Technology for the surgical management of renal stones is constantly evolving and improving. As stone surgeons, these developments are exciting, yet can be somewhat overwhelming when considering which modality to use and when. Ureteroscopy (URS) continues to have a clear role in the management of small renal stones, and standard percutaneous nephrolithotomy (PCNL) continues to be the standard of care for large renal stones greater than 2 cm in size. For intermediate-sized renal stones, however, there are many options with the development of less invasive percutaneous treatments including mini-PCNL and ultra-mini-PCNL. The risk/benefit profiles between a minimally invasive percutaneous procedure and URS are now quite comparable, bringing up the question—what is the role for URS, mini-PCNL, and ultra-mini-PCNL in today’s treatment paradigm for intermediate size renal stones?

Broadly, mini-PCNL is defined as PCNL via an access tract too small to accommodate a standard nephroscope (23Fr).¹ First published in the late 1990s, mini-PCNL techniques aim to provide improved stone-free rate (SFR) relative to retrograde approaches, but with a lower complication rate and convalescence relative to standard PCNL. Indeed, decreasing the access sheath size from 30Fr to 20Fr decreases the volume of renal parenchyma dilated by 56%, although long term impact on functional parenchyma is likely much smaller.^2,3 Regarding postoperative complications, ElSheemy et al published one of the largest studies comparing standard vs mini-PCNL and found a significantly lower complication rate for mini-PCNL (7.9%) relative to standard (20.5%).⁴ As with standard PCNL, mini-PCNL and ultra-mini-PCNL access can be obtained under fluoroscopic, ultrasound, or endoscopic guidance and in either prone or supine positioning, dependent upon patient anatomy, stone location, and surgeon preference.

Under the umbrella of mini-PCNL, further iterations with decreasing sheath size have been utilized (see Table). Ultra-mini-PCNL, first described in India by Desai et al in 2013, utilizes a tract ≤14 Fr and laser lithotripsy to dust the stones while irrigating out small fragments through the sheath, similar to mini-PCNL.⁵ Desai et al described a stone-free rate of 97.2% at 1 month postoperatively for a cohort with mean stone size of 14.9 mm. When looking at a direct comparison between URS and ultra-mini-PCNL for intermediate-sized renal stones, Demirbas et al found that stone location seems to be the biggest determinate of differences in outcomes between the 2 approaches, with comparable stone-free rate overall, but significantly higher stone-free rate in lower pole stones with ultra-mini-PCNL.⁶ Of note, both modalities were shown to be safe and effective, with fluoroscopy and recovery time in the ultra-mini-PCNL arm greater than the URS arm.⁶

Table. PCNL categories.

Regardless of the specific sheath size, mini-PCNL and ultra-mini-PCNL offer the potential to optimize treatment for mid-sized stones. As all providers who treat stones are aware, retrograde URS for stones >1 cm can be inefficient. Importantly, the success of URS also decreases with stones >1 cm with SFR around 80% and ranging from 48% to 95%, thus increasing the potential for secondary procedures or subsequent stone events.^7–9 Comparative studies evaluating mini-PCNL and URS for 1–2 cm stones have found mini-PCNL SFR to be closer to 90%–95%.^7–9 Although results have varied, multiple studies have found mini-PCNL to result in significantly decreased operative time compared to URS.^7,9 However, overall complication rates are likely at least slightly higher for mini-PCNL relative to URS, driven by a transfusion rate which ranged from 0% to 13%.^7–9

In our practice, we have found supine, mini-PCNL with ultrasound guidance and thulium fiber laser to offer a beneficial alternative to retrograde surgery for patients with 1–2 cm stones, particularly within the lower pole. In this setting, operative efficiency is improved as supine positioning is virtually the same as for retrograde surgery and stone extraction efficiency is higher given minimal to no need for basketing (see Figure). Specifically, the Venturi or “vacuum cleaner” effect allows for stone fragment removal simultaneously with laser lithotripsy.¹⁰ In addition, the procedure is performed “tubeless” with a stent left in postoperatively and patient discharge same day. Thus, we have found mini-PCNL maintains the relatively low risk complication profile and outpatient nature of URS, but with improved efficiency and higher likelihood of stone clearance.

Figure. Stone fragments from mini-PCNL.

Overall, standard PCNL and URS remain the gold standards for the surgical management of stones >2 cm and <1 cm in size, respectively. However, mini-PCNL and ultra-mini-PCNL offer promising alternatives for intermediate sized stones and may strike the balance of high SFR and lower complications. Surgeons in high-volume stone centers should really have these miniaturized techniques in their surgical armamentarium and tailor them to their preferences (supine, prone, ultrasound, fluoroscopy, thulium fiber laser, holmium, etc). Each technique likely has a role in the current era of stone management, and future research should help give some clarity into which modality to use and when....until new technology comes in to change the game again.