FIG. 1.

Graphical illustration of the concepts of how SWL and BWL are believed to fragment stones which may then be repositioned to the ureter by ultrasonic propulsion.⁵ Traditional SWL (upper image sequence) fragments stones noninvasively through sequential shock waves that create localized stress, leading to a primary fracture point within the stone. Typically, stones treated with SWL break from larger fragments into proportionally smaller fragments with repeated shockwaves, analogous to a “fragmenting” strategy in laser lithotripsy.^7,8 In contrast, BWL comminution of stones (lower image sequence) occurs through small pieces shedding off a single large stone, more analogous to a “dusting” strategy in laser lithotripsy.^4,9 This action is achieved using focused sinusoidal bursts of US waves that repetitively stress multiple regions within the stone. BWL's lower pressure amplitude avoids cavitation clouds that can shield the stone from US energy and possibly cause tissue injury.^4,10 Clinically, BWL is advantageous because a higher rate can be used compared with SWL (10–100 Hz vs 1–2 Hz) again because of lower amplitudes, which allows energy to be delivered more quickly.^2,10,11 (Image courtesy of Kim Reading of Applied Physics Laboratory.) BWL, burst-wave lithotripsy; SWL, shockwave lithotripsy.