Development of a novel percutaneous digital flexible nephroscope: its use and application

Abstract

Background

Renal calculi are one of the most frequent diseases in urology, and percutaneous nephrolithotomy (PCNL) being the gold standard for treating renal calculi larger than 2 cm. However, traditional rigid nephroscope cannot bend, presents significant limitations during PCNL. This study aims to develop a novel digital flexible nephroscope for PCNL and verify its safety and efficacy using 3D printed models and ex vivo porcine kidney models, providing new equipment for PCNL.

Methods

Based on the determined technical parameters, the novel digital flexible nephroscope was manufactured. First, 3D-printed model and ex vivo porcine kidney models were utilized to simulate the PCNL procedures. Then, the traditional rigid nephroscope and the novel digital flexible nephroscope were utilized to simulate the PCNL procedures on 10 ex vivo porcine kidneys for comparison. We observed and recorded the renal calyces visualized and accessed by both the traditional rigid nephroscope and the novel digital flexible nephroscope.

Results

In both the 3D printing and ex vivo porcine kidney models, the novel percutaneous digital flexible nephroscope smoothly entered the renal collecting system through the percutaneous renal tract. It freely changed angles to reach most target calyces, demonstrating significant advantages over the traditional rigid nephroscope.

Conclusion

The successful development of the novel percutaneous digital flexible nephroscope allows it to be used either independently or as an adjunct in complex stone cases, providing more effective and safer surgical equipment for percutaneous nephrolithotomy.

Background

Renal calculi are one of the common diseases in urology, accounting for 86% of the total number of inpatients in urology [1]. Patients with renal calculi may experience symptoms, such as abdominal pain and fever, significantly influencing their daily lives, particularly in cases of complex renal calculi. The management of complex renal calculi poses a challenge, as achieving satisfactory outcomes with a single surgical approach proves to be difficult [2].

The percutaneous removal of renal calculi was initially reported in 1976 [3] and officially named percutaneous nephrolithotomy (PCNL) in 1981 [4]. PCNL involves establishing a percutaneous tract to access the kidney for stone removal, which is characterized by minimal invasiveness, being safe, and high efficiency. The PCNL is considered as the primary treatment for high-load renal calculi in upper urinary tract [5]. Compared with other procedures, PCNL possesses notable advantages in terms of stone clearance rate, operation time, postoperative recovery time, etc., making it the gold standard for treating renal staghorn calculi [6,7,8]. In multiple renal calculi, especially staghorn calculi, involving multiple renal calyces, the effectiveness of single-tract PCNL in treating such stones is mainly suboptimal due to the limitations in puncture access and maneuverability of the intraoperative renal endoscope. To enhance the first-stage stone clearance rate, it is essential to increase the maneuvering angle of the renal endoscope intraoperatively or establish multiple tracts. However, increasing the angles can raise the risk of injury and bleeding of renal calyx neck, while employing multiple-tract punctures may lead to an elevated risk of puncture-related complications [9].

Therefore, it is imperative to improve surgical instruments and explore new treatment approaches. While enhancing stone clearance efficiency, efforts should be made to minimize maneuvering angles of the renal endoscope intraoperatively and reduce the number of puncture tracts. Hence, to improve the first-stage surgical stone clearance rate while reducing surgical risks and associated complications, a novel digital flexible nephroscope was developed in the present study. The device can be utilized alongside the existing PCNL puncture tract to enable real-time monitoring and treatment of renal calyces beyond the reach of traditional nephroscope. When the flexible ureteroscopes were first introduced into clinical practice, two types, involving fiber-optic flexible ureteroscope and digital flexible ureteroscope, were utilized, each employing different technologies. With advances in manufacturing technology, the cost of digital flexible ureteroscope has decreased significantly, and its imaging quality and operational convenience exhibited clear advantages over the fiber-optic flexible ureteroscope. The fiber-optic flexible ureteroscope has now been gradually phased out in clinical practice. Therefore, a digital flexible nephroscope was designed and manufactured in this research. Research conducted on 3D printing model and ex vivo porcine models confirmed the effectiveness and technical features of this device. This research provided equipment, technology, and methodological support for enhancing stone clearance in subsequent clinical PCNL procedures.

Methods

Technical parameters of the novel digital flexible nephroscope

Following examination of the usage parameters of the existing digital flexible ureteroscope, the technical specifications for this novel percutaneous digital flexible nephroscope were determined. The specific technical parameters were summarized as follows: the maximum outer diameter of the insertion section was 8.4 Fr, the instrument channel diameter was 3.6 Fr, the view angle was 120°, the direct viewing angle was 0°, the depth of field was 3 ∼ 50 mm, the front end of the working tube was bendable, the bidirectional bending degree was ≥ 270°, the bending radius of the flexible part was 10 mm, and the lens was 160,000 pixels.

Manufacturing the novel percutaneous digital flexible nephroscope

The manufacturing process was summarized as follows: (1) Assembly of the distal end module: The camera module, LED lights, and instrumental tube were glued and sealed onto the head cap, and the adhesion was allowed to cure. (2) Assembly and welding of the bending section and insertion section: Steel cables with head ends were threaded through the snake bones and the corresponding wire channels in the working section. The connection between the snake bone and the working section was securely welded using laser welding. (3) Assembly of operational section components: Various components of the handle assembly were glued and secured, and the circuit board and signal wires were fixed and soldered. (4) Assembly and integration of the working section and operational section: The distal end module was threaded through the snake bones and insertion section in precise sequence. Subsequently, the head cap was carefully adhered to the connection point with the snake bones, and after the adhesive was fully cured, the working section was formed. Thereafter, the working section was securely fastened to the handle assembly, and the camera module and LED lights were soldered to the circuit board. The heat-shrink covering of the working section was applied. (5) Assembly of the bending control system: the length of the steel cables was adjusted to allow the bending section to flex up and down, reaching an angle of 275° (tolerance, -15°). The adjusted-length steel cables were secured on the wire wheel. (6) Final assembly: The instrumental channel was connected, and the endoscope image processor was utilized to check images and lighting. The bending angle of the bending section was checked using the handle shaft pushrod. After ensuring that all requirements were met, the upper and lower covers, along with the conical part of the handle, were assembled. The product assembly was completed (Fig. 1).

A company was contracted to procure raw materials, process, and assemble the novel percutaneous digital flexible nephroscope.

Establishment of a human urinary system model using the 3D printing technology

Exterior dimensions of the urinary system model were summarized as follows: length, 91 mm; width, 50 mm; and height, 35 mm. Internal structures included the ureter, renal pelvis, major calyces, minor calyces, and renal columns. An opening with a diameter of 6 mm was positioned in the middle posterior group of calyces. The Fengyi Automated Parallel Arm Delta 3D printer (Shanghai, China) was utilized, and the material employed was polylactic acid (PLA).

Establishment of an ex vivo porcine kidney model

Given the similarity in size, shape, and structure between porcine and human kidneys, intact porcine kidneys (n = 10) were purchased, ensuring retention of the renal artery, vein, ureter, and complete collecting system. Moreover, porcine kidneys were utilized to verify the research and development of the novel percutaneous digital flexible nephroscope.

Results

Simulating key surgical steps using the digital flexible nephroscope in the 3D kidney model

Manufacturing the novel digital flexible nephroscope is illustrated in Fig. 1. The 3D printing technology was employed to establish a human kidney model (Fig. 2a) and a kidney-ureter-bladder model (Fig. 2b). A tract was made from the middle posterior group of calyces using the novel digital flexible nephroscope (Fig. 2c), enabling smooth access to the renal collecting system. The fabricated digital flexible nephroscope met the design requirements. The digital flexible nephroscope allowed bending in the kidney to reach the target renal calyx area, entering from the middle posterior group. The digital flexible nephroscope was capable of flexing both upward and downward to access both the upper and lower groups of renal calyces, as well as the ureter (Fig. 2d-f).

Fig. 1

The novel percutaneous digital flexible nephroscope. a The flexible scope extends. b The flexible scope bends downward by 90 degrees. c The flexible scope bends downward by over 180 degrees. d The flexible scope bends upward by 90 degrees. e The flexible scope bends upward by over 180 degrees

Fig. 2

Using the novel digital flexible nephroscope to simulate surgical steps on a 3D printing model. a Coronary section of the kidney model. b Kidney-ureter-bladder model. c-f The novel digital percutaneous flexible nephroscope creates a route from the middle posterior calyx into the renal collecting system, flexibly bending both upward and downward to access different calyces in the upper and lower groups, as well as effectively entering the ureter

Simulating key steps of percutaneous nephroscopy using the ex vivoporcine kidney model and compare the novel digital flexible nephroscope with the traditional rigid percutaneous nephroscope

Using the ex vivo porcine kidney model, key steps of percutaneous nephroscopy were further simulated. The digital flexible nephroscope entered the renal pelvis of the porcine kidney from the upper posterior group of calyces, and the front-end bending of the digital flexible nephroscope was allowed to access the middle and lower calyces inside the kidney, as well as the minor calyces (Fig. 3a-c). Then, it was inserted into the kidney through the middle posterior group of calyces of the porcine kidney, and the front-end bending of the digital flexible nephroscope allowed access to the upper, middle, and lower calyces inside the kidney, as well as the minor calyces. Additionally, it could smoothly enter the ureter (Fig. 3d-g). Similarly, entering the kidney from the lower posterior group of calyces of the porcine kidney, the front-end bending of the digital flexible nephroscope facilitated access to the middle and upper calyces inside the kidney, as well as the minor calyces (Fig. 3h-j).

Fig. 3

Using the digital flexible nephroscope to simulate surgical steps on porcine kidney models. a-c Accessing the upper posterior calyx, the digital flexible nephroscope can bend downward to reach the middle and lower calyces. d-g Accessing the middle posterior calyx, the digital flexible nephroscope can bend upward and downward to reach the upper, middle, lower calyces and ureter. h-j Accessing the lower posterior calyx, the digital flexible nephroscope can bend upward to reach the middle and upper calyces

To authentically and comprehensively reflect the situation, as presented in clinical PCNL procedures, a complete porcine kidney model was utilized (Fig. 4a) to simulate the key steps of percutaneous nephroscopy. Firstly, under direct vision, the rigid nephroscope was advanced from the renal pelvis to locate the middle posterior calyx, then a retrograde puncture was performed to establish the percutaneous renal tract. (Fig. 4b). Subsequently, we sutured the renal pelvis. (Fig. 4c).The traditional rigid nephroscope was initially utilized to simulate the entire surgical procedure (Fig. 4d-e). The traditional rigid nephroscope allowed observation of the puncture tract, sutured renal pelvis, the middle renal calyx opening, and parts of the upper and lower calyx openings, but it could not access the openings. For making comparison, the surgical procedure was entirely re-simulated using the novel digital flexible nephroscope. The surgery utilizing the novel digital flexible nephroscope revealed the puncture tract, sutured renal pelvis, and multiple groups of renal calyx openings. The front-end bending of the flexible nephroscope could not only visualize renal calyx openings, but also access renal calyces and visualize minor calyces and the renal papillae (Fig. 4f-g).

Fig. 4

Comparison between rigid nephroscope and flexible nephroscope conducted using ex-vivo models. a Isolation of the ureter, renal artery, and renal vein. b Establishment of the percutaneous renal tract by retrograde puncture through the middle posterior calyx. c Sutured renal pelvis. d A puncture tract into the collecting system was made through the middle posterior calyx, and the traditional rigid nephroscope was inserted. e While the rigid nephroscope was manipulated, observations were made of the puncture tract, sutured renal pelvis, the middle renal calyx opening, and parts of the upper and lower calyx openings f A puncture tract was made into the collecting system through the middle posterior calyx, and the novel digital flexible nephroscope was inserted g The front end of the digital flexible nephroscope was manipulated to observe the puncture tract, sutured renal pelvis, and multiple groups of renal calyx openings. Through the accessed renal calyx openings, we could visualize minor calyces and the renal papillae

In order to improve the accuracy and authenticity of experimental data, PCNL procedures were simultaneously implemented using both traditional rigid nephroscope and the novel digital flexible nephroscope in 10 porcine kidney models. Using the traditional rigid nephroscope, after entering the renal pelvis, the openings of upper and lower calyces were visible, while only one upper calyx and two lower calyces were accessible. In contrast, after entering the renal pelvis via the novel digital flexible nephroscope, the openings of upper and lower calyces were also visible. Furthermore, except for the upper calyx of kidney 4 and the lower calyx of kidney 7, the novel digital flexible nephroscope could enter other calyces, and fully enter and observe minor calyx clearly and completely (Table 1). Consequently, it could be concluded that compared with the traditional rigid nephroscope, the novel digital flexible nephroscope possesses its unique advantages, and the use of the flexible nephroscope was allowed to access most renal calyces through a single-tract approach.

Table 1 Comparison between rigid nephroscopy and flexible nephroscopy was conducted using ex-vivo models

Discussion

Urinary tract stones represent a prevalent condition in urology, holding the top position among patients admitted to urology departments. At present, the main treatment methods for renal stones include extracorporeal shock wave lithotripsy, ureteroscopy, and PCNL [10]. For multiple and stag-horn renal stones, PCNL remains the preferred treatment [11, 12]. However, because of the extensive stone burden and involvement of multiple renal calyces, addressing all the stones with a single-tract PCNL proves challenging. A large swing angle of the nephroscope increases the risk of calyx neck injury and significant bleeding, making it challenging to clear stones in parallel calyces and reducing the one-stage clearance rate. Prior research indicated that among patients with staghorn renal stones undergoing PCNL, the initial stone clearance rate was only 56.9% [13], and 60% of residual stones after PCNL required secondary surgery [14].

In order to improve surgical outcomes, current options include: (1) multi-tracts lithotripsy, which, although it increases stone clearance rates, also markedly elevates the risk of bleeding [15]. It has been demonstrated that each additional tract increases the risk of puncture-related bleeding increases by 2.77 times [16, 17]. (2) The dual-scope approach was employed, and intraoperative positioning was adjusted to either the horse-riding stance or lateral decubitus with split legs, promoting stone treatment [18]. This method is highly suggested to female patients. While combining the stone clearance efficiency of PNL with the extensive exploration range of flexible ureteroscopy has its merits [19], However, this method requires the patient to be in a special position, such as the horse-riding stance or lateral decubitus with split legs position, requires two sets of surgeons and surgical equipment, and places high demands on the skills and coordination of both sets of surgeons. This makes the surgical steps more complex, increasing operation time, costs, and the incidence of related surgical complications.

The use of a percutaneous renal single tract in combination with flexible cystoscope was previously suggested for managing complex renal stones. Although this approach can improve the one-stage stone clearance rate, the diameter of the flexible cystoscope is extremely large. Its limited flexibility in the renal collecting system and inability to enter the ureter for inspection pose a risk of residual fragments from ureteral stones [20].

Considering the shortcomings of the current nephroscope, a novel digital flexible nephroscope was fabricated in the present study by modifying and reconfiguring the parameters of the existing ureteroscopes. When used alone or in conjunction with a single-tract percutaneous nephroscope, this novel digital flexible nephroscope can give access to most target calyces. It also avoids the limitations of the bulky flexible cystoscope in bending in the renal collecting system. Furthermore, it does not require the patient to be in a special position and only needs one set of surgeons to freely switch nephroscopes during the procedure, enabling the detection and treatment of most stones. The novel digital flexible nephroscope may reduce personnel and equipment requirements, save operation time, lower surgical risks, reduce clinical costs, and decrease the occurrence of complications.

The advent of 3D printing technology has remarkably influenced clinical urology, as it promotes the development of visual and intuitive models that simulate surgical procedures [21]. Specifically, 3D-printed kidneys provide a comprehensive and simplified approach for evaluating the renal collecting system and stones. This allows for more precise selection of PCNL puncture points and tracts and facilitates better preoperative communication with patients [22]. Additionally, this technology avoids formation of large renal vessels, ensures precise needle puncture depths, and provides preoperative tract planning for PCNL [23].

To confirm the efficacy of the novel digital flexible nephroscope, 3D printing technology was employed to successfully develop a human urinary model. The novel digital flexible nephroscope could smoothly access the upper, middle, and lower calyces of the renal collecting system and freely navigate to most target calyces and the upper segment of the ureter in the renal collecting system. To further confirm the efficacy of the proposed digital flexible nephroscope, isolated porcine kidneys were utilized. Moreover, the digital flexible nephroscope could enter the renal collecting system from the upper, middle, and lower posterior renal calyces of the porcine kidney. The proposed digital flexible nephroscope could bend its tip to reach the upper, middle, and lower calyces in the kidney and most minor calyx. It was also able to navigate into the ureter. This eliminates the need of traditional rigid nephroscope for swinging the nephroscope or creating new tracts to reach the target renal calyces, thereby enhancing surgical efficiency while reducing the occurrence of surgical complications. It was hypothesized that its clinical application could substantially enhance the stone clearance rate in a single procedure.

To observe and verify the effectiveness of the novel digital flexible nephroscope, 10 intact porcine kidneys were utilized. Notably, PCNL procedures were simultaneously carried out using both the traditional rigid nephroscope and the novel digital flexible nephroscope in kidney models. Using the traditional rigid nephroscope, after entering the renal pelvis, all renal calyx openings of upper and lower calyces were visible, while only one upper calyx and two lower calyces were accessible. In contrast, the novel digital flexible nephroscope could not only visualize all renal calyx openings, but also access all calyces except for one in each of the upper and lower groups. It could visualize and access most minor calyces, highlighting its notable advantages. This confirms the efficacy and clinical relevance of the novel digital flexible nephroscope.

The present study elucidated that the novel digital flexible nephroscope provided distinct advantages that were not found in the traditional rigid nephroscope. It could effectively detect renal stones and other lesions in the kidney and allow the use of holmium laser to treat corresponding lesions. It makes PCNL procedures safer, more efficient, significantly increases the stone clearance rate, and reduces the risks of bleeding and related complications. This further confirms the clinical value of the percutaneous digital flexible nephroscope and provides theoretical and practical support for the development of new surgical equipment and techniques in clinical practice. With further advancements in manufacturing technology, this digital flexible nephroscope maybe thinner with a smaller bending radius, allowing better access to the target renal calyces, thus enhancing its safety and effectiveness.

Nevertheless, the limitations of the present study should be pointed out, as the data obtained were based on initial experience with porcine kidney models from a single center, and human kidney validation is pending. The study primarily concentrated on the technical feasibility of the novel digital flexible nephroscope using 3D printing technology and porcine kidney models, and future clinical validation is needed. In the future research, the efficacy of this digital flexible nephroscope will be assessed in stone fragmentation and compared with clinically used traditional rigid nephroscopes regarding stone clearance rate and the occurrence of complications. Further large-scale multicenter prospective clinical trials are required to verify the cost-efficacy and safety of the proposed digital flexible nephroscope.

Conclusions

In conclusion, a novel percutaneous digital flexible nephroscope was developed and its safety and effectiveness were validated using the 3D-printed model and porcine kidneys. It is anticipated that the novel digital flexible nephroscope may provide essential support in terms of equipment, technology, and procedural methods for percutaneous nephrolithotomy in clinical practice.