BERKELEY, CA (UroToday.com) - Shock wave lithotripsy (SWL) has been the most common procedure for stone treatment and is preferred by patients over other methods. It is the least invasive technique for treating stones, can be performed without entry into the urinary tract, and is relatively quick and easy for practitioners to perform. However, SWL results in lower stone-free rates compared to ureteroscopy (URS) by a significant margin. The unpredictability in the degree of stone fracture is often a concern for patients. As a result, endoscopic methods are achieving greater adoption. The necessity of repeat or follow-on procedures with SWL most often results from incomplete fragmentation of the stone or retention of stone fragments. Thus, there remains a need to develop a noninvasive treatment for stone management that more effectively and consistently disintegrates stones, leading to greater likelihood of spontaneous passage of fragments.
Recent investigations into the physical mechanisms of stone fracture with SWL have led to a new form of lithotripsy using focused ultrasound pulses rather than shock waves to achieve stone comminution. The article in the January 2015 issue of Journal of Urology presents the first investigation of this technology in vitro. Pulses or ‘bursts’ of ultrasound are emitted from a focused transducer and create stresses in the targeted stone and cavitation on the stone surface that result in fracture and subsequent disintegration.
Treating stones with ultrasound pulses rather than shock waves produces some desirable and important changes in how stones fracture. Such pulses can fracture stones with an order of magnitude lower pressure than SWL – pressure levels that can be generated by a relatively small ultrasound device. Several types of natural stones, including struvite, uric acid, cystine, and calcium oxalate monohydrate, were successfully disintegrated.
Furthermore, it was observed that the fragment size could be controlled by changing the frequency of the ultrasound pulse. While stones treated with 170 kHz ultrasound fragmented into pieces as large as 3-4 mm, stones treated at 285 kHz produced no fragments > 2 mm, and stones treated at 800 kHz produced fragments < 1mm, the size of grains of sand. The resulting fragment sizes were similar between stone types. Using this information, the ultrasound frequency can be chosen to produce uniformly small fragments that are more likely to spontaneously pass, reducing the likelihood of retreatment and the risk of renal colic symptoms as the stone fragments pass.
In summary, the discovery that ultrasound bursts can fragment stones opens an avenue to development of an improved lithotripter. The characteristics of this exciting therapy may provide a new extracorporeal method of lithotripsy that is more effective and faster than SWL while creating predictable and controllable stone fragment sizes. A major goal now is to identify the pulse parameters needed for stone comminution vs those that cause kidney tissue injury to minimize collateral effects. Preliminary experiments in an animal model with an ultrasound-image guided system are encouraging and have suggested that such a pulsing scheme exists to achieve safe and effective treatment. Future studies will refine this method towards improving the success of lithotripsy procedures.
Adam Maxwell, PhDa and Mathew Sorensen, MD, MSb as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
aCenter for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, WA USA
bDepartment of Urology, School of Medicine, University of Washington, Seattle, WA USA