The first generation clinical electromechanical impactor was a 5F device that used a 3.0F electrohydraulic probe within a coiled spring with a blunt solder end cap. Clinical trials showed that this device fragmented 70% of the calculi but failed to fragment the harder stones. Applying the principle that impact kinetic energy is most dependent on velocity (KE = 1/2 MV2), 3 major design modifications were made to improve fragmentation efficiency: 1) the spring was changed from an extension to a compression type that captured and used more of each electrical spark, 2) the tip was changed from solder to lightweight titanium and 3) the tip shape was changed from blunt to conical. The result was a 41% increase in impact kinetic energy, a 40% increase in probe longevity and successful in vitro fragmentation of the harder calculi without compromise in tissue safety as determined by membrane perforation studies.
