## 4 Discussion  
In this study, M- and D-type occlusion SEMSs showed a similar efficacy in occlusive properties in the 3D phantom study; however, superiority of M-type was observed in the ex vivo porcine urinary tract model. Considering that the intrapelvic pressure measured for hydronephrosis in clinical practice, or in the vivo porcine kidney, is about 50-60 mmHg (converted from 68-81 cm H\({}_{2}\)O) in the literature [21,22], the two types of occlusion SEMS used in this study are considered to be very effective in terms of their occlusive properties, as the burst pressures causing leakage through the occlusion SEMS ranged from 99-124 mmHg, for M-type, and 82-11 mmHg for D-type. Therefore, it is suggested that the design and manufacture of the occlusion SEMS used can be applied to the occlusion SEMS required for actual clinical use. Over recent decades, a variety of minimally invasive techniques, using different materials, have been introduced for management of ureteral leakage [6,7,10,11,17]. Despite advances in methodology and embolic materials for ureteral occlusion, the primary success rates range from 40% to 100%, and material-related complications can occur in some patients [6,9,10,11,13,12,13]. Re-intervention requiring additional embolization has been reported in 66.7% of the embolized ureters [7]. Migration of the embolic materials was noted, and recurrent symptoms were subsequently recorded [4]. As shown in previous clinical studies, and the current investigation, occlusion SEMS can easily work with other embolic materials to enhance occlusive properties, with lower complication and recurrence rates [14,15]. The notable point of our ex vivo study design is that, instead of piercing the kidney with a catheter and fixing it, retrogradely inserting a catheter to pierce the kidney can make a hole at once, without damaging the kidney. As a result, the experiment can be carried out easily. In general, in an experiment in which a catheter is inserted through the kidney, when an instrument, such as a guide wire or catheter, is moved or removed, damage to the kidney may occur, which may affect the experimental results or cause the experiment to be stopped. Therefore, through the ex vivo study, meaningful experimental results of the ex vivo study itself can be obtained, and trial-and-error problems in subsequent in vivo studies can be reduced. After the two types of occlusion SEMSs were evaluated in 13 ureters in 12 patients, with various inoperable urinary leakage or fistula, the reintervention rate to manage  
Figure 4: Results of ex vivo experiments. (**a**) Graphs showing the mean pressures at the point of leakage and blowout in the M-type and D-type occlusion self-expandable metallic stents (SEMSs). Fluoroscopic images obtained during the ex vivo experiment using the M-type occlusion SEMS showing (**b**) leakage (arrowhead) at 109 mmHg and (**c**) blowout (arrows) at 182 mmHg. Fluoroscopic images showing (**d**) leakage (arrowhead) at 94 mmHg and (**e**) blowout (arrows) at 178 mmHg in the D-type occlusion SEMS. Note: CI, confidence interval. * \(p<0.05\). leakage recurrence with the use of n-butyl cyanoacrylate and microcoil embolization was 7.7% [14]. Although leakage recurrence through placed occlusion SEMS can be managed with additional embolic materials, such as microcoils and NBCA, more robust occlusion SEMS could reduce the reintervention rate. Although there was no significant difference in the bursting pressure between the M-type and D-type in the current phantom model, M-type is preferable, as it is more superior in the bursting pressure in the current ex vivo study [14; 15]. A possible reason for the M-type being superior is because its middle portion was intensively constricted using a nylon thread, while in the case of the D-type, the distal portion was constricted at several points. This difference may influence the degree of occlusion of the two occlusion SEMSs and allow the M-type a higher bursting pressure than the D-type.