Effectiveness of high-frequency holmium:YAG laser stone fragmentation: the "popcorn effect"

To demonstrate the validity of non-contact holmium:yttrium-aluminum-garnet laser stone fragmentation in an in-vitro model and to characterize the optimal laser settings to perform this technique.

A caliceal model consisting of a glass test tube filled with 0.9% normal saline was created. Into this system, a 365-mum laser fiber was inserted after stabilization 2 mm above the surface of the stones. Laser experiments were conducted using tubes that contained four soda limestone phantoms with a total stone burden of approximately 0.2 g per tube. The laser energy was varied between 1.0 and 1.5 J, with a firing frequency of 20, 30, or 40 Hz. Ten tubes were tested at each setting combination for a duration of 2 minutes. Thus, 60 tubes were tested. The contents of each tube were filtered through a 2-mm sieve after treatment, and all remaining fragments were dried for 5 days. The fragments were weighed, and the difference from pretreatment was calculated. Differences in weights and laser settings were compared using one-tailed and two-sample t testing.

Each experiment resulted in a significant decrease in stone burden from prelaser weight (all P values = 0.00). Laser settings were manipulated between combinations of 1.0 J at a frequency of 20, 30, and 40 Hz, and an energy of 1.5 J and 20, 30, and 40 Hz. This resulted in a respective mean weight loss of -42%, -58%, and -47% at 1.0 J, and -24%, -56%, and -63% at 1.5 J. The t tests were performed to detect differences between weight loss and to determine the optimal settings. Laser settings of 1.5 J and 40 Hz for 2 minutes produced the greatest mean decrease in stone burden (63%). This loss was significantly different from that of all settings at 1.0 J and the setting of 1.5 and 20 Hz (P < 0.05). The mean percentage weight loss for each setting was then normalized to the total energy used. Settings of 1.0 J and 20 Hz were the most efficient, with a change in weight of -18% per kJ.

We duplicated the "popcorn effect" of non-contact laser use in vitro. In this model, the technique results in a significant decrease in stone burden (up to 63%) in just 2 minutes. Adequate energy and high frequency seemed to optimize the effectiveness of the method, but excessive energy and frequency produced diminishing returns.