Grade I
Solitary stone in mid/lower pole
Or solitary stone in the pelvis with simple anatomy
Grade II
Solitary stone in upper pole
Or multiple stones in a patient with simple anatomy
Or solitary stone in a patient with abnormal anatomy
Grade III
Multiple stones in a patient with abnormal anatomy
Or stones in a calyceal diverticulum
Or partial staghorn calculus
Grade IV
Staghorn calculus
Or any stone in a patient with spina bifida or spinal injury
The goal of PNL is to obtain a stone-free status with minimal morbidity and optimal cost effectiveness. Therefore, when evaluating the results of the surgery, it is necessary to assess several outcomes, which until now have been mainly stone-free status, operative time, morbidity, costs, and quality of life. However, there is a substantial variability in the reporting of such outcomes among the different studies [12, 13].
22.2.1 The Stone-Free Rate
22.2.1.1 The Cut-Off?
Regarding the stone-free status, some authors are faithful to the strict criterion of no fragments visualized on imaging, while others employ a more permissive definition tolerating small, passable, residual stone fragments (CIRF = clinically insignificant residual fragments). In these latter studies, the residual fragment size varies from 2 to 10 mm [12]. Furthermore, nearly one-third of papers evaluating surgical management of urinary calculi do not define stone-free status at all [12].
22.2.1.2 How?
A further difficulty arises from the different sensitivity and specificity of the methods employed for the assessment of residual fragments. These include intraoperative flexible nephroscopy [14], postoperative plain film of the abdomen (KUB), ultrasound (US), and computed tomography (CT). The reliability of these different methods is variable; therefore, the stone-free rate can be overestimated when using a poorly sensitive method. For example, KUB has been found to overestimate stone-free status by 35 % [14]. Although unenhanced computer tomography is the gold standard because it has the best sensitivity and specificity, it is not systematically used because of its cost and high radiation exposure.
22.2.1.3 When?
The timing at which stone-free status should be explored after PNL is also debatable. Many studies report the stone-free rate at 3 months, believing that during this period, most small fragments will pass. Others argue that in the case of PNL, all patients should undergo an immediate postoperative CT scan, before the nephrostomy is removed. This would allow selecting patients in whom a percutaneous second look would be beneficial.
22.2.2 The Operative Time
Operative time is also an ill-defined outcome variable. Many studies report it as the time between the first attempt to puncture the kidney and the suturing of the nephrostomy tube. However, this type of evaluation does not take into account the operative room occupation, which includes also patient positioning, endoscopic access to the bladder, and retrograde pyelography.
22.2.3 The Morbidity
Concerning complications, the modified Clavien classification seems to be worldwide accepted and increasingly used [15]. Although this classification demonstrates a high validity, it has been found somehow limited by its low inter-rater reliability for minor complications [16]. According to this classification system, perioperative complications are stratified into five grades (Table 22.2).
Table 22.2
Clavien classification of complications
Grade 1 | All events that, if left untreated, would have a spontaneous resolution or need a simple bedside intervention |
Grade 2 | Complications requiring specific medications, including antibiotics and blood transfusions |
Grade 3 | Complications necessitating surgical, endoscopic, or radiologic intervention: |
3a without general anesthesia | |
3b under general anesthesia | |
Grade 4 | Neighboring organ injury and organ failures |
Grade 5 | Death |
Standardization of all these criteria would be essential for a more accurate comparison of treatment modalities and outcomes. Unfortunately, at present, there is no consensus on how to report the results of PNL. Therefore, comparison of different series is able to provide only a limited level of evidence. Prospective randomized studies are also available, but only in limited number. Additionally, urologic papers never report clearly on other parameters such as intra- and postoperative anaesthesiological problems.
22.3 Comprehensive Review of Case Series
Two review articles identified a “trend in favour of better outcomes in the prone position over the supine position” [9] and recommend supine position only in “carefully selected patients” [10].
De la Rosette and collaborators conducted a Medline search for articles published during the 10-year period since the first report of supine PNL. Based on the hypothesis that supine position is more fit for physically compromised patients and complex calculi, this study focused on obese patients with a high proportion of staghorn calculi. The authors collected and analyzed 13 manuscripts, nine with supine [1, 3–5, 7, 8, 17–19] and four with prone position [20–23]. They concluded that outcomes in nonobese patients and with small-sized stones seem to favor the supine approach. To assess the outcomes in obese patients and with staghorn calculi, the authors calculated the weighted means for each position separately. This comparison showed a slightly better success rate (84.7 % vs. 81.2 %) and a significantly shorter operative time (79.1 min vs. 94.1 min) in prone position versus supine position respectively. However, a more recent study comparing prone and supine PNL in patients with a body mass index (BMI) >30 kg/m2 did not confirm any difference in stone-free rates, rather demonstrating an advantage of the supine position in terms of significantly shorter operative time and hospital stay [24].
22.4 The Global Study of the Clinical Research Office of Endourologic Society (CROES)
The largest available observational prospective database was collected in the global study on PNL organized by the Clinical Research Office of Endourologic Society (CROES). Investigators of 96 centers worldwide contributed to the database. Each center was invited to include all consecutive patients during a 1-year period. Five thousand seven hundred and seventy-five patients were eligible for the study.
22.4.1 Patient Positioning
Four thousand six hundred and thirty-seven patients (80.3 %) had their PNL in prone position, whereas 1,138 patients were operated supine (19.7 %). The distribution of supine versus prone position exhibited major regional differences worldwide. While in Europe and South America, respectively, 23.5 % and 98.5 % of the patients were operated supine, patients in North America, Asia, and Australia were almost exclusively treated in prone position (98.5, 98.1, and 100 % of patients, respectively).
22.4.2 ASA Score
Concerning patient demographics, interestingly, an ASA score of 1 was more common among patients in the prone position (54.7 % vs. 46.8 %), whereas an ASA score of 2 was less frequent when compared to supine position (33.4 % vs. 42.1 %).
22.4.3 Renal Access and Tract Dilation
In the prone group, access through the upper pole and access above the 12th rib were more often employed (11.4 % vs. 4.0 % and 17.6 % vs. 5.5 %). Multiple punctures were also more frequent in the prone than in the supine group (9 % vs. 4.1 %). This may be explained by the possibility of a simultaneous ureteroscopy in supine position, offering retrograde access to the ureteropelvic junction or the upper calyces, making an upper pole puncture unnecessary. However, the report did not mention how many patients in the supine group had simultaneous retrograde ureterorenoscopy.
There was also a significant difference in the tract dilation method with the balloon, more frequently used in the supine positioned patients (43.8 % vs. 40.3 % p = 0.04).
22.4.4 Operative Time
The mean operative time was significantly shorter in the prone group with 82.7 min versus 90.1 min in the supine (p < 0.001). This is somehow unexpected, because one of the supposed advantages of supine position is the elimination of the necessity of patient repositioning after retrograde ureteric catheter insertion, which should shorten operative room occupation. A possible explanation is the dissimilar definition of “operative time” by different investigators. Analyzing the different publications deriving from the CROES database, the operative time is determined based on the sum of the different phases of surgery by some investigators [25], but others recorded the operative time as the time from the first puncture to the completion of the stone removal [26]. If some investigators in the global study used the second definition, this would mean that patient positioning (which should be shorter in supine) was not always taken into consideration during statistical analysis.
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