This is a prospective controlled study to compare the efficacy and safety of two methods of flexible urethroscopy in patients with urolithiasis, as well as the effect of surgery on quality of life.

Retrograde flexible URS is the most modern and safe method of treating patients with bowel stones up to 20 mm. Bilateral kidney stones can be treated with bilateral retrograde flexible URS. This type of surgery allows a patient to get rid of both kidney stones in a single intervention. Technically, the operation can be performed alternately on each side or simultaneously by two surgeons if two flexible instruments and video endoscopic racks are available simultaneously. Flexible ureteroscope diameter of 7.5-9 Сh enables to place two instruments even in the male urethra simultaneously without significant traumatization. One of the disadvantages of this method is the need for bilateral prestenting, with the frequency and the frequency and severity of stent-associated symptoms may increase. Few scientific works demonstrate contradictory data concerning the safety of bilateral flexible URS; therefore, further research is advisable.

All patients meeting the selection criteria will undergo preoperative examination: history, physical, laboratory and CT scan of the kidneys.

MATERIALS AND METHODS To calculate the sample, the investigators used the method for "no lesser efficiency" studies. To confirm the hypothesis of no greater number of complications in the form of upper urinary tract mucosal damage, exit (extravasation) of irrigation solution outside the upper urinary tract, bleeding and perforation of the ureteral wall, which according to the literature are 11.9% for bilateral and 8.5% for unilateral RIRH respectively, a minimum of 82 patients would be required in order to study of 80% and a probability of first-order error of 5% to exclude a 20% difference in the number of complications, which is clinically significant. To compensate for data loss, the sample size is increased by 5% to 86 patients. Categorical variables will be reported as absolute numbers and percentages. Unadapted univariate analyses, to compare the two treatment groups, will be based on Fisher's exact test. Relative risks and 95% confidence intervals will be calculated using the two-by-two table method using a logarithmic approximation. Continuous variables will be displayed as mean ± standard deviation or median and interquartile range.Pre- and postoperative data collected will be anonymized using unique codes that patients will receive immediately after randomization.

All surgical procedures will be performed by 3 qualified surgeons. Postoperative follow-up will be performed 1 day,1 month after surgery by two investigators, a non-blinded study.

Inclusion Criteria:

• Single and bilateral kidney stones with a total 20 mm or less on each side
• Over 18 years of age
• Signed informed consent

Exclusion Criteria:

• Patients with large stones (total size greater than 20 mm) and coral stones
• Urethral and ureteral strictures
• Urinary tract abnormalities
• Unwillingness to sign informed consent
• Presence of active urinary tract infection
• Pregnant patients