Introduction

Dating back to the early 1800s, lithotripsy was conceived with the utilization of galvanic current to dissolve calculi.¹ Over the years, the technique underwent extensive refinement, progressing from galvanic current to platinum electrodes, electrohydraulic shock waves, laser, ultrasound, and other technologies capable of breaking down stones.¹ While lithotripsy has had a significant impact on the treatment of renal stones, a challenge arises with accumulation of debris resulting from stone fragmentation. Gravity tends to direct debris to the lower pole of the kidney, posing a significant issue in clearance and recurrence rates. Consequently, the development of adjunctive techniques to alleviate the accumulation of lower calyceal stone debris has progressed alongside stone-breaking techniques (see Table).

Historical Techniques

Just prior to the introduction of flexible ureteroscopy, shock wave lithotripsy (SWL) became commonplace for renal stones in the 1980s. Despite its initial popularity, its suboptimal clearance rates in lower pole stone cases eventually became apparent. Nevertheless, investigations into therapies to supplement to SWL yielded positive results. For example, percussion therapy involving vibratory flank massage is effective in improving stone-free rates and reducing stone recurrence with minimal complications.² Often percussion, diuresis, and inversion therapies are combined to enhance the passage of lower calyceal stone debris after SWL. While out of favor, the implementation of percussion, diuresis, and inversion for treating lower calyceal stone debris continues to be used selectively today in adults and children motivated to avoid further surgery.^3,4

One of the first effective adjuncts for flexible ureteroscopy was electrohydraulic lithotripsy (EHL). Initially utilized in the 1950s, Denstedt and Clayman reported their success using 1.9F EHL probes for ureteroscopy (URS) in 1996.⁵ At the time, only EHL and laser lithotripsy probes were sufficiently malleable to be used for flexible URS to gain access to lower calyceal stones, and EHL was significantly less expensive. The study reported a fragmentation rate of 94% for lower calyceal stone debris, without any intraoperative complications or damage to the ureteral or renal mucosa.⁵ The late 1990s also brought about the advent of the modern nitinol basket,⁶ which permitted not only stone removal but also transpositioning of stones from the lower pole for more effective lithotripsy.⁷

Current/Contemporary Options

A host of recent ureteroscopic innovations have improved treatment of lower pole stones. Digital and single use ureteroscopy permit not only superior visualization but also minimal concerns for scope damage during complex, unfavorable angled cases. Modern lithotripsy utilizes multiple laser options that did not exist previously. Lasers are not only delivering higher power (120 watts) but also varying pulse widths. Pulse modulation holmium:YAG technology permits superior fragmentation with minimal retropulsion compared to standard holmium:YAG lasers.⁸ Thulium fiber lasers afford another option in lithotripsy that create exceptionally tiny fragments permitting possible true dust formation, and the latest thulium:YAG may represent a hybrid of the above 2 options.⁹

Percutaneous treatment of the lower pole has also become more feasible with the miniaturization of scopes. Mini–percutaneous nephrolithotomy (PCNL) permits efficacy of standard PCNL with lower morbidity.¹⁰ An adjunct to mini-PCNL is the ClearPetra device, a disposable vacuum-assisted renal access sheath, which touts improved initial stone-free rate with decreased operative time and complication rate.¹¹

The aspiration of lower calyceal stone fragments after URS/laser lithotripsy can be performed with steerable ureteroscopic renal evacuation using the CVAC aspiration device (Figure 1). Given its ability to be steered to different target calyces (Figure 2), CVAC demonstrated improved proportion of stones removed and stone-free rates compared with standard basket extraction, with similar complication profile. In patients with lower calyceal stone debris, it was more effective and faster in removing stones.¹²

Figure 1. The CVAC device removes stone fragments via a 7.5F inner vacuum lumen and a 12F outer irrigation lumen. UAS indicates ureteral access sheath. © 2023 Calyxo, Inc. Reprinted with permission.

Figure 2. The CVAC device is deployed through a 12F/14F ureteral access sheath and guided through the renal collecting system fluoroscopically or by ultrasound. © 2023 Calyxo, Inc. Reprinted with permission.

Similarly, FANS (flexible and navigable suction ureteral access sheaths) are equipped with a flexible 10-cm proximal portion, which may be navigated to the desired calyx via flexible URS, facilitating removal of lower calyceal stone debris. In one study, the use of FANS helped decrease the systemic inflammatory response following URS, as it maintains low intrarenal pressure and temperature.¹³

The glue-clot technique involves injecting autologous patient blood through the ureteroscope and allowing it clot, acting as a biologic “glue” to adhere to stone fragments, thus facilitating easier basket removal.¹⁴ The glue-clot procedure is regarded as an elegant technique that has brought improvements in the effective clearance of lower calyceal stone fragments. Furthermore, its success has inspired the exploration of new bioadhesive techniques aimed at further enhancing the results already achieved with the glue-clot technique.

Future/Emerging Adjuncts

Emerging technologies for lower calyceal stone debris clearance can be divided into biochemical and technical advances. A novel biocompatible adhesive for intrarenal embedding and endoscopic removal of small residual fragments was recently investigated in an ex vivo porcine kidney model.¹⁵ Two liquid biocompatible polysaccharide substrates are combined endoscopically using a 3F catheter and form a gel at a temperature of 37 °C, creating an adhesive mass that surrounds and encapsulates the stone fragments. This method bears resemblance to the glue-clot technique but without the need to extract blood from the patient and has shown promising results.¹⁵

One potentially revolutionary technical advance in lithotripsy is ultrasonic propulsion and burst wave lithotripsy, which involves using an ultrasound probe (Figure 3) to manipulate, reposition, or break up kidney stones in the awake patient (Figure 4). One pilot study included 29 patients, with 19 experiencing stone movement. Burst wave lithotripsy successfully fragmented the stones in 7 cases.¹⁶ During the 2-week followup, 18 out of 21 patients (86%) with distal ureteral stones successfully passed their stones. On average, the time to stone passage was approximately 4 days.¹⁶

Figure 3. The SonoMotion ultrasonic propulsion probe is larger than a conventional diagnostic ultrasound probe but contains both diagnostic and treatment modalities. © 2023 SonoMotion Technologies, Inc. SonoMotion technologies are investigational devices that are currently limited by US law to investigational use. Reprinted with permission.

Figure 4. Burst wave lithotropsy can be performed on an awake patient using skills borrowed from diagnostic ultrasonography. © 2023 SonoMotion Technologies, Inc. Illustrated by Carol Marinelli, University of Washington. Reprinted with permission.

Lastly, robotic surgery has revolutionized laparoscopic urologic surgery over the last 20 years. A promising future direction for lithotripsy involves robot-assisted ureteroscopy, which touts an increased range of motion, instrument stability, and improved ergonomics compared to conventional ureteroscopy.^17,18 Optimization of this technology is likely to expand its role in lithotripsy and lower calyceal stone treatment in the near future.

Funding: Boston Scientific funding for joint UC San Diego- Kaiser Permanente Laparoscopic-Endourology fellowship.

Conflict of Interest Disclosures: Roger L. Sur is a Scientific advisor with equity in Calyxo, Inc., and is a Scientific advisor with equity in SonoMotion, Inc.