PCNL has a long learning curve. Studies have shown that practitioners need to perform 115 procedures before achieving excellent skills [12]. Furthermore, the overall complication rate of PCNL is nearly 15%, and its rate of Clavien-Dindo class III or worse complications, such as interventional embolization, organ damage and sepsis is 5% [2, 13]. Improved stone clearance and reduced surgical risk are associated with the selection and establishment of percutaneous renal puncture pathways. Therefore, to fully understand the structural morphology of the renal pelvis and calyces, knowledge of the relationships between the stone, pelvis, calyces, and distribution of blood vessels, rational preoperative planning of the number and sequence of puncture channels, and accurate intraoperative nipple puncture [3] are necessary to improve the stone clearance rate and reduce complications associated with complex renal stones. Researchers have attempted various methods to achieve this goal. Laser-guided puncture [14], visual puncture [15], flexible ureteroscopy-assisted puncture [16], iPad-based system tracking (Apple Inc., Cupertino, CA, USA) [17], electromagnetic tracking [18], robotic-assisted systems, and other assistive systems [19–21] have improved the efficiency and accuracy of puncture and correspondingly reduced the occurrence of postoperative complications. Augmented reality, machine learning models, artificial intelligence, and virtual reality technologies have also been used to improve simulation training, develop predictive models, and improve stone clearance, thus providing great advantages when treating complex renal stones [22, 23]. However, because these options are complex and expensive, their wide promotion is difficult.
Reportedly, 3D printing uses computer software to convert a two-dimensional image to a specific material [24]. With the maturation of 3D reconstruction and fusion technologies, 3D engineering has received increasing attention from a growing number of researchers in the medical field [25, 26], and it has been used to assist in preoperative surgical planning and surgical simulation because of its ability to accurately print organs. The use of 3D reconstruction for complex renal tumors improves the understanding of the vascular anatomy, the surgical planning, and the prediction of surgical difficulty [27]. Conventional two-dimensional information, such as that obtained with CT and ultrasound, does not provide complete anatomical images of tissues and local details and lacks the 3D morphological understanding of the renal pelvis and stones in relation to each other. Additionally, insufficient operator experience, poorly designed puncture channels, and inaccurate punctures of the target renal calyx are the major causes of complications [28]. 3D printing technology has been applied to PCNL for complex kidney stones or horseshoe-shaped kidney stones because it can create 3D solid models of structures such as renal calyces, stones, blood vessels, and surrounding organs. These models can help physicians more intuitively consider the anatomical characteristics of the target area and interrelationships of the adjoining tissues and provide puncture guidance because of their accuracy. A well-designed puncture angle and pathway can reduce renal parenchymal tears caused by oscillating mirrors, thereby reducing the risk of other complications such as hemorrhage and renal artery embolism associated with multichannel puncture and improving stone clearance [29 30]. This could simultaneously reduce the learning curve for novice physicians and improve patient understanding [31].
Sampaio et al. categorized the renal collecting system into two types, A and B, according to the mode of drainage of the middle renal calyces [1]. The type A collecting system (62.2%) includes two renal calyces, the upper pole and lower pole, through which the middle renal calyces drain urine. The calyces in the middle part of the kidney in a type A-I collecting system (45.0%) are subordinate to one of the major renal calyces in the upper or lower pole of the kidney, and the calyces in the middle part of the kidney in a type A-II collecting system (17.2%) are subordinate to the major renal calyces in the upper and lower poles of the kidney, respectively, with a calyx–neck crossover. The renal calyces in the kidneys that are not subordinate to the renal calyces in the upper and lower poles comprise type B collecting systems (37.8%). The type B-I (24.1%) collecting system includes the calyces in the middle of the kidney that drain urine to the renal pelvis through a separate large calyx. The type B-II (17.2%) collecting system includes a central part of the kidney that drains urine directly to the renal pelvis through two to four small calyces. Similar to the literature, this study found 18 (56.25%) type A-I, 5 (15.63%) type A-II, 6 (18.75%) type B-I, and 3 (0.09%) type B-II collecting systems in the affected kidneys in the experimental group using 3D reconstruction (total of 32 collecting systems). Additionally, we found that most of the staghorn stones in the type A-I collecting system occupied an enlarged renal pelvis and calyces, and that the necks of the middle calyces were wider, making it easier to reach all calyces by performing punctures in the upper calyx or middle and posterior calyces. The angle of oscillation of the mirror was larger, and it was possible to achieve complete removal of the stone in a single channel; furthermore, the stone removal rate was the highest in the experimental group (17/18 patients). Most of the middle calyceal necks of type A-II collecting systems are elongated, and the middle anterior calyceal stones should be considered subordinate to the large calyces of the unperforated upper or lower kidney pole. Access mirrors do not reach the middle anterior group of calyces, making it difficult to complete the procedure through a single channel; therefore, it may be necessary to perform punctures in both upper and middle calyces to safely remove the stones. In B-I collecting systems, the necks of both the middle and upper calyces are elongated, and the stone clearance is low; however, simultaneous punctures of both the upper and middle calyces can improve stone clearance. In our study, we found that the type B-II collecting system has two to four parallel calyces in the middle calyces of the kidney; therefore, it is not possible to completely clear the stone by performing a single pass of either the upper calyx or middle calyx because of the limited space in which the mirror can swing and reach. In the type B-II collecting system, three to four channels are commonly required for stone fragmentation, and the stone clearance rate is the lowest (1/3 patients). Although the B-II type of renal pelvis collecting system is less common, we believe it is the most complicated. Therefore, we believe that 3D technology incorporating the Sampaio collecting system fractal theory for PCNL can help physicians fully understand the complex anatomy of the renal pelvic collecting system and its relationship with renal stones and the angle between the renal calyces by providing them with more detailed anatomical information that allows them to reasonably design the number of puncture channels and puncture sequence, predict the stone clearance rate, formulate a reasonable surgical plan, optimize the puncture process during surgery, and improve the stone clearance rate, thus reducing the surgical difficulty caused by the diversity and complexity of the renal collecting system.
Study Limitations
This study had some limitations. First, it was a nonrandomized retrospective study performed at a single center with a limited number of cases; therefore, the statistical analysis of the data may have been biased. Our results should be confirmed by a multicenter, randomized, controlled study with a large sample.