PCNL in the prone position has been preferred for decades and its advantages have been highlighted in many studies. Prone positioning for PCNL provides a wide surface area for puncture and affords adequate space for manipulation of the nephroscope[7, 8]. Nevertheless, this position can be contraindicated for anesthesiology reasons in patients with circulatory or ventilatory problems. In recent years, the supine position has been increasingly investigated and used. Supine PCNL may be preferable in patients with cardiopulmonary disease and those who are obese because it allows optimal airway control by the anesthesiologist during surgery. The original prone position for PCNL described by Valdivia et al in 1998 is seldom used now because it does not allow for simultaneous RIRS and affords limited space for renal access. Various modified supine positions have recently been proposed. The Galdakao-modified Valdivia position allows combined use of retrograde instruments in the entire urinary system simultaneously without the need for repositioning. The Barts flank-free modified position has been introduced and requires a 15° tilt, which is achieved by placing a 3-l saline bag under the ipsilateral rib cage and a gel pad under the pelvis. There is no support under the flank area when using this position, which allows more space for renal access; however, the kidney is in a neutral position and less mobile, which makes the nephroscope more difficult to maneuver., A variety of other modified supine positions have been proposed, including the supine oblique position, semi-supine position, and complete supine position. Each modification has strengths and weaknesses. Several meta-analyses have reported that the SFR is better in the prone position but that the operation time is shorter with a lower blood transfusion rate in the supine position. However, some of the published findings have been inconsistent. At our center, the supine position preferred for modified flank-free lithotomy is similar to the Galdakao-modified Valdivia position described above. All our operators routinely perform puncture and tract dilation for PCNL under US guidance, which also permits simultaneous retrograde surgery or insertion of a ureteral catheter. Our modified lithotomy position is achieved by placing the patient supine with a gel pillow under the thorax and another under the hip with the leg on the operated side extended and the contralateral leg fully abducted. Most of the previous studies of PCNL included fluoroscopic guidance and there is limited information on the use of the US-guided technique, especially in patients with complex stones. To our knowledge, this is the first study to compare outcomes of US-guided PCNL for complex renal stones on the basis of whether the patient is in the standard prone position or a modified supine position.
We found that the numbers of renal access and single-session SFR were lower when PCNL was performed in the modified supine position, which is in contrast with the findings of a meta-analysis but in accordance with the conclusions reached by Yuan et al. There may be several reasons for the smaller tract number and higher SFR in our P group. First, the distance between the 11th rib and the iliac crest is short in the modified supine position, which would have limited the choice of puncture site in the S group. Second, this position results in rotation of the kidney such that the posterior calyx moves to the ventral side; therefore, puncture is difficult even under US guidance. Third, the kidney is highly mobile in this position, including during access performing. Meanwhile, the lower pole of the kidney is deeper down and the distance between the skin and calyx is greater, which affects visual clarity under US and increases the difficulty of establishing the tract [Figure 1]. Although retrograde ureteroscopy was performed simultaneously in 20 patients in the S group, the stone clearance rate was still not as good as in the P group, probably because PCNL often involves bleeding more or less during the procedure, resulting in a poor endoscopic view and limited ability to use a flexible ureteroscope. The fourth reason is also related to patient positioning. The recently developed needle-perc nephroscope is now used widely to treat complex stones at our institution. The needle-perc is presently the smallest nephroscope available worldwide and has recently been confirmed to reduce perioperative complications and improve the SFR. However, although needle-perc-assisted standard access is convenient in the prone position, puncture is more difficult even in a modified position because of the limited space and ventral movement of the kidney.
Although the operation time was longer in our S group, we found no statistically significant between-group difference in operation time. This finding is in accordance with that in a study by Knoll et al who reported that the operation time was similar, even for mini-PCNL and standard PCNL. Our present findings may be attributed to use of a flexible ureteroscope. Frequent introduction of lithotripsy instruments, such as a fiber laser, and replacement of the retrieval basket and other equipment may prolong the operation time. Furthermore, the operation time in our P group was measured from renal puncture to placement of the nephrostomy tube and did not include the time taken for placement of the ureteral catheter, which would have contributed to the short operation time in this group.
In our study, the postoperative hospital stay was significantly shorter in the P group than in the S group, probably because of the lower single-session SFR in the S group. More patients in the S group needed a second session to remove residual stones, which would have contributed to a longer hospital stay. Furthermore, the overall length of hospital stay was longer in both our study groups than in many previous studies; however, this could reflect traditional Chinese culture and aspects of the health care system. First, many patients with complex stones are referred to our institution from different regions throughout the country and are not discharged from hospital until they have made a complete recovery. Second, because of regional inconsistencies in health care facilities, many local hospitals are poorly equipped to manage postoperative complications of PCNL. Therefore, most patients want to be sure there is no risk before leaving our center.
In this study, there were no significant differences in the rates of postoperative fever and severe complications between the two groups, which is consistent with most of the previous studies. One patient in our P group developed pneumothorax after removal of the nephrostomy tube, and a thoracic drainage tube was placed for 2 days. There were no cases of sepsis or septic shock in this series, possibly because standard access was used, which may result in a lower intrapelvic pressure and reduce the possibility of pyelovenous or pyelosinus backflow and the subsequent risk of infectious complications. Moreover, there were no cases of organ damage or embolization in either of our study groups; this may reflect the routine use of US guidance, which allows tissues along the puncture path to be clearly identified. As we have reported previously, there have been no cases of visceral or intestinal injury since the introduction of PCNL at our center.
Three patients in our P group required blood transfusion postoperatively for anemia that had been present before surgery and were not included in our postoperative bleeding data. There was no significant difference in blood loss between the two groups, which is in line with the findings of a previous meta-analysis. However, there may have been some more specific between-group differences that affected blood loss, in that patients in the S group had significantly fewer tracts than those in the control group and access-related bleeding was greater in the P group during similar operation time. We speculate that this might be related to puncture of the target calyx. Given that the kidneys are more medial and have greater mobility in the supine position, there is a greater likelihood of puncturing the anterior calyx rather than the posterior calyx, which may increase the risk of vascular injury when the tract is established.
This study had some limitations. First, it had a retrospective design, which inevitably introduced a degree of selection bias. Therefore, a prospective randomized study is needed to confirm its findings. Second, although the study was performed after the surgeons had mastered the learning curve for PCNL in both the supine position and prone position, they may have been less skilled in performing the procedure in the supine position. With improved learning and experience, better results can be anticipated.