Renal stones are one of the most common urological diseases and are characterized by high recurrence rates . In the case of asymptomatic, tiny renal stones, observation can be performed without any treatment. However, if the stone causes obstruction or infection, is associated with symptoms such as pain or hematuria, or has a high possibility of size increase, treatment is recommended. Interventional treatment for renal stones may be considered if the size is greater than 1.5 cm or if removal of the stone is necessary because of the patient's social situation. The EAU guideline recommends ESWL and RIRS as first-line treatments for kidney stones < 2 cm in diameter and PCNL as the first-line treatment for stones > 2 cm . As surgical procedures, PCNL and RIRS have an anesthetic burden and their invasiveness is a disadvantage, but the stone-free rates of PCNL and RIRS are higher than those of ESWL . The development of surgical techniques and instruments continues to play a major role in the popularization of PCNL and RIRS [48, 50]. mPCNL is defined as PCNL performed using Amplantz sheaths with a diameter of 14–20 Fr [3, 51]. mPCNL has the advantage of reducing complications that may arise from larger instruments and sheaths .
The evaluation of perioperative and postoperative outcomes in the surgical treatment of renal stones is very important. The stone-free rate, operative time, and complications may be appropriate indicators for the perioperative and postoperative outcomes. Among these indicators, the stone-free rate can be one of the most important outcomes to avoid the need for auxiliary treatment as well as complications related to residual fragments. The stone-free rate is mainly correlated with the stone burden; however, most importantly, differences have been reported between different procedures . The stone-free rate is the most important parameter to estimate the efficacy of all the approaches . According to previous reports, PCNL and mPCNL have a higher stone-free rate than RIRS, although various imaging modalities were used.
In 2017, Kang et al. reported a systematic review and meta-analysis, in which updated evidence of stone-free rates of RIRS and PCNL in > 2-cm renal stones were compared with a previous report . In their meta-analysis comparing the success (stone-free) rates between PCNL and RIRS, the forest plot using the random-effects model showed a risk ratio of 1.11 (95% CI: 1.02–1.21; p = .01) favoring PCNL. In 2014, Zheng et al. reported no difference between RIRS and PCNL in > 2-cm renal stones using meta-analysis . Kang et al. concluded that their meta-analysis was performed using three additional articles compared with Zheng et al., and all three additional studies reported stone-free rates with RIRS that were relatively lower than those seen with PCNL . Zhang et al. examined the efficacy and safety of RIRS, PCNL, and SWL in the management of lower pole renal stones . They concluded that PCNL is associated with the highest stone-free rate at the expense of the longest hospital stay. In 2015, Zhu et al. performed meta-analysis of the stone-free rate between mPCNL and PCNL . They concluded that mPCNL was a safe and effective procedure with a stone-free rate comparable to that of PCNL. In addition, they found that mPCNL resulted in less bleeding, fewer transfusions, less pain, and shorter hospitalization. Another recent systematic review demonstrated that smaller tracts used in mPCNL tended to be associated with significantly lower blood loss or the need for blood transfusion, at the cost of a significantly longer procedure than standard PCNL . The results of our study support these previous study findings. In network meta-analysis of the LOS, mPCNL and RIRS were superior to PCNL. However, regarding the stone-free rate, mPCNL and PCNL were superior to RIRS, and there was no difference between mPCNL and PCNL (OR: 0.95; 95% CI: 0.51–1.9).
mPCNL could be developed because of the popularization of dilating instruments, including the recently released miniature nephroscope and irrigation system. In 1998, Jackman et al. used a 6.9-Fr rigid ureteroscope, a 7.2-Flexible ureteroscope, and a 7.7-Fr rigid offset pediatric cystoscope . MIP-M by Nagele et al. (Karl Storz GmbH & Co. KG, Tuttlingen, Germany) and Miniperc by Lahme et al. (Richard Wolff, Knittlingen, Germany) are miniature nephroscope instruments with a typical single-step dilating system [6, 50]. In addition, mPCNL often offers even higher stone-free rates than conventional PCNL perhaps because of the vacuum-cleaner effect .
The American Urological Association and EAU have not yet presented specific recommendations for the use of mPCNL to treat renal stones. However, previous reported evidence shows that mPCNL can achieve similar outcomes as standard PCNL in the treatment of renal stones > 2 cm . RIRS, which uses a flexible ureteroscope through a natural orifice, can be a competitor of mPCNL for treatment of renal stones, not Staghorn stones . However, it is clear the stone-free rate of mPCNL is superior to that of RIRS and that stones can be removed easily through the vacuum-cleaner effect. However, the long operation time of mPCNL can be a major disadvantage. In our network meta-analysis, the operation time of mPCNL was longer than that of PCNL and RIRS. No significant difference was seen between the complication rates of these three surgeries; however, the total number of complications of PCNL was higher than that of mPCNL and RIRS. Finally, a well-designed prospective study is needed to better understand the use of mPCNL, and to explore its potential to replace PCNL.