ENDOUROLOGY & LAPAROSCOPY

Evaluation of synchronous twin pulse technique for shock wave lithotripsy: determination of optimal parameters for in vitro stone fragmentation
Sheir KZ, Zabihi N, Lee D, Teichman JM, Rehman J, Sundaram CP, Heimbach D, Hesse A, Delvecchio F, Zhong P, Preminger GM, Clayman RV
From the Urology and Nephrology Center, Mansoura University (KZS), Mansoura, Egypt; Division of Urology, University of Texas Health Science Center (NZ, JMT), San Antonio, Texas; Department of Urology, College of Medicine, University of California-Irvine Medical Center (DL, RVC), Irvine, California; Division of Urology, Washington University School of Medicine (JR, CPS), St. Louis, Missouri; Department of Urology, Section of Experimental Urology, University of Bonn (DH, AH), Bonn, Germany; and Department of Mechanical Engineering and Materials Science, Duke Comprehensive Kidney Stone Center and Division of Urology/Department of Surgery, Duke University Medical Center (FD, PZ, GMP), Durham, North Carolina, USA
J Urol. 2003; 170: 2190-4

• Purpose: The Twinheads extracorporeal shock wave lithotriptor (THSWL) is composed of 2 identical shock wave generators and reflectors. One reflector is under the table and the other is over the table with a variable angle between the axes of the 2 reflectors. The 2 reflectors share a common second focal point, making it possible to deliver an almost synchronous twin pulse to the targeted stone. We studied the optimal parameters for in vitro stone fragmentation.
• Materials and Methods: Two types of 1 cm artificial stones were used, namely Bon(n)-stones of 3 compositions (75% calcium oxalate monohydrate [COM] plus 25% uric acid, struvite and cystine) and plaster of Paris. The parameters tested were shock wave number (100, 500 and 1,000), shock wave power (8, 11 and 14 kV) and angle between the reflector axes (67, 90 and 105 degrees). After the optimal parameters were determined, we studied the disintegrative efficacy of THSWL for 3 types of human urinary calculi, including COM, calcium hydrogen phosphate (brushite) and cystine. Each stone received 1,000 twin shock waves at 14 kV with an angle of 90 degrees between the reflectors. All experiments were done using a rate of 60 twin shock waves per minute. Following lithotripsy stone fragments were processed and sized. The ratio of the weight of fragments greater than 2 mm-to-total weight of all fragments was calculated.
• Results: Optimal stone fragmentation results for THSWL were obtained with the maximum number of shock waves (1,000) and full power (14 kV). There was no significant statistical difference in fragment size or the ratio of fragments greater than 2 mm with the use of different angles except for cystine and plaster of Paris calculi, for which the right angle was most effective. At application of the optimal parameters to human stones THSWL produced small fragment size for COM and cystine stones, while brushite stones were not fragmented to the same extent.
• Conclusions: The efficacy of synchronous twin pulse technology improves as the number of shock waves and power increase. A 90-degree angle between the shock wave reflectors is advantageous for certain stones (that is cystine and plaster of Paris) but it is not a factor for other stone compositions. THSWL has satisfactory disintegrative efficacy for human stones, especially COM and cystine calculi.
  
  
• Editorial Comment
  There are currently 2 dual-head lithotriptors available: the Twinheads (FMD) and the Duet (Direx). In this study, the Twinheads was used to fragment 4 types of artificial stones and 3 types of human urinary calculi. The results of dual head lithotripsy were intriguing, although the assessment of any superiority of this type of lithotripsy over other types is only through comparison of these results to those in other studies (using methods similar to the ones in this study, by many of the same investigators, brushite stones were found to be resistant to several standard lithotriptors while these same stones fragmented well with dual head lithotripsy). It is not clear if any advantage of lithotripsy with the Twinheads machine owes to the same cavitation bubble interaction investigated by Zhong and associates (1) in their studies of dual lithotripsy. Moreover, the other dual head lithotriptor currently available (Duet, from Direx) can be set to either synchronous or asynchronous firing, and thus might provide different results. It remains to be seen if dual head lithotriptors will prove better, in terms of either efficacy or safety, than standard ones, but dual lithotripsy might well be the next big thing in shock wave lithotripsy.

Reference
1. Zhong P, Cocks FH, Cioanta I, Preminger GM: Controlled, forced collapse of cavitation bubbles for improved stone fragmentation during shock wave lithotripsy. J Urol. 1997; 158: 2323-8.

Dr. J. Stuart Wolf Jr.
Associate Professor of Urology
University of Michigan
Ann Arbor, Michigan, USA