Staghorn stones are complicated tasks for surgical treatment of kidney stone disease. The most serious problems are caused by complete staghorn stones when they occupy the pelvis and all groups of calices. Initially, low invasive surgery for nephrolithiasis in patients with staghorn stones included standard percutaneous nephrolithotripsy (PCNL) as a monotherapy or in combination with extracorporeal shock wave lithotripsy (ESWL) [1-4]. During the development of the PCNL technique, wide nephroscopes (≥ 26 Ch) and sheaths (≥ 28–30 Ch) were used, which was explained by the absence of thin endoscopes and laser lithotripters. An advantage of standard PCNL was the fragmentation of a stone into large pieces that were removed via a wide sheath, significantly reducing the time of surgery. However, this type of surgery was associated with various complications that included hemorrhage. The rate of hemorrhagic complications that required blood transfusion or super-selective embolization increased when multiple and broad percutaneous approaches were used [5-10].

Due to the implementation of thin nephroscopes and laser lithotripters, new modifications of PCNL were developed. The most widespread of them was mini percutaneous nephrolithotripsy (mini PCNL). Various authors compared the effectiveness of standard and mini PCNL in patients with single large kidneys [11-13]. The main advantages of low invasive modifications of PCNL compared to its standard form included a low rate of hemorrhagic complications, weaker postoperative pains, and the possibility of tubeless PCNL [12][13]. There are conflicting published data on the outcome of standard and mini PCNL in the treatment of staghorn stones [14-16]. The data on the effectiveness and lower rate of hemorrhagic complications after mini PCNL are comparatively similar, but the data on the duration of this surgery varies. This is primarily associated with the heterogeneity of the characteristics of the patients included in the study, the volume of staghorn stones, the number of percutaneous approaches, and the surgeon's experience.

The study aimed to compare the outcomes of standard and mini percutaneous nephrolithotripsy in patients with staghorn stones.

The study included PCNL outcomes in 90 patients with staghorn stones who underwent surgery at the Urological Clinic, I.I. Mechnikov North-Western State Medical University at the facilities of the Urology Division, Municipal General Hospital No. 2 and the Urology Center with Robot-Assisted Surgery, St. Petersburg Mariinsky Hospital.

There were 48 (53.3%) men and 42 (46.7%) women. The mean age of patients was 50.4 ± 14.6 (38 to 75) years old. The patients were divided into two groups. Group 1 included 58 (64.4%) patients who underwent standard PCNL, and Group 2 included 32 (35.6%) patients who underwent mini PCNL.

All patients had laboratory tests and underwent unenhanced computed tomography (CT) to evaluate the dimensions and density of a stone and to plan the number and direction of percutaneous approaches. Renal function was evaluated with dynamic scintigraphy. Patients with upper urinary tract infection received antibacterial therapy before surgery. Postoperative complications were classified by Clavien-Dindo. The efficacy of PCNL monotherapy was evaluated before discharge from the hospital with plan urinary tract radiography for radiopaque stones and native CT for radionegative stones. The surgery was considered effective when there were no residual stones or their size was ≤ 3 mm.

Standard and mini percutaneous nephrolithotripsy. In all cases, at the beginning of surgery, a cystoscopy with ureteral catheterization was performed. A percutaneous approach to the pelvicalyceal system (PCS) was carried out under ultrasonic guidance when a patient was lying in a prone position. During standard PCNL, a puncture channel was dilated to 28 or 30 Ch and an Amplatz tube (Boston Scientific Corp., San Jose, CA, USA) was introduced of the respective size. A 24 Ch nephroscope (Karl Storz SE GmbH & Co. KG, Tuttlingen, Germany) was used. Stone fragmentation was made with an ultrasonic lithotripter (Karl Storz SE GmbH & Co. KG., Tuttlingen, Germany). Stone fragments were removed with endoscopic forceps. The surgery was finished with the installation of a 16 Ch nephrostomy balloon catheter (Boston Scientific Corp., San Jose, CA, USA), which was removed two to three days after the surgery provided there were no complications, and the patient was discharged to continue outpatient therapy. During mini PCNL, the percutaneous channel was dilated to 18–20 Ch with a one-step dilator. A 15 Ch nephroscope was used (Karl Storz SE GmbH & Co. KG., Tuttlingen, Germany). Stone fragmentation was made with a laser lithotripter (Karl Storz SE GmbH & Co. KG., Tuttlingen, Germany). Stone fragments were removed through the sheath using a vacuum cleaning effect and retrograde injection of a sterile solution during removal of the nephroscope. At the end of the surgery, a stent was installed (Boston Scientific Corp., San Jose, CA, USA) and the sheath was removed under direct endoscopic guidance without nephrostomy drainage installation if there were no significant complications.

Statistical analysis. Comparative analysis was made using the software package IBMÒ SPSS Statistics 25 (SPSS: IBM Company, IBM SPSS Corp., Armonk, NY, USA). Included parameters such as operative and hospitalization time and the rate of intraoperative and postoperative complications. Operative time was measured from the time of ureteral catheter cystoscopy to installation of the nephrostomy during standard PCNL or removal of a sheath during mini PCNL. To evaluate the significance of the parameters compared, the authors used Student's t test for parametric data and Mann-Whitney test for nonparametric data. The data was significant at p < 0.05.

The characteristics of patients from both groups are presented in Table 1. It shows that the age, sex, and body mass index (BMI) ratios was identical in both groups.

The results of PCNL in both groups regarding the preoperative parameters studied are presented in Table 2. The operative time of standard PCNL in patients with staghorn stones was significantly shorter and the rate of complications was higher than in the group with mini PCNL. Thus, operative time, rate of complications, and blood transfusions in Groups 1 and 2 were 80.0 ± 20.6 and 96.5 ± 25.0 minutes; 24.1 and 15.6%; 10.3 and 3.1%, respectively.

In Group 1, PCNL was effective in 50 (86.2%) patients out of 58, and in Group 2, in 27 (84.4%) patients out of 32; residual stones were revealed in 13.8% and 15.6% of patients, respectively. Residual stones were observed more often in the middle and lower groups of calyces. During the planning of PCNL for complete staghorn stones, especially via a mono approach, it is necessary to discuss with a patient possible complications and surgery outcomes, the high risk of residual stones not only in hard to access areas but also in the calices outside the area of the expected approach. In Group 1, PCS perforation was diagnosed in 2 (3.4%) patients; in Group 2, PCS perforation was not registered. This complication developed more frequently during the percutaneous approach via the lower calyx, wherein the puncture channel was directed via this calyx into the pelvis. In the case of deep dilation, it becomes perforated. Repeated PCNL for residual stones was not performed. In Group 1, additional interventions were required in 7 (12.1%) patients, in Group 2, in 4 (12.5%) patients. After standard PCNL, ESWL was performed in four patients, flexible ureteropyeloscopy – in two patients, and rigid ureteroscopy – in one patient. One patient with residual stones refused further therapy. In the group with mini PCNL, ESWL was performed in two cases and rigid ureteroscopy, in two cases. One patient refused further therapy.

Various complications in Group 1 were observed in 14 cases (24.1%), in Group 2 — in five cases (15.6%) (Table 2). After standard PNCL, post-operative fever was observed in three patients, and after mini PCNL in two patients. The rate of blood transfusion (grade II complication) was significantly higher during standard PCNL (10.3% versus 3.1%, p < 0.05). The grade III complication after mini PCNL included a replacement of the malpositioned stent or its early removal due to poor tolerance in two patients. During standard PCNL, two patients required drainage with an internal stent for the liquidation of urine leakage via the nephrostomy fistula and one patient with hydrothorax had pleural drainage installed. In both groups, there were no Clavien-Dindo IV and V complications.

Currently, standard PCNL remains the primary method for the surgical treatment of patients with staghorn stones recommended by various urological societies [17][18]. The evolution of nephroscopes due to a decrease in diameters and the implementation of lithotripters resulted in the appearance of various modifications of PCNL. Mini PCNL is one of the most widespread modifications. Unlike these modifications, the rate of complications after standard PCNL, including ≥ grade III by Clavien-Dindo, remains high [5][6][19]. This is associated with renal parenchyma wound during the application of a wide endoscope, which enlarges during forced manipulations and formation of several approaches that can be required in patients with staghorn stones.

New mini-invasive modifications of PCNL aim to reduce traumatization of percutaneous access and the rate of complications after this surgery. At the beginning of implementation, mini PCNL was performed in children and its application in adults was limited or rejected [20]. When experience was accumulated and good outcomes were received, surgeons started to use mini PCNL to treat small 10 –20 mm renal stones [11][15][16][21]. The authors established advantages of mini PCNL compared to standard PCNL that included a low rate of hemorrhages, weaker postoperative pains, and shorter hospitalization periods. However, the main drawback of mini PCNL was the long operative time, which is explained by the impossibility of extraction of large stone fragments that can be easily removed via a 28 or 30 Ch sheath. With the accumulation of experience, specialists started to publish successful results of mini PCNL in patients with staghorn and complicated kidney stones [14-16][22].

According to Khadgi et al. (2021), the general rate of complications during standard PCNL was twice as high as during mini PCNL (24.0%/12.0%, p = 0.048), primarily because of a lower rate of hemorrhages that required blood transfusions (2.4%/12.9%, p = 0.013) [14]. The authors associate this with a larger difference around renal parenchyma wound between 18–20 Ch and 30 Ch sheaths. Other authors also highlighted this fact when comparing the results of standard and mini PCNL in the treatment of kidney stones [7][12][13]. ElSheemy et al. (2019) compared the results of 378 mini PCNL and 151 standard PCNL surgeries in patients with stones comparatively similar stones (3.77 ± 2.21/3.77 ± 2.43 cm2) [12]. Mini PCNL operative time was longer (68.6 ± 29.09/60.49 ± 11.38 min, p = 0.434) and the hospital stay was shorter (2.43 ± 1.46/4.29 ± 1.28 days). Standard PCNL had a significantly higher rate of complications (20.5%/7.9%, p < 0.001) and effectiveness (96.0%/89.9%, p = 0.022).

Another factor in favor of mini PCNL in patients with staghorn stones was the need for several approaches. During standard PCNL, multiple approaches are used in 70.0% of cases, and during mini PCNL, only in 35.0% of patients [14]. This is associated with the possibility of accessing most renal calices with a thin nephroscope through one calyx without damaging its neck. Generally, forced manipulations with a nephroscope >24 Ch can injure the calix neck and cause severe bleeding. Several approaches increase the risk of hemorrhage, the need for transfusion, and possible superselective embolization [4-6][19].

Retrospective studies showed the safety and effectiveness of mini PCNL and encouraged more surgeons to perform it for large, complicated, and staghorn stones [11][14][15]. Khadgi et al. (2021) reported that stone-free status parameters were comparable (83.0% after mini PCNL and 88.6% after PCNL, p = 0.339) [14]. The differences between both methods were in the necessity of using several tracts and sessions during standard PCNL (p > 0.001 and p = 0.003, respectively). Zhong et al. (2011) compared the results of 29 mini PCNL surgeries and 25 standard PCNL surgeries for staghorn stones [22]. In Group 1, the effectiveness of the surgery was significantly higher (89.7% / 68.0%, p = 0.049) and the rate of repeated PCNL was lower (13.8%/28.0%, p = 0.048). The main difference between the studies by Zhong et al. (2011) and Khadgi et al. (2021) was that the first group of authors used multiple approaches during all mini PCNL surgeries and Khadgi et al. (2021) – only in 35.0% of cases. In the present study, the effectiveness of standard and mini PCNL surgeries was 86.2 and 84.4%.

Earlier studies showed a longer operative time of mini PCNL in comparison with standard PCNL [13]. In the meta-analysis by Jiao et al. (2020), the operating time was established in 12 of all included studies. Five of them showed that the mini PCNL had a longer duration compared to standard methods [23]. Due to the smaller channel and the comparatively poorer visibility of the intraoperative field with two small endoscopes, for the removal of a stone, it is necessary to fragment it into smaller pieces, which increases the operative time of mini PCNL. Khadgi et al. (2021) showed that the duration of mini and standard PCNL surgery for staghorn stones did not differ significantly (90.0/99.6 min, p = 0.071) [14]. However, they used multiple approaches during standard PCNL in 70.0% of cases and during mini PCNL — only in 35.0% of cases, which influenced the operative time. Zhong et al. (2011) performed 67 percutaneous accesses in 29 patients in Group I (2.3 on average) and 28 approaches in 25 patients in Group 2 (1.1 on average) [22]. During mini PCNL, the effectiveness of surgeries was higher (89.7%/68.0%, p = 0.049), the number of complications was significantly lower (37.9%/52.0%, p = 0.300), and the average operative time was comparable (116/103 min, p = 0.052). The factors that led to a reduction in operative time during mini PCNL included a lower BMI and spinal anesthesia, which reduced the time the patient spent in a lying position after ureter catheterization. Other factors included shorter time for the formation of the narrow percutaneous approach and PCNL surgery for a staghorn stone through one tract in 65.0% of the patients. The hospitalization period after mini PCNL was also shorter (3 days versus 6 days, p < 0.001), which is consistent with publications by other authors [7][12][24]. This is the result of a lower rate of complications, a lower rate of repeated PCNL, and the absence of a nephrostomy tube after mini PCNL. In the present study, the operative time of mini PCNL for staghorn stones was significantly longer than that of standard PCNL (96.5 ± 25.0/80.0 ± 20.6).

The American Urological Association recommends PCNL with multiple approaches as monotherapy for staghorn stones [17]. Although the safety of a puncture channel formation in the PCS is well established, there is some concern about multiple approaches because of additional complications. Although numerous studies show the effectiveness of one or multiple approaches during PCNL for complicated and staghorn stones, the surgery outcomes remain controversial. Huang et al. (2021) performed 793 (88.4%) PCNL surgeries using one approach (Group 1) and 104 (11.6%) surgeries using multiple approaches (Group 2) [10]. In Group 2, there was a significant decrease in hemoglobin level (16.0 ± 12.5/11.4 ± 11.8 g/l, p < 0.001), high postoperative fever rate (19.2%/11.9%, p = 0.034) and longer operative time (110.6 ± 39.6/97.8 ± 34.5 min, p < 0.001). Risk factors for a reduction in renal function included age, positive urine culture, decreased hemoglobin level, and embolization of a damaged artery. The meta-analysis by Jiao et al. (2021) compared the results of one or multiple approaches during percutaneous treatment in patients with staghorn stones. It included the results of 10 studies with 1844 patients [25]. The authors established that PCNL with one approach had a lower clinical effectiveness compared to the immediate outcome (p = 0.42) and the three-month outcome (p = 0.74) after PCNL with multiple approaches. However, the use of one approach was characterized by an insignificant decrease in the hemoglobin level (p < 0.001), a lower rate of blood transfusions (p < 0.001), and pulmonary complications (p = 0.02).

The meta-analysis by Jiao et al. did not reveal a significant difference in the stone-free status of standard and mini PCNL, indicating the effectiveness of mini PCNL in the treatment of renal stones [26]. At the same time, the results obtained by other authors demonstrated certain differences in the effectiveness of standard and mini PCNL [23][26]. However, the results of each study could differ due to various factors. First, there is no precise definition of stone-free status. Most often, it is defined as the presence of 0 – 4 mm residual stones. Second, the effectiveness is evaluated using various visualization methods (plan radiography, renal ultrasound investigation, or native CT). The evaluation of stone-free status with CT is the most precise because it provides better detection of small residual stones. Third, the diagnostic time for residual stones is significant (right after surgery or 1.0 to 1.5 months after, when some of them can spontaneously evacuate).

Apart from the retrospective character, the drawbacks of the present study included a small sample and heterogeneity of their characteristics in the compared groups. Often, they differed by age, BMI, stone dimensions, and other parameters. Furthermore, native CT was not always used to assess the surgery efficacy. Despite a significant number of Russian publications on PCNL results for nephrolithiasis, there are no studies on a comparative analysis of standard and mini PCNL. There is still a need for a multicenter randomized controlled study to obtain a higher level of evidence on the effectiveness of mini PCNL for staghorn stones. However, comparable parameters on the safety and effectiveness of this surgery compared to the standard method demonstrate the necessity of its application in a well-selected group of patients with staghorn stones.

An effective method of surgical treatment for staghorn stones is standard PCNL, which is still characterized by a high rate of complications. An experienced endourologist with expertise in percutaneous surgery for nephrolithiasis can safely and effectively perform mini PCNL in a certain group of patients with staghorn stones. The advantages of this intervention include a lower rate of complications and shorter hospital stay.