as best seen in fig1 handpiece 10 that may be used with the method of the present invention generally comprises ultrasonic horn 12 , typically made from a titanium alloy . horn 12 has a plurality of helical slits , which will be discussed below . a plurality ( typically 1 or 2 pairs ) of ring - shaped piezoelectric elements 14 are held by compression nut 15 against horn 12 . aspiration shaft 16 extends down the length of handpiece 10 through horn 12 , piezoelectric elements 14 , nut 15 and through plug 18 at the distal end of handpiece 10 . aspiration tube 16 allows material to be aspirated through hollow tip 20 , which is attached to horn 12 , and through and out handpiece 10 . plug 18 seals outer shell 11 of handpiece 10 fluid tight , allowing handpiece 10 to be autoclaved without adversely affecting piezoelectric elements 14 . addition grooves 22 for sealing o - ring gaskets ( not shown ) are provided on horn 12 . as best seen in fig2 , horn 12 contains a plurality of spiral slits 24 . preferably , the width of slits 24 is between 2 % and 65 % of the outside diameter of horn 12 . this , of course , will affect how many slits 24 can be made on horn 12 ( e . g ., if slits 24 are 65 % of the diameter of horn 12 , then only one slit 24 may be cut into horn 12 ). the width of slits 24 selected will depend upon the desired amount of torsional movement . the depth of slits 24 in horn 12 preferably is between 4 % and 45 % of the outside diameter of horn 12 . slits 24 may have a flat or square cut bottom , but preferably have a rounded or radiused bottom , which are easier to manufacture . the length of slits 24 preferably is between 8 % and 75 % of the length of the larger diameter of horn 12 . the pitch of slits 24 preferably is between 125 % and 500 % of the larger diameter of horn 12 . by way of example , the inventors have found that one suitable configuration of slits 24 on horn 12 with an outside diameter of 0 . 475 inches is a total of eight slits 24 , having a width of 0 . 04 inches , a depth of 0 . 140 ( with a full radius bottom ), a length of 0 . 7 inches and a pitch of 1 . 35 inches gives suitable torsional movement of horn 12 without compromising the longitudinal movement of horn 12 . as best seen in fig1 , the location of longitudinal and torsional nodal points ( the points with zero velocity of the respective mode ) is important for proper functioning of handpiece 10 . the torsional node 26 preferably is located at the proximal longitudinal node 28 , so that the torsional node 26 and the longitudinal node 28 are coincident , e . g ., both of which are located on plug 18 . handpiece 10 also contains a distal longitudinal node 30 located at reduced diameter portion 32 of horn 12 . as best seen in fig3 , drive circuit 34 that may be used with handpiece 10 of the present invention preferably is similar to that described in u . s . pat . no . 5 , 431 , 664 , the entire contents of which being incorporated herein by reference , in that drive circuit 34 provides a drive signal to handpiece 10 and tracks the admittance of handpiece 10 and controls the frequency of handpiece 10 to maintain a constant admittance . however , drive circuit 34 monitors both the torsional mode and the longitudinal mode and controls these modes in handpiece 10 using two different drive frequencies . preferably , the torsional drive signal is approximately 32 khz and the longitudinal drive signal is 44 khz , but these frequencies will change depending upon the piezoelectric elements 14 used and the size and shape of horn 12 and slits 24 . although both the longitudinal or the torsional drive signal may be supplied in a continuous manner , preferably the longitudinal drive signal and the torsion drive signal are alternated , so that the drive signal is provided in a desired pulse at one frequency and then switched to the other frequency for a similar pulse , with no overlap between the two frequencies , but no gap or pause in the drive signal . alternative , the drive signal can be operated in a similar manner as described , but short pauses or gaps in the drive signal can be introduced . in addition , the amplitude of the drive signal can be modulated and set independently for each frequency . the pause or gap between drive signals can serve various purposes . one purpose is to allow for the ultrasound movement of piezoelectric elements 14 and horn 12 to attenuate or stop so that lens fragments can once again be suctioned to tip 20 and an occlusion reestablished , thereby increasing the holding force on the lens fragment . reestablishing the occlusion will increase cutting efficiency of the following pulse of ultrasound , whether longitudinal or torsional . another purpose of the pause or gap between drive signals is to allow for the ultrasound movement of piezoelectric elements 14 and horn 12 to attenuate or stop prior to the other ( either longitudinal . or torsional ) mode being excited . such attenuation between drive signals will reduce amount of potential non - linear interactions in the system which can generate undesirable heat and lead to premature degradation of piezoelectric elements 14 or mechanical failure of the entire assembly . alternatively , there can be a slight overlap in the longitudinal and torsional drive signals . the overlap may provide relatively short time intervals when the added action of both torsional and longitudinal displacements results in especially fast rate of lens emulsification , and yet the overlap is short enough to prevent piezoelectric elements 14 from premature degradation or failure of the entire mechanical assembly as a result of excessive stress . yet another alternative if to have both longitudinal and torsional drive signals overlap completely thus resulting in applying high stress levels to the lens material when the two signals overlap , and yet leaving a pause in between for the occlusion to reestablish itself and allow for vacuum to build - up , thus improving efficiency of the following pulse application . still another alternative is to apply a continuous longitudinal signal with a pulsed torsional signal , or vice versa , a continuous torsional signal with a pulsed longitudinal signal . continuous application of torsional ultrasound does not cause repulsion because tip 20 movement is oriented perpendicular to the direction of the engagement of tip 20 with the lens , and the pulsed applications of longitudinal ultrasound are short enough to prevent overheating or mechanical damage to piezoelectric elements 14 . finally , as discussed above , both the longitudinal and torsional drive signals can be applied continuously and simultaneously , with the amplitudes of the both signals being selected such that overheating and excessive mechanical stress on the system is reduced . if such a drive scheme is to be used , two sets of piezoelectric elements 14 are preferred with the torsional signal being applied to one set , while longitudinal signal applied to the other set . as best seen in fig4 and 5 , typical duty cycle 200 includes power on portion 210 and power off portion 220 . by way of example , fig4 illustrates a 50 % duty cycle 200 , meaning that on portion 210 equals off portion 220 . the inventor has discovered that the power level ( which may be measured as a percent of the maximum possible stroke or rotation of the tip , or as a percentage of the maximum amplitude of the power signal delivered to piezoelectric elements 14 ), within on portion 210 may be further divided into high power portion 230 and low power portion 240 . by way of example , high power portion 230 may be at a 60 % power level and low power portion 240 may be at a 10 % power level . such a division of on portion 210 allows high power portion 230 to be of extremely short duration , for example , between 1 millisecond and 5 milliseconds , and preferably less than 5 milliseconds , while the overall duration of on portion 210 may remain relatively long , for example , 20 milliseconds or greater . maintaining a relatively long on portion 210 allows the feedback loop contained within drive circuit 34 sufficient time to cycle and automatically adjust the operating parameters of handpiece 10 . further , because low power portion 240 encompasses a significant percentage of on power portion 210 , on the order of 75 % to 95 %, maintaining a relatively long on power portion 210 does not significantly increase the heat generated by tip 20 . as a result , ultrasonic power pulses of less than 5 milliseconds are possible while maintaining the optimum frequency for handpiece 10 while at the same time , the potential for thermal injury to the eye is reduced . one skilled in the art will recognize that both high power portion 230 and low power portion 240 may be include longitudinal or torsional vibration of tip 20 in any combination desired . in addition , while fig4 and 5 demonstrate on power portion 210 as having one high power portion 230 and one low power portion 240 , on power portion 210 may be further sub - divided into multiple high power portions 230 and / or multiple low power portions 240 , serially , alternately or randomly . while certain embodiments of the present invention have been described above , these descriptions are given for purposes of illustration and explanation . variations , changes , modifications and departures from the systems and methods disclosed above may be adopted without departure from the scope or spirit of the present invention .
a method of operating an ultrasonic handpiece by pulsing the power supplied to the handpiece and varying the amplitude of the power during the power pulse .