Location of coupling defects influences stone breakage in shock wave lithotripsy.

Abstract

SWL is the most common treatment for kidney stones. However, compared to ureteroscopy and nephrostolithotomy, SWL is least effective—failing in ∼50% of cases. Since stone breakage is highly effective under controlled conditions, acoustic coupling between the lithotripter and patient may be the weak link. Previous in vitro studies determined that air‐pockets created in routine coupling reduce SW‐amplitude by ∼20% and defects occupying only 2% of coupling area reduced breakage 20%–40%. As a step toward determining if the position of defects influences SW delivery to the target we used styrofoam to selectively block portions of the coupling interface between a DoLi‐50 and the test tank. Stone breakage was ∼three times greater when the entire 13 cm diameter coupling interface was unblocked than when all but the center 6 cm was blocked, consistent with the reduction in surface area. However, the transition was abrupt, with ∼70% of the loss in efficacy occurring upon reduction in the aperture from 7 to 6 cm. Reducing the aperture had a greater effect on P− than P+ (P+/P− no aperture ∼41/−4.3 MPa; 7 cm ∼42/−3.5 MPa; 6 cm ∼37/−2.5 MPa). These initial findings begin to identify a region of the overall coupling interface where defects are likely to be problematic. [Work supported by Grant No. NIH‐DK43881.]