Presented by: Bingyuan Yang MB BChir
University of Oxford

Introduction

Laser lithotripsy destroys urinary stones by ablating them into dust and fine particles. Extremely small particles may remain in suspension and are presumed to wash out in the urine, while larger particles may need to be extracted intraoperatively. The distribution of the resulting stone fragments therefore has important clinical implications. We report the size distribution of calcium oxalate and struvite stones after ablation with a Ho:YAG laser.

Materials

Canine calcium oxalate (CO) and struvite stones were soaked in water for 1 hour then ablated using a Ho:YAG laser (Lumenis Pulse™ 120h, Boston Scientific, USA) using 1J 10Hz short pulse settings and a 230um fibre. Ablation was done by hand in a dish by a “painting” motion across the surface of the stone. The stone fragments were collected and separated by size by wet sieving using 2000um, 1000um, 500um, 250um, 125um and 63um sieve sizes. The solid component of each fraction was captured using 11um filter paper, dried for 24 hours and the dry weight recorded. 3 samples were used for each stone type.

Results

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Particle size has a bimodal distribution, with peaks at the smallest and largest particle sizes. The distribution is similar for both stone types, with CO having slightly larger particles on average. The proportion of particles >500um was 18.5% for struvite and 21.0% for CO.

56% of starting stone mass was recovered for struvite, compared to 90% for CO. This suggests that struvite produces more ultra-fine particles that are too small to be collected by 11um filter paper.

Conclusion

The size of stone particles after laser ablation follows a bimodal distribution with peaks at <63um and 500-2000um. This suggests that there is more than one mechanism involved in stone ablation, with one generating fine dust <250um and one generating larger particles >250um.

About 20% of stone particles are >500um, which may not wash out of the kidney spontaneously and may contribute to lower stone-free rates and higher rates of recurrence. These particles are too small to ablate further by laser, and their large number and relatively small size makes basket extraction impractical or impossible. Thus, techniques or technologies are required to remove these particles completely to achieve stone-free status.

Disclaimer: Bench test results may not necessarily be indicative of clinical performance.

Funding

Boston Scientific Corporation: research grant

Co-Authors

Aditi Ray, PhD
Boston Scientific Corporation

James Zhang, PhD
Boston Scientific Corporation

Ben Turney, MB BChir, FRCS, DPhil
University of Oxford