Mechanical Impactor Employing Nitinol Probes to Fragment Human Calculi: Fragmentation Efficiency with Flexible Endoscope Deflection

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Abstract
The Browne Pneumatic Impactor (BPI; Browne Medical Systems, Minneapolis, MN) is a new mechanical lithotripter that relies on a pneumatically driven metal probe. It can be employed through both rigid and actively deflectable flexible endoscopes. Prior in vivo studies have shown excellent fragmentation of hard human calculi while employing the BPI with active endoscope deflection of as much as 45°. In this study, we set out to define the efficiency of stone fragmentation with active endoscope deflection and to quantify the extent of retrograde stone migration that is often noted when mechanical lithotripters are employed. Pressure transducer testing of the BPI revealed an inverse relation between increased deflection of the endoscope and transduced voltage. Calculus fragmentation tests showed that as the endoscope was actively deflected to 90°, the BPI still was able to fragment human calcium oxalate monohydrate calculi into extractable fragments with as few as six pulses. The maximum active endoscope deflection was 95° with the 0.020-inch Nitinol probe passed through the working channel. At this deflection, the BPI was able to fragment an 8 mm pure calcium oxalate monohydrate calculus into two fragments after 14 pulses. Retrograde migration was evaluated by employing the BPI in a simulated ureter in a waterbath. The travel distance of a 5-mm 5-g calculus with one pulse averaged 12.2 mm (range 1-44 mm). Smaller, less dense calculi travelled farther. A rebound effect was occasionally noted, and this was associated with an off-center delivery. In conclusion, the BPI is an efficient mechanical lithotrite able to fragment hard human calculi when employed with active endoscope deflection of as much as 95°. Delivered energy decreased with active endoscope deflection, but this change did not prevent fragmentation. Retrograde migration was an important variable more notable with smaller calculi.