Utility of a novel and cost-effective biological model for percutaneous renal access training

Introduction

Nephrolithiasis is a highly prevalent and recurrent disease with 50% relapse rate within 10 years around the world (1,2). Currently, percutaneous nephrolithotomy (PNL) is recommended for treating kidney stones larger than 2 cm according to American Urological Association (AUA) and European Association of Urology (EAU) Guidelines (3). The most crucial aspect of PNL is to establish percutaneous renal access (PRA) (4). Approximately 90% of PRA in the United States is performed by radiologists, while the remaining 11% is conducted by urologists (5). In China, nearly all PRA procedures are performed by urologists, likely due to the preference among most Chinese urologists for ultrasonic guidance over fluoroscopy (6). Research has shown that PRA performed by urologists, as opposed to radiologists, is associated with a lower complication rate and a higher stone-free rate (7).

However, the learning curve of PRA is steep. Schilling et al. found a higher complication rate in the novice group compared to the expert group through the analysis of 49 PNL procedures performed on live patients by both experts and novices (8). Song et al. evaluated novice urologists performing 120 PNL procedures under total ultrasound guidance, finding that ultrasound screening time and mean operating time were significantly reduced at least after 15 cases (9). Based on current complicated doctor-patient relationships, there are many challenges on training residents with real patients (10). Therefore, in this study, we introduce a simple, self-assembled, self-designed and cost-effective biological model to support percutaneous renal access training under ultrasonic guidance and evaluate its feasibility and validity in training novices.

Methods

Percutaneous renal access model preparation

Freshly slaughtered porcine kidneys with a minimum length of 5 cm and ureters were obtained from a local market, costing 3 U.S. dollars per kilogram. Special precautions were taken while isolating porcine kidneys to preserve maximal ureteric length. Next, a catheter (12 F) was inserted into the ureter and it was ligated with sutures to make a watertight seal (Figure 1). Suture ligation integrity was tested by injecting normal saline through the ureteral catheter to check for any obvious leakage, and hydronephrosis was simulated through this procedure. A carrageenan solution was then made by adding 5% carrageenan to water at 80 degrees Celsius. The fully dissolved carrageenan solution was poured into a container with the previously prepared porcine kidney placed on a 30-degree trigonal mold, ensuring the ureteral catheter remained outside. After 2 hours of cooling, the carrageenan solution solidified, surrounding the kidney with sufficient elasticity to mimic perirenal tissue. The cost of carrageenan is 0.5 U.S. dollars per 100 grams. The container’s length should be 8 cm plus the kidney’s height to mimic the depth of regular renal access length, costing 1 U.S. dollar. This model provides a surface with elasticity similar to human skin and simulates hydronephrosis in actual renal calculi patients. The model can be kept under 4 ℃ and used for about one week.

Figure 1 The kidney of a freshly slaughtered porcine, inserted with 12 F catheter.

Study design

From September 2023 to November 2023, this model was utilized for percutaneous renal access training under the guidance of ultrasound (MindRay, Type DC70, China). An ultrasound unit equipped with a 3–6 MHz probe was utilized to image the porcine kidney before instillation of fluid to mimic hydronephrosis. Normal saline solution was injected through the ureteral catheter to create artificial hydronephrosis (Figure 2). The collecting system was punctured with an 18-gauge needle through one of the calyces (Figure 3A,3B). Success of the puncture was verified by fluid dripping from the needle end or by aspirating fluid. The porcine model was then evaluated through a questionnaire completed by experts.

Figure 2 Ultrasound image identifies artificial hydronephrosis.

Figure 3 Ultrasound-guided puncture. (A) External view. (B) Ultrasound image of needle puncture (yellow arrow).

The training model was assessed by 10 experienced urologists who had performed more than 100 PNL cases and by 30 novices who had no independent PNL experience. 10 experienced urologists were attendings in the department of urology, Sir Run Run Shaw Hospital. 17 novices came from residents and young fellows from department of urology, Sir Run Run Shaw Hospital and other 13 novices come from urological post graduate students from Zhejiang University. All experts performed PRA on the model once, and data were collected before completing the questionnaire. The 30 novices were evaluated performing PRA before and after model training. The model training for novices lasted about one hour for each participant. All procedures were performed using an 18 G needle and guided by ultrasound freehand. Collected data included total procedure time, ultrasound screening time, and the number of attempts required to achieve successful PRA. Success of PRA was evaluated by an observer and defined as PRA performed in the correct direction through the papilla, not the infundibulum or renal pelvis. The 10 experts who successfully performed PRA using the model completed a questionnaire with four questions about the PRA model (Table 1). This questionnaire was adapted from the instrument conceptualized by Dr. Vernez and Dr. Ali’s publication (11,12) and modified to suit the current research objectives. All 10 experts and 30 novices who attempted PRA using the model were evaluated based on objective parameters. After one-hour of training, 30 novices were tested again on the same objective parameters. Each novice performed PRA using the model, and post-training objective data were recorded.

The questionnaire evaluated the following items: overall appraisal of the ultrasound-guided puncture of the pelvicalyceal system, realism, and visualization of a calyx for puncture, realism of kidney anatomy evaluation using ultrasound imaging, and realism of tissue model feedback. It was based on a 5-point Likert scale, with 1 denoting very poor, 2 as below average, 3 as average, 4 as above average, and 5 as excellent. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the ethics committee of Sir Run Run Shaw Hospital (No. 20250614). And informed consent was taken from all the participants.

Statistical analysis

We used SPSS software (Version 16.0., SPSS Inc., Chicago, USA) statistical analysis. The paired t-test and independent t-test were used where appropriate, with a P value less than 0.05 considered statistically significant.

Results

All 10 experts and 30 novices completed the PRA with the model, and all experts evaluated the model using the questionnaire. Objective data were collected for all 10 experts and 30 novices. For the questionnaire (Table 1), regarding overall appraisal, nine experts rated the model as 4 (i.e., “above average”), while one rated it 5 (i.e., “excellent”). For the model’s realism and visualization of a calyx for puncture, six experts rated it 5 (i.e., “excellent”) and four rated it 4 (i.e., “above average”). As for the realism of kidney anatomy, seven experts rated it 5 (i.e., “excellent”) and three rated it 4 (i.e., “above average”). Regarding the realism of tissue model feedback, seven experts rated it 4 (i.e., “above average”) and three rated it 3 (i.e., “average”).

Tables 2,3 show the results from 10 experts and 30 novices. For all objective parameters, including total time, ultrasound screening time, and number of attempts, the experts’ results were significantly better than the novices’ pre-training results (P<0.001). A comparison of the novices’ pre- and post-training results revealed statistically significant improvements for all three parameters after training (P<0.001).

Discussion

The most crucial part of PNL is establishing PRA (13). While American urologists have traditionally favored fluoroscopy-guided for PRA, there’s a growing interest among many urologists in American in adopting ultrasound guidance (14). On the other hand, Chinese urologists prefer to perform PRA under ultrasonic guidance (15), which has the benefits of a lower complication rate and a higher stone-free rate. However, competency in PRA requires more than 30 cases of PNL during which complications such as significant bleeding and hydrothorax could occur (16). Thus, for novice urologists, a simulator for PRA is necessary for learning PNL and beneficial for patient safety. In 2018, the European Association of Urology Section of Urolithiasis (EULIS) Consensus Statement emphasized the need for incorporating a simulation training component into the current curriculum for novice urologists (17). A review by Antoniou et al. in 2023 indicated that training models can be categorized into low-fidelity simulators, high-fidelity simulators, artificial intelligence, and virtual reality (18). Each of these training models has its own advantages and disadvantages.

Virtual reality simulators offer an immersive experience that allows learners to complete the entire procedure with headsets or displays, some equipped with haptic systems. The Marion Surgical K181 PNL Simulator is a virtual reality surgical simulator that allows trainees to operate on a virtual patient in a virtual operating room, specifically for PNL training. Its simulation can predict calyceal access for PNL using computed tomography and determine the angle, depth, and puncture for renal access. While it excels in fluoroscopy-guided PRA training, it lacks software for ultrasonic guidance simulation (19). The PERC Mentor™ Simulator (Simbionix, Cleveland, Ohio, USA) is a hybrid simulator that comprises a mannequin connected to a computer. It can provide different patient scenarios where fluoroscopy-guided PRA is performed under real-time virtual fluoroscopy, and the mannequin provides tactile feedback when inserting the needle. While these virtual reality simulators offer comprehensive learning opportunities for PRA, they focus only on fluoroscopic guidance and cost around 60,000 U.S. dollars, making them difficult to obtain for hospitals in rural areas lacking funding.

Low-fidelity models refer to simulators that are low-cost, portable, and usually lack authenticity (20). Matsumoto et al. demonstrated a very cost-effective means of providing effective low-fidelity rigid URS training (21). When assessing the performance of final-year medical students, there was no significant difference in completing a task involving the removal of a mid-ureteric stone between those trained on this model and those randomized to a high-fidelity model, which was more than 150 times the price ($3,700). Sinha et al. described using a bottle gourd filled with pledgets soaked in radio-opaque dye for PRA, but such low-fidelity models struggle to simulate PRA training without “actual kidney” components, despite their low cost (22).

High-fidelity models can be divided into biological models, such as animal kidneys (usually porcine), and non-biological ones. Strohmaier et al. described a biological model using porcine kidneys and ureters covered with parts of the thoracic or abdominal wall of pigs (23). This model is utilized for PRA under fluoroscopic guidance and is low-cost, available for most hospitals; however, preservation can be challenging if not used immediately. Jutzi et al. modified this model by placing the entire rib cage covering porcine kidneys in a laparoscopy trainer (SITUS Box), allowing PRA under ultrasonic guidance. This improvement has proven effective in PRA training but requires a special container and faces similar preservation challenges (24). Hameed et al. reviewed articles regarding 3D-printed models in endourology and found that 3D-printed kidney models could better simulate patients’ kidney anatomy using designated software to obtain CT images beforehand. They also offer a complete training program including puncture, tract dilation, and lithotripsy. However, such non-biological models do not provide the realistic feedback of a biological kidney during puncture, and the dilation of the collecting system was constant due to their pre-made hydronephrosis structure. The cost for each model was 100–400 U.S. dollars, and the software required to obtain and recreate the collecting system, along with the 3D printer, costs 3200 U.S. dollars. While 3D-printing models are excellent for anatomical representation, they are still relatively high in cost, and learning to operate the 3D printer and software takes time (25).

Our model utilizes a porcine kidney with a ureter, maintaining its biological features. Carrageenan covers the kidney to mimic skin and muscle tissue, and a 30-degree trigonal mold simulates the kidney’s location in a prone position. This model offers four advantages. First, unlike non-biological models, our model uses a porcine kidney that closely resembles actual patients’ kidneys when performing PRA, providing novices with a more authentic experience when puncturing the kidney. Most simulators offer programs for PRA training under fluoroscopic guidance, but very few provide options for ultrasonic guidance practice. Second, the price of the model is much more affordable. The total cost for a porcine kidney, a catheter, and the carrageenan is about 20 U.S. dollars. This affordable price allows young urologists to practice ultrasound-guided PRA more easily than before. Third, using carrageenan as the kidney cover not only mimics the surrounding tissue but also acts as a preservative, keeping the model ready for use for at least seven days when stored in the refrigerator. Carrageenan has been added to processed foods since the 1950s, making it readily available compared to silicone and posing no health risks to model makers (26). Finally, our model is portable. The total weight of one model is approximately 1 kg, allowing for easy transport without the need for large metal boxes or heavy machinery, which simplifies PRA training. Summarization of different training models could be found in Table S1.

In our study, the questionnaire results from all 10 experts showed an above-average rating for the model overall. They specifically rated its realism and visualization under ultrasound as excellent, which validated the usefulness of the model. Before the training, the experts had significant less time and fewer attempts compared to novices. During the one-hour training, the novice has a better understanding of the anatomy of the kidney and more experience in the puncture under ultrasound guidance. The pre- and post-comparison of the novice showed significant improvement in their PRA skills including total time for PRA, ultrasound screening time and number of attempts, which showed our model to be a good training tool for PRA.

However, our training model has several limitations for PRA. While it is easy to make, lack of ribs is limitation for ultrasound access, which could present a challenge during actual PRA, and it also does not simulate the movement of the kidney. Ribs completely block the ultrasound waves, creating a “shadow” behind them. This acoustic shadowing obscures the kidney parenchyma and collecting system that lies directly beneath, and if the target calyx or the desired needle trajectory falls behind a rib, real-time visualization becomes impossible, making a safe and accurate puncture difficult (27). Respiratory movement leads to the movement of the kidney, and it transforms percutaneous renal access from a static anatomical challenge into a dynamic one which requires the surgeon to adapt accordingly (28). Another limitation of this model is the absence of realistic nearby organs or tissues, such as peri-renal fat or bowel. In obese patients, fat tissue can significantly reduce ultrasound image quality, and without clear identification of the bowel, particularly the colon, inadvertent puncture could occur (29). Additionally, a further drawback is the model’s lack of kidney stones within the collecting system, which could be overcome by squeezing stones through the ureter, but large staghorn stone may not succeed. This model solely simulates PRA but does not encompass the entire process, such as dilation of the tract or the use of a nephroscope. Future studies could compare the effectiveness of different models for PRA training under ultrasound.

Conclusions

Based on the questionnaire results, the novices had better performance after the training. In this study, we showed that our biological model is simple, cost-effective, and demonstrates effectiveness as a training tool for PRA. It provides excellent visibility under ultrasound, is portable, and can be widely used in training centers for novices.

Acknowledgments

We would like to thank Dr. Vernez and Dr. Ali for their original work.

Footnote

Data Sharing Statement: Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-315/dss

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-315/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-315/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the ethics committee of Sir Run Run Shaw Hospital (No. 20250614). And informed consent was taken from all the participants.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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