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Abstract

Objective

This study was performed to investigate the learning curve of ultrasound-guided percutaneous nephrolithotripsy (PCNL) for the treatment of complex renal calculi.

Methods

Seventy-two patients with complex renal stones who underwent ultrasound-guided PCNL by the same operator from November 2016 to September 2020 were retrospectively analyzed. The learning curve of PCNL for complex renal stones was analyzed using multifactorial cumulative sum (CUSUM) analysis and best-fit curves.

Results

The CUSUM best-fit curve peaked at Case 36, which represented the minimum number of cumulative cases required to cross the learning curve for this surgical procedure. Accordingly, the learning curve was divided into a learning improvement period (36 cases) and a proficiency phase (36 cases). Compared with the learning improvement period, the proficiency phase was characterized by a shorter puncture time and operation time, a smaller drop in the hemoglobin level, and a lower postoperative complication rate.

Conclusion

The learning curve of PCNL for the treatment of complex renal calculi can be divided into a learning improvement period and a proficiency phase, and the minimum cumulative number of cases is 36.

Keywords

Percutaneous nephrolithotripsy learning curve complex renal calculi cumulative sum analysis proficiency phase postoperative complication

Introduction

Kidney stones have been affecting the health of approximately 10% of adults worldwide for the past decade, and their incidence is increasing every year.¹ Complex renal calculi pose a serious challenge to urologists because of the high stone burden, comorbidities, and complications. Compared with traditional open surgery for complex kidney stones, which may lead to renal impairment, bleeding, and other unfavorable complications, percutaneous nephrolithotripsy (PCNL) has become the first-line treatment for complex stones based on its high stone clearance rate and low incidence of complications.^2,3 However, no reports to date have focused on the learning curve of PCNL for complex kidney calculi, and previous learning curves of PCNL have been analyzed according to subjective groupings, which is unscientific. To better demonstrate the progression of the operator from the learning improvement stage to the proficiency stage, we analyzed the learning curve of PCNL for the treatment of complex stones using a multifactorial cumulative sum (CUSUM) control chart^4,5 and determined the point at which the learning curve was crossed.

Materials and methods

Patients and variables

We retrospectively collected the data of 72 cases of PCNL for complex renal stones performed by the same attending surgeon in the same surgical team at our center from November 2016 to September 2020. The patients comprised 41 men and 31 women with a mean age of 53.18 ± 12.66 years (range, 21–78 years). The surgeon had completed nephrostomy and animal kidney puncture simulation exercises. Preoperatively, the attending surgeon carefully read each patient’s computed tomography (CT) scan, studied a surgical video and acquired theoretical knowledge of the surgery, and independently performed the PCNL procedure. The inclusion criteria for this retrospective study were (1) single or multiple stones with a maximum diameter of >2.5 cm, (2) cast or staghorn stones, and (3) treatment by PCNL. The exclusion criteria were (1) open or laparoscopic surgery, (2) severe spinal deformities or inability to tolerate prone positioning, (3) stone obstruction with high fever in patients requiring nephrostomy drainage, and (4) PCNL combined with other surgical procedures. Complex renal stones were defined as staghorn stones, multiple stones, cast stones, or stones of >2.5 cm in diameter.⁶

History taking, physical examination, and laboratory tests were completed in each patient preoperatively. A preoperative urologic CT scan; kidneys, ureters, and bladder (KUB) X-ray; and intravenous pyelogram were used to diagnose complex renal stones. If the preoperative urine culture was positive, 3 to 7 days of antibiotic therapy was given before PCNL. A residual kidney stone diameter of <4 mm on a postoperative KUB X-ray or urologic CT scan was considered clinically meaningless stone residue, and stone removal was defined as effective in such cases. Postoperative complications were assessed using the Clavien–Dindo complication grading system.⁷

For protection of patient privacy, we have de-identified all patient details. The study was approved by the Ethics Committee of Chaohu Hospital of Anhui Medical University (No. Quick-PJ 2023-12-12), and all patients provided written informed consent. The reporting of this study conforms to the STROBE guidelines.^8,9 Additionally, the study was performed in accordance with the Declaration of Helsinki (World Medical Association) and the International Ethical Guidelines for Biomedical Research Involving Human Subjects (Council for International Organizations of Medical Sciences).

Surgical procedures

After induction of anesthesia, the patient was placed in the lithotomy position and a 5-French ureteral stent tube was placed under ureteroscopy. The patient was then placed in the prone position, and the 11th intercostal or 12th subcostal space between the post-axillary line and the subscapular line was taken as the puncture point. An ultrasound-guided puncture was performed along the calyx dome into the renal pelvis, and a safety guidewire was placed. Using this guidewire, a fascial dilator was sequentially dilated to 20 or 24 French, and a percutaneous renal microchannel was established. A ureteroscope or nephroscope was passed through the working channel for stone extraction using laser therapy or pneumatic/ultrasonic lithotripsy. If the stone burden was large, another small channel could be created for lithotripsy as described above. Postoperative double-J tubes and nephrostomy tubes were routinely placed.

Postoperative treatment

The nephrostomy tube was removed 1 to 4 days postoperatively. A KUB X-ray or urologic CT scan was performed 5 to 7 postoperatively to identify any residual stones and confirm the location of the double-J tube, and the tube was removed on an outpatient basis 2 to 4 weeks postoperatively.

CUSUM analysis and learning curve equation fitting

We performed a multifactorial CUSUM analysis with 16 indicators to evaluate the learning curve of PCNL for complex renal stones: stone side (δ1), stone localization (δ2), body mass index (δ3), stone diameter (δ4), stone burden (δ5), degree of hydronephrosis (δ6), skin-to-stone distance (δ7), puncture duration (δ8), number of accesses (δ9), access size (δ10), access approach (δ11), operative time (δ12), drop in hemoglobin level (δ13), postoperative hospital stay (δ14), stone-free rate (δ15), and perioperative complications (δ16). The mean value of each group of data was taken as the target value, and the gap between each group of data and its target value was calculated according to the equation δ = Xi − $\bar{X}$ . This was followed by synthesizing the quantitative values of the 16 indicators for each case of surgery with reference to the following formula: Σ = δ1 + δ2 + δ3 + δ4 + δ5 + δ6 +δ7 + δ8 + δ9 + δ10 + δ11 + δ12 + δ13 + δ14 +δ15 + δ16. The quantitative CUSUM value for the first case was equal to the sum of the 16 indicators, the quantitative CUSUM value for the second case was the sum of the 16 indicators of the second case plus the CUSUM value of the first case, and so on up to the 72nd case. SPSS 25.0 software (SPSS Inc, Chicago, IL.USA) was used to create a polynomial fit to the CUSUM learning curve, and GraphPad Prism 9.0 (GraphPad Software, La Jolla, CA, USA) was used to draw learning curve scatterplots of multifactor CUSUM quantitative values. A P value of <0.05 indicated that the fitting curve was successful. The quality of the fit was judged using the fitting coefficient R²; a higher R² value indicated a better fit, with an R² value of 1 representing a perfect match. When the slope of the fitted curve changed from positive to negative, the corresponding number of surgical cases was the minimum cumulative number of cases across the learning curve. Based on this cut-off point, the learning curve was divided into two stages: the learning improvement stage and the proficiency stage.

Statistical analyses

When the data conformed to a normal distribution, continuous variables were expressed as mean ± standard deviation and analyzed using the independent-samples t-test, and count data were expressed as percentages and analyzed using Pearson’s chi-square test or Fisher’s exact test. A P value of <0.05 indicated a statistically significant difference.

Results

Seventy-two patients with complex renal stones were included in this study, and all underwent PCNL. The puncture duration, operative time, postoperative hospital stay, drop in hemoglobin level, stone-free rate, postoperative complications, and total CUSUM fitted curve are shown in Figure 1. With the accumulation of operative cases, we observed a gradual decrease in the operative time, puncture duration, postoperative hospital stay, drop in hemoglobin level, and incidence of operative complications and a slight increase in the stone-free rate. From the fitted curves of the total CUSUM analysis, we found that 36 cases was the critical node. Based on the above results, the learning and improvement stage was defined as Cases 1 to 36, and the proficiency stage was defined as Cases 37 to 72.

Figure 1.

Learning curve for percutaneous nephrolithotomy in the treatment of complex renal stones. (a) Puncture time. (b) Operation time. (c) Postoperative hospital stay. (d) Stone-free rate. (e) Drop in hemoglobin level. (f) Clavien–Dindo classification and (g) total CUSUM. CUSUM, cumulative sum analysis.

As shown in Table 1, there was no statistically significant difference between the two groups in terms of age, sex, body mass index, stone diameter, stone side, American Society of Anesthesiologists classification, stone burden, degree of hydronephrosis, stone location, skin-to-stone distance, or history of surgery. As shown in Table 2, there were statistically significant differences between the two groups in terms of the puncture time (P < 0.001), operative time (P = 0.001), intraoperative access size (P = 0.013), postoperative hospital stay (P = 0.006), perioperative complications (P = 0.020), and postoperative hemoglobin drop (P = 0.002) but no statistically significant differences in terms of the puncture site, number of accesses, or stone-free rate. The stone-free rate increased from 72.2% to 80.6% with the accumulation of operative cases.

Table 1.

Comparison of demographic characteristics between learning stage and proficiency stage.

Variable	Learning stage(n = 36)	Proficiency stage(n = 36)	P value
Age, years	52.97 ± 12.81	52.11 ± 12.18	0.771
Sex			0.096
Male	24 (66.7)	17 (47.2)
Female	12 (33.3)	19 (52.8)
BMI, kg/m²	23.74 ± 3.38	24.17 ± 3.19	0.581
Stone diameter, cm	3.44 ± 0.99	3.46 ± 0.91	0.951
Stone side			0.478
Right	18 (50.0)	21 (58.3)
Left	18 (50.0)	15 (41.7)
Stone area, cm²	8.74 ± 4.47	9.29 ± 4.50	0.601
ASA classification			0.093
ASA I	18 (50.0)	11 (30.6)
ASA II	18 (50.0)	25 (69.4)
Stone burden			0.841
One stones	9 (25.0)	7 (19.4)
Two stones	7 (19.4)	7 (19.4)
Three or more stones	20 (55.6)	22 (61.2)
Degree of hydronephrosis			0.189
Mild (<10 mm)	14 (38.9)	19 (52.8)
Moderate (10–20 mm)	17 (47.2)	16 (44.4)
Severe (>20 mm)	5 (13.9)	1 (2.8)
Stone localization			0.307
Calyx	2 (5.6)	0 (0.0)
Pelvis	17 (47.2)	14 (38.9)
Staghorn	17 (47.2)	22 (61.1)
Skin-to-stone distance, cm	6.59 ± 1.04	6.70 ± 1.04	0.651
Surgical history	6 (16.7)	4 (13.9)	0.743

Data are presented as mean ± standard deviation or n (%).

BMI, body mass index; ASA, American Society of Anesthesiologists.

Table 2.

Comparison of perioperative outcomes between learning stage and proficiency stage.

Variable	Learning stage(n = 36)	Proficiency stage(n = 36)	P value
Puncture duration, minutes	16.22 ± 7.43	6.56 ± 1.30	<0.001
Access approach			0.101
12th subcostal space	33 (91.7)	28 (77.8)
11th intercostal space	3 (8.3)	8 (22.2)
Number of accesses			0.199
One	31 (86.1)	35 (93.5)
Two	5 (13.9)	1 (6.5)
Access size			0.013
18 French	4 (11.1)	1 (2.8)
20 French	26 (72.2)	18 (50.0)
24 French	6 (16.7)	17 (47.2)
Operation time, minutes	137.86 ± 42.34	107.64 ± 25.54	0.001
Drop in hemoglobin level, g/L	14.56 ± 8.60	9.00 ± 5.17	0.002
Postoperative hospital stay, days	8.23 ± 2.41	7.00 ± 1.77	0.006
Stone-free status	26 (72.2)	29 (80.6)	0.405
Perioperative complication			0.020
Clavien–Dindo grade I/II	3 (8.3)	2 (5.6)
Clavien–Dindo grade III/IV	3 (8.3)	0 (0.0)

Data are presented as mean ± standard deviation or n (%).

Eight complications occurred among the 72 PCNL procedures (Table 2). Four patients (Patients 12, 25, 43, and 67) presented with postoperative hyperthermia and recovered after administration of antibiotics and antipyretic medications. One patient (Patient 21) recovered from postoperative hemorrhage after administration of a blood transfusion. Two patients (Patients 22 and 34) recovered from postoperative hemorrhage after administration of interventional therapy. One patient (Patient 29) recovered from postoperative urogenital sepsis after being transferred to the intensive care unit for treatment.

Discussion

Complex stones have always challenged urologists because of the high stone burden and susceptibility to co-infection. Currently, PCNL is the main surgical procedure for the treatment of complex stones.^10,11 Although previous articles have described the learning curve of PCNL for the treatment of kidney stones, real mastery of the PCNL technique should span the learning curve for treatment of complex kidney stones.

Most scholars historically adopted the grouping method to explore the learning curve, which led to a certain degree of subjectivity and lack of rigor in their studies.^12,13 CUSUM analysis is a graphical method of quality control that determines trends over time by examining a continuous series of procedures.¹⁴ Recently, increasingly more scholars have applied CUSUM analysis to the study of learning curves. This analysis can scientifically distinguish the critical points across the learning curve based on clinical data and show the progress of the operators from the learning improvement stage to the proficiency stage. However, CUSUM analysis is still not rigorous enough to analyze the learning curve and the learning process only according to the operation time. This is because the learning process is not only reflected by the operation time but also by the stone size, stone burden, body mass index, puncture duration, puncture site, access size, number of accesses, degree of hydronephrosis, postoperative hospital stay, operative blood loss, incidence of perioperative complications, and other factors. Only by combining these factors and quantifying them is it possible to obtain convincing results. Therefore, in the present study, the residual differences between the 16 indexes and their mean values were calculated in each case, and the cumulative sum of the 16 indexes was determined using CUSUM analysis; the resultant fitted curves objectively presented the overall learning process of the operators who completed PCNL for complex renal stones. The tipping point in the learning curve of this technique was found to be 36 cases, and the difficulties to be crossed in this process include accurate puncture, dilatation to establish the channel, and the stone clearance process.

The use of PCNL has dramatically improved the outcome of complex stones, and the establishment of an ideal percutaneous renal access is a crucial step.¹⁵ The optimal access should have a short distance from the skin to the calyx dome and should allow for reaching most of the target calyxes. The middle or upper calyx of the dorsal kidney is the preferred site for puncture, and ultrasound-guided puncture of the calyx dome facilitates reduction of surgical bleeding and efficient stone clearance. Both ultrasound-guided in-plane and out-of-plane punctures are effective in accessing the target renal calyces. When the ultrasound fan plane finds the target renal calyx, the ultrasound probe is fixed and the angle of needle entry is adjusted so that the tip of the needle enters the fan plane to complete the puncture; this is a process of the needle finding the plane. If a hydronephrosis-free kidney stone is encountered, the puncture needle can enter the calyx to push the stone, thus reserving space for placing the guidewire and minimizing its displacement during dilatation. If the guidewire is displaced during dilatation and the access is missed, the access to the target calyx is retrieved by careful visualization of the puncture hole or by injection of methylene blue into the ureteral stent tube. With the accumulation of operative cases, the accuracy of the puncture increases and the puncture time is shortened accordingly. Taguchi et al.¹⁶ recently reported that the duration of puncture and the median operative time for robot-assisted ultrasound-guided PCNL were 8 minutes and 112 minutes, respectively. In the present study, we found that the mean puncture duration and procedure time were significantly shorter in the proficiency phase than in the learning improvement phase (16.22 vs. 6.56 minutes (P < 0.001) and 137.86 vs. 107.64 minutes (P = 0.001), respectively).

The size of the working channel not only affects the efficiency of the surgery but also the incidence of surgical complications. Compared with a 16- to 18-French channel size, 20- to 24-French PCNL, which is currently the most frequently used, meets the requirement for rapid stone removal and helps to reduce the incidence of infection in patients with preoperative urinary tract infection.¹⁷ Laser, pneumatic ballistic, and ultrasonic lithotripsy stone systems are currently available. A laser or ultrasound system is preferred for lithotripsy of renal stones with a small burden and soft texture. For infected stones or stones with positive urine cultures, ultrasonic lithotripsy systems can help reduce the incidence of postoperative sepsis. For staghorn or cast stones, ultrasound combined with pneumatic ballistic lithotripsy can increase the efficiency of stone removal and reduce the incidence of perioperative complications. Generally speaking, the only way to improve the stone-free rate and the safety of the procedure is to be flexible in choosing different methods of lithotripsy and stone extraction during the operation.

The present study had several limitations. First, this was a retrospective analysis, and selection bias was unavoidable. Second, the present study does not reflect the real-life environment of the operators and patients; thus, the learning curve has limited reproducibility. Third, postoperative mixing of X-ray and CT examinations may have affected the reliability of the stone clearance rates. More operators, more cases, and longer follow-up periods may be needed to validate the accuracy of the results of learning curve studies.

Conclusions

Overall, as the proficiency in PCNL for complex renal stones gradually increased, several important indicators of the technique gradually stabilized. The fitted curves of the multifactorial CUSUM analysis clearly showed that after performing 36 PCNL procedures for complex renal calculi, the learning curve from the learning improvement stage to the proficiency stage, puncture duration, operative time, stone-free rate, and postoperative complication rate all tended to stabilize. The minimum cumulative number of cases to cross the PCNL learning curve may be 36.

Supplemental Material

sj-pdf-1-imr-10.1177_03000605241239026 - Supplemental material for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi

Supplemental material, sj-pdf-1-imr-10.1177_03000605241239026 for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi by Zhaoxiang Lu, Wentie Yang and Wei He in Journal of International Medical Research

Supplemental Material

sj-zip-2-imr-10.1177_03000605241239026 - Supplemental material for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi

Supplemental material, sj-zip-2-imr-10.1177_03000605241239026 for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi by Zhaoxiang Lu, Wentie Yang and Wei He in Journal of International Medical Research

Supplemental Material

sj-pdf-3-imr-10.1177_03000605241239026 - Supplemental material for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi

Supplemental material, sj-pdf-3-imr-10.1177_03000605241239026 for Learning curve of ultrasound-guided percutaneous nephrolithotripsy in the treatment of complex renal calculi by Zhaoxiang Lu, Wentie Yang and Wei He in Journal of International Medical Research

Footnotes

Acknowledgements

We are grateful to the staff of the Department of Urology, Chaohu Hospital, Anhui Medical University, Anhui Province, China.

Author contributions

Investigation: Wentie Yang, Wei He.

Methodology: Zhaoxiang Lu.

Supervision: Wei He.

Validation: Wentie Yang.

Writing – original draft: Zhaoxiang Lu, Wentie Yang.

Writing – review & editing: Zhaoxiang Lu.

The authors certify that each author participated sufficiently in this study and approved the final version of the manuscript.

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

Funding

This work was supported by the Open Subject Foundation of Anhui Provincial Key Laboratory of Genitourinary Diseases (No. 2022APKLGUDO3), the Health Research Program of Anhui Province (No. AHWJ2023A20532), and the Research Fund Program of Anhui Institute of Translational Medicine (No. 9816025201).

ORCID iD

Zhaoxiang Lu

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