## 4 Discussion  
A double-J stent is used along with metallic stents in ureteral stenosis or occlusion [4; 5; 6; 7; 8]. The goal in using a double-J stent is to obtain a route for urine flow and to achieve a sufficient and acceptable urine flow through the stented ureter. The indwelling pigtail ureteral stent has proven its efficacy in maintaining ureteral patency in many cases, and it is widely used as an adjunct to extracorporeal shock wave lithotripsy. Here, we limited the study to cases of ureter stones. Although malignant ureteral obstruction is treated with a double-J stent, the patency rate has been poor [20]. Normal urine flow through the ureter is formed with the help of peristalsis and hydrodynamic forces [1; 2]. Peristalsis occurs about five to six times per minute in a normal condition, but it is affected by physical changes in the ureter or by the placement of a double-J stent. Initially, peristalsis can be activated to overcome a limited urine output to the bladder, but as time goes on, the frequency of peristalsis may lessen and finally zero-out [21; 22; 23]. This condition is a great obstacle in maintaining urine flow through the ureter. The critical evaluation for the clinical application of a double-J stent should be whether the stent could get a sufficient and acceptable urine flow through a stented ureter and which stent size is the best in achieving this. No fluid dynamics studies have been conducted yet on the proper size in the application of a double-J stent. There is a question of whether we could achieve a sufficient urine flow through a stented ureter with only a luminal flow. The peak values of the luminal flow rates in this study were 16 mL/h and 24 mL/h in the 4.57 mm and 5.39 mm ureters, respectively, obtained with a 74% ureteral stenosis and an 8-Fr double-J stent. The urine output over 24 h was 384 mL and 576 mL with the peak flow values, which were not sufficient or acceptable urine flows in normal daily living. Achieving the goal in the clinical application of a double-J stent is difficult with only a luminal flow. The extraluminal flow was greater than the luminal flow in mild ureteral stenosis, and it was devoted to the total flow rate even in severe stenosis, in which the luminal flow rate was greater than the extraluminal flow. In severe ureteral stenosis, achieving a sufficient and acceptable total urine flow is difficult with a high luminal flow and a low extraluminal flow. The total flow rate in the 4.57-mm ureter with a 74% ureteral stenosis was 16-23 mL/h and in the 5.39-mm ureter with 74% ureteral stenosis the flow rate was 26-43 mL/h. The urine output over 24 h was 384-552 mL and 624-1032 mL. Achieving a sufficient urine flow through a stented ureter in severe ureteral stenosis is not easy. In mild ureteral stenosis or no stenosis, achieving a sufficient and acceptable urine flow was relatively easy through a stented ureter, especially with a smaller-sized double-J stent. In the 4.57-mm ureter, the peak total flow rates with 5-, 6-, 7-, and 8-Fr double-J stents were 63, 47, 35, and 26 mL/h, respectively, and the urine output totals over 24 h were 1512, 1128, 840, and 624 mL, respectively. In the 5.39-mm ureter, the peak total flow rates with 5-, 6-, 7-, and 8-Fr double-J stents were 147, 116, 92, and 71 mL/h, respectively, and the urine output totals over 24 h were 3528, 2784, 2208, and 1704 mL, respectively. [PERSON] et al. [12] reported correlation between inner and outer diameters of the ureter stent and urine flow in the stented ureter in an in vivo study. Our study showed similar results, direct proportion, in luminal flow, but partially different results, inverse proportion, in extraluminal flow. In their study, the stent smaller than or equal to 7 Fr showed a direct proportion and the stent bigger than 7 Fr showed an inverse proportion. Our study supports their study findings regarding a decrease in extraluminal flow with bigger ureteral stents. [PERSON] et al.