Extracorporeal Shockwave Lithotripsy Versus Flexible Ureteroscopy for the Management of Upper Tract Urinary Stones in Children

Abstract

Objective:

To compare the efficacy and morbidity of extracorporeal shockwave lithotripsy (SWL) and flexible ureteroscopy (F-URS) for the management of upper tract urinary stones in children.

Methods:

All SWL and F-URS performed in children in a single institution between 2000 and 2014 were reviewed retrospectively. Only procedures performed to treat upper tract urinary stones (upper ureter or kidney) were included in this study. Preoperative and perioperative outcomes were compared between the SWL and F-URS groups. Univariate and multivariate logistic regression analyses were used to evaluate predictors of stone-free (SF) status.

Results:

Over the study period, 100 SWL and 46 F-URS were conducted in 69 children. The SWL and F-URS groups were comparable in terms of stone size (14.6 vs 13.2 mm, p = 0.32), but there were more multiple stones (31% vs 57%; p = 0.003) and lower pole calculi (14% vs 37%; p = 0.003) in the F-URS group. The SF rate after one procedure was almost two times higher in the F-URS group compared with the SWL group (37% vs 21%; p = 0.04) without increasing the complication rate (21.7% vs 16%; p = 0.31). Similar results were observed in the subgroup of single renal stones <20 mm (SF rates: 78.6% vs 50%; p = 0.06). In multivariate analysis, the use of F-URS vs SWL was a predictor of an SF status (odds ratio = 3.7; p = 0.02).

Conclusion:

F-URS provides a higher single-session SF rate, despite more complex urinary stones (multiple, lower pole, etc.) and without increasing morbidity.

Introduction

The incidence of urolithiasis in children has increased over the past 20 years and affects 2% of the pediatric population.¹ There is a higher rate of stone recurrence in children, which stresses the importance of preventive dietary measures and the need for efficient and low morbidity treatments.^1,2 Extracorporeal shockwave lithotripsy (SWL) is currently recommended as the first-line treatment for kidney and proximal ureteral stones <2 cm in children.³ Flexible ureteroscopy (F-URS) has emerged as an option to treat pediatric stones. Several series have shown the safety and efficacy of F-URS in pediatric patients.^4
–6 However, to date, only one study compared F-URS with SWL: Mokhless et al.⁷ prospectively evaluated 60 patients and reported a trend toward higher stone-free (SF) rates in the F-URS group compared with SWL.

Our objective was to compare the efficacy and morbidity of SWL and F-URS for the management of upper urinary tract stones in children in a large single-center series.

Patients and Methods

Study design

We retrospectively reviewed the charts of all children aged between 0 and 18 years who were treated with SWL or F-URS at our center between 2000 and 2014. Only procedures that were performed for upper tract urinary stones (upper ureter or kidney) were included. During the first era (2000–2007), F-URS was not available at our institution and SWL was used as the first-line treatment for all upper tract stones <2 cm. During the second era (2008–2014), F-URS became available and was used as the first-line treatment in selected patients: those with multiple stones, stone density >1000 HU or patients referred from other centers because of failure of one or more SWL procedures. All the other patients underwent one or two SWL as first-line therapy during this second era.

Stone burden was assessed on preoperative imaging by using the cumulative stone diameter method.⁸ All operative and perioperative reports and charts were reviewed to collect patients' characteristics, operative data, and perioperative outcomes (notably postoperative complications graded according to the Clavien-Dindo classification⁹).

The primary endpoint was the SF rate. A patient was considered to be SF in the absence of a stone fragment >4 mm on the imaging performed 6 weeks to 3 months after surgery (i.e., SF level 4 according to the recent Somani et al.¹⁰ classification). SF rates were analyzed after each procedure, since a majority of patients during the second era received both SWL and F-URS. The choice of the follow-up imaging modality (either kidney, ureter, and bladder radiograph [KUB] and ultrasonography or unenhanced CT scan) was left at the surgeon's discretion. Secondary endpoints were postoperative complications and length of hospital stay.

Extracorporeal shockwave lithotripsy and F-URS techniques

All patients had a negative preoperative urine culture and received 50 mg/kg of Cefuroxime at the beginning of the F-URS (no antibiotic prophylaxis was given in case of SWL). Patients with a positive preoperative urine culture (≥10⁴ CFU/mL) received antibiotics according to isolated bacteria for a minimum period of 48 hours before the SWL or F-URS.¹¹

SWL was mostly performed under general anesthesia or under simple sedation if the child was old enough to stay still during the procedure. SWL was performed with the Modulith^® SLK 2000 lithotripter (Storz^®). Patients were placed in the prone position. Stones were mostly spotted by fluoroscopy (ultrasound was used only in case of radiolucent stones). Power was increased gradually to a maximum of 60 W, with a 2 Hz shockwave frequency for a total of 2500 to 4000 shocks according to age and pain tolerance. All SWLs were performed by two surgeons (O.A., K.B.).

F-URS were all performed under general anesthesia. A Terumo^® “working” 0.035 inch polytetrafluoroethylene hydrophilic guidewire was systematically placed in the ureter up to the renal pelvis under fluoroscopic guidance. No second “safety” guidewire was used. We used the Olympus^® URF-P5 8.4F (distal end 5.3F) flexible ureteroscope in every case. The ureteroscope was placed up to the renal pelvis over the working guidewire under fluoroscopic guidance, and the guidewire was then removed. No access sheath was used unless the endoscope could not pass the ureteral orifice. Stones were fragmented with a 273-μm holmium: YAG laser fiber. The “hand irrigation” technique¹² was used to limit the increase of intrarenal pressure intraoperatively. A Double-J stent was placed preoperatively only in patients who had renal colic imperfectly relieved by analgesics or obstructive pyelonephritis or failure to access the upper tract with the flexible scope. A Double-J ureteral stent or ureteral catheter was left at the end of the procedure only in case of significant residual fragments to prevent renal colic. Double-J stent and ureteral catheter were removed after 1 week and before patient discharge, respectively. An experienced adult urologist (K.B.) trained two pediatric surgeons (O.A., A.A.) during the first 20 F-URS cases. Then, all cases were done by these two pediatric surgeons.

Statistical analysis

SF and complications rates were compared between F-URS and SWL groups. Further analyses were performed in the subgroup of patients with a single stone <20 mm. Means and standard deviations were reported for continuous variables, and proportions were reported for nominal variables. Comparisons between groups were performed by using the χ ² test or Fisher's exact test for discrete variables, and Student's t-test or Mann–Whitney test for continuous variables as appropriate. Statistical analyses were performed by using JMP v. 10.0 software (SAS Institute, Inc., Cary, NC). All tests were two sided, with a level of p < 0.05 considered statistically significant. Univariate and multivariate logistic regression analyses were used to evaluate predictors of SF status.

Results

Patients' characteristics

Over the study period, 100 SWL and 46 F-URS were performed in 69 children. Patients and stone characteristics are summarized in Table 1. Patients underwent an average of 2.2 procedures. In the F-URS group, 22 patients underwent primary F-URS (47.8%, with 18 of these 22 patients who had stones >1000 HU), 12 underwent F-URS after failure of one SWL (16.1%), and 12 underwent F-URS after failure of at least 2 SWL (16.1%). Children were older in the F-URS group (9.1 vs 6.7 years; p = 0.02). SWL and F-URS groups were comparable in terms of stone size (14.6 vs 13.2 mm; p = 0.32), but there were more multiple stones (31% vs 57%; p = 0.003) and lower pole calculi (14% vs 37%; p = 0.003) in the F-URS group. Seventeen patients in the F-URS group had a preoperative Double-J stent that had been placed (14 due to renal colic and 1 due to failure to access the upper tract with the scope) compared with 15 in the SWL group (37% vs 15%, respectively; p = 0.004).

Table 1.

Patients' Characteristics

	SWL n = 100	F-URS n = 46	p-Value
Age (years)	6.7 ± 0.6	9.1 ± 0.9	0.02^*
Maximum diameter of the largest stone (mm)	14.6 ± 0.8	13.2 ± 1.1	0.32
Mean number of stones per patient	1.6 ± 0.1	2.2 ± 0.1	0.004^*
Stone burden (mm)	19.5 ± 1.5	21.6 ± 2.0	0.42
General anesthesia	81 (81%)	46 (100%)	0.007^*
Multiple stones	31 (31%)	26 (57%)	0.003^*
Body mass index (kg/m²)	18.1 ± 0.7	18.3 ± 0.6	0.97
X-ray characteristics	98 (98%)	46 (100%)	0.55
Radiopaque radiolucent	2 (2%)	0 (0%)
HU stone density	1175 ± 316	898 ± 262	0.70

indicates statistically significant values.

F-URS = flexible ureteroscopy; HU = Hounsfield unit; SWL = extracorporeal shockwave lithotripsy.

General anesthesia was administered in 100% of F-URS and in 81% of SWLs (p = 0.007). Patients who underwent SWLs under local anesthesia were older than SWL patients treated under general anesthesia (mean ages: 14.1 year-old [range 9–17] vs 4.7 year old [range 0–17]; p < 0.0001). Twelve and 15 children had a positive preoperative urine culture in F-URS and SWL groups, respectively (26% vs 15%; p = 0.21). Positive preoperative urine culture had no impact on SF rates and complications rates in any of the two groups.

The preoperative imaging modalities used were as follows: KUB in 19 patients in the F-URS group (42.1%) and in 57 patients in the SWL group (57%); and CT scan in 27 patients in the F-URS group (57.9%) and in 43 patients in the SWL group (43%) (p = 0.07). The postoperative imaging modalities used were as follows: KUB in 39 patients in the F-URS group (84.9%) and in 88 patients in the SWL group (88%); and CT scan in 7 patients in the F-URS group (15.2%) and in 12 patients in the SWL group (12%) (p = 0.59).

Metabolic disorders facilitating stone formation were found in 12 and 8 patients in the SWL and F-URS groups, respectively (12% vs 17.4% respectively; p = 0.38). Stones composition did not differ significantly between both groups, the most common stone types being struvite (31.1% vs 42.5%), calcium phosphate (36.4% vs 20%), and calcium oxalate (28.6% vs 32.5%; p = 0.32). Metabolic disorders and stones composition are summarized in Table 2.

Table 2.

Stones Composition and Metabolic Disorders in Each Group

	SWL n = 100	F-URS n = 46	p-Value
Stones composition			0.32
Calcium oxalate	22 (28.6%)	13 (32.5%)
Calcium phosphate	28 (36.4%)	8 (20%)
Struvite	24 (31.1%)	17 (42.5%)
Uric acid	1 (1.3%)	1 (2.5%)
Cystine	1 (1.3%)	1 (2.5%)
2,8-Dihydroxyadenine	1 (1.3%)	0
Unknown	23	6
Metabolic disorders facilitating stone formation			0.38
Primary hyperoxaluria	2	3
Enteric hyperoxaluria	3	2
Cystinuria	1	1
2,8-Dihydroxyadeninuria	1	0
Renal tubular acidosis	3	1
Nephrocalcinosis	2	1
Unknown	88 (88%)	38 (82.6%)

Efficacy and morbidity of SWL versus F-URS

SF rate after one procedure was almost twice higher in the F-URS group compared with the SWL group (37% vs 21%; p = 0.04, see Table 3), and complication rates were similar (21.7% vs 16%; p = 0.31). Length of stay was longer in the F-URS group (1.4 vs 0.9 days; p = 0.02). There were three major complications (i.e., Clavien grade ≥3) in the F-URS group (two urinomas requiring ureteral stenting and one acute urinary retention due to urethral stone fragment) and one in the SWL group (emergency placement of a Double-J stent due to a renal colic refractory to analgesics; i.e., 7% in the F-URS group vs 1% in the SWL group; p = 0.2). Minor complications are detailed in Table 3. In the SWL group, mean number of shocks per session was 3203.1. The number of shocks was not associated with SF status (odds ratio [OR] = 1.3; p = 0.74). Mean operative time was longer in the F-URS group (105.5 vs 28.9 minutes; p < 0.001). There were difficulties in accessing the upper tract in five patients. Two were managed by using a ureteral access sheath, and two were managed by dilatating the ureteral orifice. In the last case, a Double-J stent was placed for 2 weeks before rescheduling URS. Five ureteroscopes were damaged (i.e., one every 9.2 procedures). A Double-J stent was left postoperatively after 34 of the F-URS (73.8%). After a mean of 2.2 procedures in each group (p = 0.94), the overall monotherapy SF rates were 76.1% and 60% in the F-URS and SWL groups, respectively (p = 0.06).

Table 3.

Results for the Whole Cohort

	SWL n = 100	F-URS n = 46	p-Value
SF rates after one procedure	21 (21%)	17 (37%)	0.04^*
Complications	16 (16%)	10 (22%)	0.31
Clavien grade 1	13 (10 renal colics, 1 fever resolving spontaneously, 1 hematuria, 1 discomfort due to Double-J stent)	3 (3 discomfort due to Double-J stent)
Clavien grade 2	2 (2 urinary tract infections)	4 (1 hematuria, 3 urinary tract infections)
Clavien grade 3	1 (renal colic refractory to analgesics requiring Double-J stent placement)	2 (1 urinoma, 1 acute urinary retention due to stone fragment)
Clavien grade 4	0	1 (1 urinoma)
Clavien grade 5	0	0
Major complications (Clavien ≥3)	1 (1%)	3 (7%)	0.2
Length of stay (days)	0.9 ± 0.1	1.4 ± 0.2	0.02^*

indicates statistically significant values.

SF = stone free.

Subgroup analysis: single stone <20 mm

In the subgroup of patients with a single stone <20 mm, 40 children underwent SWL and 14 were treated with F-URS (Table 4). In this subgroup, patients from the SWL group were younger (6.1 vs 10 years; p = 0.02), and had a lower rate of lower pole calculi (23% vs 57%; p = 0.02) but mean stone size was similar (12.1 vs 11.9 mm; p = 0.84). The single-session SF rate was higher in the F-URS group (78.6% vs 50%; p = 0.06), with comparable complications between both groups (35.7% vs 23.7%; p = 0.39). As observed in the whole cohort, hospital stay and operative time were longer in the F-URS group (1.6 vs 0.7 days; p = 0.02 and 28 vs 96 minutes; p = 0.001).

Table 4.

Subgroup Analysis Limited to Single Stones <20 mm

	SWL n = 40	F-URS n = 14	p-Value
Mean stones size (mm)	12.1 ± 0.7	11.9 ± 1.1	0.84
Lower pole calculi	9 (23%)	8 (57%)	0.02^*
SF rates	20 (50%)	11 (78.6%)	0.06
Complications	9 (23.7%)	5 (35.7%)	0.39
Length of stay (days)	0.7 ± 0.2	1.6 ± 0.3	0.02^*

indicates statistically significant values.

Multivariate analysis

In multivariate analysis adjusting for number of stones per patient, maximum diameter of the largest stone, and lower pole stone location, the use of F-URS vs SWL was a predictor of SF status (OR = 3.7; 95% confidence interval [CI]: 1.3, 11.9; p = 0.02; see Table 5). Number of stones per patient (OR = 0.3; 95% CI: 0.1, 0.8; p = 0.02) and maximal diameter of the largest stone (OR = 0.8; 95% CI: 0.7, 0.9; p < 0.001) were also associated with SF status.

Table 5.

Predictors of Stone-Free Status in Multivariate Analysis

				SF status
	Univariate analysis				Multivariate analysis
		95% CI				95% CI
Variables	OR	Lower	Upper	p-Value	OR	Lower	Upper	p-Value
Lower pole stone	0.4	0.2	1.1	0.06	0.5	0.2	1.5	0.21
Maximal diameter of the largest stone	0.8	0.7	0.9	<0.001	0.8	0.7	0.9	<0.001
Number of stones	0.5	0.2	1.2	0.08	0.3	0.1	0.8	0.02
Treatment
SWL	1 [ref]	—	—	0.04	1 [ref]	—	—	0.02
F-URS	2.2	1.1	4.8		3.7	1.3	11.9

CI = confidence interval; OR = odds ratio.

Discussion

In this large series comparing SWL and F-URS in children, we found that F-URS was associated with a higher SF rate without increasing morbidity. These results support those from the only reported study to date that compares these two techniques in the pediatric population: Mokhless et al. found SF rates of 86.6% and 70% for F-URS and SWL, respectively. However, probably because of a small sample size, the difference was not statistically significant (p = 0.11). Over the past few years, technological improvement of scopes, increasing knowledge, and refinement of surgical techniques have led many urologists to use F-URS as first-line treatment for renal stones in adult patients.^13,14 There are numerous retrospective series reporting better SF rates of F-URS in adults.^15,16 Even though data from randomized trials comparing F-URS with SWL are limited and conflicting,^17,18 the European Association of Urology (EAU) recommended F-URS as first-line treatment in adults with renal stones <20 mm.¹⁹ In contrast, current pediatric urology guidelines do not recommend F-URS as a possible first-line treatment.⁴ The more frequent spontaneous passage of stones in children has traditionally led pediatric surgeons to recommend SWL over other treatments.^3,20 However, one could assume that spontaneous passage of residual fragments could also be beneficial for F-URS, but no conclusion has been drawn due to the lack of comparative data between SWL and F-URS in children. Our results suggest that F-URS could be a more effective first-line treatment option in pediatric populations, especially for kidney stones <20 mm.

Another important point to emphasize is the relatively low SF rates we observed in the F-URS group (37%). To date, pediatric series of F-URS have reported SF rates ranging from 50%^5,21 to >90%.^4,20 It could be partly explained by a center-effect in our early F-URS era. Indeed, our institution was the first in the west of France to purchase flexible ureteroscopes, which yielded several centers to refer complex cases with multiple SWL failures. As a result, our patients had bigger stones compared with previous series (stone burden: 21.6 mm in our series compared with 8.8 to 11.5 mm in a recent systematic review²⁰). Moreover, most patients in our study had multiple calculi (57%), whereas the percentage of multiple stones was rarely mentioned in previous series.^{4

–7,19
–21} The 79% SF rate in the subgroup of single stones <20 mm supports the assumption that our overall SF rates were affected by the complexity of the calculi we treated. Another possible explanation could be related to our learning curve of F-URS in children, although data regarding the impact of surgeon's experience on F-URS outcomes are scarce.²²

We also report a poor SF rate in the SWL group (50%). In the literature, average rates of 60% to 80% have been reported for renal stones <2 cm.^3,23 A possible explanation could also be the complexity of treated stones (23% rate of lower pole calculi and 31% rate of multiple stones, which is higher than in previous series²³), which, conversely to the F-URS, is difficult to explain as SWL was available in most surrounding institutions over the study period. Another possible hypothesis is that our lithotripter settings could have been suboptimal with 2500 to 4000 shocks delivered, a power of 15 to 60 W, and a frequency of 2 Hz.^3,23

An important finding is the high rate of successful retrograde access to the upper ureter and renal pelvis (28 out of 29 patients after exclusion of those with preoperative Double-J stent, i.e., 96.5%). Indeed, inability to obtain retrograde access to the upper tract has been reported to be as high as 57%.²⁴ Our high rate of access to the ureter could be related to our sheathless technique, whereas most of reported series use a ureteral access sheath.^19,24 One could argue that this sheathless technique could have diminished our SF rate. However, to our knowledge, no study has compared the results of F-URS with or without access sheath in children.¹⁹ In adults, while waiting for the results of the “dusting” vs “basketting” prospective trial (NCT01619735), retrospective series have suggested similar safety and efficacy of the two techniques.^25,26 Moreover, serious concerns have been raised regarding the safety of ureteral access sheaths use in children.²⁷

Our study has several limitations. First, its retrospective and nonrandomized design could have biased our results. For that reason, we made a “per procedure” analysis, as most patients during the second era received both SWL and F-URS, which would make it difficult to perform a “per patient” analysis. This could explain the lack of comparison between SWL and F-URS and the non-controlled design of most series in the literature. Despite this difficulty, we believe that the comparative data we provide are of value whereas the best management of upper tract stones in children remains to be determined. Our sample size was relatively small, which could be regarded as a shortcoming as it has limited the statistical power of our analyses. It is also important to note that we compared F-URS and its outcomes during the learning curve of our physicians, whereas SWL was well established in our center for several years. We included patients from 0 to 18 years, and the techniques were the same for everyone although the anatomical features completely change during child growth. Another substantial point to emphasize is that our two groups were not comparable for each preoperative variable and for levels of experience, which could have introduced some biases. We tried to minimize the impact of these differences in patients' characteristics by using subgroup and multivariate analyses. No attempt of oral dissolution was made for these two stones, which could be regarded as a drawback. Finally, there was no standardization regarding follow-up imaging modalities and no long-term data were available regarding stone regrowth rate and new stones fragmentation rate, which could be regarded as a shortcoming, especially when considering the “dusting” technique used.

Conclusion

In this relatively large series, F-URS provided a higher single-session SF rate than SWL without increasing morbidity. Further, large randomized controlled trials are needed to assess the role of F-URS for upper tract stones <20 mm in children.

Authors' Contributions

Freton: data collection or management, data analysis, article writing/editing; Peyronnet: protocol/project development, data analysis, article writing/editing; Arnaud: protocol/project development; Tondut: data collection or management; Hascoet: data collection or management; Pradère: data collection or management; Verhoest: protocol/project development; Habonimana: protocol/project development; Azzis: protocol/project development; Fremond: protocol/project development; Bensalah: protocol/project development, article writing/editing.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

References

Tasian

, Copelovitch

. Evaluation and medical management of kidney stones in children. J Urol, 2014; 192:1329–1336.

Wood

, Stanasel

, Koslov

, et al. Changing stone composition profile of children with nephrolithiasis. Urology, 2013; 82:210–213.

Tekgül

, Dogan

, Hoebeke

, Kocvara

, Nijman

, Radmayr

, et al. EAU guidelines on paediatric urology. 2015. Available at: https://uroweb.org/guideline/paediatric-urology/

Resorlu

, Unsal

, Tepeler

, et al. Comparison of retrograde intrarenal surgery and mini-percutaneous nephrolithotomy in children with moderate-size kidney stones: Results of multi-institutional analysis. Urology, 2012; 80:519–523.

Azili

, Ozcan

, Tiryaki

. Retrograde intrarenal surgery for the treatment of renal stones in children: Factors influencing stone clearance and complications. J Pediatr Surg, 2014; 49:1161–1165.

Erkurt

, Caskurlu

, Atis

, et al. Treatment of renal stones with flexible ureteroscopy in preschool age children. Urolithiasis, 2014; 42:241–245.

Mokhless

, Abdeldaeim

, Saad

, et al. Retrograde intrarenal surgery monotherapy versus shock wave lithotripsy for stones 10 to 20 mm in preschool children: A prospective, randomized study. J Urol, 2014; 191(5 Suppl):1496–1499.

Merigot De Treigny

, Bou Nasr

, Almont

, et al. The cumulated stone diameter: A limited tool for stone burden estimation. Urology, 2015; 86:477–481.

Dindo

, Demartines

, Clavien

. Classification of surgical complications: A new proposal with evaluation in a cohort of 6336 patients and results of a survey. Ann Surg, 2004; 240:205–213.

10.

Somani

, Desai

, Traxer

, et al. Stone-free rate (SFR): A new proposal for defining levels of SFR. Urolithiasis, 2014; 42:95.

11.

Bruyère

, Sotto

, Escaravage

, et al. Recommendations of the Infectious Disease Committee of the French Association of Urology (AFU): Antibiotic prophylaxis for urological procedures. Prog Urol, 2010; 20:101–108.

12.

Johnson

, Grasso

. Exaggerated primary endoscope deflection: Initial clinical experience with prototype flexible ureteroscopes. BJU Int, 2004; 93:109–114.

13.

Dauw

, Simeon

, Alruwaily

, et al. Contemporary practice patterns of flexible ureteroscopy for treating renal stones: Results of a worldwide survey. J Endourol, 2015; 29:1221–1230.

14.

Estrade

, Bensalah

, Bringer

, et al. Place of the flexible ureterorenoscopy first choice for the treatment of kidney stones. Survey results practice committee of the AFU lithiasis completed in 2011. Prog Urol, 2013; 23:22–28.

15.

Orywal

, Knipper

, Tiburtius

, et al. Temporal trends and treatment outcomes of flexible ureteroscopy for lower pole stones in a tertiary referral stone center. J Endourol, 2015; 29:1371–1378.

16.

Ito

, Kuroda

, Kawahara

, Makiyama

, et al. Preoperative factors predicting spontaneous clearance of residual stone fragments after flexible ureteroscopy. Int J Urol, 2015; 22:372–377.

17.

Sener

, Bas

, Sener

, et al. Asymptomatic lower pole small renal stones: Shock wave lithotripsy, flexible ureteroscopy, or observation? A prospective randomized trial. Urology, 2015; 85:33–37.

18.

Kumar

, Vasudeva

, Nanda

, et al. A prospective randomized comparison between shock wave lithotripsy and flexible ureterorenoscopy for lower caliceal stones ≤2 cm: A single-center experience. J Endourol, 2015; 29:575–579.

19.

Türk

, Petřík

, Sarica

, et al. EAU guidelines on interventional treatment for urolithiasis. Eur Urol, 2016; 69:475–482.

20.

Ishii

, Griffin

, Somani

, et al. Flexible ureteroscopy and lasertripsy (FURSL) for paediatric renal calculi: Results from a systematic review. J Pediatr Urol, 2014; 10:1020–1025.

21.

Tanaka

, Makari

, Pope

4th , et al. Pediatric ureteroscopic management of intrarenal calculi. J Urol, 2008; 180:2150–2153.

22.

Ito

, Kuroda

, Kawahara

, et al. Clinical factors prolonging the operative time of flexible ureteroscopy for renal stones: A single-center analysis. Urolithiasis, 2015; 43:467–475.

23.

, Wang

, Song

, et al. The clinical efficacy of extracorporeal shock wave lithotripsy in pediatric urolithiasis: A systematic review and meta-analysis. Urolithiasis, 2015; 43:199–206.

24.

Kim

, Kolon

, Canter

, et al. Pediatric flexible ureteroscopic lithotripsy: The children's hospital of Philadelphia experience. J Urol, 2008; 180:2616–2619.

25.

Berquet

, Prunel

, Verhoest

, et al. The use of a ureteral access sheath does not improve stone-free rate after ureteroscopy for upper urinary tract stones. World J Urol, 2014; 32:229–232.

26.

Johnson

, Portela

, Grasso

. Advanced ureteroscopy: Wireless and sheathless. J Endourol, 2006; 20:552–555.

27.

Wang

, Huang

, Routh

, et al. Use of the ureteral access sheath during ureteroscopy in children. J Urol, 2011; 186(4 Suppl):1728–1733.