In the modern world, ultrasound diagnostic is an integral part of the practical activities of most surgeons. Ultrasound is used not only for the diagnosis of various diseases, but also for performing invasive procedures, such as vascular puncture, tissue biopsy, and access during surgical interventions. In this article, the new model of the simulator for puncture of the renal cavity system under ultrasound control will be demonstrated in detail. Ultrasound-guided puncture is one of the basic skills for a urologist. It is used for various diseases of the genitourinary system, including percutaneous nephrostomy ― the installation of drainage into the renal cavity system in case of kidney stone disease, cancer, ureteral strictures and other conditions that disrupt the outflow of urine from the kidney [1]. At the same time, the puncture of the pelvicalyceal system is the first stage of percutaneous nephrolithotripsy [2]. Training on a live patient, from viewpoint of law and ethics, is a controversial topic. Most of the training process can and should be carried out not with respect to the patient, but the training model. According to the research, practice on simulators reduces the learning curve and helps in planning and preparing for surgery [3]. Nowadays, a urologist has an opportunity to work with an extensive range of training models, which include virtual simulators, animal simulator models, corpses simulator models, non-biological simulator-polymer models. However, each simulator has both advantages and disadvantages. Most of the models presented on the market have such disadvantages as short service life and low wear resistance [4]. Animal organs and existing simulator systems do not reproduce the detailed morphology and physical properties of human organs at the appropriate level [5]. Modern models made of polymer materials are characterized by a high cost (starting from 200 thousand rubles), require special storage conditions (are to be kept in the refrigerator), and also have a short service life (up to 6 months, and with active use-up to 7 days). The study aimed to develop a simulator model that will realistically reproduce the ultrasound picture of the kidney and its cavity system and will be available at a price and operating conditions for almost any clinic and university.

The development of the simulator model for the renal cavity system puncture was organized in several stages.

The first stage was the selection of materials for creating the model. Compositions based on gelatin have the most similar acoustic characteristics to the human body since they are a component of many organic tissues. The protein that is part of gelatin is completely denatured, which allows it to be used as a gelatinous material. Glycerin is an organic compound, the simplest representative of triatomic alcohols, a viscous transparent liquid with a sweet taste; it is a non-toxic matter. A two-component silicone was used to imitate the simulator skin. Silicones are widely used due to their special properties — from medical equipment to food packaging.

The second stage was the study of the sound speed in the composition. One of the main characteristics of the ultrasound simulator is the identity of the model echogenicity and the real object [6]. The limited number of polymers makes it difficult to obtain a high-quality response to ultrasound exposure. The most promising materials are based on animal proteins, one of which is gelatin. A cell filled with compositions based on gelatin and glycerin was made to assess the effect of the liquid on the speed of sound conduction in the compositions. After the gelatinization process was completed, the sound velocity was measured in the obtained samples.

Therefore, three samples with different gelatin and water contents were made (Table 1).

Table 1. Gelatin and water content in the samples

Sample number Content of gelatin, mass. h. Gelatin weight, g Water weight, g 1 15 33 217 2 20 42 208 3 25 50 200

It was figured out that the sound speed transmission is affected only by a continuous medium, which is a high-molecular compound (gelatin) (Fig. 1).

The authors have developed a model of a simulator for puncture of the renal cavity system under ultrasound control with realistic anatomical structures, physical and acoustic properties, as close as possible to the natural ones. The polymer base of such a model is a composition of gelatin and glycerin in an anhydrous medium. This made it possible to increase the service life and the wear resistance of the model [6]. The model material has a density similar to one of the human tissues (approximately 10 units, Shore A). The developed simulator can be used for training both students and young medical specialists at the stage of mastering puncture skills.

The created model of the puncture simulator is almost identical to the human kidney cavity system [1]. The simulator allows you to perform more than 300 punctures and has a service life of more than 1 year in the case of being stored at room temperature.

Testing of the created simulator model took place based on the Department of Urology No. 2 of the Pavlov First St. Petersburg State Medical University. Eighteen doctors were offered to fill out questionnaires based on the Likert scale to assess the fitness of the simulator for teaching the puncture skills under ultrasound control (Table 4). The results of this questionnaire are demonstrated in Fig. 8.

Table 4. Questionnaire for doctors

Questions Excellent Good Average Poor Very poor The visualization quality of kidney model during ultrasound examination 5 4 3 2 1 The visualization quality of the needle and PCS during puncture 5 4 3 2 1 The visualization quality during repeated punctures (taking into account the tracks from previous punctures) 5 4 3 2 1

Puncture of the renal cavity system is an integral part of percutaneous nephrostomy and percutaneous nephrolithotripsy. Many simulators have been developed for this specialists’ skill practicing. For example, pig kidneys covered with tissues imitating the human body tissues were previously used. Such simulators are relatively inexpensive and allow you to work out the skills necessary for percutaneous nephrolithotripsy, such as puncture and augmentation of the puncture course [5]. However, they have various disadvantages, such as short service life and the inability to perform 2 or more manipulations; at the same time, the anatomy of animal kidneys differs from the anatomy of a human organ [7][8]. Virtual reality programs have also been developed for practicing various surgical skills. In particular, PERC Mentor™ (Simbionix; Lod, Israel) is a virtual reality simulator designed specifically for training percutaneous puncture of the renal cavity system [9]. A comparative evaluation of the effectiveness of the VR simulator and practice on live pigs was carried out. The research proved that, despite the high efficiency of these methods, both options are really expensive. In the case of training on live pigs, it is the cost of medicines, veterinarian aid, vivarium presence, etc. On the other hand, it may be compared with the purchase of the PERC Mentor simulator (more than $100,000) taking into account the cost of consumables and maintenance of the simulator (Fig. 9) [10].

The simulator developed by the authors can be used for the training of young specialists. In addition, it is possible to use the simulator to assess the practical and theoretical skills of graduates within the framework of the accreditation. The use of this simulator for continuous professional development of specialists and while planning the surgical intervention in a particular patient will increase the effectiveness of the surgery and its safety.