Since Fernstrom et al. and Castaneda-Zuniga et al., who introduced and performed PCNL in the prone position and reported excellent results with minimal complication rates(15, 16), PCNL has become the gold standard procedure for the treatment of large or staghorn kidney stones, and all surgeons have positioned their patients in the classic prone position for more than 10 years. Because of the practical advantages of the prone technique, which is easier identification of the correct calyx while theoretically minimizing injuries to adjacent structures, the prone position remains the dominant position for PCNL till today. Over the past few decades, various modifications of the classic prone position have been made, including, but not limited to, reverse lithotomy position(10), PSL position(9, 17) and prone flexed position(18), which not only provides a percutaneous renal access area as large as TP position, but also provides the possibility to perform PCNL and ureteroscopic procedures simultaneously.
In fact, the reverse lithotomy position(10) is the early form of the PSL position. Two universal stirrups were used to fix the patient's thighs and knees bilaterally only for female patients. As for the common PSL position(9, 17), the lower extremities are placed on two padded adapters in the plane of the operative table that allows the extremities to be split by as much as 45°. The drawbacks of this position have been described above, and flexible ureteroscopes should usually be used for retrograde management. The prone-flexed modification(18) increases the area for puncture and flattens the flank, but it may further increase airway pressure in obese patients and may increase intraocular pressure, resulting in ischemic optic neuropathy, necessitating a reduction in patient flexion. Generally, the disadvantage of these positions is that they tend to cause excessive abduction of the hip joints and that the patient's thighs would interfere with the manipulation torque of the ureteroscopy. Therefore, based on the reverse lithotomy position and the operational requirements for retrograde ureteroscopy, we modified it to focus on the flexion of only the contralateral knee joint. The patient's ipsilateral leg showed slight splitting but did not need to be too abductive. Through the flexibility of the fixing system to be adjusted in real time by the scrubbed surgeon, the abduction and external rotation of the bilateral hip joints were significantly reduced, which was more consistent with the physiological state of the lower extremities. Kwon et al. (19) examined the effects of knee flexion angle on hip extensor muscle activity and suggested that the maximum torque in hip extension and muscle activities of the biceps femoris and semitendinosus were significantly higher at 0° but decreased at knee flexion angles of more than 60°. We maintained contralateral knee flexion angles of approximately 60° in the prone position. We also recorded the postoperative Braden scale scores of the two groups, which did not differ significantly. After retrograde URS ureter catheterization, the support arm could be adjusted to relax the leg freely by the scrubbed surgeon if needed.
In our study, the main advantage of KF-PSL over TP is the reduction in operative time. Statistically significant reductions in the total operative time and pre-lithotripsy time were evident in favor of the KF-PSL group with a mean time gain of 27 min. Prolonged operation time is frequently associated with increased complication rates and is thus a crucial factor in choosing a surgical technique(20). Some studies have shown that operative time is an intraoperative factor that increases the risk of sepsis(21, 22). Hence, the KF-PSL position seems to decrease the rate of postoperative urosepsis, but more cases need to be included in further studies to prove this deduction. Actually, the real-time savings came from the pre-lithotripsy time, which can be attributed to patients’ repositioning, re-prepping, and re-draping as well as surgeons’ re-scrubbing and gowning. Flipping-over patients into the TP position after induction of anesthesia consumes time and effort and has some potential for complications, which is unwelcome to both Anesthesiologists and Urologists(3, 8, 23), it might be risky for patients with respiratory or cardiac problems and markedly obese patients. Another time-saving factor of the pre-lithotripsy time was that retrograde URS uretericatheterization and anterograde percutaneous renal puncture could usually be performed simultaneously. In the early stages, performing retrograde URS simultaneously with the percutaneous procedure requires experience, because the rotation of the trunk produces a relatively unfamiliar position and direction for ureteroscopy. However, in our experience, the learning curve of retrograde access with a ureteroscope in the KF-PSL position is not long, and a junior attending physician can master the technique after two to three operations.
The SFRs of the two groups were similar (p > 0.05), even though the KF-PSL group seemed to be higher (84.3% vs. 72.2%). More data have been collected, and further studies are needed to confirm this conclusion. This SFR was also similar to other studies on common PSL positions, from which Liu et al. (3) reported 72.5% or Hamamoto et al. (9, 24) as 81.7% with total ECIRS. Similarly, Ahmed et al. (8)]compared PCNLs performed in split-leg (SL)-modified lateral position(MLP) and those performed in TP and showed that the SFR was comparable between groups: 75.4% in the SL-MLP group and 77.8% in the TP group (p = 0.755). Even for the supine versus prone position, two earlier meta-analyses by Liu et al. (25) and Wu et al. (26) showed no difference in SFRs in these positions. Thus, it seems that both prone and supine PCNLs have similar SFRs, but more randomized clinical trials are needed. The nine patients who needed re-look PCNL were patients with a large stone burden and required secondary surgery for the sake of controlling operation time. Three cases of ECIRS were performed, and no ancillary URS was needed in the KF-PSL group, while three cases of secondary URS lithotripsies needed to be performed in the TP group.
In the study by Liu et al. (3), there were still two cases of retrograde access for ureteral catheterization requiring the help of flexible cystoscopy in the common PSL position group because of the difficulty in locating the ureteric orifice with the ureteroscope. In our study group, however, no fURS was needed for the ECIRS procedure because the KF-PSL provided sufficient space for the torque to manipulate the semi-rigid scopes. In some cases with ureteral stones, the stones could be pushed back to the renal pelvis for anterograde lithotripsy. Coincidentally, the ureter was found straighter in the prone position. Hamamoto et al. (9)performed three-dimensional CT to determine anatomic variations of the ureteral location in both the supine and prone positions and showed that the normally curved ureter between the orifice and the UPJ straightens when the patient is in the prone position as a result of ventral movement of the kidney and ureter due to gravity. In contrast, the ureter in the UPJ may bend toward the back in the supine position. Therefore, the straightened ureter facilitated the insertion and retrograde proceeding of the ureteroscope more smoothly and pushing back of the ureteral stones more easily in the KF-PSL group than in the TP group. Regardless of cost, fURS is, of course, suitable for ECIRS performed in this position, especially when disposable fURS has been increasingly used. However, cost problems can be a limitation of real-time ECIRS using flexible scopes. In most cities of China or other countries such as South Korea, there is a restriction that the same renal stone treatment in unilateral cases cannot require the cost of PCNL and RIRS simultaneously (27). Therefore, it is possible that the operation cost problem will become a burden to the hospital if more cases of ECIRS are performed(27). However, in the KF-PSL position, the commonly used semi-rigid URS could present a cost-effective advantage that it can easily be adopted for ureteral lithotripsies if needed.
In the later stages of the surgical procedure, the simultaneous anterograde-retrograde approach afforded some other advantages. There were 8 cases with CIRF in the TP group and only 2 cases in the KF-PSL group, although the difference was not statistically significant (Table 3). Since both semi-rigid or flexible instruments could be used in the KF-PSL position, if necessary, retrograde irrigation might help to easily flush out small stone fragments through the PCNL tract. This simultaneous antero-retrograde approach afforded better visibility because of the use of ureteroscopic and nephroscopic irrigation.
Retrograde access was also helpful in confirming the position of the distal end of the double-J stent when the bladder could be expediently re-looked by URS, which could ensure good maintainability of urinary drainage and avoid ureteric submucosal placements. In the present study, all the distal loops of the stents were in the right circle state in the KF-PSL group because we checked or adjusted them at the end of the operation. However, the TP group had no such conditions. Abdelaal et al. (28)studied medication on double J stent-related symptoms and showed that the position of the stent with respect to the midline remains the most significant factor after analyzing different stent factors, including the use of different stent sizes, lengths, and position of the distal end of the stent, which shows that correct stent placement is essential to minimize stent-related symptoms. In addition to symptomatic issues, good stent placement avoids the risk of stent retraction or misplacement of the stent, which requires further ureteroscopic extubation under anesthesia.
The complication rates of the KF-PSL and TP groups were not significantly different. The three patients who required blood transfusion were all due to preoperative anemia caused by chronic renal insufficiency before surgery. We also used the Braden scale to see if the knee-flexion position would cause compression injury of the limb joints, which proved to be very safe. Overall, as no statistically significant difference was observed in most of the results and no obvious disadvantage of KF-PSL PCNL was found while saving time for surgery, the benefits of the KF-PSL can be said to be that the time of surgery can be reduced, and the commonly used semi-rigid ureteroscope is sufficient and more convenient for simultaneous antero-retrograde access. In addition, the surgeon can freely change between PCNL, URL, or ECIRS when required.
A major limitation of this study was that it was a retrospective review and not a randomized, double-blind study. Second, the number of cases included was relatively small. Otherwise, the two modalities were adopted asynchronously at different time intervals. Therefore, although we recommended that patients undergo procedures based on the CUA or AUA guidelines, patient selection bias may still be present. However, the allocation of KF-PSL cases to TP cases with similar S.T.O.N.E. scores may have minimized some of this selection bias. Further studies with more cases are needed to compare the effectiveness of the KF-PSL position to the TP position.