Conversion of the electrohydraulic electrode to an electromechanical stone impactor: basic studies and a case report

A 3.3F electrohydraulic electrode (Wolf 2137.23) has been confined within a spring with a metal end cap, irrigated with water and covered with a 0.003-inch metal sheath (outside diameter 5F). The electrohydraulic lithotripsy discharge (Wolf Generator 2137.50) at E1 causes the metal cap to extend 3 mm. at 1,500 cm. per second and creates an impact pressure of 600 to 800 bar. Stone fragmentation efficiency of the electromechanical impactor was equivalent to unshielded electrohydraulic lithotripsy (gallstone 2.83 mg. per pulse, struvite/apatite 1.41 mg. per pulse, cystine 0.41 mg. per pulse, uric acid 1.48 mg. per pulse and 100% calcium oxalate monohydrate 0.10 mg. per pulse). Studies of the discharge of the electromechanical impactor within the pig ureter showed that minimal ureteral submucosal edema and hemorrhage occurred at 300 shocks discharged at a single point, and disruption of the mucosa and partial injury to the muscle layer occurred after 600 shocks given at the site of a pinched pig ureter. Pushing the electromechanical impactor perpendicular to the wall of the pig bladder will create a mechanical perforation within 35 shocks (electrohydraulic lithotripsy within 2 shocks). One patient had excellent fragmentation of a lower ureteral mixed monohydrate and dihydrate stone under direct vision performed with the electromechanical impactor passed via a 9.5F ureteroscope. There was no evidence of mucosal injury with 500 shocks. The electromechanical impactor has been developed to provide a safe and inexpensive method of ureteral stone fragmentation or disimpaction. These studies were performed to establish limits of safety that may allow use of the electromechanical impactor for stone fragmentation in the ureter without the need for ureteroscopy.