Minimally invasive percutaneous nephrolitholapaxy (PCNL) as an effective and safe procedure for large renal stones


Mohamed F. Abdelhafez, Department of Urology, Tuebingen University, Konrad-Adenauer Street 48, Tuebingen 72072, Germany. e-mail:


Study Type – Therapy (case series)

Level of Evidence 4

What's known on the subject? and What does the study add?

The minimally invasive percutaneous nephrolitholapaxy (MIP) has shown high efficacy and safety for the management of small renal stones. It was initially developed to overcome a gap between the minimally invasive extracorporeal shockwave lithotripsy and invasive conventional percutaneous nephrolitholapaxy (PCNL) in the management of low stone burden but there is debate as to whether the MIP is also effective for larger stones.

The present study shows the high efficacy and safety of MIP, which is comparable to conventional PCNL in the treatment of stones of >20 mm, including complex staghorn stones.


  • • To evaluate the safety and efficacy of minimally invasive percutaneous nephrolitholapaxy (MIP) in the management of large and complex renal calculi.


  • • From January 2007 to March 2011, 73 patients with 83 renal units with large renal stones (>20 mm in diameter) were retrospectively evaluated.
  • • Stones were classified into simple (isolated renal pelvis or isolated calyceal stones) or complex (partial or complete staghorn stones, renal pelvis stones with accompanying calyceal stones).
  • • Stone-free rate, complications according to the modified Clavien system, decrease in haemoglobin, creatinine level, operative duration and hospital stay were compared for simple and complex renal calculi.


  • • The mean (sd) stone size was 36.7 (23.37) mm and mean operative duration was 99.2 (48.3) min.
  • • In all, 65 cases (78.3%) were stone-free after the first procedure and another 14 needed an auxiliary procedure (four second-look percutaneous nephrolitholapaxy, nine ureterorenoscopy, and one extracorporeal shockwave lithotripsy) to become stone-free, resulting in a 95.2% stone-free rate.
  • • Complications occurred in 22 procedures (26.5%), 17 of them were Clavien Grade 1 or 2 (20.5%), five were Grade 3 (6%). There were no Grade 4 or 5 complications.
  • • The only significant difference between complex and simple stones was the stone-free rate (96.9% vs 66.7%, P= 0.001).


  • • The MIP technique is effective and safe for larger stones with low morbidity, good success rate and reasonable operative duration.

miniaturized percutaneous nephrolitholapaxy


extracorporeal shockwave lithotripsy




minimally invasive PCNL


Clinical Research Office of the Endourological Society


operating time (time from puncture to closure of the access tract)


Miniaturized percutaneous nephrolitholapaxy (miniPCNL) was first introduced by Jackman et al. [1] as an alternative treatment method compared with conventional large bore (24–34 F) PCNL in a paediatric population. Using a miniaturized nephroscope and an 11 F access sheath, they treated seven patients with a median stone burden of 1.2 cm2. In the subsequent years the technique was adopted for adult patients and was initially mainly used for smaller lower calyceal and diverticular stones, filling the therapeutic gap between extracorporeal shockwave lithotripsy (ESWL), flexible ureterorenoscopy (URS) and conventional PCNL [2]. Meanwhile the technique has further been modified towards a ‘minimally invasive PCNL’ (MIP), characterised by the use of a 12 F nephroscope and an 18 F access sheath, a continuous low-pressure irrigation stream allowing for quick stone clearance and immediate closure of the access tract without placing a nephrostomy tube [3]. Schilling et al. [4] 2010 reported that this technique had completely replaced conventional PCNL in their department.

The efficacy and safety of MIP in treating patients with a large stone burden and complex staghorn stones has been questioned, mainly due to the limited diameter of the miniaturised access sheath, putatively leading to reduced visibility, prolonged operative duration and reduced stone-free rate [5]. Not only stone size, but also stone composition, distribution, renal function, renal abnormalities as well as associated UTI influence treatment success [6]. In the present study stones were classified as simple (isolated renal pelvis or isolated calyceal stones) or complex (partial or complete staghorn stones, renal pelvis stones accompanying calyceal stones), regardless of their size, as described by Tefekli et al. [7] 2008.

The goal of this retrospective analysis was to report our experience in treating patients with complex large renal stones (>20 mm) using the MIP technique, focusing on stone clearance, complications and retreatment rate.


All patients treated for large renal stones (greatest diameter on plain X-ray or CT >20 mm) using MIP technique at the Department of Urology of the University Hospital Tuebingen from January 2007 to March 2011 were included in the study.

All patients underwent MIP prone using the modular miniature nephroscope system with automatic pressure control by Nagele (Karl Storz, Germany) as follows [8,9]. After retrograde placement of a ureteric balloon occlusion catheter proximal to the PUJ in lithotomy position, patients were placed prone with an inflatable cushion placed just caudally to the xiphoid. Percutaneous access was obtained under ultrasonographic and fluoroscopic guidance. Single-step dilatation with a 16 F metal dilator was applied and then an 18 F metal Amplatz sheath was introduced. Using the 12 F nephroscope, stones were fragmented by pneumatic ballistic lithotripsy (Swiss Lithoclast-EMS, Switzerland) under vision and stone fragments were evacuated under continuous irrigation without additional pressure or suction using the hydrodynamic effects of the PCNL system. Only if fragments adhered to the parenchyma, a 2.4-F tipless nitinol basket (Zero Tip, Boston Scientific, USA) was used for stone retrieval. At the end of the procedure, the balloon occlusion catheter was removed and a JJ stent was placed antegradely. The Amplatz sheath was withdrawn and usually the tract was closed using a gelatine matrix haemostatic sealant (GMHS; Baxter, Germany). The skin incisions were closed with adhesive skin closures.

Generally renal access was gained through the posterior lower calyx. If complete stone clearance via this access was not achievable, either flexible nephroscopy (Flex X2, Karl Storz, Germany) and laser lithotripsy through the 18 F sheath was performed or, in case of a remaining large stone burden, a second access using a 15 F Amplatz sheath was placed under ultrasonographic and fluoroscopic control in the middle or upper calyx. In cases of remaining large fragments at the end of the manoeuvre, the access tract was not closed but a 12 F nephrostomy tube was placed to allow for a second-look PCNL 2–3 days later.

Patients were examined with plain X-ray (of the kidneys, ureters and bladder) and abdominal ultrasonography on the first postoperative day. Haemoglobin level, serum creatinine and electrolytes were monitored pre- and postoperatively.

The clinical records were retrospectively reviewed for the following clinical parameters: stone complexity, operative duration (defined as the time from puncture to closure of the access tract), fluoroscopy time, decrease in haemoglobin level, hospital stay, stone-free rate and complications.

Stone complexity was recorded according to the scoring system described by Tefekli et al. [7] in 2008. Stones were classified as simple (isolated renal pelvis or isolated calyceal stones) or complex (partial or complete staghorn stones, renal pelvis stones with accompanying calyceal stones) regardless of size. Patients were considered ‘stone-free’ in the absence of any detectable stone fragment upon nephroscopy at the end of the procedure and on postoperative X-ray and ultrasonography. A ‘complication’ was defined as any adverse event intraoperatively or ≤30 days after the procedure. The grade of complication was determined on the basis of the Clavien classification and its modification for percutaneous procedures [7].

Data are expressed as mean (sd) and P < 0.05 was considered to indicate statistical significance.


Between January 2007 and March 2011, 73 patients (83 renal units) with renal stones of >20 mm were treated with MIP in the Department of Urology, University Hospital Tübingen. The mean (sd, range) stone size was 36.7 (23.37, 20–145) mm. In all, 51 stones (61.4%) were classified as complex stones and 32 (38.6%) as simple stones.

In 73 cases (87.9%) a single tract procedure was carried out. In eight cases (9.6%) two tracts were used and in two cases (2.4%) three access sheaths were inserted simultaneously. Access was gained through the posterior lower calyx in 82 cases (98.8%). Puncture of the middle calyx was performed in nine cases (10.8%) and of the upper calyx in four cases (4.8%).

All punctures of the lower and the middle calyx were subcostal; none of the punctures was above the 11th intercostal space.

The mean (sd) operative duration for all 83 renal units was 99.2 (48.3) min and fluoroscopy time 214 (147.4) s. The mean (sd) operative duration in complex stones was not significantly longer than in simple stones, at 104.7 (51.6) vs 90.7 (42.1) min (P= 0.2) and fluoroscopy time did not differ significantly between the two groups, at 210 (137.8) vs 222.4 (163.4) s for complex and simple stones, respectively (P= 0.4).

The mean (sd, range) decrease in haemoglobin level was 1.7 (1.2, 0.1–5.3) g/dL, with no significant difference between the two groups, at 1.7 (1.3) vs 1.7 (1.1) g/dL for complex and simple stones, respectively (P= 0.5). Only one patient with complex stones received blood transfusion after a second-look PCNL.

In all, 65 of the 83 cases (78.3%) were stone-free after the first procedure (primarily stone-free). In 14 (16.9%) cases auxiliary treatment methods rendered the patients stone-free. Four patients (4.8%) underwent a second-look MIP, nine (10.4%) flexible URS and one (1.2%) ESWL, thus the total stone-free rate with an auxiliary procedure was 79/83 renal units (95.2%). Four patients (4.8%) required more than one auxiliary procedure to be stone-free. There was a highly significant difference in the primary stone-free rate for complex stones 66.7% (34/51 renal units) vs 96.9% (31/32 renal units) in simple stones (P= 0.001; Table 1).

Table 1. Univariate analysis of patients' characteristics and treatment results for simple and complex stones
VariableSimple stonesComplex stones P
  • *

    Statistically significant.

Total number3251 
Mean age, years4847.430.345
Gender, male: female, n20:1228:230.469
Multiple access MIP, n190.08
Mean OR-time, min90.7104.70.243
Mean fluoroscopy time, s222.0210.00.50
Mean decrease in haemoglobin, g/dL1.71.70.594
Stone free, n (%)   
 Primary31 (96.9)34 (66.7)0.001*
 With second-look MIP4 (74.5)
 With any auxiliary procedure32 (100)47 (92.2)0.9
Complications, n (%)12 (37.5)10 (19.6)0.07

In all, 22 complications occurred during 83 MIP procedures (26.5%, Table 2). In 17 cases (20.5%) complications were Clavien grade I or II and were managed conservatively, five complications (6.0%) necessitated endoscopic intervention (Clavien grade IIIb). There were no grade IV or V complications (Table 2). Grade I complications were more likely to occur in simple stones (33.3% vs 5.8%, P < 0.01). However, grade IIIb complications were more likely to occur in complex stones (13.9% vs 3.1%, P < 0.6). Table 2 gives an overview of the complications.

Table 2. Intra- and postoperative complications for simple and complex stones
 Clavien gradeSimple stone, nComplex stone, nTotal, n
Gross haematuriaII101
Fever >38 °CI8311
Bladder haematoma requiring cystoscopic evacuationIIIb112
Urinoma requiring JJ stentIIIb011
Extravasation treated conservativelyII213
Obstruction requiring JJ stentingIIIb022
Perinephric haematomaII011
Total 121022


Percutaneous access to the renal collecting system leads to a high stone-free rate and is therefore recommended as the treatment of choice for renal stones measuring >20 mm in diameter [10]. PCNL generally is considered a demanding procedure although safe and effective in experienced hands [11]. Conventional PCNL is usually carried out through a renal access with a diameter of 24–34 F and a semi-closed irrigating system. Reducing the diameter of the renal access sheath led to the implementation of the miniPCNL-technique [12]. Meanwhile, it has been shown that miniPCNL can reduce blood loss and transfusion rate compared with conventional PCNL [13]. In an attempt to further reduce the morbidity of the procedure, the MIP has been established, characterized by a small-bore renal access (18 F), one-step dilatation technique, a continuous low-pressure irrigation allowing for rapid stone retrieval without the use of stone forceps or baskets and finally the direct closure of the renal access without the placement of a nephrostomy tube. The safety, feasibility and efficacy in the treatment of small renal and lower calyceal calculi has been shown earlier [3].

Although the MIP concept leads to complete stone clearance in 92.9% of patients with renal stones of <20 mm [3], the application in patients with a larger stone burden has been questioned. It has been argued that the smaller access and putatively reduced irrigation flow leads to poorer visibility, difficulties in handling endoscopic graspers and therefore reduced stone clearance [5,14]. The goal of this retrospective analysis was to determine the safety and efficiency of the MIP concept in treating renal stones with a diameter >20 mm.

The primary stone-free rate for all MIP procedures in the present patient population was 78.3%, with a total stone-free rate of 95.2% with an auxiliary procedure (need for auxiliary manoeuvre: 16.9%). These results are similar to the stone-free rate in a large multi-institutional international prospective trial carried out by the Clinical Research Office of the Endourological Society (CROES). In 5803 patients undergoing conventional PCNL the stone-free rate was 75.7% with a re-intervention rate of 15.5% [15]. However, the patient collective is very heterogeneous, including different surgical techniques, e.g. prone and supine PCNL, as well as small stones with a diameter of <20 mm and complete staghorn stones. Several complicating factors besides stone size influence the success of the PCNL procedure: Tefekli et al. [7] suggested a classification of simple and complex renal calculi, with complex renal stones representing any stone with staghorn formation or pelvic stones and accompanying calyceal stones. In the present study population, 61.4% cases had complex renal calculi. Not surprisingly the primary stone-free rate in patients with complex stones was significantly lower than in patients with simple renal stones (66.7% vs 96.9%, P= 0.001). This is also in accordance with the aforementioned CROES study in which the stone-free rate in patients with staghorn stones was 56.9%; conversely, when all patients with staghorn stones were excluded from the analysis, the stone-free rate increased to 82.5% [15]. To meet the challenge of large stone burden or complex stone disease, various treatment approaches have been reported. In an analysis of 127 consecutive patients with a mean stone size of 23.8 mm undergoing combined retrograde and percutaneous stone treatment, Scoffone et al. [16] achieved an 81.9% primary stone-free rate. After a second-look PCNL using the same renal access during the same hospital stay, 87.4% of their patients were rendered stone-free. In comparison with contemporary studies, the concept of MIP shows comparable stone-free rates, even for complex renal stones. Two comparative studies between miniaturized and conventional PCNL failed to show a significant difference in stone-free rate between the two methods [13,17]. Considering that the mean stone size in the present analysis (36.7 mm) was higher than in the aforementioned studies, it can be assumed that stone clearance is not affected by the smaller diameter of the access tract. The stone retrieval using the ‘vacuum cleaner effect’ of continuous low-pressure irrigation [18] without the need for endoscopic manipulation with stone graspers might in fact contribute to an effective stone clearance and accelerate the procedure.

There is conflicting data as to whether miniaturization of the access sheath necessarily leads to longer operating time (OR-time) for larger stones. A comparative trial in 180 patients undergoing either conventional or miniPCNL found significantly longer OR-times for miniPCNL in simple (89.4 vs 77.0 min), staghorn (134.3 vs 118.9 min) as well as multiple stones (113.9 vs 101.2 min) [17]. In contrast, in a prospective comparative study between conventional and miniPCNL of 50 consecutive patients, Knoll et al. [13] noted no significant difference in OR-time between patients operated with an 18 F access sheath than with a 26 F sheath (miniPCNL 48 min vs PCNL 57 min). However, the authors argue that this fact might be attributed to differences in stone burden between the two groups. The mean operating duration in the present study was 99.2 min. Several aspects might lead to decreased OR-time in MIP, namely the one-step dilatation of the access tract and the ‘vacuum cleaner effect’ allowing stone retrieval without the need for stone graspers. The so called ‘vacuum cleaner effect’ is a hydrodynamic effect due to the difference between the inner diameter of the access sheath and the outer diameter of the nephroscope. During continuous irrigation a laminar low-pressure flow occurs through the access sheath. Turbulences occurring at the tip of the nephroscope allow suction of stone fragments from the renal collecting system.

One major concern in PCNL is significant blood loss and the need for blood transfusions. In the present investigation only one of the patients had to receive transfusions (transfusion rate 1.2%). This patient underwent multiple accesses MIP for a staghorn calculus. Contemporary studies state transfusion rates of ≈4.5% and 9% for non-staghorn and staghorn stones, respectively [19]. A reason for the lower transfusion rate in MIP might be the smaller parenchymal trauma and the reduced risk of injuring larger segmental renal vessels with a small-bore dilator during establishment of the access tract. The reduced transfusion rate has also been reported previously [17]. Although blood loss seems to depend on stone size and stone complexity, in the present study there was no significant increase in blood loss with increase in stone size (P= 0.4) and no significant difference between simple and complex stones (P= 0.5).

During 83 MIP procedures, 22 complications occurred (26.5%). Most of the complications were modified Clavien grade I or II and could be managed conservatively. Grade I complications (mostly temperature >38.0 °C) were significantly more frequent in simple stones. However, there was a tendency to higher grade complications in complex stones. The incidence of urinoma and haematoma was higher in complex stones and these complications were frequently associated with fever; however, these complications were graded >grade I. This might explain the statistical difference in frequency of complications in these two entities. The five grade III complications (6%) could be managed endoscopically. Severe complications (grade IV of V) did not occur throughout the 83 procedures. The complication rate reported here is in accordance with published rates. Although the overall complication rate in PCNL is high, generally higher grade complications (>grade II) are rare [11].

The present study investigated the feasibility of MIP for treating large renal stones. Although the data indicate that a minimally invasive approach results in a similar stone-free rate with an acceptable OR-time and a low morbidity, the limitations of a retrospective study without a comparative control population have to be considered. However, based on the relatively large number of patients with a large stone burden, a comparison with contemporary publications can help to judge the advantages and disadvantages of the minimally invasive approach.

In conclusion, the present retrospective analysis of 83 MIP procedures shows that this approach is not only effective in small stones but also in patients with a large stone burden and complex stones. The method has a stone-free rate comparable with large bore conventional PCNL and is similarly effective in complex renal calculi. There is a tendency to a longer operative duration; however, one major advantage is the low blood loss and transfusion rate, as well as the low rate of high grade complications. The MIP may be equally effective as conventional PCNL independent of stone size. Further prospective, controlled, randomised studies to properly evaluate the method are recommended.


The authors thank Miriam Germann for critically proof reading and correcting the manuscript.


None declared.