Fiber-to-stone distance and fragment size with holmium laser lithotripsy.

Control of fragment size during laser lithotripsy has taken on increasing importance with adoption of ureteroscopic aspiration technologies. The objective of this study was to characterize the relationship between laser-fiber-to-stone distance (FSD) and the size of stone fragments produced when using Ho:YAG laser lithotripsy with fragmentation settings. Brushite crystalline aggregate model stones were treated with Ho:YAG laser with MOSES™ distance (MD) and short pulse (SP) modes. A 200 μm laser fiber was positioned at 0, 0.5, 1.0, 1.5, or 2.0 mm above the stone surface. Pulsed laser energy (0.8 J x 10 Hz) was delivered continuously while the fiber was automatically translated through a predetermined grid of points to cover the entire stone surface. Multiple passes were performed until ≥ 50% of each stone was ablated. Stone fragments were passed through sequential sieves to separate the fragments into size ranges of 2–4, 1–2, 0.5-1, and 0.25–0.5 mm. A statistically significant inverse relationship between FSD and the distribution of fragment sizes was discovered when using MD mode. For example, the risk of creating a fragment > 1 mm with FSD of 0 mm was approximately 4, 7, 27, and 27 times greater than FSD of 0.5, 1.0, 1.5, and 2.0 mm respectively. With SP mode there was no statistical difference in fragment size distribution for any FSD tested. This discovery has potential impact on laser lithotripsy techniques and provides motivation for technological developments to measure (and better manage) FSD during laser lithotripsy.