Advantage of a Broad Focal Zone in SWL: Synergism Between Squeezing and Shear.

Objective: The focal zone width appears to be a critical factor in lithotripsy. Narrow focus machines have a higher occurrence of adverse effects, and arguably no greater comminution efficiency. Manufacturers have introduced new machines and upgrades to broaden the focus. Still, little data exists on how focal width plays a role in stone fracture. Thus, our aim was to determine if focal width interacts with established mechanisms known to contribute to stone fracture. Method: A series of experiments were undertaken with changes made to the stone in an effort to determine which is most important, the shock wave (SW) reflected from the back end of the stone (spallation), the SW ringing the stone (squeezing), the shear wave generated at surface of the stone and concentrated in the bulk of it (shear), or SWs generated from bubble collapse (cavitation). Shock waves were generated by a Dornier HM3-style lithotripter, and stones were made from U30 cement. Baffles were used to block specific waves that contribute to spallation, shear, or squeezing, and glycerol was used to suppress cavitation. Numerical simulation and high-speed imaging allowed for visualization of specific waves as they traveled within the stone. Results: For brevity, one result is explained. A reflective baffle was placed around the front edge of a cylindrical stone. The proximal baffle prevented squeezing by preventing the SW from traveling over the stone, but permitted the SW entering the stone through the proximal face and did not affect the other mechanisms. The distal baffle behaved the same as no baffle. The proximal baffle dramatically reduced the stress, and the stone did not break (stone broke after 45±10 SWs without the baffle and did not break after 400 SWs when the experiment stopped). The result implies that since removing squeezing halted comminution, squeezing is dominant. However, there is much more to the story. For example, if the cylindrical stone was pointed, it broke with the point on the distal end but not with the point on the proximal end. In both cases, squeezing was the same, so if squeezing were dominant, both stones should have broken. But the pointed front edge prevents the shear wave. The squeezing wave and its product — the shear wave — are both needed and work synergistically in a way explained by the model. Conclusions: A broad focus enhances the synergism of squeezing and shear waves without altering cavitation’s effects, and thus accelerates stone fracture in SWL. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]