ENDOUROLOGY AND STONE DISEASE Efficacy and Safety of NTrap® Stone Entrapment and Extraction Device for Ureteroscopic Lithotripsy Kehua Jiang1†, Musa Male2†, Xiao Yu2, Zhiqiang Chen2, Fa Sun1*, Huixing Yuan2** Purpose: NTrap® stone entrapment and extraction device (NTrap®) is a device used to extract and remove stones from the urinary tract and to minimize retrograde stone migration during ureterolithotripsy (URS). This study aimed to evaluate the efficacy and safety of NTrap® in URS. Methods: From Jan 2014 to June 2017, 148 patients underwent URS with the aid of NTrap® (Group A), and 209 patients underwent standard URS without any anti-retropulsion device (Group B). Their demographics, operation time, complications, stone migration rate, and stone-free rate (SFR) were recorded for comparison. Results: Compared with group B, Group A had a significantly shorter operative and lasering time (P = 0.003, P<0.001, respectively). There was no significant difference between the 2 groups in overall complications, a de- crease in mean hemoglobin, and length of stay (LOS) (P = 0.426, P = 0.097, P = 0.058, respectively). The inci- dence of stone migration was significantly lower in Group A than Group B (P = 0.035). The postoperative auxiliary procedure rate (in patients with stones retropulsion during the operation) was significantly lower in Group A com- pared to Group B (P = 0.024). The SFR was considerably higher in Group A than Group B (P = 0.009). Conclusion: URS, with the aid of NTrap®, is an effective and safe method for treating ureteric stones. It may prevent stones from retropulsion and shorten the operative time. Keywords: ureteroscopic lithotripsy; NTrap®; ureteric stones; stone free rate INTRODUCTION Retropulsion migration of stone fragments into the renal pelvis, calyces or both during ureterolitho- tripsy (URS) is a persistent problem that increases the chances of re-treatment or auxiliary procedures and subsequent cost. It is one of the challenges to deal with during ureteric stone management, especially proximal ureteric stones(1-3). Migration of stone fragments is in- fluenced by several factors such as the pressure of irri- gation fluid, degree of proximal ureteral dilation, stone site, the degree of stone impaction, lithotripter type and experience of the surgeon(4). An estimate of 5% to 40% of retropulsion of stone fragments occurs during intra- corporeal lithotripsies(5). However, some other studies reported that stone migration rate might reach up to 60% when patients undergo URS by pneumatic litho- tripsy (URS-PL)(6-8). With the advancement in technology, several devices have been developed to prevent stone retropulsion and facilitate fragments extraction during URS(5,9-16). These traverse from stone trap devices such as NTrap®, Stone Cone™, Accordion™ BackStop™, Escape™, and Lithocatch™ to suction devices (Lithovac™) and even balloon catheters (Passport™)(17-23). The NTrap® is a relatively novel device designed to minimize retrograde 1Department of Urology, Guizhou Provincial People’s Hospital, Guiyang, 550002, China. 2Department of Urology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China. *Correspondence: Department of Urology, Guizhou Provincial People’s Hospital, Guiyang, China. Phone: 86-851-85922979. Fax: 86-851-85922979. Email: sfgmc@sina.com. ** Department of Urology, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China. Phone: 86-27-836-65208. Fax: 86-27-836-65208. Email: tjmwyhx@163.com. Received September 2019 & Accepted February 2020 migration of ureteral stones and enables extraction and removal urinary stone fragments plus other foreign bodies from the urinary tract during URS (laser, ultra- sonic, electrohydraulic, or pneumatic lithotripsy). We performed this study to evaluate the safety and efficacy of NTrap® during holmium laser URS for the manage- ment of ureteral stones. PATIENTS AND METHODS Study population From January 2014 to June 2017, patients diagnosed with ureteric stone were retrieved from the archives of Tongji Hospital of Huazhong University of Science and Technology. Patients were divided into 2 groups, Group A contained 148 patients undertaking holmi- um laser (Lumenis, USA) URSL and with the aid of NTrap® (Cook Urological, Bloomington, IN, USA), while Group B contained 209 patients undertaking the standard holmium laser URSL without the aid of any anti-retropulsion device. All patients were diagnosed with ureteral stones by computed tomography (CT) and intravenous urography (IVU). The demographic char- acters, including age, gender, BMI, stone size, stone location, stone laterality, and hydronephrosis severity were recorded. Routine blood examinations, urine anal- Urology Journal/Vol 18 No. 2/ March-April 2021/ pp. 160-164. [DOI: 10.22037/uj.v0i0.5584] ysis and culture, serum biochemistry, abdominal ultra- sonography, CT, and IVU were evaluated. The inclusion criteria were: patients with ureteric stones (as diagnosed and measured by multi-slice spiral CT and IVU) who undertook standard URS or URS with the aid of NTrap stone extractor, and age >18 years. The Exclusion criteria were: patients with ureteral stric- ture, ureteral stones combined with ipsilateral intrarenal stones, sepsis, age < 18 years, history of open surgery, congenital anomalies or pregnancy. Surgical technique NTrap® stone entrapment and extraction device consist of a 2.8-Fr flexible sheath 145-cm in length, a remova- ble handle, and a 7 mm basket design. It is made from tightly woven nitinol wires with resilient shape memory characteristics that allow the basket to retain its shape after deployment. All the procedures were performed in lithotomy posi- tion under general anesthesia using semirigid uretero- scopic (8F/9.8 F Wolf) combined with holmium laser (Lumenis, USA) to disintegrate the stones(24,25). In group A, the basket of NTrap® bypassed the stone to entrap stones in place for laser disintegration and prevent re- tropulsion migration of stone fragments into the renal pelvis. All fragments were extracted from the ureter under direct vision with the NTrap® and released into the bladder. If the stone was embedded inside the ure- teric mucosa (polypoid or edema), laser polypectomy was done first to create a channel through which the NTrap® device was passed. In group B, laser lithotrip- sy was conducted to fragment stones into small pieces. Stone fragments were retrieved from the ureter with the help of ureteroscopic forceps. Surgery was concluded when no fragments remained in the whole ureter. Dou- ble-J stents were placed in those patients with ureteric injuries in either group. Variables in observation were both clinical and surgical characteristics, which included lasering time, overall operative time, ureter stent insertion, intra- and post-op- erative complications according to Clavien–Dindo clas- sification systems, and stone-free rates (SFR). Postop- erative CT was performed after 6 weeks to evaluate the SFR. Ancillary procedures such as SWL, and flexible URS were recorded. No residual stones or presence of any asymptomatic fragments ≤ 4 mm on CT at 6 weeks after the operation was considered as successful out- comes. Postoperative follow-up lasted for at least 3 to NTrap® Device for Ureteroscopic Lithotripsy-Jiang et al. Table 1. Baseline data of selected patients Variable Group A (n=148) Group B (n=209) P value Age, year 44.9 ±.0.8 42.5 ± 14.2 0.084 Gender, n 0.280 Male 103 134 Female 45 75 BMI, kg/m2 24.2 ± 3.8 23.4 ± 3.4 0.069 Stone laterality 0.188 Left 61 102 Right 87 107 Mean stone size, mm 16.0 ± 3.2 16.7 ± 3.6 0.081 Stone site 0.178 Proximal 132 176 Distal 16 33 Hydronephrosis 0.197 No or Mild 52 60 Moderate or Severe 96 149 Urinary infection 21 30 0.965 Positive urinary culture 6 11 0.597 Surgery history SWL 19 26 0.911 URS 12 18 0.866 Group A: URS with the aid of NTrap stone extractor; Group B: URS without any anti-retropulsion device; BMI: body mass index. Group A: URS with the aid of NTrap stone extractor; Group B: URS without any anti-retropulsion device Abbreviations: LOS: length of hospital stay; SFR: stone-free rate; SWL: shockwave lithotripsy; FURS: flexible ureteroscope. Variable Group A (n=148) Group B (n=209) P value operative time, min 41.8 ± 8.7 44.8 ± 9.3 0.003 Mean lasering time, min 10.6 ± 3.7 16.9 ± 5.0 0.000 Mean hemoglobin reduced, g/dL 0.88 ± 0.42 0.80 ± 0.39 0.097 Stone migration rate (%) 5(3.4%) 19(9.1%) 0.035 Overall complications, n (%) 4 9 0.426 bleeding 0 2 postoperative fever 2 3 ureteric injury 2 4 LOS 4.0 ± 0.6 4.2 ± 0.7 0.058 Ureter stent remove, d 30 30 SFR at 6 weeks follow up 95.9% (142/148) 88.0% (184/209) 0.009 Auxiliary procedures 5 20 0.024 SWL 1 4 Immediately FURS 4 16 Table 2. Operative and postoperative data statistics. Endourology and Stones diseases 161 6 months. The ethics committee approved the study, and all pa- tients were informed about this study and a signed written consent were obtained. Statistical analysis Statistical Package for the Social Sciences (SPSS) Ver- sion 16 was utilized for statistical analysis. Descriptive statistics were used to present the general data. The Chi- squared test and Fisher exact test were utilized to com- pare the differences between the 2 groups. A P < 0.05 was considered statistically significant. RESULTS Characteristics of all patients are summarized in Table 1. Both groups had comparable preoperative parameters such as age, gender, BMI, stone size, stone location, stone laterality, degree of hydronephrosis, urinary tract infection (UTI) rate, and surgical history (P > 0.05; Ta- ble 1). Compared with group B, patients who underwent URS with the aid of NTrap (group A) had a significantly shorter operative time and lasering time (P = 0.003, P=0.000, respectively) (Table 2). In group A, 2 patients suffered from postoperative fever and 2 from ureteric injury. In group B, 2 patients presented with hemor- rhage, 3 with postoperative fever, and 4 with a ureteric injury. The overall complications in group A and group B were comparable (P = 0.426; Table 2). There was no significant difference between the 2 groups with re- gards to mean hemoglobin reduction and length of hos- pital stay (LOS) (P = 0.097, P = 0.058, respectively; Table 2). The instantaneous success rate of stone fragmenta- tion during the operation was significantly higher in group A (only 5 patients with stone retropulsion) than in Group B (19 patients with stone migration into the pelvic or calyx). The incidence of stone migration was significantly lower in group A compared to group B (P = 0.035; Table 2). Regarding subsequent treatment, 1 patient in group A underwent SWL and 4 FURS while in group B, 4 patients underwent SWL and 16 FURS. The rate of requiring a postoperative auxiliary proce- dure was significantly lower in group A than group B (P = 0.024; Table 2). No residual stones or the presence of asymptomatic fragments ≤ 4 mm on CT at 6 weeks after the operation was considered as a successful outcome. The SFR was 95.9% (142/148) and 88.0% (184/209) in group A and group B, respectively which was statistically significant (P = 0.009; Table 2). DISCUSSION Both the American Urological Association (AUA) and the European Association of Urology (EAU) rec- ommend URS as the first-choice treatment for ureter- al stones > 10 mm. With the advancement of surgical technology, an increase in higher SFR and low morbid- ity have been achieved in URS(2,3,26,27). Lam et al. re- ported that URS had achieved higher SFR and lower complication rates analogous to those of SWL when managing large upper ureteric stones(2). Moreover, the miniaturization of ureteroscopy and improved intracor- poreal lithotripsy technology have made it possible to successfully access and manage any stone within the ureter by relatively atraumatic fashions(28). However, some limitations remain, including incom- plete fragmentation, lack of stone extraction, stone mi- gration and residual fragments. Some studies reported that stone migration rate might reach as high as 60% after ureteroscopic lithotripsy(6,29,30) indicating that ure- teral stone migration is one of the most significant chal- lenges during URSL. Knispel et al.(7) reported that 40% and 5% of ureteric stone migrations occurred from the proximal and distal ureter, respectively during URS. Stone retropulsion might increase operative time along with complication rate as a result of a change from semi-rigid to flexible ureteroscope(31). Migrated stones might require an auxiliary procedure even after the sur- gical procedure(6,8,32). Migration of stone fragments is influenced by many factors, which include pressure of irrigation fluid, stone location, degree of stone impac- tion, lithotripter, and experience of the surgeon(4). Various strategies have been employed to obviate retro- grade migration of stone fragments during intracorpor- eal lithotripsy. Fortunately, the new emerging occlusive instruments may primarily overcome this great existing dilemma. Amongst the commercially available novel devices (Accordion™, Stone Cone™, and NTrap®, etc.), Stone Cone™ has been reported to be highly ef- fective in preventing stone retropulsion in several stud- ies with 100% success rate during URS for proximal ureteral stones(19-22,33). The Accordion™ device, on the other hand, is among the most recent development and now is under investigation in clinical trials(34). There is, however, a limited number of studies concerning the effectiveness of NTrap® in endourological practice. The NTrap® stone occlusion device is also a relatively new ureteral occlusive device that prevents migration of stone fragments during URS. The NTrap® is composed of a tightly woven mesh of nitinol wires that mainly consists of the inner wire and the outer radio-opaque carrying catheter. The inner wire is a shape memory alloy that has a 7 mm sized umbrella designed basket. NTrap® device has 2.8 Fr diameter with a total length of 145 cm. Lee et al. reported the efficacy of the NTrap for managing ureteric stones with a 98.7% success rate(5). Ouwenga et al. reported that the difference in strength for inner wire advancing was clinically insig- nificant between Stone Cone™ and NTrap®(35). A me- ta-analysis demonstrated that NTrap® stone occlusion device was efficient in halting stone retropulsion during URSL for proximal ureteric stones(17). Nevertheless, this meta-analysis included only included 3 studies with a small sample size. Therefore, occlusive devices repre- sent a new generation of technology that can minimize proximal ureteric stone migration. Our study demonstrated a significantly lower incidence of stone migration (Table 2) with the use of NTrap® device than without any anti-retropulsion device, es- pecially for proximal ureteric stones. The NTrap® not only prevents stone migration but may also function as a useful tool for stone fragment extraction. Economic efficiency can be another strong reason for choosing NTrap®, which can save time and cost by low- ering the stone retropulsion rates. Stone retropulsion in- volves unnecessary procedures, for instance, prolonged operative time, rigid-flexible ureteroscope alteration, besides additional operations. Our study showed that patients who underwent URSL with the aid of NTrap® had a significantly shorter operative time and lasering time (Table 2). Furthermore, our study showed that the NTrap® Device for Ureteroscopic Lithotripsy-Jiang et al. Vol 18 No 2 March-April 2021 162 rate of requiring a postoperative ancillary procedure for the management of stone retropulsion was also signifi- cantly lower in NTrap group® (Table 2). Although we have achieved promising results with the use of NTrap® device during URS, the limited sample size might have thwarted an ultimate conclusion in fa- vor of NTrap® stone entrapment and extraction device. Therefore, prospective randomized control studies with larger sample sizes as well as multicenter trials are still necessary. There were some limitations in our study. The major limitation is that our study is a retrospective study, pre- operative data evaluation is insufficient, selective bias and data heterogeneity may exist in our study. Second- ly, the sample size in our study was relatively small that had limited impact on the outcomes. Some varia- bles were influenced by the heterogeneities of patients’ conditions, surgeon's surgical skills and the sample size of studies. Therefore, multicentre, larger sample size, randomized control studies are very necessary in the future. CONCLUSIONS The NTrap® Stone Entrapment and Extraction Device is an effective and safe tool for minimizing retrograde stone migration or stone retropulsion together with fa- cilitating the extraction of stone fragments from the uri- nary tract during URSL. ACKNOWLEGEMENT This study was funded by the Natural Science Founda- tion of Hubei Province of China (Number: 2017CFB516, 2017CFB638), National Natural Science Foundation of China (Number: 81873608), Foundation of Health and Family Planning Commission of Guizhou Prov- ince (Number: gzwjkj2019-1-127) and Doctoral Foundation of Guizhou Provincial People’s Hospital (GZSYBS[2018]02). CONFLICT OF INTEREST The authors declare no conflicts of interest. REFERENCES 1. Hollenbeck B, K., Schuster T, G. , Faerber G, J . , Wolf J, Jr . . Comparison of outcomes of ureteroscopy for ureteral calculi located above and below the pelvic brim. Urology. 2001;58:351-6. 2. Lam JS GT, Gupta M. 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