Characteristics of Kidney Stones that Lead to High Stone-Free Rates in Supine Ultra-Mini Percutaneous Nephrolithotomy

Background: The increasing frequency of health checkups has led to an increasing diagnosis of renal stones. There exist various treatments for renal stones, but the less invasive ultra-mini percutaneous nephrolithotomy (UMP) treatment is gaining wide attention. The present study aimed to confirm the characteristics of renal stones that help achieve a high stone-free rate through UMP regardless of the complications. Methods: A retrospective study was conducted on 100 consecutive patients from May 2016 to February 2021. The study used a 7.5 Fr nephroscope and an irrigation pump. Renal stones were pulverised into as small particles as possible using a holmium laser discharged through an 11/12 Fr operating sheath. Kidneys, ureters, and bladder (KUB) and stone computed tomography (CT) scans were performed four weeks after surgery to confirm the stone-free state in patients. Results: The present study enrolled 72 men and 28 women who underwent UMP. The mean age of the patients was 61.5 years. The average size of the stones was 2.7 +/- 1.1 cm. The average operation time was 74.7 +/- 38.5 minutes. Three collecting system injuries occurred; However, no damage to other organs or bleeding was observed. At the one-month follow-up, the overall stonefree rate of UMP was 66%, lower than the target stone-free rate of 87%. The multivariate logistic regression analysis revealed that a high surgical success rate could be predicted in the cases of a single calcium oxalate or uric acid stone with a stone size <3 cm. Conclusions: UMP can be considered the first treatment option for single renal calcium oxalate or uric acid stones with a size <3 cm.