In vitro investigations of repulsion during laser lithotripsy using a pendulum set-up

Ureteroscopic laser lithotripsy is a commonly used technique to treat ureteral calculi.The type of energy source used is one of the main influences of retrograd calculi propulsion. Using a momentum pendulum under-water set-up the induced momentum and the initial velocity were investigated. Pulsed laser light from three different clinically available laser systems, including a Ho:YAG laser, a frequency-doubled double-pulse (second harmonic generation, SHG) Nd:YAG laser and a flash-lamp pumped dye (FLPD) laser, were transmitted via flexible fibres of different core diameter to the front of the pendulum sinker. Single pulses at variable pulse energy, according to the clinical laser parameter settings, were applied to the target sinker, thus causing a repulsion-induced deflection which was documented by video recording. The maximum deflection was determined. Solving the differential equation of a pendulum gives the initial velocity, the laser-induced momentum and the efficiency of momentum transfer. The induced deflection as well as the starting velocity of the two short-duration pulsed laser systems (SHG Nd:YAG, FLPD) were similar (s _max = 2–3.6 cm and v ₀ = 150–200 mm/s, respectively), whereas both values were lower using the Ho:YAG laser with a long pulse duration (s _max = 0.9-–1.6 cm and v ₀ = 60–105 mm/s, respectively). The momentum I induced by the Ho:YAG laser was only 50% and its transfer efficacy η _Repuls was reduced to less than 5% of the values of the two short-pulsed laser systems. This investigation clearly showed the variable parts and amounts of repulsion using different pulsed lasers in an objective and reproducible manner. The momentum transfer efficiency could be determined without any physical friction problems. Further investigations are needed to compare stone fragmentation techniques with respect to laser repulsion and its clinical impact.