Abstract:
A non-linear cut-rate multiplier for a vitreous cutter is provided whereby a signal from a host drive system may be multiplied in non-linear fashion to achieve significantly higher cut-rates for the vitreous cutter. Depending upon the cut-rate received from the host drive system, the multiplier may be configured to generate a subsequent cut-rate which is, potentially, linear for lower cut-rates of the host drive system and variably non-linear for higher cut-rates of the host drive system.

Description:
RELATED APPLICATIONS 
       [0001]    The present patent application claims priority to U.S. Provisional Application No. 61/181,199, filed on May 26, 2009, the content of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to devices for performing micro-surgical procedures in the posterior portion of the eye. More particularly, the present invention relates to a cut-rate controller for a vitreous cutter. 
         [0003]    The instrument most commonly used, and generally preferred, for vitreous surgery is a pneumatically-operated axial guillotine cutter. A typical pneumatically-operated guillotine cutter system includes a handpiece (sometimes called a “cutter”) that includes a needle with a cutting/aspiration port located near the needle&#39;s distal end. The handpiece receives pneumatic power from a vitreoretinal surgical system (sometimes called a “drive unit” or a “console”). Often, the system also provides aspiration and illumination functions. 
         [0004]    Although numerous improvements have been made over the years, the fundamental aspects of vitreous cutters are known and taught by O&#39;Malley and Heintz in U.S. Pat. Nos. 3,884,238 and 3,815,604, respectively. In its modern form, the axial guillotine cutter is relatively small, lightweight, durable, inexpensive, and exhibits good cutting characteristics. 
         [0005]    One improvement made in vitreous cutting has been the ability to operate at higher cut-rates (the number of cuts made per minute). This generally results in better-controlled and safer cutting. Operation at high cut-rates requires improvements both to the vitreous cutter and to the drive unit. U.S. Pat. No. 6,575,990, issued to Wang et al., discloses a means of adding a separate drive unit as an accessory to an existing surgical system (referred to as the “host system”) in order to provide for higher cut-rates without modification of the host system. Examples of accessory drive units which embody this patent (referred to as the “accessory drive unit”) include the AVE and the VIT Enhancer units (see, http://www.midlabs.com/ave.htm) sold by Medical Instrument Development Laboratories, Inc. of San Leandro, Calif. An accessory drive unit may include electronics and pneumatic components, such as tubing and a pneumatic valve controlled by the electronics. Some accessory drive units may include internal air compressors. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In available accessory drive units, the cut-rate of the guillotine cutter is set using controls on a front panel. There are some deficiencies in setting the cut-rate in this manner. First, the controls add complexity and cost to the accessory drive unit. Second, because the controls are used to set the cut-rate, the accessory drive unit must be designed so that the controls can be readily accessed by the user of the cutter (e.g., a surgeon). Lastly, the cut-rate can not be varied using the controls of the host surgical system, although in some instances it is possible to provide means for the surgeon to vary the cut-rate using a foot pedal control. 
         [0007]    Accordingly, in one embodiment, the invention provides an accessory drive unit, in the form of a non-linear cut-rate multiplier, so that the cutter can be driven at a frequency or rate that is different than the rate available from the host surgical system. In this embodiment, the cut-rate varies as a non-linear function of the cut-rate set on the host surgical system. 
         [0008]    In another embodiment, the invention provides a non-linear cut-rate multiplier that includes an input sensor that senses an input signal with a first frequency provided by a drive unit, and a non-linear frequency multiplier circuit that receives the input signal and outputs an output signal at a second frequency that is a non-linear multiple of the first frequency. The non-linear cut-rate multiplier also includes a trigger circuit that receives the input signal and outputs a trigger signal. A drive circuit receives the output signal and the trigger signal and outputs an actuation signal with a third frequency. The third frequency is substantially equal to the second frequency. 
         [0009]    In another embodiment, the invention provides a method of controlling the cut-rate of a vitreous cutter. The method includes detecting a drive signal produced by a drive unit. The drive signal drives the vitreous cutter at a first frequency. The method also includes sensing the drive signal with an input sensor of a non-linear cut-rate multiplier; processing the drive signal into an output signal with a second frequency that is a non-linear multiple of the first frequency; and producing an actuation signal to drive the vitreous cutter at a third frequency that is substantially equal to the second frequency. 
         [0010]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0011]      FIG. 1  is a schematic view of a high-speed vitreous cutting system that includes a host system, a non-linear cut-rate multiplier, and a vitreous cutter; 
           [0012]      FIG. 2  is a schematic diagram of one embodiment of a non-linear cut-rate multiplier circuit suitable for use in the present invention; 
           [0013]      FIG. 3  is a circuit diagram illustrating additional detail of a non-linear frequency multiplier circuit, which is part of the cut-rate multiplier circuit shown in  FIG. 2 ; 
           [0014]      FIG. 4  is a schematic diagram of a second embodiment of a non-linear cut-rate multiplier circuit suitable for use in the present invention; 
           [0015]      FIG. 5  is a circuit diagram illustrating additional detail of a track and hold circuit, which is part of the cut-rate multiplier circuit shown in  FIG. 2 ; 
           [0016]      FIG. 6  is a schematic diagram of one embodiment of a non-linear cut-rate multiplier microcontroller unit suitable for use in the present invention; 
           [0017]      FIG. 7  is an example of a host system selection table used in the vitreous cutting system of  FIG. 1 ; 
           [0018]      FIG. 8  is an alternative example of a host system selection table used in the vitreous cutting system of  FIG. 1 ; 
           [0019]      FIG. 9  is a graph illustrating several functions for adjusting the cut-rate frequency of the vitreous cutting system using the non-linear cut-rate multiplier of  FIG. 1 ; 
           [0020]      FIG. 10  is a flow chart of a method for operating the non-linear cut-rate multiplier of  FIG. 1 ; and 
           [0021]      FIG. 11  is a perspective view of a switch mechanism used with the high-speed vitreous cutting system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0023]      FIG. 1  illustrates a high-speed vitreous cutting system  10  that includes a vitreoretinal surgical host system  14  (herein referred to as a “drive unit”), an accessory in the form of a non-linear cut-rate multiplier  18 , and a pneumatically-operated axial guillotine cutter  22  (herein referred to as a “cutter”). The drive unit  14  may be any unit typically used in vitreoretinal surgery that is operable to output a signal to drive a pneumatically or electrically actuated cutter. 
         [0024]    The cutter  22  is a pneumatically driven, axial guillotine-type vitreous probe or cutter. One such cutter is described in detail in U.S. Pat. No. 6,575,990, filed Oct. 20, 2000, the contents of which are herein incorporated by reference. The illustrated cutter  22  contains a generally cylindrically shaped housing  26  designed to be held in a human hand. The housing has a first end  30  and a second end  34 . A needle  38  is coupled to the first end  30 . The needle  38  includes a cutting or aspiration port near the distal end for use in removing vitreous. 
         [0025]    Tubing, having two adjacent tubes  42  and  46 , is connected to the cutter  22 . One end of the aspiration tube  42  is connected to a port on the second end  34  of the cutter  22 . Actuation tube  46  is also coupled to the second end  34  of the cutter  22 . The other end of the actuation tube  46  is connected to a fitting  50 , such as a male, luer-lock fitting, located on a front panel  54  of the non-linear cut-rate multiplier  18 . In addition to the pneumatically driven cutter described below, in other embodiments, the cutter may be an electrically actuated cutter. (The electric signal could be provided to an electrically-controlled valve to control pneumatic pulses or to a solenoid that produces linear motion to drive an electrically-controlled cutter). 
         [0026]    The non-linear cut-rate multiplier  18  is illustrated as a separately housed accessory in the vitreous cutting system  10 . A tubing  58  is connected to an output port  62  on the drive unit  14  and to an input port  66  on the non-linear cut-rate multiplier  18 . The non-linear cut-rate multiplier  18  also includes an output port  70 . The fitting  50  is connected to the output port  70 . The non-linear cut-rate multiplier  18  may include a pneumatic drive system (e.g., an electric air compressor) for providing pneumatic energy or compressed air to the cutter  22 . A back panel of the non-linear cut-rate multiplier  18  includes a socket for connecting an electrical cord with a plug for supplying electric power to the transformer and a power switch for turning the non-linear cut-rate multiplier  18  on and off. 
         [0027]    In some drive units, the available frequencies may range from approximately 400 cuts/min to 750 cuts/min. In other drive units, the available frequencies may range from 400 cuts/min to 2,500 cuts/min. With the use of the non-linear cut-rate multiplier  18 , the range of available frequencies increases. For example, the available frequencies can range from 400 cuts/min to 12,000 cuts/min. The range of frequencies available from the non-linear cut-rate multiplier  18  is determined by a non-linear function (illustrated as Equation 1 and described below), which is determined by the circuit parameters of the non-linear frequency multiplier circuit  94  ( FIG. 2 ). Thus, the user can achieve cut-rates that are significantly greater than the cut-rates available from the drive unit  14  alone. In alternative embodiments, the range of frequencies from the multiplier  18  also can be determined by a linear function. This is often desirable when performing vitreoretinal surgery because lower cutting rates are used for rapid removal of vitreous in the center part of the eye while higher cutting rates are provided for more controlled removal of vitreous near the retina. In other constructions, the non-linear cut-rate multiplier  18  may provide options for adjusting the function or choosing a predefined function using a switch or other selection mechanism. Alternately, a two or more position rotating dial may be rotated by the user to choose one of two or more different functions for adjusting the frequency. Thus, two drive units, with two different ranges of available frequencies (e.g., 400 cuts/min to 750 cuts/min and 400 cuts/min to 2,500 cuts/min) can be used with the non-linear cut-rate multiplier  18  to each produce the same range of frequencies (e.g., 400 cuts/min to 12,000 cuts/min) by choosing an appropriate function. 
         [0028]    In operation, the non-linear cut-rate multiplier  18  receives a signal from the drive unit  14  through tubing  58 , which is connected to the input port  66 . The front portion of the non-linear cut-rate multiplier  18  includes a front panel button  72  and a display  74 . A user can select the type of drive unit  14  (host system) connected to the non-linear cut-rate multiplier  18  from a variety of drive/host systems shown on the display  74  by pressing the button  72 . The display  74  can show the available host systems, along with their corresponding cut rates and cut rate control forms (footpedal, panel, or other). As indicated above, the front panel button  72  also can be used to adjust or select a specific function. The back panel of the non-linear cut-rate multiplier  18  can include a rear power switch (not shown) that activates the front panel button  72 . The non-linear cut-rate multiplier  18  processes the signal and outputs an actuation signal that drives the cutter  22 . The frequency of the actuation signal is a non-linear multiple of the frequency of the signal from the drive unit  14  such that the cutter  22  may be operated at a faster speed or rate than possible using the drive unit  14  alone. In addition, the non-linear cut-rate multiplier  18  allows switchability between a non-linear cut-rate and a linear cut-rate, and the multiplied signal may be outputted at both non-linear and linear cut-rates to the vitreous cutter. 
         [0029]    To illustrate the advantages and/or improvements of non-linear multiplication over linear multiplication in operating the high-speed vitreous cutting system  10 , the following example should be considered. The host system  14  in this example has a practical control range of 150 cuts/min to 800 cuts/min. The accessory device (multiplier  18 ) can be used to give cut rates up to 8000 cuts/min, but is mostly used in the 2000 to 4000 cuts/min range. In addition, there are certain situations where it might be desirable for a user to have a cut rate as low as 300 cuts/min. At the upper limit of the range (800 cuts/min input, 8000 cuts/min output) a multiplication factor of 10 is required. At the lower limit of the range (150 cuts/min input, 300 cuts/min output) a multiplication factor of 2 is required. With linear (fixed) multiplication, it would be necessary to stop at various times during the surgical procedure to select different multiplication factors. In contrast, the non-linear cut-rate multiplier  18  can easily be set up to have the required multiplication factors of 2 at 150 cuts/minute input and 10 at 800 cuts/minute input. Moreover, the non-linear cut-rate multiplier  18  would have an effective multiplication factor of approximately 4.8 at the center of the most-commonly-used range. 
         [0030]      FIG. 2  illustrates one embodiment of a non-linear cut-rate multiplier circuit  78  (shown in schematic form) that may be used by the non-linear cut-rate multiplier  18 . An input sensor  82  senses the (pneumatic or electrical) signal  86  from the drive unit  14  and outputs an alternating current (AC) signal  90  with the same frequency to the non-linear frequency multiplier circuit  94  and to a trigger circuit  98 . 
         [0031]    The cut-rate multiplier circuit  78  includes a non-linear frequency multiplier circuit  94 . The circuit  94  includes a comparator  102 , a latch  106 , a sawtooth waveform generator  110 , a track and hold circuit  114 , and a voltage controlled oscillator (“VCO”)  118 . The latch  106  receives the AC signal  90  and a signal from the comparator  102 . The latch  106  produces a square-wave output  122  in response to the received signals. The sawtooth waveform generator  110  receives the square-wave signal  122  and generates a sawtooth waveform  126 . The track and hold circuit  114  receives the sawtooth waveform  126  and tracks a minimum value of the sawtooth waveform  126 . The track and hold circuit  114  produces a direct current (DC) output  130 , which is provided to the VCO  118 . The VCO  118  produces an AC output  134  with a frequency that varies under the influence of (or is based upon) the signal  130  received from the track and hold circuit  114 . (The AC output  134  is also the output of the circuit  94 .) 
         [0032]    The trigger circuit  98  generates a DC signal  138  which is provided to a waveform shaping circuit  142 . The DC signal  138  is a logical signal that is a high voltage, typically about 5 volts, when the signal  90  is detected by the trigger circuit  98  and a low voltage, typically about 0 volts, when the signal  90  is not detected by the trigger circuit  98 . The DC signal  138  enables the waveform shaping circuit  142  when it is a high voltage and disables the waveform shaping circuit  142  when it is a low voltage. The waveform circuit  142  also receives the AC output  134  of the VCO  118 . One example of a waveform shaping circuit is described in U.S. Pat. No. 6,575,990, filed on Oct. 20, 2000, which is incorporated herein. The waveform shaping circuit  142  processes the signals  134  and  138  to produce a square-wave, output signal  146 . The output signal  146  is provided to a solenoid output valve  150 . The solenoid output valve  150  is a three-way valve and receives pressurized gas  154  (or pressurized air) from a pressurized gas source  158 . The signal  146  causes the solenoid valve  150  to open and close, which results in the generation of a gas pulse train (or pneumatic signal)  162 . The gas pulse train is provided to the cutter  22  to pneumatically operate or drive the cutter  22  at a frequency that approximates the frequency of the output signal  134  of the non-linear frequency multiplier circuit  94 . The solenoid output valve  150  is vented when not actuated. 
         [0033]      FIG. 3  illustrates the non-linear frequency multiplier circuit  94  in more detail. Although one particular example is illustrated and explained herein, other circuits can be designed that perform similar functions. The non-linear frequency multiplier circuit  94  receives the AC input signal  90  from the input sensor  82 . The signal  90  has a frequency F 1 . The output signal  134  has a frequency F 2 . Equation 1 is a non-linear function that describes the relationship between the frequencies F 1  and F 2 , where α and β are positive constants determined by the circuit parameters of the non-linear frequency multiplier circuit  94 , as described below. 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                      
                     
                         
                     
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                     2 
                   
                   = 
                   
                     F 
                      
                     
                         
                     
                      
                     1 
                      
                     
                       ( 
                       
                         1 
                         
                           α 
                           - 
                           
                             β 
                              
                             
                                 
                             
                              
                             F 
                              
                             
                                 
                             
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                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
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                   1 
                 
               
             
           
         
       
     
         [0034]    The non-linear frequency multiplier circuit  94  is designed such that when the frequency F 1  of the input signal  90  is low, the frequency F 2  of the output signal  134  is approximately equal to the frequency F 1  of the input signal  90 . As the frequency F 1  of the input signal  90  increases, the frequency F 2  of the output signal  134  increases at a faster, non-linear rate that is determined by the circuit parameters such as resistance, capacitance, input bias voltage, etc. Such a relationship has certain benefits for a surgeon performing vitreoretinal surgery. For example, when a surgeon is operating a cutter at a low frequency (i.e., a low cut rate), which is commonly used for rapidly removing vitreous, the surgeon can make more controlled adjustments to the output frequency because the output frequency is nearly equal to the input frequency at low cut rates. When the surgeon is operating at a high frequency, less vitreous is removed over time. In general, the higher the frequency, the slower the removal of vitreous. Thus, the non-linear frequency multiplier circuit  94  allows the surgeon to operate at higher frequencies (i.e., higher cut-rates) than achievable from the drive unit  14  alone, thereby providing the surgeon with better control over the removal of vitreous. 
         [0035]    The specific aspects of the circuit  94  that help achieve the advantages noted above include the sawtooth generator circuit  110 . As shown in  FIG. 3 , the sawtooth waveform generator circuit  110  includes an open-collector driver  166 , a first resistor  170 , a second resistor  174 , a capacitor  178 , and an operational amplifier  182 . The open-collector driver  166  (alternatively an open-drain driver could be used) acts similar to a switch to short its output  126  to ground when activated and to allow its output  126  to float when not activated. When the open-collector driver  166  is not activated, the output  126  of the operational amplifier  182  integrates downward at a rate which is determined by the values R of the resistors  170  and  174 , the value C of the capacitor  178 , and the values V A  and V B  of the voltages at nodes  190  and  194 , respectively. When the open-collector driver  166  is activated, the output  126  of the operational amplifier  182  integrates upward at a rate determined by the values R of the resistors  170  and  174 , the value C of the capacitor  178 , and the values V A  and V B  of the voltages at nodes  190  and  194 , respectively. 
         [0036]    The latch  106  receives the input signal  90  from the input sensor  82 . The latch  106  is set when the input signal  90  rises above a first threshold, which is determined by the latch and is typically about 5 volts. When the latch  106  is set, the output  122  of the latch  106  is high and the open-collector driver  166  is activated. The latch  106  also receives a signal  202  from a comparator  206 . When the output signal  126  of the sawtooth waveform generator circuit  110  rises above a voltage V 3 , the comparator  206  outputs a high voltage, typically about 5 volts, to reset the latch  106 . When the latch  106  is reset, it outputs a low voltage, typically about 0 volts, which is received by the open-collector driver  166 . When the open-collector driver  166  receives a low voltage, the open-collector driver  166  is not activated and the output of the open-collector driver  166  floats. Thus, the output  126  of the sawtooth waveform generator circuit  110  begins to integrate downward at a fixed rate, until the latch  106  is again set by the input signal  90 . When the input signal  90  to the latch  106  is a signal such as a periodic pulse train with a frequency F 1 , the output  126  of the sawtooth waveform generator circuit  110  is a sawtooth waveform with approximately the same frequency as the input signal  90 . The sawtooth waveform oscillates between a maximum voltage equal to V 3  and a minimum voltage arbitrarily denoted as V x . The minimum voltage V x  depends on the frequency of the input signal  90  and the rates at which the signal  126  integrates upward and downward (i.e., the slopes of the rise and fall of the sawtooth waveform). 
         [0037]    The track and hold circuit  114  receives the output  126  from the sawtooth waveform generator circuit  110  and outputs a DC voltage  130  that tracks with the minimum voltage V x  of the output  126  of the sawtooth generator circuit  110 . One example of a track and hold circuit is illustrated in  FIG. 5 , where the track and hold circuit  114  receives the output  126  from the sawtooth waveform generator circuit  110  as an input signal V 5  and receives the square wave output  122  from the latch  106  as an input signal V 4 . The track and hold circuit  114  outputs the DC voltage  130  as an output signal V 6 . The minimum voltage V x  of the output  126  of the sawtooth generator circuit  110  can be determined from Equation 2, where A is a constant determined by the component and bias voltage values in the sawtooth waveform generator circuit  110 . 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     x 
                   
                   = 
                   
                     
                       V 
                       3 
                     
                     - 
                     
                       A 
                        
                       
                         1 
                         
                           F 
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                   2 
                 
               
             
           
         
       
     
         [0038]    The square wave signal  122  transitions from a low voltage to a high voltage when the signal  126  is at the minimum voltage V x . The track and hold circuit  114  samples the voltage of signal  126  on the low-to-high transition of the square wave signal  122 . Thus, the output  130  of the track and hold circuit  114  is approximately equal to the minimum voltage V x  of signal  126 . The voltage controlled oscillator  118  receives the minimum voltage V x  and produces an output signal  134  with a frequency F 2  that can be determined according to Equation 3, where B is a constant determined by the circuit parameters of the VCO  118 . 
         [0000]    
       
         
           
             
               
                 
                   
                     1 
                     
                       F 
                        
                       
                           
                       
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                       2 
                     
                   
                   = 
                   
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                      
                     
                       ( 
                       
                         
                           V 
                           1 
                         
                         - 
                         
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                           x 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
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                    
                   
                       
                   
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                   3 
                 
               
             
           
         
       
     
         [0039]    More specifically, the VCO  118  includes two comparators  218  and  226 , a latch  210 , and a sawtooth generator circuit  234 . The comparator  218  outputs a high voltage when the output  134  of the sawtooth waveform generator circuit  234  is below the voltage of the output signal  130  of the track and hold circuit  114 . The comparator  226  outputs a high voltage when the output  134  of the sawtooth waveform generator circuit  234  is above V 1 . The latch  210  receives the signal  214  output by comparator  218  and the signal  222  output by comparator  226 . The latch  210  is set and outputs a high voltage  230  when the signal  214  from comparator  218  is high (typically about 5 volts). The latch  210  is reset and outputs a low voltage  230  when the signal  222  from comparator  226  is high (typically about 5 volts). 
         [0040]    The sawtooth waveform generator circuit  234  is similar to the sawtooth waveform generator circuit  110  and operates in a similar manner. As illustrated, the circuit components have the same nominal values (e.g., R and C), are connected in a similar manner, and perform similar functions. Like components have been given like reference numbers of the 300 series. When the open-collector driver  366  receives a high input voltage  230 , the output  134  of the sawtooth waveform generator circuit  234  integrates upward, or increases linearly. When the open-collector driver  366  receives a low input voltage  230 , the output  134  of the sawtooth waveform generator circuit  234  integrates downward, or decreases linearly. 
         [0041]    Thus, the output  134  of the sawtooth waveform generator circuit  234  increases until the output  134  becomes greater than V 1 , at which point, the output of the comparator  226  becomes high and resets the latch  210 . Then, the output  134  of the sawtooth waveform generator circuit  234  begins to decrease until the output  134  becomes less than the output  130  of the track and hold circuit  114 , at which point the comparator  218  outputs a high voltage to set the latch  210 . When the latch  210  is set, the output  134  begins to increase again. The cycle repeats itself such that the output of the non-linear frequency multiplier circuit  94  outputs a sawtooth waveform that oscillates between V x  and V 1  with a frequency F 2  described by Equation 3. 
         [0042]    Equation 3 can be simplified by substituting Equation 2 into Equation 3 and simplifying. After simplification, Equation 3 can be rewritten as follows. 
         [0000]    
       
         
           
             
               
                 
                   
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                     F 
                      
                     
                         
                     
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                         1 
                         
                           AB 
                           - 
                           
                             
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                                     3 
                                   
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                                     1 
                                   
                                 
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                   4 
                 
               
             
           
         
       
     
         [0043]    As can be seen by comparing Equation 1 to Equation 4, α is equal to AB and β is equal to B(V 3 −V 1 ). Equations 1 and 4 may be used interchangeably. For simplicity, Equation 1 is disclosed to the user of the cut-rate multiplier  18 . In some constructions, the cut-rate multiplier  18  may include user options that allow the user to select the desired value of β, perhaps by adjusting a switch on the front panel  54  of the non-linear cut-rate multiplier  18  or otherwise selecting values for f 3 . For example, a switch may be provided on the front panel  54  to allow the user to select a voltage for V 3 , which is applied to the negative input of the comparator  206 . Alternatively, a switch may be provided on the front panel  54  to allow the user to select a voltage for V 1 , which is applied to the negative input of the comparator  226 . 
         [0044]      FIG. 4  illustrates a second embodiment of a non-linear cut-rate multiplier circuit  400  (shown in schematic form). An input sensor  401  senses the signal  86  from the drive unit  14  and outputs an AC signal  402  with the same frequency to a non-linear frequency multiplier circuit  403  and to a trigger circuit  405 . 
         [0045]    The multiplier circuit  403 , which is similar to the non-linear frequency multiplier circuit  94  of  FIG. 2 , includes a comparator  407 , a latch  409 , a sawtooth waveform generator  410 , a track and hold circuit  414 , and a VCO  418 , which all operate in a similar way as described with respect to  FIG. 2 . The VCO  418  generates an output signal  420 , which is an AC signal with a frequency that is a non-linear multiple of the frequency of the input signal  86 . A drive system  422  receives an output signal  424  from the trigger circuit  405  and the AC signal  420 . The drive system  422  converts the signals received into an electrical signal  430  that drives an electrically actuated vitreous cutter. 
         [0046]    In an alternative embodiment, instead of using the analog frequency multiplier circuit  78 , the variable non-linear cut-rate multiplier  18  is driven or operated with digital components by a microcontroller unit (“MCU”)  88 . As shown in  FIG. 6 , the microcontroller unit  88  includes an input counter-timer hardware  76 , a processor  84 , and an output counter-timer hardware  92 , where all components are located on a single chip. In other embodiments, the MCU  88  can include additional components that are not located on a single chip. An input sensor  82  senses the signal  86  from the drive unit  14  and outputs an alternating current (“AC”) signal  90  with the same frequency to the MCU  88 . 
         [0047]    In addition, the processor  84  is connected to the display  74 , which displays various types of hosts systems (drive units  14 ) and corresponding cut rates that can be selected by a user via the front panel button  72 .  FIG. 7  shows an example of a host system selection table that is available to a user via the display  74  and the front panel button  72 . Additional data or elements can be included in the table, as will be apparent to those skilled in the art. Further, the processor  84  of the MCU  88  is connected to a cutter connect sensor  80  that can transmit various conditions of the cutter  22 . Using the information received from the cutter connect sensor  80 , the processor  84  can determine whether the cutter  22  is working properly by executing a cutter test, which is described in more detail below. 
         [0048]    During the cutter test, the processor  84  receives signals from the cutter connect sensor  80  and uses various calculations to determine whether the cutter  22  is defective in any way and thus can be dangerous to the patient. For example, by measuring the pressure and the volume of circulating gas, the cutter test looks for leaks in tubes  42  and  46  that lead to a potential malfunction of the cutter  22 . In addition, the cutter test can be used to determine if any other or all components of the cutter  22  function properly. 
         [0049]    Conventional methods for initiating the cutter test require that the operator of the cutter  22  performs an action involving controls/switches in order to start the test (e.g., press a button). In an embodiment of the invention, the cutter test is initiated automatically from the act of connecting the cutter  22  to the multiplier  18 . This automatic initiation of the cutter test helps to avoid a potential mistake or neglect by a human operator and verifies that there are no hazards associated with the cutter  22 . 
         [0050]    The cutter test is initiated by a mechanical switch  265  coupled to a switch mechanism  260  ( FIG. 11 ), where the switch  265  is automatically “pressed” based on the motion of connecting the cutter  22  to the multiplier  18 . As shown in  FIG. 11 , the switch mechanism  260  includes the switch  265 , a switch block  270  that engages and supports the switch  265  in a position relatively adjacent to the switch block  270  and in a position connected to a luer shaft  275 , which pneumatically links a vitrectomy probe luer connector  295  to the multiplier  18 . The switch mechanism  260  further includes a switch actuator  285  that slides inwardly on the top surface of the luer shaft  275  and includes a flange (not shown) that actuates the switch  265 , a spring  280  that releases the switch actuator  285  from the switch  265  when the probe luer connector  295  is not connected to the multiplier  18 , and a luer lock  290  that engages with the luer shaft  275  to secure the switch actuator  285  and the spring  280  on the luer shaft  275 . The vitrectomy probe luer connector  295  is positioned into the luer lock  290  and includes an inwardly opening that engages the luer head of the luer shaft  275 , where the probe luer connector  295  provides pressure to “push” the switch actuator  285  to activate the switch  265  when the luer connector  295  is connected to the multiplier  18 . Additional embodiments and elements of the switch mechanism  260  can also be used and will become apparent to those skilled in the art. In an embodiment, the cutter test is performed by the processor  84  of the microcontroller unit  88  and the switch mechanism  260  is located within the body of the multiplier  18 . In an alternative embodiment the multiplier  18  and the drive unit  14  can be consolidated into a single unit that is connected to and controls the cut-rate of the cutter  22 . In that situation, the cutter test is performed by the controller of the consolidated unit and the switch mechanism  260  is located within the body of the consolidated unit. 
         [0051]    In an embodiment of the invention, the input counter-timer hardware  76  receives an initial AC signal  90  sent from the input sensor  82  and measures the period (P in ) between pulses received from the input sensor  82  in order to determine the frequency of the received signal. The counter-timer hardware  76  then transmits a signal  200  to the processor  84  that modifies the frequency of the signal  200  using non-linear multiplication. The modification (multiplication) of the frequency in the processor  84  is based on different functions for adjusting frequency that are inputted by a user via the front panel button  72 . Any host system (drive unit  14 ) can have a predetermined cut rate range as shown in  FIG. 7 . In addition, during a surgical procedure a user can adjust the cut rate by changing the Output Maximum Cut Rate or the Cut Rate Multiplier of the host device (drive unit  14 ). Examples of different Output Maximum Cut Rates and Cut Rate Multipliers are shown in  FIG. 8  but other variations will be apparent to those skilled in the art. 
         [0052]    After the processor  84  multiplies the received cut-rate signal, the processor  84  transmits the modified frequency in the form of two signals/commands to the output counter-timer hardware  92 . The 205 signal/command determines the output period (P out ) by which the counter-timer hardware  92  controls the timing between electrical pulses (signals) transmitted to the cutter  22 . The output period P out  is based on the non-linear frequency calculations performed in the processor  84 . The 215 signal/command determines the waveform shaping of the signal transmitted to the cutter  22  via output counter-timer hardware  92  in the same way as previously described with respect to  FIG. 2 . 
         [0053]    The output counter-timer hardware  92  produces an output signal  146  based on the signals received from the processor  84 . The output signal  146  is provided to a solenoid output valve  150 . The solenoid output valve  150  receives pressurized gas  154  (or pressurized air) from a pressurized gas source  158 . The signal  146  causes the solenoid valve  150  to open and close, which results in the generation of a gas pulse train (or pneumatic signal)  162 . The gas pulse train is provided to the cutter  22  to pneumatically operate or drive the cutter  22  at a frequency that approximates the frequency of the output signal  162  of the microcontroller unit  88 . 
         [0054]      FIG. 9  shows a graph  255  that represents several functions for adjusting cut-rate frequency that can be executed by the processor  84  of the microcontroller unit  88 . The front panel button  72 , or any other type of user control, can be used to configure the processor  84  to execute these functions for adjusting frequency (transformation functions). In  FIG. 9 , the x-axis of the graph  255  represents the input rate of the signal from the drive unit  14  and the y-axis represents the output rate of the signal after the non-linear multiplication performed by the processor  84 . Function (1) of the graph represents the equation P out =A*P in −B (where A and B are constants) that is mathematically equivalent to the non-linear frequency multiplication of the input signal as described in this application. Function (2) represents the equation P out =(where K is a constant) that is mathematically equivalent to a fixed-constant (linear) frequency multiplication of the input signal. Therefore, the microcontroller unit  88  allows switchability between non-linear and linear multiplication, and the multiplied signal can be outputted to the vitreous cutter at both non-linear and linear cut-rates. Function (3) represents the equation P out =C*P in   2 +D*P in +E (where C, D and E are constants) that provides a somewhat improved non-linear frequency multiplication algorithm, with a higher effective multiplication factor in the mid-range of input cut rates, which gives the surgeon a better control of the cutter  22 . 
         [0055]    Thus, the digital microcontroller unit  88  allows further refinement and control of the outputted cut rate signal. For example, below a certain threshold input rate (i.e., for input pulse periods longer than a certain threshold limit) a single output pulse can be generated by the processor  84  in response to each input pulse. In other words, the multiplication factor used by the processor  84  is one. Alternatively, above the threshold input rate (for input periods shorter than the threshold limit) either the equation from line ( 3 ) or line ( 4 ) of  FIG. 8  may be applied. The benefit of this approach is the ability to smoothly control the cutter  22  down to very low cut rates, as well as up to very high cut rates. 
         [0056]      FIG. 10  illustrates a method  100  for operating the non-linear cut-rate multiplier  18  to control the cut-rate of the vitreous cutting system  10 . The first step in the method is to turn on the power of the non-linear cut-rate multiplier  18  (step  105 ). The next step in the method  100  is to verify whether the front panel button  72  of the non-linear cut-rate multiplier  18  is “on” (step  115 ). If the panel button is not “on”, the method goes back to step  105 . If the panel button is “on,” a user can select the type of “host” or drive unit  14  that is connected to the non-linear cut-rate multiplier  18  (step  120 ). The display  74  shows the available host types and the corresponding cut rate ranges and cut rate control form (footpedal, panel, or other) for each host type. 
         [0057]    In the next step, the method  100  verifies whether the cutter  22  is connected to the multiplier  18  and ready for operation (step  125 ). If the cutter  22  is ready for operation, the processor  84  performs the cutter test (step  135 ). If the cutter  22  fails the cutter test (step  140 ), the processor  84  checks whether the cutter  22  is connected to the multiplier  18  and the drive unit  14  (step  145 ). The function of the multiplier  18 , generally, is to allow the cutter  22  to be driven at a rate much higher than the frequency rate inputted from the drive unit  14 , where the drive unit  14  is still controlling the non-linear cut-rate multiplier  18 . When the cutter  22  fails the cutter tests and is still connected to the multiplier  18 , the method loops in step  145  until the user disconnects the cutter  22  from the multiplier  18 . When the cutter  22  is disconnected the method goes back to step  125  where a user connects a new cutter to the multiplier  18 . 
         [0058]    When the cutter  22  passes the cutter test (step  140 ), a user can select and change the desired cut rate by using the front panel button (step  155 ). The processor  84  then determines whether the drive unit  14  is outputting pressure pulses (signal) to the non-linear multiplier  18  (i.e. to check if it is “on”) (step  160 ). If the drive unit is not “on”, the method goes back to step  155 . If the drive unit is “on”, the processor  84  determines the input frequency rate transmitted from the drive unit  14  (step  165 ). In the next step, the processor  84  calculates the output frequency rate using multiplication based on the desired input of the user (step  175 ). In step  180 , the processor  84  of the MCU  88  sets the output signal of the output counter-timer hardware  92  that is used to drive the high-speed vitreous cutter system  10 . Finally, the vitreous cutting system  10  will continue working as long as the cutter  22  is connected to the multiplier  18  and to a power source. When the cutter  22  is connected (step  185 ), the user can adjust the frequency of the cut-rate at any time by going back to step  155 . 
         [0059]    Thus, the invention provides, among other things, an accessory in the form of a non-linear cut-rate multiplier that is operable to receive a signal from a drive unit, process the signal, and output an actuation signal to drive a high-speed vitreous cutter at a frequency or rate that is a non-linear multiple of the frequency output by the drive unit to, among other things, provide the user with a plurality of previously unachievable cut-rates.