neonatal outcomes . We tried to consider even this important element even if, without a doubt, in simulation training, many parameters can differ from the clinical reality: the number of pushes, maternal contractions, psychological pressure, etc. However, in our study, we demonstrated a significant reduction in the duration of operative vaginal delivery.
Also, to make the simulation more likely and to bypass the exclusively technical aspects, a specific scenario was built. Trainees, also bases on their technical skills acquired and beyond their success rates, showed to have a different attitude with the vacuum application procedure. The operator's experience therefore would not only be important to reduce maternal  and neonatal  adverse outcomes but also to modulate the decision-making skills of the operators themselves.
This issue constitutes the second original element compared to previous studies. For the first time, the concept of operator experience and, above all, the self-confidence on vacuum application is addressed as a pivotal issue in the decision-making process.
This point of view is innovative compared to literature. Even if the technical skills of each operator play an important role in carrying out the procedures and in doing it with confidence, however, the use of simulation can improve the overall performance even in less experienced operators.
After 8 weeks, the simulation training determines a statistically significant improvement on all the performance of the trainees, including the successful rate and the time taken to complete the procedure.
The choice to evaluate trainees directly at 8 weeks and not after the first training session is not causal. Many studies confirm that the importance of simulation as a type of learning is in the immediate but, above all, in the "knowledge retention" which diminishes with time especially with traditional teaching . In our work, we demonstrate a statistically significant improvement in the technical skills of operators that remains at high levels after about 2 months from the training.
However, our work presents some limitations. For the simulation session, we used a simulator called "Lucy and her Mum". This model was born with the aim of teaching and allowing to learn the correct application of the kiwi-cup, and it is the same used by Prof. Vacca in his exhibitions. In our humble opinion, it has technical characteristics that partially influence the pelvic-perineal evaluation, which is instead possible in other models. Even the same quantity of lubricant used can interfere with the correct application and the successful rate. These elements are well known for those who do simulation and can be easily overcome in a common training session. However, in our case, since we used a very accurate evaluation scale, data may only partly be affected by these small details. For this reason, it would be desirable that this evaluation scale could be validated with other types of mannequins and in a different scenario to truly develop a standardized evaluation scale beyond the system used.
In conclusion, our work supports the use of simulation for learning the correct procedures for applying kiwi-cup. Besides, it provides a significant step towards introducing specific scales of evaluation in training sessions to standardize the assessment of individual technical skills of trainees.