**4.3. Calculus migration/retropulsion**

A few investigations of the URS treatment of upper ureteral calculi have revealed that the main reason for calculus‐free failures can be due to recoil motion and less frequently to inabil‐ ity to track or seek the stones [35–37]. Recoil‐dislocated calculus could lead to longer opera‐ tion period, the necessity for another process to deal with recoiled parts and as such reduced stone‐free level. The stone recoil motion results in additional patient morbidity and health‐ care expenses [30, 33]. Besides, left‐behind stone debris can act as a seed for calculus growing back, renal colic, and persistent infection.

Recoil motion investigations in the past revealed the relation between retropulsion displace‐ ment and laser power, frequency, and fiber core size [28–31]. Recoil motion is proportional to the laser power and the fiber core size. Furthermore, another research claimed that the recoil motion decreased with a longer laser pulse without compromising dusting effectiveness sig‐ nificantly [32]. A conventional experimental method to characterize calculus migration uti‐ lized a hosting container (e.g., a "V" groove, a flat and smooth surface, or a test tube). These methods, however, demonstrated large variation and poor detectability, possibly attributing to friction between the calculus and the container on which the calculus was situated. Our earlier study of retropulsion [38] and Blackmon et al. [44] showed more than 100% the peak‐ to‐peak retropulsion variation with the conventional experimental method.

Sroka et al. [39] used a ball‐shaped lead sinker fixed to a nylon string to study the retropulsion during laser lithotripsy with a regular CCD camera. In this in vitro study, a high‐speed camera was used to study the movement of the calculus which covered displacement, speed, and acceler‐ ation. Our study shows that the combination of a pendulum and a high‐speed camera provides a very useful tool for retropulsion characterization. The apexes of a 200 mm–10 mm<sup>3</sup> phantom pen‐ dulum by a 10‐Hz Holmium laser are 1.25 ± 0.10, 3.01 ± 0.52, and 4.37 ± 0.58 mm for 0.5‐, 1.0‐, and 1.5‐J energy per pulse, respectively (peak‐to‐peak variation is less than 50%). And the average initial force by 10 of 0.5‐J pulses is 3.1 × 10−5 Newton or 3.1 Dyne. These data conclude that utiliz‐ ing a pendulum method to get rid of the friction enhanced the detectability and repeatability, and the high‐speed camera provides a better understanding of laser‐calculus interaction, especially the recoil motion of the calculus and its particles, cavitation bubble forming and burst, and so on.

Even though URS is now the top treatment choice for urolithiasis, further investigation should be done to gain a thorough knowledge of the detailed processes during the laser‐water and laser‐stone interactions. At least four processes play a role in the URS: (1) heat (super‐heated water or sometimes plasma formation); (2) acoustic or pressure wave (cavitation bubble form‐ ing and burst); (3) chemical (disintegrate of mechanical and chemical bond between the calcu‐ lus particles); (4) physical kinetic (recoil motion of the calculus and scattering of the particles). The high‐speed camera combined with a calculus pendulum can provide a better understand‐ ing of items 2 and 4. More investigation should be conducted on all of the four processes of the laser‐calculus interactions in the URS.

In summary, combining a high‐speed camera with other tools/method: hydrophone, pen‐ dulum, and Schlieren imaging method, provides not only a very useful tool for cavitation bubble, shock wave, fiber burn‐back, and retropulsion characterization but also a great insight into laser‐calculus interaction in regard to acoustic and kinetic processes.
