Supine Versus Prone Position During Percutaneous Nephrolithotomy: A Report from the Clinical Research Office of the Endourological Society Percutaneous Nephrolithotomy Global Study

Abstract

Purpose:

To determine differences in patients' characteristics, operative time and procedures, and perioperative outcomes between prone and supine positioning in percutaneous nephrolithotomy (PCNL) using the Clinical Research Office of the Endourological Society (CROES) PCNL Global Study database.

Patients and Methods:

Between November 2007 and December 2009, prospective data were collected on a total of 5803 consecutive patients who were treated over a 1-year period at each of 96 participating global centers. Patients with data on body position were dichotomized into prone or supine PCNL.

Results:

The majority of PCNL treatments were performed in the prone position (n=4637; 80.3% of sample). Differences in patient characteristics included in the prone group: A greater proportion of males (57.4% vs 52.2%); younger age (48.8 y vs 51.0 y); less frequent history of shockwave lithotripsy (19.5% vs 28.6%); greater frequency of American Society of Anesthesiologists score of 1 (54.7% vs 46.8%); and a Clavien grade of 2 or more (10.0% vs 7.2%). The mean operative time was significantly lower for prone vs supine PCNL (82.7 min vs 90.1 min) regardless of the method of tract dilation, while the stone-free rate was significantly higher (77.0% vs 70.2%). Compared with supine patients, prone patients exhibited higher rates of blood transfusions (6.1% vs 4.3%) and fever (11.1% vs 7.6%), but lower rates of failed procedures (1.5% vs 2.7%).

Conclusions:

Since operative time and stone-free rates favor prone PCNL, but patient safety favors supine PCNL, the choice of patient position should be tailored to individual patient characteristics and the surgeon's preference.

Introduction

S ince the first successful removal of a renal calculus via a nephrostomy tract in 1976,¹ percutaneous nephrolithotomy (PCNL) has become the preferred method of treating patients with a large or complex stone burden.² Despite advantages over traditional open stone surgery,³ as well as shockwave lithotripsy (SWL),⁴ PCNL is not without limitations, with perioperative complications occurring in up to one-third of patients who are undergoing the procedure.⁵ Accordingly, the determination of optimal PCNL techniques remains a challenge. Although PCNL has been widely adopted for some time, it presently lacks a standardized technique.² Such methodologic variability has implications for operative time and costs, postoperative success, and patient morbidity.

During the first decade of PCNL treatment, the procedure was performed exclusively with patients in the prone position.¹ In the ensuing decade, a number of variations in PCNL patient positioning have been proposed, although rarely adopted en masse, including the modified prone,⁶ flank,⁷ or prone split-leg position.⁸ In 1998, Valdivia Uria and associates⁹ reported on the first series of patients who were undergoing PCNL in the supine position, suggesting a number of advantages of this technique over the classic prone position. Since that landmark report, a number of investigators have published case reports documenting their experience with the technique.^10,11 A further development has been the Valdiva Galdakao variant developed in 2006, which introduces some rotation to the supine position by positioning the contralateral leg in flexion and the ipsilateral leg in extension.¹² In addition, comparative studies^13,14 and comprehensive reviews^15,16 assessing differences between prone and supine patient positioning during PCNL have been published. Despite the accumulating literature, however, it remains unclear whether one method of patient positioning is optimal over the other.

The Clinical Research Office of the Endourological Society (CROES) is a recently established unit of the Endourological Society that is responsible for organizing, structuring, and facilitating a global network for endourologic research.¹⁷ The first major initiative by the CROES involved the development of the PCNL Global Study, a prospective global database of indications and outcomes of PCNL on more than 5800 patients from 96 centers worldwide. Previous publications emanating from this study have assessed stone free-outcomes,¹⁸ tract dilation methods,¹⁹ and operative complication rates.²⁰ An ongoing analysis is examining the treatment of patients with staghorn calculi. Using the large database of the PCNL Global Study, the current analysis sought to determine differences in perioperative outcomes and patients' characteristics between patients receiving PCNL while in the prone vs the supine position.

Patients and Methods

The background for the development of CROES¹⁷ and the organizational details of the CROES PCNL Global Study have been reported previously.¹⁸ Briefly, the authors were invited by the CROES council to form a globally representative steering committee responsible for directing the CROES PCNL Global Study. The objective of the steering committee was to recruit 100 centers worldwide with an assortment of sites, particularly with different PCNL treatment volumes. Each center was invited to include all patients who were treated consecutively for 1 year, with the study period at each site starting with the treatment of the first included patient.

Patients

Between November 2007 and December 2009, consecutive patients who were treated over a 1-year period at 1 of 96 participating global centers were included in the PNCL Global Study. Patients who were eligible for inclusion in the study were all those who were candidates for PCNL treatment as the primary indication or after failure of previous treatment for renal stones. For the current analysis, patients were dichotomized into two distinct groups based on their body position during the PCNL procedure: Supine or prone.

Treatment protocol

Treatment of patients with PCNL was based on the presence of symptoms of pain in the flank, hematuria, fever and/or sepsis, and/or dilatation of the upper urinary tract. As part of the PCNL procedure, patients were positioned in either the supine or prone position, as decided by the treating physician. Patients in the supine position were angled based on individual patient characteristics and preference of the operating surgeon. Access to the upper tract was guided by ultrasonography and/or radiography in combination with (retrograde) intrarenal contrast injection. Once access was obtained, a guidewire was inserted and maneuvered toward the ureter. Dilation was performed with balloon or telescopic dilators (metal or polyurethane variety) along with an application of an Amplatz sheath. In follow-up, the system was inspected by the rigid nephroscope and the stones were either disintegrated by laser, ultrasound, or ballistic devices or removed completely with graspers. The procedure was considered to have been completed when all removable stones had been taken out. Certain centers confirmed this through the use of a flexible nephroscope. Internal and/or external drain(s) were positioned according to the judgment of the surgeon.

Patient follow-up

Postoperative renal assessment was performed by ultrasonography, radiography, or CT scan based on availability or local clinical practice. Success of treatment was defined as the patient being stone free (on radiography, renal ultrasonography, or CT scan) by 30 days post-treatment. Severity of bleeding was assessed by the treating physician according to clinical judgment and the necessity for blood transfusion based on local clinical practice guidelines. Perioperative complications were assessed and graded according to the modified Clavien system as applied to PCNL.⁵

Data collection

Data were collected in a central database held at the CROES office. All data were encrypted to guarantee confidentiality. At each participating center, Institutional Review Board approval was obtained if needed; otherwise, the lead investigator was responsible for ensuring the quality of clinical data collected. Each center appointed a member of staff to coordinate the collection, handling, and to provide regular updates of their center's data to the central database.

Data analysis

The current analysis consists of a comparison of various patient characteristics and treatment outcomes according to patient positioning during PCNL treatment (prone vs supine). Comparisons between continuous variables were performed using the Student t test while comparisons between categorical variables were performed using a Pearson chi-square test. The level of significance was defined as P<0.05. All analyses were performed using SPSS version 16.0.

Results

Global characteristics

Of the 5803 patients in the CROES PCNL Global Study database, a total of 5775 patients had information regarding body positioning during the PCNL procedure and were included in the current analysis. The vast majority of patients undergoing PCNL were in a prone position during the procedure (n=4637; 80.3%), while the remaining 1138 patients (19.7%) were in a supine position. As illustrated in Table 1, patients undergoing PNCL in participating centers in North America (n=16), Asia (n=16), and Australia (n=1) were almost exclusively treated while in the prone position (98.5%, 98.1%, and 100.0% of patients, respectively). Although the prone position also predominated in the 60 European centers, being used in 76.5% of cases, the reverse was true in the three centers in South America, where 98.5% of PCNL patients were positioned in the supine position during treatment. A majority (n=761) of the patients who underwent supine PCNL came from 38 centers in Europe followed by 339 patients from 3 centers in South America. Twelve and 26 patients from North America and Asia respectively were operated on in the supine position.

Table 1.

Continental Differences in Patient Position During Percutaneous Nephrolithotomy

Continent	No. of centers	Supine n (%)	Prone n (%)	Total
Asia	16	26 (1.9)	1340 (98.1)	1366
Australia	1	0 (0.0)	9 (100.0)	9
Europe	60	761 (23.5)	2479 (76.5)	3240
North America	16	12 (1.5)	804 (98.5)	816
South America	3	339 (98.5)	5 (1.5)	344
Total	96	1138 (19.7)	4637 (80.3)	5775

Patient characteristics

Table 2 outlines the patient characteristics according to body position during PCNL. The ratio of male to female patients was significantly different between the two body positions, with a greater proportion of patients in the prone position being male (57.4% vs 52.2%; P=0.002). PCNL patients in the prone position group were younger than those in the supine position (48.8 vs 51.0 years; P<0.0001). In addition, a smaller proportion of prone-positioned patients had a history of SWL treatment in comparison with those in the supine position (19.5% vs 28.6%; P<0.0001). Furthermore, 0.8% of the patients who underwent supine PCNL had ectopic kidneys as compared to 0.4% of the patients who were operated on in the prone position. On the contrary, 1.2% of the patients who were operated in the supine position had horseshoe kidneys while 2.0% of those in the prone position had horseshoe kidneys.

Table 2.

Patient Characteristics

Patient characteristics	Supine	Prone	P value
Sex (% male/% female)	52.2%/47.8%	57.4%/42.6%	0.002
Age (years) mean (SD)	51.0 (14.7)	48.8 (15.7)	<0.0001
Diabetes (%)	12.8%	13.6%	0.497
BMI mean (SD)	26.6 (4.6)	26.7 (5.4)	0.944
Anticoagulant use (%)	6.5%	5.2%	0.092
Prednisone use (%)	1.8%	1.1%	0.070
Cardiovascular disease (%)	23.4%	23.2%	0.907
Renal anomalies			0.009
Ectopic	0.8%	0.4%
Horseshoe	1.2%	2.0%
Malrotation	2.0%	1.2%
Previous procedures (%)
PCNL	12.4%	14.8%	0.129
SWL	28.6%	19.5%	<0.0001
Pyelolithotomy	7.0%	8.3%	0.161
URS	9.5%	9.8%	0.776
Nephrostomy	8.5%	7.5%	0.312
ASA (%)			0.0001
1	46.8%	54.7%
2	42.1%	33.4%
3	10.3%	11.0%
4	0.8%	0.9%
Stone burden (mm²) mean (SD)	470.6 (386.4)	449.1 (359.0)	0.232
Staghorn stone (%)	25.4%	27.9%	0.097

BMI=body mass index; SD=standard deviation; PCNL=percutaneous nephrolithotomy; SWL=shockwave lithotripsy; URS=ureteroscopy; ASA=American Society of Anesthesiologists score.

Finally, the distribution of American Society of Anesthesiologists (ASA) scores differed with an ASA score of 1 being more common among patients in the prone position (54.7% vs 46.8%; P=0.0001), whereas an ASA score of 2 was more common among patients in the supine position (42.1% vs 33.4%; P=0.0001). No significant differences between patients in the two body positions during PCNL were seen for rates of diabetes, cardiovascular disease, anticoagulant or prednisone use, staghorn calculi, median body mass index (BMI), or stone burden.

Operative time and procedures

The differences in operative procedures between patients in the supine and prone positions during PCNL are listed in Table 3. The mean operative time was significantly higher for patients in the supine position, regardless of the method of tract dilation (90.1 vs 82.7 min; P<0.0001). In contrast to supine-positioned patients, prone-positioned patients were more likely to receive a puncture at upper pole of the kidney (11.4% vs 4.0%) as well as punctures at multiple sites (9.0% vs 4.1%), but less likely to receive a lower kidney pole puncture (63.8% vs 74.8%; p<0.0001). Access to the kidney was predominantly obtained via a route below the 12th rib. Above the level of the 12th rib, however, supracostal access was more common among prone-positioned patients (17.6% vs 5.5%), whereas the reverse was true of infracostal access (80.8% vs 93.3%; P<0.0001). Guidance during PCNL was more commonly achieved solely via ultrasonography in prone-positioned patients (12.2% and 3.3%), but less commonly via fluoroscopy alone or in combination with ultrasonography (76.1% and 88.3%; P<0.0001). Where combination guidance was used, the specific guidance used during different phases of the procedure was not documented. Lastly, there was a statistically significant, albeit modest, difference in the types of tract dilators used during PCNL between prone- and supine-positioned patients, with balloon dilation being more common among supine patients (43.8% vs 40.3%; P=0.04).

Table 3.

Intraoperative Differences Between Patients Treated with Percutaneous Nephrolithotomy in the Supine and Prone Positions

Intraoperative characteristics	Supine	Prone	P value
Operative time (min) mean(SD)	90.1 (43.3)	82.7 (44.1)	<0.0001
Balloon mean (SD)	106.3 (44.5)	98.7 (45.1)	<0.0001
Telescopic mean (SD)	81.6 (36.6)	71.7 (40.0)	<0.0001
Puncture site (%)			<0.0001
Upper	4.0	11.4
Midpolar	17.1	15.8
Lower	74.8	63.8
Multiple	4.1	9.0
Location of access (%)			<0.0001
Above 11th rib	1.2	1.6
Above 12th rib	5.5	17.6
Below 12th rib	93.3	80.8
Guidance (%)			<0.0001
US	3.3	12.2
Fluoroscopy	67.8	62.5
Fluoroscopy + US	20.5	13.6
Others	8.5	11.7
Dilation (%)			0.040
Balloon	43.8	40.3
Telescopic dilators	56.2	59.7

SD=standard deviation; US=ultrasonography.

Perioperative outcomes

Differences in perioperative patient outcomes between PCNL patients who were treated in the prone vs the supine position are listed in Table 4. The stone-free rate at 30 days post-PCNL was significantly higher among prone-positioned patients (77.0% vs 70.2%; P<0.0001). Overall, perioperative morbidity occurred rather infrequently, regardless of the positioning of the patients, ranging in frequency from 1.4% to 11.1%. Nevertheless, the rate of failed procedures, in which access to the kidney was not accomplished, was slightly higher among supine-positioned patients (2.7% vs 1.5%; P=0.01). Conversely, patients in the prone position exhibited higher rates of blood transfusions (6.1% vs 4.3%; P=0.026) as well as fever (11.1% vs 7.6%; P=0.001). There was no difference in the mean hospital stay between patients who were operated on in the supine (4.2 d) and prone (4.3 d) positions.

Table 4.

Perioperative Outcomes Based on Patient Position; Percentage Values Shown

Outcome measure	Supine	Prone	P value
Overall stone-free rate (%)	70.2	77.0	<0.0001
Imaging modality based stone-free rates			<0.001
CT scan(%)	62.5	58.9
Ultrasonogrphy (%)	70.9	83.8
KUB radiography (%)	71.6	78.7
Blood transfusion (%)	4.3	6.1	0.026
Fever (%)	7.6	11.1	0.001
Perforation (%)	3.4	3.3	0.958
Hydrothorax (%)	1.4	1.9	0.242
Failed procedure (%)	2.7	1.5	0.010
Hospital stay (d) mean (SD)	4.2 (3.8)	4.3 (3.3)	0.429

CT=computed tomography; KUB=kidneys, ureters, and bladder; SD=standard deviation.

As illustrated in Table 5, approximately 80% of patients who were treated in either the supine or prone position had no perioperative complications, while another 10% to 12% had a modified Clavien grade of 1. A slightly greater proportion of prone-positioned patients, however, had a Clavien grade of 2 or more in comparison with supine-positioned patients (10.0% vs 7.2%; P=0.064), indicating a greater probability of necessitating pharmacologic, surgical, or other treatment post-PCNL in the prone-positioned patients.

Table 5.

Clavien Score and Patient Position

	Supine		Prone
Clavien score	n	%	n	%	Total
None	910	80.8	3631	79.2	4541
I	136	12.1	495	10.8	631
II	43	3.8	257	5.6	300
IIIA	21	1.9	112	2.4	133
IIIB	9	0.8	67	1.5	76
IVA	3	0.3	15	0.3	18
IVB	3	0.3	7	0.2	10
V	1	0.1	1	0.0	2
Total	1126	100.0	4585	100.0	5711

Two patients died: One because of a cardiac arrest after a supine PCNL and one in the context of colonic perforation after prone PCNL.

Supine vs prone only centers

A further analysis was performed on centers using only supine (n=538) or only prone (n=1675) positions for PCNL. The trends that were observed in the total study database were repeated in this subanalysis (Table 6) in that stone-free rates were significantly lower and mean operative time was significantly longer for supine-positioned patients 100.0 (standard deviation [SD]=45.6) minutes vs 89.0 (SD=46.2) minutes with a level of significance at P<0.0001. While blood transfusion rate was also lower in the supine group, this did not reach significance.

Table 6.

Comparison of Outcomes Between Centers Conducting Percutaneous Nephrolithotomy Solely in the Supine or the Prone Positions

Outcome measure	Supine only (n=538)	Prone only (n=1675)	P value
Stone-free rate (%)	63.3	76.9	<0.0001
Blood transfusion (%)	4.2	6.3	0.085
Operative time (min) mean (SD)	100.0 (45.6)	89.0 (46.2)	<0.0001

SD=standard deviation.

Discussion

The CROES PCNL Global Study is the largest prospective database created to date of patients who were treated with PCNL. This most recent analysis from the database including a total of 5775 patients represents the largest investigation of differences in patients' characteristics and perioperative outcomes between prone and supine PCNL. Indeed, the largest trial to previously investigate the differences between prone vs supine PCNL consisted of only 130 patients.²¹ Despite the introduction of various techniques of patient positioning since inception of the PCNL procedure in 1976, the results of this investigation suggest that the prone position remains the most popular option among urologists. In fact, more than 80% of the patients who were undergoing PCNL in this study were treated in the prone position. This trend was true in most of the continents surveyed, including Asia, Australia, Europe, and North America, where the proportions of PCNL cases performed in the prone position ranged from 76.5% to 100.0%. Somewhat surprisingly, the opposite was true of the participating South American PCNL centers, with 98.5% of patients being treated in the supine position. Nevertheless, on a global scale, supine PCNL has not yet gained widespread use in the urologic community.

The apparent resistance of urologists worldwide to adopt supine patient positioning during PCNL as standard practice is unexpected given the often advocated advantages of the technique over prone positioning. For example, it has been suggested that supine PCNL is less demanding and time consuming than prone PCNL.^15,22 This notion has been supported by results of previous small-scale comparative studies, both of which report significantly shorter operative time for supine vs prone PCNL (75 min vs 107 min¹³ and 43 min vs 68 min¹⁴). The current findings are contradictory, with mean operative times being actually 7.4 minutes longer during supine than prone PCNL, a finding that remained true regardless of the method of tract dilation (balloon or telescopic). It was also valid for the subanalysis of centers performing only supine- or only prone-positioned PCNL, where supine mean operative time was 11.0 min longer. It should be noted that operative time is determined based on the sum of the different phases of surgery, and certain centers have been able to reduce the time spent during individual phases.

Theoretically, supine PCNL should save time by eliminating the need to reposition the patient from supine to prone after administering general anesthesia and ureteral catheterization, as needed during prone PCNL. A more accurate evaluation of the time differences between the two positions would involve a study of the times of the different operating phases. This may very well not have been captured correctly in the present study. Another factor that may influence operative time and stone-free rate is the possible use of ureterorenoscopy during the procedure. Because this was not captured in the database, we were not able to include this in our analysis. Furthermore, different centers may have used different methods and protocols to assess stone-free status. A detailed analysis of stone-free rate will be presented in a separate manuscript. There are also other confounding factors in the current study, such as differences in ASA score, which should be taken into consideration. Finally, the differences in operative time, although statistically significant in this study, may not necessarily be clinically significant.

In the current study, the use of fluoroscopy alone or in combination with ultrasonography was greater among supine- vs prone-positioned PCNL patients, whereas the opposite was true of unaccompanied ultrasonography use. Another facet of PCNL treatment approach that was shown to vary according to the patient positioning was the puncture site as well as the location of access. Indeed, both puncture at the upper pole, as well as supracostal access at the level of the 12th rib were all less common among supine- vs prone-positioned patients. These findings are in line with the espoused difficulties of upper pole puncture in supine patients.^15,22

Given that perioperative complications can occur in up to one-third of patients undergoing the PCNL,⁵ selecting the optimal technique for each patient can be a key step toward a successful and complication-free procedure.²³ The lack of significant differences between PCNL patients who were treated in the prone vs supine position in outcomes such as BMI, rates of diabetes or cardiovascular disease, the use of anticoagulants, median stone burden, or the rate of staghorn calculi suggests that these patient characteristics rarely influence the choice of patient position. On the other hand, a larger proportion of prone-positioned PCNL patients had a preoperative ASA score of 1—ie, were deemed to be healthy—whereas the reverse was true for the proportion of patients with mild systemic disease (ASA score 2). The only other significant differences between supine and prone PCNL patients were a lower age, a greater proportion of females, and greater rate of previous SWL in the former group—findings of questionable clinical relevance. In unison, these findings do not follow what is commonly argued in the literature; ie, that supine PCNL may be a more appropriate option for patients who are obese, have compromised cardiopulmonary status, and are generally at higher anesthesiology risk.^9,24

In the current study, the 30-day stone-free rates were found to be significantly lower among supine vs prone PCNL patients. As noted above, the mean stone burden was higher (not significantly, P value 0.2324) in patients who were treated in the supine vs prone positions (470.6 mm² vs 449.1 mm²). In addition, prone patients underwent more multiple punctures than supine patients, increasing access and stone clearance. It must be acknowledged, however, that these stone-free rates fall below what is often reported in the PCNL literature (80%–90%).^{10,11,13,14,21} In addition, the finding of lower stone-free rates among supine vs prone PCNL patients contradicts the equivalent rates found in previous comparative studies (89% vs 84%, 88.7% vs 91.6%, and 77.5% vs 80.0%, for the comparison of supine vs prone PCNL, respectively).^13,14,21 Moreover, in the present study, the centers performing supine PCNL may have had less access to the best lithotripters for stone disintegration and thus produce lower stone-free rates.

In terms of patient morbidity, although it has been suggested that supine PCNL is a safer technique, previous studies have failed to show any such advantage over prone PCNL.^13,14,21 In fact, in all previous comparative studies conducted, the rates of complications were comparable between supine and prone PCNL. While postoperative complications occurred at a low frequency regardless of patient position in the current investigation, however, rates of blood loss necessitating blood transfusion and fever were lower among supine vs prone PCNL patients. While these differences are statistically significant, they are of modest magnitude. One possible explanation for the increased transfusion rates in prone patients was that this patient group underwent significantly more multiple punctures than supine patients. Thus, it remains possible that because of the significantly smaller sample sizes and lower statistical power of previous comparative studies, these differences were not easily detected.

Further modest differences were observed in the distribution of patients according to Clavien score. Specifically, a smaller proportion of supine PCNL patients had a Clavien score of 2 or 3 in comparison with their prone PCNL counterparts. These differences indicate a slightly lower probability of supine PCNL patients needing pharmacologic, surgical, or other postoperative treatment. Conversely, the rates of failed procedures, where access to the kidney was not accomplished, were slightly greater in supine vs prone PCNL, a potential artifact of the relative novelty and infrequent use of supine PCNL within endourology. Two patients died because of postoperative complications, including bowel perforation after a prone PCNL. Although it is speculated that the risk of this complication is higher in the prone than in the supine position, this cannot be supported by data derived from the evidence-based literature.

One of the major limitations of this study was that the data obtained were not homogenous because the centers had different methods to assess stone-free rates and operative times. This may potentially have resulted in a lack of a unified reporting methodology.

Conclusion

The current study represents the largest investigation of differences in patients' characteristics, operative time and procedures, and perioperative morbidity between two common methods of patient positioning during PCNL: Supine vs prone. Despite the advantage of a large sample size derived from numerous centers worldwide, the results of the investigation do not lend themselves to readily favor one position over another. While operative time and stone-free rates are in favor of prone PCNL, lower rates of patient morbidity favor supine PCNL. The study also did not measure the individual experience of surgeons with the different positions.

Footnotes

Acknowledgment

The PCNL Global Study was supported by an unrestricted educational grant from Olympus.

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

References

Fernström

, Johansson

. Percutaneous pyelolithotomy. A new extraction technique. Scand J Urol Nephrol, 1976; 10:257–259.

Skolarikos

, Alivizatos

, de la Rosette

. Percutaneous nephrolithotomy and its legacy. Eur Urol, 2005; 47:22–28.

Al-Kohlany

, Shokeir

, Mosbah

et al. Treatment of complete staghorn stones: A prospective randomized comparison of open surgery versus percutaneous nephrolithotomy. J Urol, 2005; 173:469–473.

Albala

, Assimos

, Clayman

et al. Lower pole I: A prospective randomized trial of extracorporeal shock wave lithotripsy and percutaneous nephrostolithotomy for lower pole nephrolithiasis—initial results. J Urol, 2001; 166:2072–2080.

Tefekli

, Ali Karadag

, Tepeler

et al. Classification of percutaneous nephrolithotomy complications using the modified clavien grading system: Looking for a standard. Eur Urol, 2008; 53:184–190.

Lehman

, Bagley

. Reverse lithotomy: Modified prone position for simultaneous nephroscopic and ureteroscopic procedures in women. Urology, 1988; 32:529–531.

Kerbl

, Clayman

, Chandhoke

et al. Percutaneous stone removal with the patient in a flank position. J Urol, 1994; 151:686–688.

Scarpa

, Cossu

, De Lisa

et al. Severe recurrent ureteral stricture: The combined use of an anterograde and retrograde approach in the prone split-leg position without X-rays. Eur Urol, 1997; 31:254–256.

Valdivia Uria

, Valle Gerhold

, López López

et al. Technique and complications of percutaneous nephroscopy: Experience with 557 patients in the supine position. J Urol, 1998; 160:1975–1978.

10.

Scoffone

, Cracco

, Cossu

et al.

Endoscopic combined intrarenal surgery in Galdakao-modified supine Valdivia position: A new standard for percutaneous nephrolithotomy?

Eur Urol, 2008; 54:1393–1403.

11.

Steele

, Marshall

. Percutaneous nephrolithotomy in the supine position: A neglected approach? J Endourol, 2007; 21:1433–1437.

12.

Daels

, González

, Freire

et al. Percutaneous lithotripsy in Valdivia-Galdakao decubitus position: Our experience. J Endourol, 2009; 23:1615–1620.

13.

De Sio

, Autorino

, Quarto

et al. Modified supine versus prone position in percutaneous nephrolithotomy for renal stones treatable with a single percutaneous access: A prospective randomized trial. Eur Urol, 2008; 54:196–202.

14.

Falahatkar

, Moghaddam

, Salehi

et al. Complete supine percutaneous nephrolithotripsy comparison with the prone standard technique. J Endourol, 2008; 22:2513–2517.

15.

de la Rosette

, Tsakiris

, Ferrandino

et al. Beyond prone position in percutaneous nephrolithotomy: A comprehensive review. Eur Urol, 2008; 54:1262–1269.

16.

, Wang

. Supine versus prone position in percutaneous nephrolithotomy for kidney calculi: A meta-analysis. Int Urol Nephrol, 2011; 43:67–77.

17.

de la Rosette

. A platform for global endourological research. J Endourol, 2009; 23:1551–1553.

18.

de la Rosette

, Assimos

, Desai

et al. The Clinical Research Office of the Endourological Society Percutaneous Nephrolithotomy Global Study: Indications, complications, and outcomes in 5803 patients. J Endourol, 2011; 25:11–17.

19.

Lopes

, Sangam

, Alken

et al. The Clinical Research Office of the Endourological Society Percutaneous Nephrolithotomy Global Study: Tract dilation comparisons in 5537 patients. J Endourol, 2011; 25:755–762.

20.

Yamaguchi

, Skolarikos

, Buchholz

et al. Operating times and bleeding complications in percutaneous nephrolithotomy: A comparison of tract dilation methods in 5537 patients in the Clinical Research Office of the Endourological Society Percutaneous Nephrolithotomy Global Study. J Endourol, 2011; 25:933–939.

21.

Shoma

, Eraky

, El-Kenawy

et al. Percutaneous nephrolithotomy in the supine position: Technical aspects and functional outcome compared with the prone technique. Urology, 2002; 60:388–392.

22.

Autorino

, Giannarini

. Prone or supine: Is this the question? Eur Urol, 2008; 54:1216–1218.

23.

Michel

, Trojan

, Rassweiler

. Complications in percutaneous nephrolithotomy. Eur Urol, 2007; 51:899–906.

24.

Manohar

, Jain

, Desai

. Supine percutaneous nephrolithotomy: Effective approach to high-risk and morbidly obese patients. J Endourol, 2007; 21:44–49.