6.3Fr Versus 7.5Fr Mini-Flexible Ureteroscopy for Renal Stones (MINI-FURS)

Renal stone disease is a common condition with an increasing global incidence. Flexible ureteroscopy has become one of the standard treatments for renal stones because it provides excellent stone clearance with a minimally invasive approach. Recent technological advances have led to the development of mini-flexible ureteroscopes with smaller diameters, which may facilitate easier access to the kidney while reducing ureteral trauma and the need for preoperative ureteral stenting.

Although both 6.3Fr and 7.5Fr mini-flexible ureteroscopes are currently available for clinical use, there is limited high-level evidence comparing their clinical performance and cost-effectiveness. This prospective, randomized, single-blinded, non-inferiority trial aims to compare these two ureteroscope sizes in adult patients undergoing flexible ureteroscopy for renal stones.

Eligible participants will be randomized in a 1:1 ratio to undergo flexible ureteroscopy using either a 6.3Fr or a 7.5Fr mini-flexible ureteroscope. All procedures will be performed using standardized surgical techniques by experienced endourologists.

The primary outcomes are stone-free rate at one month after surgery and successful primary renal access without preoperative ureteral stenting. Secondary outcomes include operative time, fluoroscopy utilization, intraoperative ureteral trauma, postoperative ureteral stenting, postoperative pain, complications classified using the Clavien-Dindo system, emergency department visits, hospital length of stay, need for secondary procedures, and cost-effectiveness from the hospital payer perspective.

The investigators hypothesize that the 6.3Fr mini-flexible ureteroscope will improve primary renal access while reducing ureteral trauma, fluoroscopy exposure, postoperative stenting, and overall healthcare costs without compromising stone-free rates. The findings of this study may help establish evidence-based recommendations for selecting mini-flexible ureteroscopes and optimize the surgical management of patients with renal stone disease.

• 6.3Fr Mini-Flexible Ureteroscope (Device): Use of a disposable 6.3Fr mini-flexible ureteroscope during flexible ureteroscopy for renal stone treatment
• 7.5Fr Mini-Flexible Ureteroscope (Device): Use of a disposable 7.5Fr mini-flexible ureteroscope during flexible ureteroscopy for renal stone treatment

Primary outcomes

• Stone Free Rate [1 month after surgery]: Stone-free status defined as the absence of residual stone fragments greater than 2 mm on non-contrast CT KUB
• Primary Renal Access Success Rate [Perioperative/Periprocedural]: Successful passage of the flexible ureteroscope into the renal collecting system without the need for preoperative ureteral stenting

Secondary outcomes

• Operative Time [During surgery]: Total operative time measured from insertion of the ureteroscope until completion of the procedure
• Intraoperative Fluoroscopy Use [During surgery]: Fluoroscopy utilization during the procedure, including fluoroscopy time
• Intraoperative Ureteral Trauma as Assessed by the Traxer Ureteral Injury Scale [Perioperative/Periprocedural]: Ureteral trauma will be assessed using the Traxer Ureteral Injury Scale, graded from 0 to 3, where Grade 0 indicates no observable ureteral injury and Grade 3 indicates the most severe ureteral injury. Higher scores indicate greater severity of ureteral injury.
• Postoperative Ureteral Stenting Rate [Immediately after surgery]: Percentage of participants requiring ureteral stent placement at the end of surgery
• Postoperative Pain as Assessed by the Visual Analog Scale [6, 24, and 48 hours after surgery]: Postoperative pain will be assessed using the Visual Analog Scale, ranging from 0 to 10, where 0 represents no pain and 10 represents the worst possible pain. Higher scores indicate greater pain.
• Postoperative Complications [Up to 30 days after surgery]: Complications graded using the Clavien-Dindo Classification
• Emergency Department Visits [Within 2 weeks after surgery]: Emergency department visits related to the procedure
• Hospital Length of Stay [Perioperative/Periprocedural]: Length of hospital stay measured in hours from admission for the index ureteroscopy procedure to hospital discharge.
• Need for Secondary Procedures [Up to 1 month after surgery]: Requirement for additional stone-related procedures
• Incremental Cost-Effectiveness Ratio Comparing 6.3Fr and 7.5Fr Mini-Flexible Ureteroscopes [From surgery through 30 days after surgery]: The incremental cost-effectiveness ratio (ICER) will compare the additional direct medical cost associated with the 6.3Fr versus 7.5Fr mini-flexible ureteroscope relative to the incremental clinical effectiveness. Direct medical costs will include ureteroscope acquisition costs, repair costs, procedure-related disposables, operating room time, hospital stay, management of complications, readmissions, and secondary procedures. Clinical effectiveness will be assessed primarily using stone-free and successful primary access outcomes.

Sex: All, ages: 18 Years to any

Inclusion Criteria: * Adults aged 18 years or older.

* Scheduled to undergo flexible ureteroscopy for renal stones.
* Renal stone size ≤15 mm in the greatest dimension.
* No indwelling ureteral stent at the time of surgery.
* Able and willing to provide written informed consent.

Exclusion Criteria: * Active urinary tract infection.

* Anatomical abnormalities of the urinary tract (e.g., ureteral stricture, horseshoe kidney, ectopic kidney).
* Solitary kidney.
* Pregnancy.
* Stones located within a calyceal diverticulum.
* Any condition that, in the opinion of the investigator, would interfere with study participation or follow-up.

• Ebner B, Blajan I, Westphal JR, Papadopoulos I, Ivanova T, Karatas D, Happe M, Volz Y, Stief CG, Apfelbeck M, Chaloupka M. Feasibility and Safety of Primary Ureteroscopy with Single-Use Flexible Ureteroscope HU30M (6.3 Fr, HugeMed): An Initial Experience. Diagnostics (Basel). 2025 Oct 5;15(19):2522. doi: 10.3390/diagnostics15192522. PMID 41095741
• Geavlete B, Ene C, Iordache V, Geavlete P. Initial Experience with the New Super Thin Single-use Pusen Flexible Ureteroscope 7.5 Fr in Renal Stones Endoscopic Treatment. Chirurgia (Bucur). 2021 May-Jun;116(3):354-360. doi: 10.21614/chirurgia.116.3.354. PMID 34191717
• Hu H, Qin M, Yang L, Hu H, Qin G, Qiu M, Gao Y, Dou J, Wu T, Long P, Zhang W. Analysis of the effectiveness and safety of 7.5 Fr ultra-thin flexible ureteroscope combined with a tip-flexible suctioning ureteral access sheath for the treatment of kidney stones. Int Urol Nephrol. 2025 Mar;57(3):817-823. doi: 10.1007/s11255-024-04269-1. Epub 2024 Nov 9. PMID 39521733
• Geavlete PA, Bulai CA, Multescu RD, Ene CV, Militaru A, Punga AA, Geavlete BF. Early clinical outcomes using a 6.3 Fr single use ureteroscope compared to a 7.5 Fr device. Sci Rep. 2025 May 2;15(1):15362. doi: 10.1038/s41598-025-00515-3. PMID 40316636
• Alharran AM, Alajmi SA, Hussain AA, Alajmi MS, Hashim S, Alwazzan LT, Alaradi H. Efficacy and Safety of 6.3 Fr Versus 7.5 Fr Single-Use Flexible Ureteroscopes for Upper Urinary Tract Stones: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Medicina (Kaunas). 2025 Nov 26;61(12):2103. doi: 10.3390/medicina61122103. PMID 41470105
• Assimos DG. Urolithiasis/Endourology. J Urol. 2021 Sep;206(3):765-767. doi: 10.1097/JU.0000000000001904. Epub 2021 Jun 11. No abstract available. PMID 34111940
• Zhang L, Zhang X, Pu Y, Zhang Y, Fan J. Global, Regional, and National Burden of Urolithiasis from 1990 to 2019: A Systematic Analysis for the Global Burden of Disease Study 2019. Clin Epidemiol. 2022 Aug 15;14:971-983. doi: 10.2147/CLEP.S370591. eCollection 2022. PMID 35996396

• Yasser A Noureldin, MD, PhD, Principal investigator, Urologist, Sault Area Hospital, Sault Ste. Marie, ON, Canada, and Assistant Professor of Urology, Northern Ontario School of Medicine University, ON, Canada