since the first introduction of percutaneous nephrolithotomy ( pcnl ) in 1976   it has become the gold standard surgical treatment for large kidney stones . in order to reduce the morbidity associated with pcnl , several attempts have been made to modify the standard technique and reduce the size of instrumentation used for pcnl . the first significant size reduction has been proposed by helal et al .   in 1997 , who described the  mini - pcnl technique in a 2-year - old child using 15 fr . further advances led to the development of metallic amplatz sheath with nephroscope , where one of the prominent advantages was vacuum cleaner effect , consistent with hydrodynamic bernoulli 's principle . according to this principle , during mini - pcnl , the front of a low pressure called pseudocavity is produced at the tip of nephroscope , which can extract the stone fragments through an access sheath without any additional grasping devices . at a closer look , we have noticed that the horizontal plane , being one of the necessary conditions for bernoulli equation , possibly for negation of gravitational influence on the flow velocity , was for some reason neglected . in this study , we evaluated whether bringing the access sheath during mini - pcnl to the horizontal plane , which we called bernoulli maneuver, by table manipulation and/or gentle hand assistance , can be beneficial regarding nephroscopy time . we compared the 2 groups of patients operated either in a standard prone or tilted prone position . we obtained institutional review board approval to perform a single center prospective , randomized and controlled study ( approval number :  - 72345 ) . all patients undergoing mini - pcnl procedure between february 2015 and april 2016 for solitary medium - sized kidney stones ( 12 cm ) of high density ( more than 1,000 hu ) were consented to be included in the study . choice of treatment modality was dictated by existing guidelines   and local insurance coverage protocols . randomization was performed via a web - based generator of true random numbers , in which the randomness comes from atmospheric noise ( www.random.org ) and results were assigned to the control ( c ) and bernoulli ( b ) groups in numbered envelopes . patients with multiple or complex renal stones , stones in anomalous kidneys as well as patients with active infection or coagulation disorders were excluded from the study . all pcnl procedures were performed by 2 competent surgeons   under general anesthesia . with the patient in the lithotomy position , a ureteral catheter then the patient was turned to the prone position and strapped down at the upper body and buttocks to the operating table ( alphamax , maquet , germany ) to prevent sliding during after calyceal puncture , a 0.035 hydrophilic guidewire ( roadrunner , cook medical , bloomington , in , usa ) is inserted into the pelvicalyceal system ( pcs ) . single - step tract dilation , with a 16.5/19.5 fr metal access sheath , is performed ( karl storz , tuttlingen , germany ) . after identification of the stone , according to the group assignment , the patient was operated in either standard prone ( fig . fragmentation of the stone mass instead of  laser painting  with the formation of fitting the access sheath particles , which were immediately evacuated via bernoulli effect , was accomplished with laser lithotripter ( versapulse 100 w , lumenis , israel ) and 365 micron laser fiber , under the following initial parameters : 0.5 j and 20 hz . nor basket neither grasping device was needed during all operations . evaluated parameters in the study were nephroscopy time i.e. , time from stone identification till the end of the evacuation of the last fragment , stone free and complication rates . low - dose computed tomography ( ct ) imaging was obtained on all patients on postoperative day 1 to evaluate residual stones . data acquisition was performed using a 64-row ct unit somatom definition as ( siemens , berlin , germany ) . it is rather fruitful because it combines not only confidence interval estimates for parameters , for the standardized effect size dc , for the achieved power ( 1 ) , and planned sample sizes , but prediction intervals for them and for p - values , as well as estimates of posterior probabilities for the null hypothesis p(h0d ) and bayes factors bf10 . for parameter estimation and hypothesis testing the complementary combination of frequentist ( orthodox ) and nonparametric bootstrapping and permutation ( monte carlo ) procedures were realized with the number of replicates and/or random permutations as large as possible ( but no less than 1 million ) . for measurable variables ( software past , boxplotr ( http://boxplot.tyerslab.com/ ) , jasp ( https://jasp-stats.org/ ) , gpower   and leprep ( http://lmrs.univ-rouen.fr/persopage/lecoutre/pac.htm ) have been involved . we obtained institutional review board approval to perform a single center prospective , randomized and controlled study ( approval number :  - 72345 ) . all patients undergoing mini - pcnl procedure between february 2015 and april 2016 for solitary medium - sized kidney stones ( 12 cm ) of high density ( more than 1,000 hu ) were consented to be included in the study . choice of treatment modality was dictated by existing guidelines   and local insurance coverage protocols . randomization was performed via a web - based generator of true random numbers , in which the randomness comes from atmospheric noise ( www.random.org ) and results were assigned to the control ( c ) and bernoulli ( b ) groups in numbered envelopes . patients with multiple or complex renal stones , stones in anomalous kidneys as well as patients with active infection or coagulation disorders were excluded from the study . all pcnl procedures were performed by 2 competent surgeons   under general anesthesia . with the patient in the lithotomy position , a ureteral catheter then the patient was turned to the prone position and strapped down at the upper body and buttocks to the operating table ( alphamax , maquet , germany ) to prevent sliding during after calyceal puncture , a 0.035 hydrophilic guidewire ( roadrunner , cook medical , bloomington , in , usa ) is inserted into the pelvicalyceal system ( pcs ) . single - step tract dilation , with a 16.5/19.5 fr metal access sheath , is performed ( karl storz , tuttlingen , germany ) . after identification of the stone , according to the group assignment , the patient was operated in either standard prone ( fig . fragmentation of the stone mass instead of  laser painting  with the formation of fitting the access sheath particles , which were immediately evacuated via bernoulli effect , was accomplished with laser lithotripter ( versapulse 100 w , lumenis , israel ) and 365 micron laser fiber , under the following initial parameters : 0.5 j and 20 hz . nor basket neither grasping device was needed during all operations . evaluated parameters in the study were nephroscopy time i.e. , time from stone identification till the end of the evacuation of the last fragment , stone free and complication rates . low - dose computed tomography ( ct ) imaging was obtained on all patients on postoperative day 1 to evaluate residual stones . data acquisition was performed using a 64-row ct unit somatom definition as ( siemens , berlin , germany ) . it is rather fruitful because it combines not only confidence interval estimates for parameters , for the standardized effect size dc , for the achieved power ( 1 ) , and planned sample sizes , but prediction intervals for them and for p - values , as well as estimates of posterior probabilities for the null hypothesis p(h0d ) and bayes factors bf10 . for parameter estimation and hypothesis testing the complementary combination of frequentist ( orthodox ) and nonparametric bootstrapping and permutation ( monte carlo ) procedures were realized with the number of replicates and/or random permutations as large as possible ( but no less than 1 million ) . for measurable variables ( software past , boxplotr ( http://boxplot.tyerslab.com/ ) , jasp ( https://jasp-stats.org/ ) , gpower   and leprep ( http://lmrs.univ-rouen.fr/persopage/lecoutre/pac.htm ) have been involved . total of 67 patients were included in the study ; of these , 40 patients were randomized to group c and 27 patients to group b. their ratio 1.5 with 95% confidence limits from 0.9 to 2.4 did not differ statistically from the expected 1:1 ( p=0.14 ) . data in group c appeared not agree with a normal distribution ( table 1 ) . so all statistical estimations and tests used were nonparametric . in group b the nephroscopy time was significantly shorter than in group c ( fig . the time difference between studying groups was 12.6 minutes ( with 95% confidence interval from 8 to 18 minutes ) . bayes factor bf10=2,340 means that the probability to receive the observed effect under the alternative hypothesis is 2,340 times larger than under the null hypothesis . at the same time , it means that the posterior odds in favor of the alternative hypothesis against the null hypothesis are 2,340 times larger then prior odds . this result agrees well with the high value of achieved power ( pow=0.998 ) and with a high probability of replication psrep=0.97 . prediction analysis shows that in replication the cohen effect size dst will be not less than 0.49 and p - value will not exceed 0.027 with the probability 95% ( table 2 ) . moderate positive correlation was observed between stone size and nephroscopy time , where linear pearson correlation coefficient appeared to be r=0.53 ( 4.510 ) and spearman correlation coefficient was rs=0.55 ( 1.410 ) ( fig . 4b ) , r=0.66 ( 1.510 ) and rs=0.55 ( 510 ) . no intraoperative complications such as profuse bleeding , pcs rupture etc . there were no statistically significant differences between groups in terms white blood cell count , creatinine level and stone - free status defined by ct on the first postoperative day . though we did find statistically significant difference in hemoglobin ( hb ) drop between groups ( less in group b ) , it was not clinically significant ( table 3 ) . there are many factors that have been shown to impact the surgical outcomes of pcnl procedures . due to the challenging nature of the procedure , there have been many modifications suggested to improve patient related outcomes and stone free rates . in this study we evaluated a simple yet reproducible maneuver to improve stone free rates and nephroscopy time . bernoulli maneuver, to our best knowledge , has never been evaluated before during pcnl procedures and this is the first study to quantitatively demonstrate the benefits of this simple technique . , we saved 12 minutes just on nephroscopy time when the  bernoulli maneuver was used . although this difference seems small and not very clinically significant , cumulatively makes a huge difference at tertiary centers such as our institution where we daily perform a high volume of surgical procedures for urolithiasis . since this maneuver requires tilting the table , one should pay close attention for patient positioning and securing with tapes on the table to prevent slipping during the procedure . if the table tilting is not enough to bring access sheath to the horizontal plane then gentle hand assistance may be used , but not zealously  not to tear the kidney . although we did not assess surgeons ' subjective opinion , we believe that the maneuver makes the stone fragmentation and removal significantly easy . possible explanation for the observed pattern may be reduced gravitational influence with the implementation of the  bernoulli maneuver. moreover , one can observe a comparative easiness of fragments washout during standard pcnl in the supine position , though this effect does not occur so prominently in standard pcnl as compared to mini - pcnl . our study demonstrated positive correlations between the stone size and operative time , nephroscopy time and decrease in hb level . . today definition of mini - pcnl comprises utilization of access sheaths of size 1420 fr . according to this definition the first mention of mini pcnl was made by helal et al .   in 1997 using 15 fr . hickman peel away catheter , followed by jackman et al .   in 1998 utilizing an 11 fr . peel - away access sheath , with comparable to standard pcnl results . the first description of standard  minimally invasive pcnl  with specialized 12-fr rigid nephroscope and 15-fr access sheath came out in 2001 . however , according to the authors ' note , despite the fact that no hemorrhage has occurred in a group of 19 patients , miniaturized instruments were associated with significant operative time increase with larger stone masses , due to the fact that utilization of a smaller sheaths are associated with more difficult stone fragment retrieval and need of additional tools such as disposable retrieval baskets . it was not until nagele et al .   in 2008 reported on a  newly designed amplatz sheath ( karl storz ) with the vacuum cleaner effect , which was nicely depicted in a study by nicklas et al . . the basis of this effect lies in bernoulli 's principle , according to which an increase in the speed of a fluid in the constriction zone occurs simultaneously with a decrease in pressure , also known as pseudocavity which is being located at the tip of the nephroscope during mini - pcnl able to extract stone fragments through access sheath without any additional stone extracting devices . besides vacuum cleaner effect passive or active wash out as well as purging effect can also be utilized to gain maximum stonefree rate during mini - pcnl , though were not used in our study . the former has been shown to be associated with reduced morbidity , pain and blood loss compared to standard pcnl . in a series of more than 1,000 patients using a 16-fr access sheath bleeding complications occurred in less than 1.5% . for medium sized stones ( 12 cm ) no significant differences between mini- and standard pcnl regarding stone free and complication rates were observed . the only parameter was in favor to standard pcnl - the operative time , which was 1214 minutes shorter in average . suggested in our study bernoulli maneuver with its additional 12 minutes possibly could offset this time difference making mini - pcnl an attractive option for medium sized stones ( 12 cm ) . one can raise a fair question , if swl is not an option due to unfavorable stone factors , why not to use an endoscopic removal in medium - sized stones instead of mini - pcnl as existing guidelines dictate ? . as was nicely depicted even miniscule residuals < 1 mm could affect an early recurrence , possibly to a delayed nidus effect . not speaking about time and expenditures needed to leave a patient absolutely stone - free with the endoscopic route . first , the study is a single - surgical - center study thus generalizability of this data is limited . moreover during mini - pcnl in control group to render patient stone - free the sheath is often moved in all types of directions , which is also should be considered . finally , surgeon 's subjective feeling was not measured and future studies with validated questionnaires would help to elucidate if the maneuver makes clinically significant difference to the surgeon during the procedure . bernoulli maneuver during the mini - pcnl significantly reduces the nephroscopy time without any compromise in stone - free and complication rates .
purposeto evaluate the effect of 
 bernoulli maneuver ( bringing the access sheath to horizontal plane ) on operative time and stone free rates in patients undergoing mini - percutaneous nephrolithotomy ( pcnl).materials and methodsall consecutive patients with a solitary kidney stone undergoing a mini - pcnl between 2015 and 2016 were included into this study . 
 patients were randomized either to standard prone or control ( c ) group patients or to tilted prone with 
 bernoulli maneuver group ( b ) patients . 
 pre- , intra- , and postoperative characteristics of these 2 groups were recorded and analyzed.resultsa total of 67 patients were included in the study . of these , 
 40 patients were randomized to group c and 27 to group b. the mean ( 95% confidence limits ) stone size ( mm ) in group c and b was 14 ( 13 , 15 ) and 13 ( 11 , 14 ) , respectively ( p=0.26 ) . 
 nephroscopy time was shorter in bernoulli group ( 35 minutes vs. 23 minutes , p=1.510 - 5 , and bayes factor bf10=2,340 , and cohen standardized effect size dst=1.2 ) . 
 the difference made it up 12 minutes ( with 95% confidence interval from 8 to 18 minutes ) . 
 there were no statistically significant differences between groups regarding white blood cell , creatinine level and stone - free status defined by computed tomography on the first postoperative day.conclusionsin our study the  bernoulli maneuver led to a shorter nephroscopy time in mini - pcnl . 
 this maneuver can significantly reduce nephroscopy time and save significant amount of operative time , especially in tertiary referral centers with high - volume mini - pcnl procedures .