Abstract:
A method of operating a microsurgical instrument, such as a vitrectomy probe, is disclosed. The probe includes a port for receiving tissue and an inner cutting member. A flow of tissue is induced into the port with a vacuum source, and the member is actuated in a cyclic manner to open and close the port over a plurality of cut rates. A duty cycle of the member is varied with the cut rate to vary the flow of the tissue into the port. The method yields the ability to adjust flow rate into the probe and aperture of the probe for a given cut rate so as to effectively perform a wide variety of vitreoretinal surgical objectives.

Description:
This application is a continuation of U.S. application Ser. No. 09/386,003, filed Aug. 30, 1999, now U.S. Pat. No. 6,514,268. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally pertains to a method of operating microsurgical instruments. More particularly, but not by way of limitation, the present invention pertains to a method of operating microsurgical instruments used in posterior segment ophthalmic surgery, such as vitrectomy probes, so as to optimize the performance of the instruments for a variety of surgical objectives. 
     DESCRIPTION OF THE RELATED ART 
     Many microsurgical procedures require precision cutting and/or removal of various body tissues. For example, certain ophthalmic surgical procedures require the cutting and/or removal of the vitreous humor, a transparent jelly-like material that fills the posterior segment of the eye. The vitreous humor, or vitreous, is composed of numerous microscopic fibers that are often attached to the retina. Therefore, cutting and removal of the vitreous must be done with great care to avoid traction on the retina, the separation of the retina from the choroid, a retinal tear, or, in the worst case, cutting and removal of the retina itself. 
     The use of microsurgical cutting probes in posterior segment ophthalmic surgery is well known. Such vitrectomy probes are typically inserted via an incision in the sclera near the pars plana. The surgeon may also insert other microsurgical instruments such as a fiber optic illuminator, an infusion cannula, or an aspiration probe during the posterior segment surgery. The surgeon performs the procedure while viewing the eye under a microscope. 
     Conventional vitrectomy probes typically include a hollow outer cutting member, a hollow inner cutting member arranged coaxially with and movably disposed within the hollow outer cutting member, and a port extending radially through the outer cutting member near the distal end thereof. Vitreous humor is aspirated into the open port, and the inner member is actuated, closing the port. Upon the closing of the port, cutting surfaces on both the inner and outer cutting members cooperate to cut the vitreous, and the cut vitreous is then aspirated away through the inner cutting member. U.S. Pat. Nos. 4,577,629 (Martinez); 5,019,035 (Missirlian et al.); 4,909,249 (Akkas et al.); 5,176,628 (Charles et al.); 5,047,008 (de Juan et al.); 4,696,298 (Higgins et al.); and 5,733,297 (Wang) all disclose various types of vitrectomy probes, and each of these patents is incorporated herein in its entirety by reference. 
     Conventional vitrectomy probes include “guillotine style” probes and rotational probes. A guillotine style probe has an inner cutting member that reciprocates along its longitudinal axis. A rotational probe has an inner cutting member that reciprocates around its longitudinal axis. In both types of probes, the inner cutting members are actuated using various methods. For example, the inner cutting member can be moved from the open port position to the closed port position by pneumatic pressure against a piston or diaphragm assembly that overcomes a mechanical spring. Upon removal of the pneumatic pressure, the spring returns the inner cutting member from the closed port position to the open port position. As another example, the inner cutting member can be moved from the open port position to the closed port position using a first source of pneumatic pressure, and then can be moved from the closed port position to the open port position using a second source of pneumatic pressure. As a further example, the inner cutting member can be electromechanically actuated between the open and closed port positions using a conventional rotating electric motor or a solenoid. U.S. Pat. No. 4,577,629 provides an example of a guillotine style, pneumatic piston/mechanical spring actuated probe. U.S. Pat. Nos. 4,909,249 and 5,019,035 disclose guillotine style, pneumatic diaphragm/mechanical spring actuated probes. U.S. Pat. No. 5,176,628 shows a rotational dual pneumatic drive probe. 
     With each of the above-described conventional vitrectomy probes, the inner cutting member is always actuated from a fully open port position, to a fully closed port position, and back to a fully open port position. It is believed that certain conventional guillotine style, pneumatic/mechanical spring actuated probes are physically capable of being operated at cutting speeds that do not allow the port to return to its fully open position in each cut cycle. However, the surgical systems with which such probes have been operated have not allowed this mode of operation to occur. This is because the ophthalmic surgical community has historically believed that a fully open port is critical to maximize fluid flow into and inclusion of vitreous within the port and to expedite vitreous cutting and removal. 
     Most conventional probes are sized to have a relatively large fully open port size (e.g. 0.020 inches to 0.030 inches) for use in a variety of surgical objectives. Operating at relatively low cut rates (e.g. up to 800 cuts/minute), these probes may be used to remove large amounts of vitreous in a single cut cycle, such as in core vitrectomy, and to cut physically large vitreous tissue, such as traction bands. In addition, these probes are also used to perform more delicate operations such as mobile tissue management (e.g. removing vitreous near a detached portion of the retina or a retinal tear), vitreous base dissection, and membrane removal. However, the combined effect of large port size, large cut stroke, and relatively slow cut rate of these probes sometimes creates unwanted turbulence of the vitreous and retinal tissues and a large peak to peak fluctuation of intraocular pressure within the eye. Both of these limitations cause difficulty for the surgeon and can be detrimental to the patient. 
     Specialized vitrectomy probes have been developed. For example, probes with relatively smaller fully open port sizes (e.g. 0.010 inches) have been used to perform more delicate surgical objectives near the retina. An example of such a specialized probe is the Microport® probe available from Alcon Laboratories, Inc. of Fort Worth, Tex. However, these probes are not highly effective for core vitrectomy, and thus the surgeon is often forced to use and repeatedly insert multiple vitrectomy probes within a patient&#39;s eye, complicating the surgery and increasing trauma to the patient. As another example, U.S. Pat. Nos. 4,909,249 and 5,019,035 disclose probes with manually adjustable port sizes. However, repeated manual adjustment of port size is time consuming and awkward. Relatively high cut rate probes have been developed by Storz Instrument Company of St. Louis (the “Lightning” probe) and Scieran Technologies, Inc. of Laguna Hills, Calif. (the “Vit Commander” probe). However, it is believed that these probes are somewhat limited in flow rate, rendering them less effective for core vitrectomy. 
     Therefore, a need exists for an improved method of performing all of the fundamental aspects of vitrectomy surgery—core vitrectomy, mobile tissue management, vitreous base dissection, and membrane removal—that does not suffer from the above-described limitations. As is explained in greater detail hereinbelow, this method would automatically control cut rate, port open duty cycle, and port open size or aperture as needed during a procedure to achieve a broad range of surgical objectives. An improved method is also needed for operating microsurgical instruments other than vitrectomy probes. Ideally, the improved methods would be safe for the patient, easy for the surgeon to use, and economically feasible. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention comprises a method of operating a microsurgical instrument. The microsurgical instrument includes a port for receiving tissue and a member. A flow of tissue is induced into the port with a vacuum source, and the member is actuated in a cyclic manner to open and close the port over a plurality of cycle rates. A duty cycle of the member is varied with the cycle rate to vary the flow of the tissue into the port. 
     The microsurgical instrument may comprise a vitrectomy probe, an aspiration probe, or other cutting probe. In the vitrectomy probe embodiment, the present invention yields the ability to adjust flow rate into the probe and aperture of the probe for a given cut rate so as to effectively perform a wide variety of vitreoretinal surgical objectives. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and for further objects and advantages thereof, reference is made to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side sectional view of a first vitrectomy probe preferred for use in the method of the present invention shown in the fully open port position; 
     FIG. 2 is a side sectional view of the probe of FIG. 1 shown in a closed port position; 
     FIG. 3 is a side, partially sectional view of a second vitrectomy probe preferred for use in the method of the present invention shown in a fully open port position; 
     FIG. 4 is a cross-sectional view of the probe of FIG. 3 along line  4 — 4 ; 
     FIG. 5 is a cross-sectional view of the probe of FIG. 3 along line  4 — 4  shown in a closed port position; 
     FIG. 6 is a block diagram of certain portions of a microsurgical system preferred for use in the method of the present invention; 
     FIG. 7 shows a flow profile for the probe of FIG. 1 according to a preferred embodiment of the present invention compared to a conventional flow profile for the probe of FIG. 1 and a conventional flow profile for the Microport® probe; 
     FIGS. 8 and 9 are top views of the probe of FIG. 1 illustrating the ability to vary open port size with the size of tissue to be cut and aspirated according to a preferred method of the present invention; 
     FIG. 10 is an exemplary electrical signal diagram for creating a pneumatic waveform for conventional operation of the probe of FIG. 1; 
     FIG. 11 is an exemplary pneumatic waveform for conventional operation of the probe of FIG. 1; and 
     FIG. 12 shows a collection of pneumatic waveforms for operation of the probe of FIG. 1 according to a preferred method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 12 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     Referring first to FIGS. 1 and 2, a distal end of a microsurgical instrument  10  is schematically illustrated. Microsurgical instrument  10  is preferably a guillotine style vitrectomy probe and includes a tubular outer cutting member  12  and a tubular inner cutting member  14  movably disposed within outer cutting member  12 . Outer cutting member  12  has a port  16  and a cutting edge  18 . Port  16  preferably has a length of about 0.020 inches along the longitudinal axis of probe  10 . Inner cutting member  14  has a cutting edge  20 . 
     During operation of probe  10 , inner cutting member  14  is moved along the longitudinal axis of probe  10  from a position A as shown in FIG. 1, to a position B as shown in FIG. 2, and then back to position A in a single cut cycle. Position A corresponds to a fully open position of port  16 , and position B corresponds to a fully closed position of port  16 . In position A, vitreous humor or other tissue is aspirated into port  16  and within inner cutting member  14  by vacuum induced fluid flow represented by arrow  22 . In position B, the vitreous within port  16  and inner cutting member  14  is cut or severed by cutting edges  18  and  20  and is aspirated away by vacuum induced fluid flow  22 . Cutting edges  18  and  20  are preferably formed in an interference fit to insure cutting of the vitreous. In addition, positions A and B are conventionally located somewhat outside the ends of port  16  to account for variations in the actuation of inner cutting member  14  in specific probes  10 . 
     Referring now to FIGS. 3 through 5, a distal end of a microsurgical instrument  30  is schematically illustrated. Instrument  30  is preferably a rotational vitrectomy probe and includes a tubular outer cutting member  32  and a tubular inner cutting member  34  movably disposed within outer cutting member  32 . Outer cutting member  32  has a port  36  and a cutting edge  38 . Port  36  preferably has a length of about 0.020 inches along the longitudinal axis of probe  30 . Inner cutting member  34  has an opening  40  having a cutting edge  41 . 
     During operation of probe  30 , inner cutting member  34  is rotated about the longitudinal axis of probe  30  from a position A as shown in FIG. 4, to a position B as shown in FIG. 5, and then back to position A in a single cut cycle. Position A corresponds to a fully open position of port  36 , and position B corresponds to a fully closed position of port  36 . In position A, vitreous humor or other tissue is aspirated into port  36 , opening  40 , and inner cutting member  34  by vacuum induced fluid flow represented by arrow  42 . In position B, the vitreous within inner cutting member  34  is cut or severed by cutting edges  38  and  41  and is aspirated away by vacuum induced flow  42 . Cutting edges  38  and  41  are preferably formed in an interference fit to insure cutting of the vitreous. In addition, position B is conventionally located somewhat past the edge of cutting surface  38  of outer cutting member  32  to account for variations in the actuation of inner cutting member  34  in specific probes  30 . 
     Inner cutting member  14  of probe  10  is preferably moved from the open port position to the closed port position by application of pneumatic pressure against a piston or diaphragm assembly that overcomes a mechanical spring. Upon removal of the pneumatic pressure, the spring returns inner cutting member  14  from the closed port position to the open port position. Inner cutting member  34  of probe  20  is preferably moved from the open port position to the closed port position using a first source of pneumatic pressure, and then moved from the closed port position to the open port position using a second source of pneumatic pressure. The first source of pneumatic pressure is pulsed, and the second source of pneumatic pressure may be pulsed or fixed. Alternatively, inner cutting members  14  and  34  can be electromechanically actuated between their respective open and closed port positions using a conventional linear motor or solenoid. The implementation of certain ones of these actuation methods is more fully described in U.S. Pat. Nos. 4,577,629; 4,909,249; 5,019,035; and 5,176,628 mentioned above. For purposes of illustration and not by way of limitation, the method of the present invention will be described hereinafter with reference to a guillotine style, pneumatic/mechanical spring actuated vitrectomy probe  10 . 
     FIG. 6 shows a block diagram of certain portions of the electronic and pneumatic sub-assemblies of a microsurgical system  50  preferred for use in the present invention. For example, system  50  could be the Accurus® surgical system sold by Alcon Laboratories, Inc. of Fort Worth, Tex. or another conventional ophthalmic microsurgical system. System  50  preferably includes a host microcomputer  52  that is electronically connected to a plurality of microcontrollers  54 . Microcomputer  52  preferably comprises an Intel® 486™ microprocessor, and microcontrollers  54  preferably comprise Intel® 80C196™ microprocessors. Of course, other conventional microprocessors having equivalent or superior performance can be utilized for microcomputer  52  and microcontrollers  54 , if desired. Microcontroller  54   a  is electronically connected with and controls an air/fluid module  56  of system  50 . Air/fluid module  56  preferably includes a source of pneumatic pressure  58  and a source of vacuum  60 , both of which are in fluid communication with probe  10  or probe  30  via conventional PVC tubing  62  and  64 . Air/fluid module  56  also preferably includes appropriate electrical connections between its various components. Although both probes  10  and  30  may be used with system  50 , the remainder of this description of system  50  will only reference probe  10  for ease of description. 
     Pneumatic pressure source  58  provides pneumatic drive pressure to probe  10 , preferably at a pressure of about 57 psi. A solenoid valve  66  is disposed within tubing  62  between pneumatic pressure source  58  and probe  10 . Solenoid valve  66  preferably has a response time of about 2 to about 3 milliseconds. System  50  also preferably includes a variable controller  68 . Variable controller  68  is electronically connected with and controls solenoid valve  66  via microcomputer  52  and microcontroller  54   a . As is later explained in greater detail, variable controller  68  preferably provides a variable electric signal that cycles solenoid valve  66  between open and closed positions so as to provide a cycled pneumatic pressure that drives inner cutting member  14  of probe  10  from its open port position to its closed port position at a variety of cut rates. Although not shown in FIG. 6, air/fluid module  56  may also include a second pneumatic pressure source and solenoid valve controlled by microcontroller  54   a  that drives inner cutting member  34  of probe  30  from its closed port position to its open port position. Variable controller  68  is preferably a conventional foot switch or foot pedal that is operable by a surgeon. For example, variable controller  68  may be the foot pedal sold as part of the Accurus® surgical system mentioned above. Alternatively, variable controller  68  could also be a conventional hand held switch or “touch screen” control, if desired. 
     FIG. 7 shows flow rate versus cut rate for three, exemplary vitrectomy probes. Profile  80  shows a preferred flow profile for a pneumatic/mechanical spring actuated probe  10  actuated according to the preferred method of the present invention. Profile  82  shows a conventional flow profile for a pneumatic/mechanical spring actuated probe  10 . Profile  84  shows a conventional flow profile for the Microport® probe. As shown in FIG. 7, flow profile  80  is preferably substantially linear. 
     At constant aspiration of 150 mmHg vacuum, flow profile  84  is approximately 40% that of profile  82  at all cut rates. Although the probe of profile  84  achieves the 1-2 cc/min flow rates that are desired by the ophthalmic surgical community when performing delicate retinal work, this same probe cannot achieve the higher 8-10 cc/min flow rates that are called for when performing core vitrectomy. 
     FIG. 7 reveals a ratio of 0 cpm (cuts/minute) vs. maximum cpm flow of approximately 2.5:1 for each of profiles  82  and  84 . In contrast, the flow ratio for profile  80  is greater than 50:1. By using the method of the present invention to modulate flow through probe  10  more completely by the application of various cut rates, port open duty cycles, and port apertures, flow profile  80  well exceeds that of profiles  82  and  84  combined. Such improved range of flow greatly reduces or eliminates the need for insertion of multiple probes into a patient&#39;s eye for different surgical objectives, reduces the complexity of the surgery, and reduces the associated trauma to the patient. 
     The improved performance of probe  10  in flow profile  80  is achieved by dynamically varying the port open duty cycle of the probe with cut rate. At high cut rates, such variation of the duty cycle also facilitates the variation of the “open” size or aperture of port  16 . One of the important discoveries of the present invention is that it is preferable to vary the open size of port  16  according to the size of the vitreous or other tissue targeted for cutting and removal. For example, FIG. 8 shows inner cutting member  14  of probe  10  being actuated from a fully open position A of port  16 , to a fully closed port position B, and back to a position A in a single cut cycle, as is conventional. In this mode of operation, the aperture of port  16  is constant. Due to the differential in cross-sectional area between a relatively small piece of vitreous tissue  86  and fully open port  16 , vacuum source  60  does not always efficiently aspirate tissue  86 . However, as shown in FIG. 9, inner cutting member  14  of probe  10  is actuated from an open position A′ of port  16 , to a fully closed port position B, and back to position A′, according to the preferred method of the present invention. In this mode of operation, the aperture of port  16  can be varied, for example to position A′, according to the size of vitreous tissue  86 . The similar cross-sectional areas of vitreous tissue  86  and open port  16  allow for higher effective vacuum pressure from vacuum source  60  and a more efficient aspiration of tissue  86  into port  16 . The concepts of dynamically varying the duty cycle and/or the open port size with cut rate according to the preferred methods of the present invention, and their resulting benefits, will now be discussed in more detail in connection with FIGS. 10,  11 , and  12 . 
     FIG. 10 shows an exemplary electrical signal supplied by microcontroller  54   a  to solenoid valve  66  so as to actuate inner cutting member  14  of probe  10  via pneumatic pressure source  58  and tubing  62 . The closed position of valve  66  is preferably assigned a value of 0 volts, and the open position of valve  66  is preferably assigned a value of 5 volts. For a given cut rate, probe  10  will have a period τ representative of the time to open valve  66 , plus the time valve  66  is held open, plus the time to close valve  66 , plus the time valve  66  is held closed until the next signal to open valve  66  occurs. τ is the inverse of cut rate. For example, at a cut rate of 800 cpm, τ=75 milliseconds (ms)/cut. For the purposes of this document, the duration of the electrical signal that holds valve  66  in the open position is defined as the pulse width PW. As used in this document, port open duty cycle, or duty cycle, is defined as the ratio of PW to τ(PW/τ). 
     As shown in FIG. 11, τ also represents the time between respective pneumatic pulses generated by air/fluid module  56  in response to the electrical signal of FIG.  10 . The pneumatic signal lags the electrical signal at valve  66  by approximately 9 ms (about 2 ms of delay in opening valve  66  and about 7 ms of transmission delay along PVC tubing  62 ). It has been discovered that an exemplary pneumatic/mechanical spring actuated probe  10 , the Accurus® probe available from Alcon Laboratories, Inc. of Fort Worth, Tex., is at the fully closed port position B at a pressure Pc of about 21 psi, and is at the fully open port position B at a pressure Po of about 4 psi. This exemplary probe is driven by air/fluid module  56  with pressure pulses having a maximum pressure Pmax of about 34 psi and a minimum pressure Pmin of about 3 psi. Pc, Po, Pmax, and Pmin may vary for different probes. 
     As mentioned above, the cut rate of probe  10  or cycling rate of the electrical signal at valve  66  is equal to 1/τ. Thus, increased cut rate results in decreased period τ. If PW is held constant, this decrease in τ results in an increase in duty cycle, which causes the DC or bias level of the pneumatic waveform in FIG. 11 to shift upwards. Independent of PW, increased cut rate gives rise to reduced peak-peak pneumatic excursion between Pmax and Pmin. 
     The motion of inner cutting member  14  is directly related to the pressure applied to drive probe  10 . Combining this understanding with the previously described effects of PW and cut rate on the pneumatic signal, an increase in cut rate with PW held constant has the net effect of creating inner cutting member  14  motion that is both reduced in amplitude and shifted in the direction of port closure (i.e., toward line B of FIG.  2 ). 
     FIG. 11 also shows excess pneumatic drive of Pmax beyond Pc, which provides for probe  10  actuation variations as well as minor tolerances in other system components, including valve  66 , PVC tubing  62 , and pressure source  58 . By reducing these variations and tolerances, much of the excess time and pressure in establishing Pmax is eliminated. In other words, if Pmax is set to Pc, cutting edge  20  of inner cutting member  14  is actuated just past cutting edge  18  of outer cutting member  12  and no more. The time for the pneumatic drive of probe  10  to return to Po is also reduced, thereby allowing for further reduction in period τ and, therefore, a further increase in cut rate. 
     FIG. 12 shows a collection of pneumatic waveforms as measured for a pneumatic/mechanical spring actuated probe  10 . Waveform  90  represents the pneumatic drive that is conventionally applied at probe  10 , and waveforms  92 ,  94 , and  96  represent examples of pneumatic drive applied according to a preferred method of the present invention. Pressure levels of Pc=21 psi for full port closure and Po=4 psi for full port open are indicated. The electrical signal at valve  66  for conventional waveform  90  is shown at top. The 9 ms delay from electrical signal  90  for pneumatic waveform  90  is also indicated. 
     Waveform  90  depicts the conventional 800 cpm pneumatic drive for probe  10 . In this case, inner cutting member  14  travels past each end of port  16  as Pmax=34 psi and Pmin=3 psi provide for full excursion. In contrast, waveforms  92 ,  94 , and  96  yield inner cutting member  14  travel that extends to cutting edge  18  but which do not result in a fully open port  16 . More specifically, waveform  92  yields a 75% open port  16  in each cut cycle, waveform  94  yields a 50% open port  16  in each cut cycle, and waveform  96  yields a 25% open port  16  in each cut cycle. For these waveforms, each cut rate is established for the desired range of inner cutting member  14  excursion, and then pulse width PW is increased or decreased as required to establish Pmax substantially equal to Pc for inner cutting member  14  travel just past cutting edge  18 . This adjustment of pulse width PW also varies the duty cycle (PW/τ). 
     Referring again to flow profile  80  of FIG. 7, pulse width PW is preferably lower at higher cut rates (e.g. above 800 cpm) than at lower cut rates (e.g. below 800 cpm). The lower pulse width PW at higher cut rates allows probe  10  to be operated with sufficient flow through port  16  at cut rates above the conventional range. Lowering pulse width PW at higher cut rates also results in the duty cycle being lower that it would have been if PW had been held constant. By varying pulse width PW or duty cycle, the flow rate through port  16  can be varied to any desired amount. 
     At lower cut rates, inner cutting member  14  preferably moves from a fully open position of port  16 , to a fully closed position of port  16 , and back to a fully open port position in each cut cycle. After a certain threshold cut rate, the open port size of port  16  preferably begins to decrease with increasing cut rate. By varying pulse width PW or duty cycle as described above, any desired amount of port open size or port aperture may be established. The threshold cut rate at which the open port size of port  16  begins to decrease may vary for different probes. 
     For each incremental cut rate on flow profile  80 , the cut rate and the pulse width PW (or duty cycle PW/τ) corresponding to the cut rate are preferably associated with a position on variable controller  68 . This association is preferably made by software and/or hardware resident in microcomputer  52  or microcontroller  54   a.    
     Variable controller  68  is preferably a conventional foot pedal having a range of motion in a generally vertical plane. The highest value of cut rate (and thus the lowest value of flow rate and the smallest aperture of port  16 ) is preferably assigned to the uppermost position of foot pedal  68 . Decreasing values of cut rate are preferably assigned to increasingly depressed positions on foot pedal  68 . The lowest value of cut rate (and thus the highest value of flow rate and a fully open aperture of port  16 ) is preferably assigned to the fully depressed position of foot pedal  68 . Therefore, before a surgeon depresses foot pedal  68 , probe  10  operates in the highest cut rate, smallest port aperture, and lowest flow rate mode. This mode of operation is especially useful for performing delicate operations near the retina, such as mobile tissue management, vitreous base dissection, or membrane removal. As the surgeon depresses foot pedal  68 , the cut rate decreases and the flow rate increases, according to flow profile  80  of FIG. 7, until the lowest cut rate, fully open port aperture, and highest flow rate is reached. This lower cut rate mode of operation is especially useful for core vitrectomy or the removal of large vitreous tissue such as traction bands. Alternatively, an opposite procedure may be followed so that before a surgeon depresses foot pedal  68 , probe  10  operates in the lowest cut rate, fully open port aperture, highest flow rate mode. As the surgeon depresses foot pedal  68 , the cut rate increases and the flow rate decreases, according to flow profile  80  of FIG. 7, until the highest cut rate, smallest port aperture, and lowest flow rate is reached. 
     Although the method of dynamically varying the port open duty cycle and/or port aperture has been described above with reference to a pneumatic/mechanical spring actuated probe  10 , it will be apparent to one skilled in the art that it is equally applicable to a dual pneumatically actuated probe  30 . In addition, it is believed that duty cycle and/or port aperture can also be varied so as to extend the range of flow and cut rates for a probe that is actuated using a conventional linear electrical motor, solenoid, or other electromechanical apparatus. 
     From the above, it may be appreciated that the present invention provides an improved method of performing all of the fundamental aspects of vitrectomy surgery that provides significant benefits to both the surgeon and the patient. The present invention is illustrated herein by example, and various modifications may be made by a person of ordinary skill in the art. For example, although the methods of dynamically varying the port open duty cycle and/or port aperture with cut rate are described above in connection with the operation of vitrectomy probes, the methods are equally applicable to the operation of microsurgical aspiration probes, or other microsurgical probes used to cut and remove body tissue in a similar manner. Of course, in an aspiration probe the inner cutting member would be replaced with a sealing member, and cycle rate would replace cut rate. As another example, although the preferred flow profile of the present invention is substantially linear, the method of the present invention is equally applicable to nonlinear flow profiles. As another example, although the preferred flow profile of the present invention is illustrated using an exemplary aspiration of 150 mmHg vacuum, the method of the present invention is equally applicable to flow profiles at different levels of aspiration. As a further example, alternative techniques may be used to control flow rate, other than by adjusting cut rate, duty cycle, and pulse width as described hereinabove in connection with probe  10 . 
     It is believed that the operation and construction of the present invention will be apparent from the foregoing description. While the apparatus and methods shown or described above have been characterized as being preferred, various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the following claims.