Wednesday, May 24, 2006
Stone Disease: SWL & Invasive Therapy Including Ureteroscopy (I)
The benefits of a broad focal zone in extracorporeal shockwave lithotripsy (ESWL) were demonstrated in two excellent presentations during this session.
In a very original and illustrative presentation, Michael R. Bailey from Seattle, WA described a series of in vitro experiments to study the mechanisms involved in stone fragmentation during ESWL. Shock waves generated with a Dornier HM-3-style lithotripter and artificial stones (U-30 cement) were used. Shock waves reflected from the back end of the stone (spallation), external "squeezing" waves, shear waves and waves generated by bubble collapse (cavitation) were all shown to have a role in stone fragmentation. When waves around the stone were blocked, stone fragmentation was significantly reduced. This is equivalent to a small focal zone, because in this setting the shock waves predominantly acting on the stone are those within the stone and not those around the stone. A broader focal zone will generate more waves around the stone. These seem to act in synergism with the "internal" shock waves, thus leading to better stone fragmentation.
Andrew P. Evan and his coworkers from Indianapolis, IN looked at the effects of broad focal zone and low-pressure ESWL on tissue damage and stone fragmentation in a porcine model. The XX-ES lithotripter, which generates shock waves of low amplitude (21 MPa at 9.3 kV) and has an 18 mm focal zone, was used in six pigs at 27 shock waves/minute. Stone fragmentation equivalent to that provided by a Dornier HM3 lithotripter at 2 Hz was obtained with the XX-ES lithotripter. Furthermore, renal hemorrhagic lesions were virtually absent when using this machine. In contrast, the Dornier HM3 produces a hemorrhagic lesion occupying around 6% of the renal parenchyma when used at clinically recommended settings. In conclusion, lithotripters with a broad focal zone and low amplitude shock waves appear to be equally effective for stone fragmentation and probably safer than traditional lithotripters.