Edge targeting reduces the number of shock waves required for biliary ESWL in vitro.

In vitro experiments were conducted to determine if differences in targeting would effect stone fragmentation. Ten pairs of twin gallstones were used. The stones in each pair were identical in volume, diameter, radiolucency, and gross shape. One stone from each pair was subjected to shock waves focused at the center of the stone; the other was treated with shock waves targeted at the edge. Lithotripsy was terminated when all fragments were less than 5mm in diameter. The total number of shock waves used for each stone was recorded. In 7 of 10 pairs, fewer shock waves were required to fragment the edge targeted stone than the center targeted stone. In two of the remaining three pairs, equal numbers of shock waves were required for complete fragmentation. The difference between edge targeting and center targeting was shown to be statistically significant using the nonparametric Wilcoxin Signed Rank Test. (1 tailed = p less than 0.02, 2 tailed = p less than 0.04). These findings suggest that the outcome of biliary lithotripsy may be improved by targeting the edge of the stone.

Biliary lithotripsy can be enhanced with proper ultrasound probe position.

We have demonstrated in our in vitro system that an extracorporeal lithotripter utilizing a movable ultrasound probe can fragment gallstones more effectively when the ultrasound probe is not partially blocking shock waves. Using a pressure transducer we measured the pressures in the focal volume of a Wolf Piezolith 2300 lithotripter with the ultrasound probe fully extended and fully retracted. We also chose 12 pairs of twin gallstones, each taken from the same gallbladder. One stone from each pair was subjected to shock waves while the ultrasound probe was fully extended and the other treated while the probe was fully retracted. Shock wave pressures (which are converted to a measurable voltage output by our transducer) were clearly lower when the ultrasound probe was extended (5.45 volts; SEM = 0.10 volts) as compared to when the ultrasound scanner was retracted (6.7 volts: SEM = 0.08 volts). Significantly more shock waves were required to completely fragment stones when the ultrasound scanner was extended than when it was retracted (p = 0.01 using the nonparametric Wilcoxon's signed rank test). These results show that, in the lithotripter tested, an extended in-line ultrasound scanner can partially block shock waves. Retraction of an extendible ultrasound probe may enhance stone fragmentation when operating at the highest shock wave intensity.