Recently, the importance of RPP during PCNL has been implicated as a potential cause of complications including fever, pain, and kidney injury [6,7,8]. Although the study of RPP is in its infancy, the upper threshold for RPP is often set at 30 mmHg (sometimes reported as 40 cmH2O), as pyelovenous backflow is estimated to occur when pressures exceed this level [1, 6, 9]. In our previous clinical study, we measured RPPs in patients undergoing PCNL, and found that postoperative pain scores and hospital lengths of stay were significantly higher if RPPs exceeded 30 mmHg [6]. Similarly, Wu et al. and Zhong et al. reported that an accumulated time of 40–60 s with RPPs > 30 mmHg led to a significantly higher incidence of postoperative fever after mini-PCNL [7, 8].

Although there has been research on elevated RPP, the significance and implications of extremely low RPP are not well elucidated. Having extremely low RPPs can lead to collapse of the pelvicalyceal system, which may limit the surgeon’s visibility and prevent adequate stone clearance [4]. In addition, a collapsed system confers a higher risk of urothelial mucosal injury during ultrasonic or laser lithotripsy and during suction. Any ensuing bleeding would also be more difficult to control due to less tamponade on bleeding capillaries and venules. Therefore, in PCNL, the optimal RPP would be high enough to allow safe and effective surgery, but not too high to cause complications resulting from pyelovenous or pyelolymphatic backflow.

In our current study, we investigated patient position during PCNL to determine the effect upon RPP. We demonstrated that under controlled settings, RPPs are indeed higher during prone PCNL compared to supine PCNL, but in all conditions tested, pressures never exceeded 30 mmHg. This study also found that in either position, the site of renal access (upper, middle, or lower) does not lead to significant alterations in RPP. Additional findings of our study include a 30–70% lower RPP when utilizing a flexible nephroscope compared to a rigid nephroscope. These lower pressures with flexible nephroscopy are in line with prior studies measuring RPPs during prone PCNL in both porcine models and patients during PCNL [4, 6].

Supine percutaneous renal access was first performed and described in 1987 by Valdivia Uria, who subsequently published on a series of 520 patients who underwent supine PCNL [10]. Over the years, many urologists have published their experience with supine PCNL and compared surgical and patient outcomes to prone PCNL. Notably, a prospective multicenter global study compared outcomes between prone and supine PCNL in 5775 patients and reported significantly higher stone-free rates with prone PCNL (77.0% vs 70.2%; p < 0.0001). However, they also found that prone PCNL had significantly higher fever rates (11.1% vs 7.6%; p < 0.001) [11]. Similarly, Kasap et al. reported that infective complications occurred at a significantly higher rate after prone PCNL compared to supine PCNL (18% vs 7.5%; p = 0.034), and prone position was found to be an independent risk factor for postoperative infections (OR = 4.5; p = 0.02) [12].

These infections are hypothesized to be due to higher RPPs in prone PCNL causing more pyelovenous backflow and leading to increased bacterial translocation into the bloodstream [12]. Although pressures never exceeded 30 mmHg in our benchtop study, we have indeed confirmed that RPP is higher in prone PCNL when compared to supine and this may partially explain the higher rates of infections with prone PCNL. As previously mentioned, our understanding of RPP is still in its infancy and other conflicting studies have not demonstrated increased fever with prone PCNL. A prospective randomized study by Al-Dessoukey et al. compared 102 prone PCNL patients to 10 l supine PCNL patients and reported no significant difference in fever rates (5.9% vs 5%; p = 0.77) [13]. Similarly, Wang et al. found no difference in postoperative fever between prone and supine PCNL [14].

With regards to stone-free rates, Al-Dessoukey et al. did not find a difference between prone and supine PCNL (87.3% vs 88.1%; p = 0.85) [13], however, Wang et al. reported that stone-free rates were significantly lower with supine PCNL compared to prone PCNL (73.3% vs 88.7%; p = 0.03), with 10% of patients in the supine group requiring a second look [14]. Conflicting results have also been reported by several meta-analyses. In one analysis comparing prone and supine PCNL, 13 studies were included, and stone-free rates were significantly lower in supine PCNL (OR = 0.74; p < 0.001) [15]. However, two other meta-analyses showed no difference in stone clearance between prone and supine PCNL [16, 17]. One recently published study specifically compared mini-PCNL in the prone and supine positions and did not find any differences in either stone-free rates or complication rates, but found that both 6-h and 24-h pain scores were significantly lower in supine mini-PCNL compared to prone (p < 0.001) [18].

Performing PCNL in the supine position has several reported advantages [19,20,21]. Patient positioning is less complicated and less time-consuming as there is no need to turn patients over. This is particularly important in patients with contractures or mobility issues and in morbidly obese patients. Having patients in the supine position also facilitates airway management and maintenance of cardiovascular function. In supine PCNL, the renal access tract is more horizontal in position allowing for spontaneous stone fragment evacuation and more fluid drainage, explaining the lower RPPs observed in our study. This also explains the faster operative times reported by some studies [15, 16]. In addition, less fragments will migrate down the ureter in supine PCNL due to the lower position of the kidney in relation to the ureter, also explaining the shorter operative times.

Additional advantages that have been described for supine PCNL over prone PCNL include the feasibility of endoscopic combined intrarenal surgery (ECIRS), as well as having the surgeon’s hands outside of the primary radiation beam due to the location of the access site and tract [21]. However, these can be successfully overcome in prone PCNL. In one study, prone split-leg position with ureteroscopy allowed for successful renal access using an ECIRS approach in a series of patients [22]. Laser-guided access can significantly reduce fluoroscopy time, allowing needle insertion in the prone position without the need for continuous fluoroscopy [23]. Use of needle holders can also successfully guide needle insertion during prone PCNL with significantly reduced radiation exposure to the surgeon’s hand and to the patient [24].

Conversely, supine PCNL has several drawbacks over prone PCNL. Having patients in the supine position leads to a narrower surgical field and window of access into the kidney. Upper pole access is particularly challenging in supine PCNL, due to its more medial and posterior location, and there is a higher risk of spleen and liver injury [19, 25]. In another prospective study reporting specifically on 1311 patients with staghorn calculi, there was significantly higher upper pole access in the prone group (12.6% vs 3.6%; p < 0.001), significantly longer operative times with supine (123.1 vs 103.2 min; p < 0.001), and higher stone-free rates in prone PCNL (59.2% vs 48.4%; p < 0.001) [26]. The challenges with limited upper pole access, together with the low pressures and system collapse seen in supine PCNL, may explain these longer durations and lower clearance rates in patients with complex staghorn stones. Additional implications include longer tract lengths in supine PCNL that may limit maneuverability, which becomes even more difficult in obese patients due to more adipose tissue and longer tracts [27, 28].

Ultimately, the decision to perform PCNL in the prone or supine positions depends on both surgeon and patient factors. These include surgeon preference, training, and experience as well as patient habitus, stone burden and location, and the presence of certain congenital anomalies that may preclude the ability to perform PCNL in a specific position. Despite the increasing popularity of supine PCNL, prone PCNL still remains the most common position, with 47.5% of urologists utilizing prone position exclusively, 16.3% using supine exclusively, and 36% using both as reported by the recent Endourological Society global census [29]. Regardless of their chosen position, it is important that surgeons remain cognizant of the advantages and drawbacks of either position, and how the differences in RPP may affect outcomes.

Limitations of this study include its benchtop design and reliance on kidney models which do not accurately reflect the distensibility of the human kidney and pelvicalyceal system. Outcomes that are influenced by the differences in pressure could not be assessed in our study, including impaired visibility, more bleeding, stone clearance, and infectious complications. In addition, patient factors that could influence positioning or RPPs were not accounted for in this study, such as body habitus, stone burden, and anatomic variations of the kidney. Finally, technical differences and the use of various instruments that can impact RPP were not investigated in this study. Despite these limitations, our study was able to accurately document the magnitude of RPPs in both prone and supine positions for PCNL, utilizing different access sites and different types of endoscopes in a controlled setting with fixed measurements, irrigation settings, and testing environment.