STONE DISEASE

Preventing Migration of Stones during Fragmentation with Thermosensitive Polymer
Sacco D, McDougal WS, Schwarz A
Department of Urology, Massachusetts General Hospital, Boston, Massachusetts
J Endourol. 2007; 21: 504-7

• Purpose: To define a method of stabilizing stones during extracorporeal (SWL) and intracorporeal lithotripsy with a thermosensitive polymer.
• Materials and Methods: Using a thermosensitive polymer that is either a liquid or a gel, depending on the temperature, both calcium oxalate and plaster of Paris phantom stones were placed in the polymer gel or saline, and SWL was performed. Comparisons were made between the effectiveness of the fragmentation in the two media. Also, in-vivo studies using the polymer to prevent migration of ureteral stones were performed in swine. Electrohydraulic lithotripsy was used on a small stone implanted in the distal ureter with the polymer instilled proximally. Once in the ureter, the polymer converted to a gel. After completion of the procedure, the polymer was restored to a liquid form by infusion of cold saline and expelled from the ureter. Three of the pigs underwent treatment of the stone, convalesced for 7 days, and then had urine collections from both ureters to compare the glomerular filtration rates, fractional sodium excretion, urine/plasma creatinine ratio, and urine/plasma urea ratio on the treated and the contralateral (control) sides.
• Results: The polymer did not enhance fragmentation when used with SWL but prevented stone migration in the in-vivo studies. The physiologic parameters were not significantly different on the treated and the control sides. The polymer was easily removed from the ureter by infusing cold water.
• Conclusion: The use of this thermosensitive polymer proximal to ureteral stones prevents migration and is not traumatic to the ureter.
  
  
• Editorial Comment
  The authors describe the use of a novel thermosensitive polymer to stabilize calculi during endourological procedures. The technique was not successful in an in vitro model of ESWL. Previous studies have demonstrated inhibition of stone fragmentation by ESWL when a mineral oil medium surrounds the stone. It is thought to occur by interference with the cavitation effects of ESWL, and a similar effect may occur with the new polymer evaluated in this study.
  The authors did not evaluate the upper collecting system acutely after intracorporeal lithotripsy in the porcine model. This is an important step to be able to conclude that the polymer did not migrate into the renal pelvis or calyces. Was this to occur, the patient may experience transient obstruction until the polymer dissolved 2 hours later.
  Clinical applications of this polymer would require the use of warmed endoscopic irrigation fluid to prevent dissolution of the polymer. It would be important to evaluate the toxicity of combustion by-products of the polymer to determine what might be anticipated if it were inadvertently targeted with the holmium laser. It would also be important to evaluate interactions of the polymer with a ureteral stent left post-operatively – could this impact polymer dissolution?
  As such, the polymer holds promise as a method to prevent stone migration during ureteroscopy. One would need to quantify the volume of polymer required, which may vary based on the degree of ureteral dilation above the calculus. One might propose that the polymer could facilitate stone fragment retrieval during PCNL.

Dr. Manoj Monga
Professor, Department of Urology
University of Minnesota
Edina, Minnesota, USA
E-mail: endourol@yahoo.com