The Moses technology for the Ho:YAG laser introduces a pulse-shape modulation that optimizes energy delivery through water and can be utilized for lithotripsy at a distance from the target. In light of this advance, we undertook an in vitro study to assess the effect of fiber tip to stone distance on fragmentation, incorporating the use of a variety of pulse modes.
Experiments were conducted with a three-dimensional (3D) positioning system, a 30 mm flat plate BegoStone, and a 230 μm core laser fiber connected to a 120W holmium laser utilizing short pulse (SP), long pulse (LP), Moses contact (MC), and Moses distance (MD) modes. Ablation crater volume was measured by 3D confocal microscopy, after a single pulse (1.0 J) was activated with the fiber tip positioned at 0, 0.5, 1, 2, and 3 mm from the stone. Fragmentation efficiency (1 J × 10 Hz) was assessed with the fiber tip at 0 and 1 mm distance, programmed to fragment the stone over 3 minutes. Fragmentation was defined as difference in stone mass before and after each experiment.
For all tested pulse modes, ablation crater volume and fragmentation were greatest when the fiber tip was in contact with the stone. Ablation declined as the working distance increased with no ablation occurring at 3 mm. At 1 mm distance, the ablation crater volume using MD mode was significantly higher when compared with SP, LP, and MC modes (p < 0.05). Compared with all pulse modes tested, MD resulted in 28% and 39% greater fragmentation at both 0 and 1 mm working distances, respectively (p < 0.05).
Holmium laser lithotripsy is significantly affected by fiber working distance with the greatest ablation obtained with the fiber in contact with the stone. At 0 and 1 mm distance, MD had the greatest fragmentation efficiency, suggesting this mode may have advantages during ureteroscopy.