Percutaneous Nephrolithotomy in Obese Patients: Comparison Between the Prone and Total Supine Position

Abstract

Background:

Percutaneous nephrolithotomy (PCNL) can be performed in the prone or in the supine position. Comparisons between the two techniques in obese patients are rare in the current literature.

Methods:

The records of obese patients (body mass index >30) who underwent PCNL in the prone or complete supine positions were reviewed. All patients had a noncontrast CT before and after the procedure. Stones were graded according to the Guy stone score and complications according to the Clavien grading. The stone-free rates, operative time, surgical complications, and hospital stay were analyzed.

Results:

A total of 56 PCNL were performed in 42 patients. Twenty-four PCNL were performed in the prone and 32 in the total supine position. Stone-free rate on the first postoperative day was 50% in the prone and 46.9% in the supine position (P=1.0). Final stone-free rates were 83.3% and 78.1%, respectively (P=0.74). Mean operative time was 164.6 minutes in the prone and 120.3 minutes in the supine position (P=0.0017), and hospital stay was 4.38 and 2.68 days (P=0.014), respectively. The transfusion rate was 20.8% in the prone and zero in the supine position patients (P=0.01). Excluding Guy IV stones, transfusion rate was 8.3% in the prone position (P=0.1). Significant surgical complications rate was 12.5% in the prone and 3.1% in the supine position (P=0.302).

Conclusion:

PCNL performed in the prone or in the complete supine position in obese patients presents similar outcomes. The supine decubitus position has the advantages of a significantly shorter operative time and hospital stay.

Introduction

T he prevalence of obesity is increasing throughout the world and its association with lithiasis is well known.¹ Obesity and weight gain increase the risk of kidney stone formation because of hyperinsulinemia and insulin resistance, leading to low urinary pH, hypocitraturia, hyperoxaluria, and hypercalciuria.² The surgical management of renal and ureteral calculi present sunique challenges in obese patients because of poor radiographic visualization, obscure anatomic landmarks, difficult renal access, and inferior stone-free rates.^3,4

Percutaneous nephrolithotomy (PCNL) is the gold-standard technique to treat patients with complex renal stones. Although some studies have demonstrated no adverse influence of obesity on PCNL outcomes,^4

–8 others suggested obesity to be an independent predictor factor for longer hospital stays, lower stone-free rates, and higher complication rates.^9
–11 PCNL has been traditionally performed with the patient in the prone position according to the original description by Fernström and Johansson.¹²

More recently, the supine decubitus position with its variations has gained wide acceptance because of reported good results allied with higher comfort to the patient, surgical, and anesthetic teams.^13,14 Compared with the classic prone decubitus position, PCNL in the supine position offers potential advantages: Less handling of patients who need to be draped only once, better control and rapid access to the airway during the procedure, particularly in patients with compromised cardiopulmonary function, reduction in overall operative time, more ergonomic position for the surgeon, no density overlap with vertebra, and easier ability to perform concomitant rigid cystoscopy or ureteroscopy.^15

–18

To our knowledge, to date no studies have compared both techniques considering only obese patients and using CT to evaluate immediate postoperative outcomes. Based on this, we present a study that compares PCNL outcomes in obese patients who were operated on in the prone or complete supine position using CT as standard image modality to define stone-free rates.

Methods

We performed a chart review in our prospectively collected database searching for all patients with body mass index (BMI) >30 kg/m² who underwent PCNL between April 2008 and February 2012. PCNL indications were: Renal stones >2 cm, or smaller stones after failed shockwave lithotripsy (SWL). Patients underwent PCNL in the prone or complete supine position under general anesthesia according to the surgeon's preference. One surgeon was responsible for all surgeries performed in the prone decubitus (EM) and another (FCV) for all procedures performed in the supine position. Both surgeons have good experience with each procedure. All patients had a noncontrast low-dose CT before and in the first (or eventually in the second) postoperative day (POD). Stones were classified according to the Guy stone score¹⁹ and complications according to the Clavien modified grading system.²⁰

Preoperatively, all patients received a prophylactic third-generation cephalosporin at anesthesia induction or therapeutic antibiotics according to urine culture initiated 7 days before surgery. Patients with staghorn stones had antibiotics started 24 hours before surgery even with negative urine culture results. All tracts were obtained by the urologist under radiographic guidance. Flexible nephroscopes and ureteroscopes were not available at our institution before 2010, and rigid instruments were used in PCNLs and ureteroscopies performed in 2008 and 2009.

PCNL technique

The technique used in patients who were operated on in the prone decubitus position followed the classic steps: Patients were positioned in the lithotomy position, and a 6F ureteral catheter was inserted to contrast the collecting system. If ureterolithotripsy was indicated for patients with both ureteral and renal stones, the procedure was performed before catheter insertion. The patient was then positioned in the ventral decubitus position with pads under the shoulders. The desired calix was punctured under fluoroscopic guidance, a guidewire was inserted, and the tract was dilated to 30F using Amplatz or metallic coaxial dilators. After positioning the Amplatz sheath, the rigid nephroscope was introduced, and stone removal itself began.

Patients who were operated on in the complete supine decubitus position were placed with the edge of the body lying just out of the border of the operating table and the ipsilateral arm hanging over the thorax. No pads were used. The contralateral leg was flexed, allowing concomitant ureteroscopy when needed. A 6F ureteral catheter was inserted with the patient in the supine position. The selected calix was then punctured between the middle and the posterior axillary line under fluoroscopic guidance. A guidewire was inserted after the renal axillary punctured, and tract dilation was performed. The Amplatz sheath was positioned and the nephroscope inserted. Rigid nephroscopes were routinely used.

Flexible nephroscopes were used if necessary in both groups of patients at the end of each procedure for final inspection. Stones were removed using forceps, and lithotripsy was obtained using the LithoClast master^® or the CyberWand^® ultrasonic lithotriptor. A nephrostomy tube was left after surgery at surgeon discretion. Briefly, criteria for tubeless PCNL were absence of significant bleeding, residual fragments, or perforation of the collecting system at the end of the procedure. In all tubeless PCNLs, a ureteral stent was left for 24 to 48 hours after surgery or a Double-J stent was left longer if indicated. Patients with residual stones were submitted to a second-look PCNL, flexible ureteroscopy, or SWL according to residual stone burden and location. A new low-dose CT or ultrasonography was performed 60 to 90 days in those patients who underwent additional procedures.

Stone area was calculated multiplying the biggest stone diameters by 0.78. Success was defined as fragments <4 mm on the POD1 CT. Operative time was considered from the beginning of cystoscopy for ureteral catheter insertion until the end of nephrostomy location. Stone-free rates, operative time, surgical complications, transfusion rate, and time of recovery at the hospital were analyzed.

Statistical analysis was performed with SPSS version 19. Chi-square test and Fisher exact test were applied to compare the success rates, postoperative complications, and blood transfusion rates. Student t test was used to compare continuous variables. Statistical significance was defined as P<0.05.

Results

A total of 56 PCNLs were performed in 42 obese patients in the 4-year analyzed period. Of these, 24 surgeries were performed in the prone position and 32 in the total supine position. Two patients operated on in the prone position had bilateral stones, and the procedures were analyzed separately. Previous procedure for kidney stone retrieval was more common among patients operated on in the prone group (four vs one patient; P=0.007). There was a predominance of female patients in both groups, and patients in the supine group were older than those in the prone group (49.0 vs 38.3 years; P=0.0001). No statistical differences were found concerning sex, BMI, and comorbidities. Patient characteristics are shown in Table 1.

Table 1.

Patient Demographic Data

	Prone position	Supine position	P
Number of patients	12	30	-
Male/female	2/10	12/18	0.14
Age range/median (years)	24–56/38.3	28–68/49	0.0001
BMI range/mean	30.1–45.4/34.2	30.1–46.8/34	0.48
Number of procedures	24	32	-
Previous PCNL/open surgery	1/3	1/zero	0.007
Patients with comorbidities (%)	7 (58.3)	14 (46.6)	0.46

BMI=body mass index; PCNL=percutaneous nephrolithotomy.

Mean stone area was 11.28 cm² for patients in the prone group (1.87–39.02 cm²) and 10.2 cm² (0.63–38.90 cm²) among those in the supine group. Although stone burden was similar among groups (P=0.43), patients operated on in the supine position had more simple stones than those operated on in the prone position according to the Guy’ stone score (P=0.048; Table 2).

Table 2.

Stone Characteristics

	Prone position	Supine position	P
Stone area range/mean (cm²)	1.87–39.80/11.28	0.63–38.90/10.2	0.43
Left/right/bilateral	6/14/2	12/20/0	0.19
Guy classification (%)			0.048
Guy I	3 (12.5)	9 (28.1)
Guy II	7 (29.1)	8 (25.0)
Guy III	4 (16.7)	11 (34.4)
Guy IV (complete staghorn stones)	10 (41.7)	4 (12.5)

Total operative time ranged from 40 to 300 minutes (mean 164.6 min) for patients operated on in the prone position and 30 to 360 minutes (mean 120.3 min) among those operated on in the supine decubitus position (P=0.0017). The number of multiple tracts did not differ significantly between the two techniques (seven in each group; P=0.66). The rate of tracts above the 12th rib was 25% in the prone position and 6.3% in the supine position (P=0.06). One patient in each group successfully underwent concomitant ureteroscopy for ureteral stone removal. Although tubeless PCNL were more frequent in patients operated on in the supine decubitus position (25% vs 8.3%), the difference was not statistically significantly (P=0.16).

Transfusion was necessary in five procedures performed in the prone position (20.8%); 60% of them (three transfusions) were used in patients with complete staghorn stones (Guy IV). No blood transfusion was needed for patients operated on in the supine position (P=0.01). Transfusion rate excluding Guy IV stones was 8.3%, nonsignificant when compared with patients operated on in the supine position (P=0.1). Hospital stay ranged from 2 to 16 days in the prone group and 2 to 5 days in the supine group (P=0.014). These data are shown in Table 3.

Table 3.

Perioperative Data

	Prone position	Supine position	P
Operative time range/mean (min)	40–300/164.6	30–360/120.3	0.0017
Number of tracts			0.66
One (%)	17 (70.8)	25 (78.1)
Two (%)	6 (25)	5 (15.6)
Three (%)	1 (4.2)	2 (6.3)
Access above 12th rib	6 (25)	(6.3)	0.06
Concomitant rigid ureteroscopy (%)	1 (4)	1 (3)	1.0
Tubeless PCNL (%)	2 (8.3)	8 (25)	0.16
Transfusion (%)	5 (20.8)	Zero	0.01
Transfusion in Guy I, II, or III stones (%)	2 (8.3)	Zero	0.1
Hospital stay (days) mean (range)	4.38 (2–16)	2.68 (2–5)	0.014

Stone-free rate after one procedure (POD1) was 50% for patients operated on in the prone position and 46.9% for those operated on in the supine decubitus position (P=1.0). This rate increases to 54.1% and 68.8% if patients with residual fragments smaller or equal to 4mm are included (P=0.28). Four (16.7%) patients operated on in the prone position and five (15.6%) in the supine position underwent adjuvant SWL to achieve complete stone-free status. Nine patients operated on in the prone position underwent a second-look PCNL in comparison with two patients among those operated on in the supine position (P=0.005). These results are summarized in Table 4.

Table 4.

Results of Percutaneous Nephrolithotomy

	Prone position	Supine position	P
Stone-free rate – POD1 (%)	12 (50)	15 (46.9)	1.0
Success rate – POD1 (%)	13 (54.1)	22 (68.8)	0.28
Final stone-free rate (%)	20 (83.3)	25 (78.1)	0.74
Stone-free rate according to Guy
Guy I (%)	3 (100)	8 (88.9)	0.84
Guy II (%)	5 (62.5)	5 (62.5)	1.0
Guy III (%)	3 (75)	6 (54.5)	0.6
Guy IV (%)	2 (20)	3 (75)	0.09
Auxiliary procedures – SWL (%)	4 (16.7)	5 (15.6)	1.0
Second-look PCNL	9 (37.5)	2 (6)	0.005

POD=postoperative day; SWL=shockwave lithotripsy; PCNL=percutaneous nephrolithotomy.

Surgical complications were similar among groups (Table 5; P=0.77). No pulmonary compliance issues were seen, and patients in both groups behaved alike intraoperatively.

Table 5.

Surgical Complications of Percutaneous Nephrolithotomy

Clavien classification	Prone position	Supine position	P
1	5 (20.8)	8 (25)
2		1 (3.1)
3a	2 (8.3)
3b
4b	1 (4.2)	1 (3.1)
Total (%)	8 (33.3)	10 (31.3)	0.77

Complications graded as Clavien I were constituted basically by fever in the first two PODs and needed no intervention other than usual medication. Among patients operated on in the prone position, three major complications occurred: Two hydrothorax that necessitated drainage under local anesthesia and one hyponatremia that necessitated intensive care support. Among patients operated on in the supine position, one patient had a laceration of the collecting system leading to retroperitoneal infiltration that necessitated intensive care support. All patients had successful outcomes after treatment. Significant surgical complications occurred in three (12.5%) cases in the prone group and one (3.1%) in the supine group, and the difference was not significant (P=0.302).

Discussion

PCNL is accepted as the preferred method for treatment of patients with renal stones larger than 2 cm. The procedure has been traditionally performed in the prone position, and the supine position with its variations is less popular maybe because of deficiency in training.^16
–18,21 In a recent publication from the Clinical Research Office of the Endourological Society (CROES), only 20% of all PCNLs performed around the world were done in the supine position.²² The advantages of one or the other technique are very debatable. The supine position has theoretical advantages over the prone: Possibility of performing concomitant ureteroscopy, less handling of the patient, reduced overall operative time, dependent Amplatz sheath drainage facilitating the evacuation of stone fragments, and more rapid access to the airway, especially in obese patients.^16
–18,21 In addition, the surgeon operates in a more comfortable and ergonomic position.

Neto and associates¹⁶ reported a 70.5% stone-free rate using a supine decubitus position with the ipsilateral leg crossed over the contralateral leg and using a cushion below the ipsilateral flank to provide a 30-degree inclination.¹⁶ Falahatkar and colleagues¹⁵ advocate complete supine positioning for all patients, obtaining a success rate of 77% with a minimal complication rate. On the other hand, adepts of the prone position states that the supine decubitus access to the upper pole calices is more difficult because of the more medial and posterior location. Also, access to the anterior calices may be restricted by the side of the operating table.^21,23 Moreover, the distance between the 12th rib and the iliac crest is greater in the prone position and could facilitate nephroscope mobility.

Stone-free and complication rates may vary among positions. The risk of a potential colonic injury is approximately 0.3% for both decubitus positions and seems to be related to patient age, being more frequent in older persons.^23,24 In the initial CROES study, comprising 5803 patients, the prone position achieved a 77% stone-free rate against 70% in the supine position. The small difference reached statistical significancy within the large cohort studied.²² The CROES study is a multicenter trial in which data from several institutions were collected prospectively into a common database from which most information is retrospectively processed. Therefore, some important facts deserve comment: The study is not prospective and is vulnerable to several different kinds of selection bias. Moreover, there was no standardization for initial stone burden characterization, measurement of surgery parameters, evaluation of perioperative complications, and, what is more important, imaging modality for stone-free rate definition. Therefore, the attempt to group several institutions' series to gather a large cohort can lead to unadvertised assemblage of heterogeneous populations. We believe that the CROES study enriches us with a global picture of PCNL outcomes worldwide. Nevertheless, CROES findings have to be analyzed with some criticism because methodologic issues are unquestionably present and quantity may be gained in detriment to quality.

Previous clinical studies pointed out that wound infection, deep vein thrombosis, pulmonary embolism, cardiac arrhythmia, myocardial infarction, respiratory infection, and even death are more likely to develop in obese patients undergoing PCNL.^25,26 Other authors, however, showed an overall stone-free rate of 76% to 88%, similar to those of normal weight patients.^4,8,9 Just recently, Fuller and coworkers²⁷ published the results from the CROES study regarding the influence of BMI in PCNL outcomes. They analyzed 747 patients with BMI >30 and compared them with normal or overweight patients. Obese patients significantly had more comorbidities and complex calculi than nonobese patients. Staghorn stone was found in 29% of their cohort, similar to our figures (overall 24%) and were more common in the obese group. However, they did not apply the Guy classification to further evaluate stone burden in patients with nonstaghorn calculi. Besides, 82% of patients were operated on in the prone position, and no comparison between supine and prone technique was performed. They found no difference between groups with respect to pulmonary or hemorrhagic complications. Intraoperatively, obese patients had more supracostal accesses and lower multiple tracts.

We compared the complete supine position, a variation of the Valdivia position that is gaining preference among urologists, with the traditional prone position. If we consider both groups in our cohort, the percentage of patients with a supracostal access was similar to theirs (14% vs 16%, respectively). We found that patients in the prone position were more likely to need a supracostal tract (25% vs 6.3%, P nonsignificant). That is in accordance with the literature that shows the total supine position to be related to a higher number of punctions in the lower calices.²² Nevertheless, multiple tracts were needed in more patients in our study (24% vs 6.9%, respectively). The reason for that is hard to establish, but it may be related to more complex cases in our cohort. This is not a rare scenario when a flexible nephroscope is not available. Our mean operative time was 164.6 minutes for patients operated on in the prone position and 120.3 minutes among those operated on in the supine decubitus position. As we can see, mean operative time is not significantly higher than that reported by other authors in obese patients.²⁷ Patients with longer surgeries were those with more complex renal stone and concomitant procedures.

A strong aspect of our investigation was the use of CT on POD1, an imaging modality with higher accuracy than ultrasonography or radiography for smaller stone fragments, especially among obese patients, to define stone-free rates. CT is considered the gold standard technique for lithiasis diagnosis, and now is gaining in popularity also for stone-free control after PCNL. Still, grounded concern regarding radiation risks impede it from being widely performed worldwide. Stones were classified according to the Guy stone score, a recently published renal stone classification, which is reproducible and stratifies renal stones according to complexity.¹⁹ Patients operated on in the prone position were older, and previous open surgeries were more frequent among them. The stone burden was similar among groups, but complete staghorn stones (Guy IV) were more frequent among patients operated on in the prone position.

A success rate on POD1 of 54% and 68% in the prone and supine decubitus position was observed, respectively. The final stone-free rate was 83% and 78% for patients operated on in the prone and supine position, respectively. The CROES study showed a stone-free rate of 78% for the obese cohort.²⁷ Our overall final stone-free rate of 77% is very similar to that, and we found no significant differences between the two techniques. If we compare only stones classified as Guy I, II, or III, we find virtually no differences between the two groups. The supine position achieved better results among more complex stones (Guy IV), but the number of patients involved in this category in both groups, especially patients operated on in the supine position, is small. We observed that procedures performed in the prone position presented a significantly higher number of second-look procedures and a transfusion rate of 20%.

Our overall transfusion rate was 8% and is a little higher than the 4.5% reported in the multi-institutional CROES study.²⁷ Yet, transfusion rates of up to 24% are observed in the literature after PCNL.5,^21,22,28 Again, we believe both findings were because of the occurrence of more complex stones in this group and not to the technique itself. If we exclude Guy IV patients, no significant difference in transfusion rate between the two groups is observed. Operative time and hospital stay were significantly shorter with the supine position. This is a great advantage of this technique. Positioning these patients is a demanding and risky maneuver. Physical strength from the operating room staff is needed; the risk of orotracheal intubation loss and peripheral nerve lesion is a reality even with great care by the anesthesiologist.²⁹ A shorter hospital stay contributes to reducing costs. This way, the supine position offers a great advantage over the prone position concerning these two aspects.

Regarding surgical complications, they were similar and had low incidence. Most were minor. Two pleural effusions were observed among patients operated on in the prone position, and this was because of a great number of supracostal accesses, which is used more when dealing with staghorn stones. Serious complications occurred in one patient of each group and were managed clinically with satisfactory outcome.

Our work presents some weak points. First, it is a retrospective study and has potential for section bias. Second, the number of patients is small and albeit being randomly distributed among surgeons, the stones managed in the prone position were significantly more complex. Therefore, the results should be interpreted for that specific population. On the other hand, we must emphasize strong aspects as well: It is the first to compare outcomes between the two decubitus positions in an obese cohort; all patients were evaluated with low-dose CT pre- and postoperatively; although not being conducted in a classic prospective fashion, some kind of randomization takes place when patients are divided according to surgeon preference of one technique over the other; both surgeons are experienced, avoiding learning curve effect; we used the Guy stone score, which is validated for stone complexity comparison and is predictive of PCNL success.

Conclusion

PCNL performed in the prone or in the complete supine position in obese patients presents good and similar outcomes. The supine decubitus position has the advantages of a significantly shorter operative time and hospital stay.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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