during the 3-year period from july 1999 to june 2002 , the " mini - perc " technique of percutaneous nephrolithotomy on pyelocaliceal stones was performed in 72 consecutive patients at our hospital . this study was approved by the institutional review board , and written informed consent was obtained from each patient or their family members at the time of enrollment in the study , after the nature of the procedure and possible complications had been fully explained . there were 27 women and 45 men , and the age range was 1.4 years to 73 years ( mean age , 49.5 years  14.7 ) . the indications included failed shock wave lithotripsy and/or ureteroscopy in 19 subjects , a large cumulative stone burden ( > 6 cm ) in 25 , renal pelvis and/or lower pole calyceal stones in 62 , upper calyceal stones in two , stone in calyceal diverticulum in one , and one child . medical records of each patient were reviewed to determine the duration of symptoms , previous medical history , pre- and post-"mini - perc " interventional procedures , and hospital stay after the " mini - perc " . radiologic examinations were also reviewed in all patients , including abdominal radiography , intravenous urography , antegrade pyelography , renal ultrasound , abdominal computed tomography , and abdominal magnetic resonance imaging . data on the stone number , location , longest stone diameter , the cumulative longest diameter of stones , cumulative stone burden , and stone density were also determined on a preoperative plain abdominal radiograph . the stone burden was defined as the two - dimensional area determined by multiplying the length by the width of the stone . stone density was graded on three levels based on comparison with the adjacent rib ( 3 : more dense , 2 : isodense , 1 : less dense ) . the immediate stone - free rate , complications after the " mini - perc " , and final stone - free rate after additional procedures were determined . we also assessed the effect of the longest stone diameter , the cumulative longest diameter , cumulative stone burden , and stone density on the immediate stone - free rate . routine percutaneous renal access was performed in the interventional radiology suite on the day or one day prior to the planned " mini - perc " . local anesthesia was used at the puncture site with the exception of a 17-month - old boy , who underwent general anesthesia ( fig . the definitive puncture was performed into the appropriate calyx with a 22-gauge chiba needle ( cook , bloomington , il ) under real - time ultrasound and fluoroscopic guidance . the approach most commonly was done through a lower pole calyx , or the postero - inferior aspect of the renal stones ( fig . the nephrostomy was created in an upper pole calyx in two patients who had upper calyceal stones . a 0.035-inch radifocus guide wire ( terumo , tokyo , japan ) was advanced through the nephrostomy tract into the bladder as a safety guide wire . an amplatz super stiff guide wire ( boston scientific , miami , fl ) was inserted into the bladder as a working guide wire ( fig . nephrostomy tract dilation was then performed to the 14 fr with serial dilators ( cook , bloomington , il ) , a 14-fr peel - away sheath ( cook , bloomington , il ) ( fig . one urologist ( s.s.j ) , who had 8-years of experience in urology and had started using the " mini - perc " in 1998 , utilized the 8/9.8 fr rigid ureteroscope with the coherent ho : yag laser lithotrite ( coherent medical group , new york , ny ; 365 m or 550 m fiber , 10 to 20 hz frequency , 0.5 to 1 j power ) . small fragmented stones were then extracted by suction , an n - circle tipless basket ( cook urological , spencer , ia ) , and three - pronged grasping forceps ( cook urological , spencer , ia ) . a 14-fr malecot nephrostomy catheter ( cook , bloomington , il ) was routinely left in the renal pelvis upon the completion of the procedure , and was removed within 48 hours . imaging studies were not obtained prior to the nephrostomy tube removal , unless intraoperative findings suggested significant extravasation or residual calculi . otherwise , nephrostomy tubes were clamped when the urine was clear and removed two hours later unless the patient reported flank pain . the mean follow - up time was 7.2 months  8.3 ( range 0.5 - 37 months ) . at these intervals , follow - up radiographic images were used to evaluate the recurrence of stones . a fisher exact test was used to assess the difference in the stone free rate according to the longest diameter , the cumulative longest diameter , the cumulative stone burden and the stone density . during the 3-year period from july 1999 to june 2002 , the " mini - perc " technique of percutaneous nephrolithotomy on pyelocaliceal stones was performed in 72 consecutive patients at our hospital . this study was approved by the institutional review board , and written informed consent was obtained from each patient or their family members at the time of enrollment in the study , after the nature of the procedure and possible complications had been fully explained . there were 27 women and 45 men , and the age range was 1.4 years to 73 years ( mean age , 49.5 years  14.7 ) . the indications included failed shock wave lithotripsy and/or ureteroscopy in 19 subjects , a large cumulative stone burden ( > 6 cm ) in 25 , renal pelvis and/or lower pole calyceal stones in 62 , upper calyceal stones in two , stone in calyceal diverticulum in one , and one child . medical records of each patient were reviewed to determine the duration of symptoms , previous medical history , pre- and post-"mini - perc " interventional procedures , and hospital stay after the " mini - perc " . radiologic examinations were also reviewed in all patients , including abdominal radiography , intravenous urography , antegrade pyelography , renal ultrasound , abdominal computed tomography , and abdominal magnetic resonance imaging . data on the stone number , location , longest stone diameter , the cumulative longest diameter of stones , cumulative stone burden , and stone density were also determined on a preoperative plain abdominal radiograph . the stone burden was defined as the two - dimensional area determined by multiplying the length by the width of the stone . stone density was graded on three levels based on comparison with the adjacent rib ( 3 : more dense , 2 : isodense , 1 : less dense ) . the immediate stone - free rate , complications after the " mini - perc " , and final stone - free rate after additional procedures were determined . we also assessed the effect of the longest stone diameter , the cumulative longest diameter , cumulative stone burden , and stone density on the immediate stone - free rate . routine percutaneous renal access was performed in the interventional radiology suite on the day or one day prior to the planned " mini - perc " . local anesthesia was used at the puncture site with the exception of a 17-month - old boy , who underwent general anesthesia ( fig . the definitive puncture was performed into the appropriate calyx with a 22-gauge chiba needle ( cook , bloomington , il ) under real - time ultrasound and fluoroscopic guidance . the approach most commonly was done through a lower pole calyx , or the postero - inferior aspect of the renal stones ( fig . the nephrostomy was created in an upper pole calyx in two patients who had upper calyceal stones . a 0.035-inch radifocus guide wire ( terumo , tokyo , japan ) was advanced through the nephrostomy tract into the bladder as a safety guide wire . an amplatz super stiff guide wire ( boston scientific , miami , fl ) nephrostomy tract dilation was then performed to the 14 fr with serial dilators ( cook , bloomington , il ) , a 14-fr peel - away sheath ( cook , bloomington , il ) ( fig . one urologist ( s.s.j ) , who had 8-years of experience in urology and had started using the " mini - perc " in 1998 , utilized the 8/9.8 fr rigid ureteroscope with the coherent ho : yag laser lithotrite ( coherent medical group , new york , ny ; 365 m or 550 m fiber , 10 to 20 hz frequency , 0.5 to 1 j power ) . small fragmented stones were then extracted by suction , an n - circle tipless basket ( cook urological , spencer , ia ) , and three - pronged grasping forceps ( cook urological , spencer , ia ) . a 14-fr malecot nephrostomy catheter ( cook , bloomington , il ) was routinely left in the renal pelvis upon the completion of the procedure , and was removed within 48 hours . imaging studies were not obtained prior to the nephrostomy tube removal , unless intraoperative findings suggested significant extravasation or residual calculi . otherwise , nephrostomy tubes were clamped when the urine was clear and removed two hours later unless the patient reported flank pain . the mean follow - up time was 7.2 months  8.3 ( range 0.5 - 37 months ) . at these intervals , follow - up radiographic images were used to evaluate the recurrence of stones . a fisher exact test was used to assess the difference in the stone free rate according to the longest diameter , the cumulative longest diameter , the cumulative stone burden and the stone density . the mean duration of the symptomatic period was 19.5 months  51.1 ( range , 0.5 - 360 months ) in 57 of 72 patients . fifteen patients were asymptomatic , and one of them was incidentally identified during work - up of cervical cancer . previous renal or ureter stones in another location was present in 28 subjects , and urinary tract infections were found in four subjects . previous interventional procedures included shock wave lithotripsy in 19 subjects , laparoscopic or open ureterolithotomy in five , traditional percutaneous nephrolithotomy in one and vesicolithotripsy in one . the mean total hospital stay was 7.68 days  3.21 ( range 4 - 22 days ) . on average , the hospital stay after the " mini - perc " was 3.97  2.72 ( range , 1 - 13 days ) . fifty - seven percent of the patients ( 41 of 72 ) had stones in the left kidney , and 43% ( 31 of 72 ) in the right kidney . the total number of calculi was 89 , with 40 of the renal pelvis , 29 of the lower calyx , eight of the upper calyx , four of the middle calyx , and eight of staghorn stones . the mean number of calculi per patient was 1.44  0.82 ( range 1 - 5 ) . the mean value of the longest diameter was 2.83 cm  1.61 ( range , 0.3 - 8.9 cm ) , and that of the cumulative longest diameter was 3.42 cm  1.99 ( range , 0.6 - 10.2 cm ) . the mean value of the cumulative stone burden was 7.18 cm  7.93 ( range , 0.24 - 39.5 cm ) . the remaining 41 patients had more dense stones than the adjacent rib density . in no case was it necessary to convert from the " mini - perc " to conventional percutaneous nephrolithotomy . the total number of residual fragments was 21 ( mean , 1.5  0.65 , range , 1 - 3 ) in 14 patients . the location was as follows : seven in the lower calyx , six in the upper calyx , and four in the middle calyx . the longest diameter , the cumulative longest diameter , and the cumulative stone burden affected the probability of the immediate stone - free rate ( table 1 ) . the immediate stone - free rate was significantly higher in patients with a longest stone diameter of less than 2.5 cm or a cumulative stone burden of less than 6 cm . however , there were no statistically significant differences in stone density ( table 1 ) . the only major complication was hemorrhage , which occurred in one ( 1.4% ) of 72 patients who was diagnosed previously with child a liver cirrhosis . after embolization with a coil and gelatin sponge , no further bleeding occurred ( fig . lung , intestine , liver , and spleen ) or loss of a kidney did not occur . mild fever occurred in seven ( 9.7% ) of the 72 patients , which subsided spontaneously within one day . fever due to toxic hepatitis occurred as a postoperative complication in one patient ( 1.4% ) . oozing at the puncture site occurred in two patients ( 2.8% ) and abdominal paralytic ileus in another two ( 2.8% ) . additional procedures after the " mini - perc " were performed in 11 ( 15.3% ) of 72 patients , including extraction of the smallest residual stone fragments . there were shock wave lithotripsy in nine , shock wave lithotripsy and conventional nephrolithotomy in one , and ureteroscopy in one . after additional procedures , the final stone - free rate was 87.5% ( 63 of 72 ) . the mean follow - up time was 7.23 months  8.29 ( range , 0.5 - 37 months ) . the final follow - up modality was kub ( kidney , ureter , and bladder ) in 44 patients , ivp ( intravenous pyelogram ) in 24 , us in three and ct in one . these images were used to establish that two ( 3.2% ) of 63 patients were finally stone - free . the mean duration of the symptomatic period was 19.5 months  51.1 ( range , 0.5 - 360 months ) in 57 of 72 patients . fifteen patients were asymptomatic , and one of them was incidentally identified during work - up of cervical cancer . previous renal or ureter stones in another location was present in 28 subjects , and urinary tract infections were found in four subjects . previous interventional procedures included shock wave lithotripsy in 19 subjects , laparoscopic or open ureterolithotomy in five , traditional percutaneous nephrolithotomy in one and vesicolithotripsy in one . the mean total hospital stay was 7.68 days  3.21 ( range 4 - 22 days ) . on average , the hospital stay after the " mini - perc " was 3.97  2.72 ( range , 1 - 13 days ) . fifty - seven percent of the patients ( 41 of 72 ) had stones in the left kidney , and 43% ( 31 of 72 ) in the right kidney . the total number of calculi was 89 , with 40 of the renal pelvis , 29 of the lower calyx , eight of the upper calyx , four of the middle calyx , and eight of staghorn stones . the mean number of calculi per patient was 1.44  0.82 ( range 1 - 5 ) . the mean value of the longest diameter was 2.83 cm  1.61 ( range , 0.3 - 8.9 cm ) , and that of the cumulative longest diameter was 3.42 cm  1.99 ( range , 0.6 - 10.2 cm ) . the mean value of the cumulative stone burden was 7.18 cm  7.93 ( range , 0.24 - 39.5 cm ) . the remaining 41 patients had more dense stones than the adjacent rib density . in no case was it necessary to convert from the " mini - perc " to conventional percutaneous nephrolithotomy . the total number of residual fragments was 21 ( mean , 1.5  0.65 , range , 1 - 3 ) in 14 patients . the location was as follows : seven in the lower calyx , six in the upper calyx , and four in the middle calyx . the longest diameter , the cumulative longest diameter , and the cumulative stone burden affected the probability of the immediate stone - free rate ( table 1 ) . the immediate stone - free rate was significantly higher in patients with a longest stone diameter of less than 2.5 cm or a cumulative stone burden of less than 6 cm . however , there were no statistically significant differences in stone density ( table 1 ) . the only major complication was hemorrhage , which occurred in one ( 1.4% ) of 72 patients who was diagnosed previously with child a liver cirrhosis . after embolization with a coil and gelatin sponge , no further bleeding occurred ( fig . lung , intestine , liver , and spleen ) or loss of a kidney did not occur . mild fever occurred in seven ( 9.7% ) of the 72 patients , which subsided spontaneously within one day . fever due to toxic hepatitis occurred as a postoperative complication in one patient ( 1.4% ) . oozing at the puncture site occurred in two patients ( 2.8% ) and abdominal paralytic ileus in another two ( 2.8% ) . additional procedures after the " mini - perc " were performed in 11 ( 15.3% ) of 72 patients , including extraction of the smallest residual stone fragments . there were shock wave lithotripsy in nine , shock wave lithotripsy and conventional nephrolithotomy in one , and ureteroscopy in one . after additional procedures , the final stone - free rate was 87.5% ( 63 of 72 ) . the mean follow - up time was 7.23 months  8.29 ( range , 0.5 - 37 months ) . the final follow - up modality was kub ( kidney , ureter , and bladder ) in 44 patients , ivp ( intravenous pyelogram ) in 24 , us in three and ct in one . these images were used to establish that two ( 3.2% ) of 63 patients were finally stone - free . percutaneous nephrolithotomy , with its introduction in 1976 , is a well - established technique for the treatment of large renal calculi , and is performed either alone or in conjunction with extracorporeal shock wave lithotripsy ( 13 - 15 ) . the disadvantages of extracorporeal shock wave lithotripsy are overcome by percutaneous nephrolithotomy , which has higher stone - free rates , shorter treatment times , and a reduced risk of febrile infection of the urinary tract ( 14 , 16 - 18 ) . nevertheless , the main risk of percutaneous stone therapy is hemorrhaging that requires a blood transfusion 11 - 14% of the time , and an increased risk of kidney loss ( 14 , 15 ) . therefore , conventional percutaneous nephrolithotomy is not justified as the primary form of urinary stone treatment for smaller lower pole concrements , although it is recommended as an effective method in children ( 19 ) . the recent development of smaller sheaths suggested that percutaneous nephrostomy tract formation can be performed with minimal damage to the involved renal parenchyma , thereby reducing the procedure - related morbidity without diminishing its therapeutic efficacy ( 1 , 7 - 12 ) . the use of the 15-fr hickman peel - away sheath was described by helal et al . ( 7 ) as the first development of minimally invasive percutaneous nephrostomy in a 2-year - old child . ( 9 ) described a novel percutaneous renal access technique using an 11-fr peel - away vascular access sheath in seven infants and preschool age children . they defined the " mini - perc " as a percutaneous nephrolithotomy achieved through a sheath too small to accommodate a standard rigid nephroscope . the same group of authors ( 8) adapted the " mini - perc " to adult patients utilizing a 13-fr ureteroscopy sheath for percutaneous access in seven patients . monga ( 11 ) reported similar results using a 20-fr nephrostomy sheath in 21 patients . ( 12 ) also described this technique using a 15-fr amplatz sheath in 19 patients . however , the " mini - perc " technique of percutaneous nephrolithotomy in a large group , to our knowledge , has not been evaluated . we believe that our study of 72 patients is the most substantial assessment of this procedure since its initial description in 1997 . first , the immediate stone free rate was evaluated according to the longest diameter , the cumulative longest diameter , the cumulative stone burden , and the stone density . we noted that if the longest diameter of the stone was greater than 2.5 cm , the immediate stone free rate was significantly lower than that achieved with stones with the longest diameter of less than 2.5 cm . the cumulative longest diameter and cumulative stone burden also significantly influenced immediate stone - free rates , showing a sharp drop at diameters greater than 2.5 cm and at stone burden greater than 6 cm . therefore it might be wise to exclude patients with large diameter calculi or large stone burdens . the 14-fr peel - away sheath was applied to both adults and a 17-month - old boy in our study . there was a 78.2% reduction in the volume of renal parenchyma that was dilated by the 14-fr sheath compared to traditional 30-fr sheaths , and a 51% reduction compared to the 20-fr sheath used by monga ( 11 ) . the immediate stone - free rate was 80.6% , slightly lower than those of previous reports - 85% in children and 89% in adults by jackman et al . ( 8 , 9 ) , 90% by monga ( 11 ) , and 100% by lahme et al . however , the average stone burden was 7.18 cm in our study , while those of previous reports were much smaller in comparison -1.2 cm and 1.5 cm by jackman et al . ( 8 , 9 ) , 2.8 cm by monga ( 11 ) , and 2.4 cm by lahme et al . ( 12 ) . in our study , the only major complication was hemorrhaging that occurred in a patient with early child a liver cirrhosis , and therefore predisposed to bleeding . there were no major complications in the previous studies , but they had comparatively fewer cases ( 7 - 12 ) . moreover , the low rate ( 1.4% ) of major complications enabled us to use the 14-fr peel - away sheath instead of the 11-fr peel - away vascular access sheath for children ( 9 ) and the 13-frureteroscopy sheath for adults ( 8) . theoretically , the smaller diameter of the device for the percutaneous tract should decrease perioperative bleeding , pain , and postoperative parenchymal scarring ( 10 ) . when serious complications developed , the radiologist , skilled in interventional techniques , played an integral role in managing them . a pediatric series of the " mini - perc " reported six days of hospital stay on average ( 9 ) , and the same group of authors estimated 1.7 days of that for adults ( 8) . these previous results are promising in terms of shortening the hospital stay for future patients . first , though nephrolithiasis is a rare disease in children that accounts for only 2% to 3% of all stone patients ( 20 ) , we treated only one child , a 17-month - old boy , without complications . therefore , the safety and usefulness of this technique has not been thoroughly evaluated in children . further investigation is warranted to clarify whether the " mini - perc " technique using 14-fr peel - away sheath could offer improved results in children . second , there was no comparison study between the " mini - perc " technique and other treatment modalities for renal calculi . our three - year study described the first results of the " mini - perc " technique of percutaneous nephrolithotomy using a sufficient number of patients . also , we obtained high immediate stone free rates , which were influenced by stone diameter and burden . an interventional radiologist played an important role in making and dilating the nephrostomy tract , and managing complications after the procedure . the treatment of renal calculi with the " mini - perc " requires the collaborative effort of interventional radiologists , urologists and genitourinary radiologists .
objectiveto assess the efficacy and safety of a " mini - perc " technique of percutaneous nephrolithotomy using a 14-fr peel - away sheath for the removal of pyelocaliceal stones , and to determine appropriate inclusion criteria.materials and methodsfrom july 1999 to june 2002 , the medical records and radiographic images of 72 patients who underwent the " mini - perc " technique of percutaneous nephrolithotomy with a 14-fr peel - away sheath , were reviewed to determine clinical history , stone characteristics , immediate stone free rate , final stone free rate after additional procedures , complications , and hospital stay . 
 we also analyzed the effect of the longest stone diameter , the cumulative longest diameter of stones , the cumulative stone burden , and the stone density on the immediate stone free rate using a fisher exact test.resultsthe only major complication , arterial bleeding , occurred in a patient with child a liver cirrhosis and was successfully treated by embolization with coils and a gelatin sponge . 
 the immediate stone free rate was 80.6% , which was significantly influenced by stone diameter but not stone density . 
 the mean hospital stay after the procedure was 3.97 days.conclusionthe " mini - perc " technique of percutaneous nephrolithotomy , which uses the 14-fr peel - away sheath , is a safe and effective modality for treating renal calculi .