In vitro assessment of ultrasonic lithotriptors

Purpose: Ultrasonic lithotriptors are commonly used to fragment and remove stones during percutaneous nephrolithotomy. To date a comparative assessment of current units has not been accomplished without potential operator bias. An objective testing environment is required for optimal appraisal of the efficiency of ultrasonic lithotriptors. Materials and Methods: An in vitro test system was devised to evaluate the ability of ultrasonic lithotriptors to core through artificial stones. The system consisted of an irrigation sheath (Cook Urological, Spencer, Indiana) through which ultrasonic probes were placed. Ultrasonic hand pieces and probes were secured in an upright position. An Ultracal-30 (U.S. Gypsum, Chicago, Illinois) stone cylinder (mean length 12.8+/-0.6 mm, mean diameter 7.6+/-0.07 mm) was centered on the probe tip. A weight (62.7 gm) was placed atop the stone to provide a constant force. We evaluated the Olympus LUS-1 and LUS-2 (Olympus, Melville, New York), Circon-ACMI USL-2000 (Circon-ACMI, Southborough, Massachusetts), Karl Storz Calcuson (Karl Storz, Culver City, California) and Richard Wolf model 2271.004 (Richard Wolf, Vernon Hills, Illinois). All probes had outer diameters of 3.4 mm except for the Circon-ACMI unit (3.8 mm). Using 100% power settings times for complete stone penetration were assessed for all units. Differences in mean stone penetration times were compared using ANOVA. Results: The Olympus LUS-2 had the fastest mean stone penetration time (28.8+/-2.7 seconds). This value was used to normalize the data into efficiency ratios, where other unit times were expressed as multiples of the LUS-2 time: Olympus LUS-2 (1.0+/-0.1) equals Circon-ACMI USL-2000 (1.1+/-0.3) greater than Karl Storz Calcuson (1.4+/-0.3) greater than Olympus LUS-1 (2.1+/-0.5) greater than Richard Wolf (3.6+/-0.8). Efficiencies of the LUS-2 and USL-2000 units were essentially equivalent, with all others significantly less efficient (p<0.05). Conclusions: This new in vitro testing model provides an objective, reproducible method for evaluating the efficiency of intracorporeal lithotriptors. Of the units tested the Olympus LUS-2 and Circon-ACMI USL-2000 were the most efficient.