Abstract:
A digital fluid delivery and aspiration apparatus with a mechanical de-amplifier for the delivery or removal of discrete volumes of fluidic material from a surgical site. The apparatus has a syringe with a plunger designed to be powered by a pneumatic air supply system. A ratcheting mechanism attached to a pneumatic piston by a mechanical linkage causes the plunger of the syringe to inject discrete doses of the fluidic material into the surgical site or remove discrete amounts of fluidic material from the surgical site. The mechanical linkage increases the force provided to the plunger by the movement of the piston and also de-amplifies the movement of the piston into a lesser movement of the plunger. Adjusting the waveform of the pneumatic air supply, coupled with the mechanical de-amplification of the movement of the pneumatic piston, reduces the jetting of the fluidic material being delivered, and allows the delivery or removal of precise volumes of fluidic material from the surgical site at precise rates without damaging the tissue into which or from which the fluidic material is being delivered or removed. The irrigation and aspiration features may be incorporated into one device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 08/940,189, now U. S. Pat. No. 6,102,895, filed Sep. 30, 1997, issued Aug. 15, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to devices for the delivery of fluidic material to, and the removal of fluidic material from, body tissue. More particularly, the present invention relates to pneumatically controlled devices capable of delivering or removing precise volumes of fluidic material at a selected site. 
     BACKGROUND OF THE INVENTION 
     The delivery or injection of fluidic materials to and removal of fluidic materials from a selected site may be performed in a number of different medical procedures. In the field of ophthalmology, for example, intraocular injections may be administered for many reasons. Some of these reasons include: (1) the injection of antibodies to treat endothalmitis or prevent its onset; (2) the injection of Transforming Growth Factor Beta (TGFB) or other growth factors to treat macular disorders; (3) the injection of Tissue Plasminogen Activator (tPA) into the subretinal space to dissolve blood clots; (4) the injection of liquids and gases into the subretinal space to facilitate subretinal surgery; (5) the injection of viscoelastic substances to dissect preretinal membranes; and (6) the injection of gases into the vitreous cavity for pneumatic retinal pexy. 
     When injections are administered to delicate tissue, e.g., intraocular injections, the surgeon must control the following: injection rate, total volume administered, and location of the injected substance. Similar concerns exist for aspiration of fluids from delicate tissues. The case of viscodissection is described below to illustrate these requirements. 
     Viscodissection is a technique where preretinal membranes are hydraulically separated from the retina using a viscoelastic substance. This substance, typically sodium hyaluranate, is delivered between the preretinal membrane and the retina using a syringe and a small gauge bent needle. The fluid creates a working space underneath the retina. Many surgeons find it difficult to hold the needle tip steady while injecting the fluid and inadvertent motion of the needle can cause damage to the retina or other surrounding tissues. Further, injecting too much fluid between the preretinal membrane and the retina, or injecting the fluid too fast, can also cause retinal damage which could lead to retinal detachment. Similarly, aspiration of unwanted fluid from these delicate tissues requires steady and measured suction. 
     There are some devices which facilitate the delivery of fluidic materials to delicate tissue. For example, U.S. Pat. No. 5,370,630 discloses a device that uses pneumatic energy to cause the injection of fluidic material into body tissue. The plunger of this device is driven by pneumatic pressure instead of finger pressure, thereby allowing the surgeon to better control the injection rate, volume, and location. A number of other syringe adapters and pneumatic pressure sources are currently available. Such devices typically have a piston displacement v. time curve as shown in FIG.  13 . 
     While more effective than manual instruments, existing pneumatic fluid delivery devices often cannot meet the requirements of the surgeons for precision because they cannot control the “jetting” of material emitted from the needle, require a relatively high amount of pneumatic pressure to operate, and cannot be precisely controlled for very low doses. “Jetting,” or turbulent flow of the fluidic material, occurs when the fluid emitted through the hole in the needle is forced out under relatively high pressure by a rapidly accelerating plunger motion. Jetting is undesirable because it may damage the tissue to which the fluidic material is being delivered. Similarly, existing aspiration devices do not meet the needs of surgeons for aspiration of delicate tissue because they do not offer adequate control of the suction force in strength, location, and volume. 
     The major deficiencies of existing devices are caused by internal friction. As with any dynamic system, friction is present in devices designed to deliver fluids. With air cylinders, as in existing injectors, friction due to o-rings rubbing against the walls of the cylinder can be very difficult to control. All o-ring type piston-cylinder assemblies have an inherent problem with initial static friction created by at least two sources. One is static friction due to material properties; the other is commonly referred to as “stiction.” Stiction is the frictional force due to a compression of the o-ring incurred when the piston-cylinder assembly has been sitting unused for some time. The ideal control for injection is a constant velocity, linear displacement travel of the piston. In prior-art devices, the stiction and static friction in the air cylinder result in uncontrollable motion, which is illustrated in FIG.  13 . As pressure is increased to initiate motion of the piston, initially nothing happens. Then, there is an almost instantaneous movement of the piston (from zero to a level indicated by reference numeral A) as the friction and stiction forces are overcome. This jump in motion results in the jetting of the fluid being delivered. This initial jump can be as much as 75% of the total stroke (reference numeral B) of the piston. Thus, no matter how well the delivered air supply is controlled, the result is a quick, pulse-like delivery of fluid and potential damage to the tissue into which the fluidic material is being injected. 
     Existing aspiration devices typically do not offer precise control of the rate and volume of fluid being aspirated and thus are not very useful around sensitive tissues. Specifically, current aspiration devices used in intraocular surgery, such as those used in vitrectomy procedures, use vacuum control. In such devices, the vacuum level is controlled, not the rate or volume of material aspirated. Aspiration devices typically used in cataract surgery suffer from several limitations. First, such devices cannot accurately remove fluids in the sub-microliter range and thus cannot be used around delicate tissue such as the retina. Second, such devices are flow-controlled rather than volume-controlled, that is, the surgeon can control the suction rate at which material is removed, but not the volume. Thus, existing devices cannot be used to remove a precise volume of material as may be required in surgeries such as the treatment of sub-retinal hemorrhages. 
     Accordingly, a need has arisen for a device capable of delivering and removing fluidic materials from delicate body tissue. Further, it would be desirable if the device could deliver a user-settable volume of fluid; deliver and remove fluids at a volume rate precisely controlled by the operator; allow the use of a relatively low pressure pneumatic source; reduce jetting of fluid emanating from the needle; and further minimize the risk of tissue damage that results from manually operated syringes, existing pneumatic syringes, and existing aspiration devices. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a fluid delivery and aspiration apparatus that prevents the jetting of fluidic material through the hole in the end of the needle. 
     It is a further object of the present invention to provide a fluid delivery and aspiration apparatus that can deliver or remove precise volumes of fluidic material from a surgical or therapy site (“selected site”). 
     It is an additional object of the present invention to provide a fluid delivery and aspiration apparatus that can deliver fluidic material to and remove fluidic material from a selected site at a precise rate and volume. 
     It is yet another object of the present invention to provide a fluid delivery and aspiration apparatus capable of delivering fluidic material to and removing fluidic material from a selected site without damaging delicate tissue. 
     The present invention provides the above-identified and many additional objects by providing a digital fluid delivery and aspiration apparatus with a mechanical de-amplifier wherein a pneumatically driven piston mechanically coupled to a plunger pushes the fluidic material out or sucks the fluid into a syringe at a rate and volume that can be precisely controlled. 
     The invention includes a syringe and housing. The syringe includes a barrel housing a plunger, teeth formed at least partially about the periphery of the plunger, the multiple teeth running along at least a portion of the length of the plunger. The plunger travels within and along the barrel to either force the fluidic material out of the barrel of the syringe or draw fluid into the barrel of the syringe depending on the direction of the motion of the plunger. The teeth on the plunger engage a ratcheting mechanism inside the housing to provide step-wise, or digital, control of the injection or aspiration rate and volume. 
     The housing includes a base which provides a place to attach the syringe and a cap enclosing a piston, a mechanical linkage, and the ratcheting mechanism. The ratcheting mechanism engages the teeth on the plunger to move the plunger within the syringe. The ratcheting mechanism is connected to the mechanical linkage which is further connected to the pneumatically driven piston. When the apparatus is used to deliver fluid to a site, the piston is preferably biased toward the upper housing piece by a coil spring and is driven toward the base by an external pneumatic source capable of overcoming the spring&#39;s bias. This motion engages the mechanical linkage connected to the ratcheting mechanism. The motion of the piston, shown in the piston displacement v. time curve of FIG. 14, is thereby translated to a displacement of the plunger towards the end of the open syringe (thus, FIG. 14 also charts the plunger displacement). When the pneumatic source is removed, the bias of the spring returns the piston to its original position and a leaf spring retains the position of the syringe plunger and prevents it from returning to its original position as the ratcheting mechanism returns to its original position in anticipation of another cycle. As should be evident to one skilled in the art, a similar, but oppositely oriented device is used for aspiration. Furthermore, it should be evident that other biasing means including pneumatic or hydraulic force may be used to return the piston to its original position. 
     The use of mechanical linkage provides several distinct advantages over existing pneumatically driven injectors. First, the use of mechanical linkage to provide mechanical leverage to the ratcheting mechanism permits the use of much lower pressure than is required by existing systems. This allows the use of small, compact compressors or other air sources, resulting in overall cost and size reductions. Second, the use of the mechanical linkage results in a motion de-amplification. That is, for every unit of displacement the piston travels, the ratcheting mechanism, and hence the plunger, travels a shorter distance in a ratio equal to the leverage provided by the mechanical linkage. This permits much more precise control of the rate of the fluid delivery or removal because the acceleration and speed of the plunger is reduced. Third, the use of mechanical linkage to provide motion de-amplification also results in a decreased influence of friction on the control of the plunger, resulting in a linearly displaced, constant velocity injection or aspiration. In existing pneumatic injectors, a much greater percentage of the plunger motion is adversely affected by the jump in the plunger after the friction in the system had been overcome by the air pressure. (See FIG.  13 ). Finally, the mechanical linkage allows the use of a stronger return spring or other biasing means, thereby further reducing the adverse effects of friction on the motion of the plunger. 
     The forces opposing the motion of the piston are friction and the biasing force. The biasing force is much more controllable than the friction and by increasing the biasing force, the friction resistance force represents a much smaller component of the overall resistance force than in existing pneumatic injectors. Thus, the ability to precisely control the rate and volume of fluidic material delivered is greatly increased and tissue damage from jetting is reduced in the present invention. Similarly, the ability to control the plunger motion allows the surgeon to precisely control the rate and volume of fluidic material aspirated from a site because the device is volume-controlled rather than vacuum controlled. 
     In the preferred embodiment, when it is necessary to fill the syringe with fluidic material for delivery or empty fluidic material that has been aspirated, the plunger can be rotated to disengage the teeth from the ratcheting mechanism. In this position, the plunger rod can be moved freely in either direction by manually pulling or pushing the plunger as in the operation of a standard syringe. In an alternative embodiment, the syringe is filled or emptied using the pneumatic control. 
     The external pneumatic actuation force is supplied as a series of controlled pulses. For each pulse delivered to the apparatus, the plunger rod travels one unit of length, the length of that unit being determined by the distance between the edges of the teeth on the plunger. Thus, the distance traveled is independent of the externally supplied pressure or the fluidic material being delivered or removed. Existing pneumatically driven syringes are typically powered by a series of sharp pulses of compressed air. This rapid, pulsing excitation of the piston contributes to undesirable jetting or turbulent flow of fluid from the tip of the syringe needle. Existing aspiration devices typically provide a constant level of suction, and control of volume aspirated is difficult to achieve. The present invention uses a pulse train having a waveform substantially as shown in FIG.  15 . The air pressure is precisely controlled to reduce the occurrence of jetting during delivery. There is a small, initial jump to a low pressure which has the effect of overcoming the friction in the system. Beyond the low pressure level, smoothly increasing pressure to a maximum level results in the smooth plunger motion. In exciting the piston in this fashion, the acceleration of the plunger is reduced, thereby reducing jetting during delivery. Although pneumatic control of the piston is preferred, it should be understood by those of skill in the art of the invention that hydraulically driven pistons could be used. Accordingly, as used herein the terms “pneumatically,” “air pressure,” “pneumatic,” and other gaseous references should be read and interpreted to include liquids and hydraulic systems. Furthermore, it should be evident that the piston may be sealed against the housing by means other than an O-ring. One such design incorporates a rolling diaphragm connected to the piston. 
     These are just some of the features and advantages of the present invention. Many others will become apparent by reference to the detailed description of the invention taken in combination with the accompanying drawings. It should be noted that while the detailed description frequently refers to use of the invention as a fluid delivery instrument, the invention may be used for aspiration by reversing the direction of stepwise plunger control, and a device in accordance with the invention may incorporate both irrigation and aspiration in the same device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a digital fluid delivery and aspiration apparatus with a mechanical de-amplifier constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a perspective exploded view of the digital fluid delivery and aspiration apparatus with a mechanical de-amplifier in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a side elevational view, shown mostly in cross-section, of the apparatus as shown in FIG. 1, taken generally along the plane  3 — 3 ; 
     FIG. 4 is a partial top plan view of the apparatus shown in FIG. 1, having the lever arm oriented such that the teeth of the plunger do not engage the ratchet spring; 
     FIG. 5 is an enlarged detail view a portion of the cross section shown in FIG. 3; 
     FIG. 5A is a partial top plan view, shown partially in cross-section, of the apparatus as shown in FIG. 5 taken generally along the line  5 A— 5 A in FIG. 5; 
     FIG. 6 is a top plan view, partially in cross-section, of the apparatus as shown in FIG. 3 taken generally along the line  6 — 6 ; 
     FIG. 7 is a side elevational view, partially in cross-section, of the apparatus as shown in FIG. 3, reflecting movement of the plunger such that fluidic material is drawn into the barrel of the syringe prior to delivery to a surgical site; 
     FIG. 8 is a partial top plan view of the apparatus shown in FIG. 7 having the lever arm oriented such that the teeth of the plunger engage the ratchet spring; 
     FIG. 9 is a side elevational view, partially in cross-section, of the apparatus shown in FIG. 7 having the plunger oriented such that the teeth engage the ratchet spring; 
     FIG. 10 is a top plan view, partially in cross-section, of the apparatus as shown in FIG. 5 taken generally along the line  10 — 10  in FIG. 9; 
     FIG. 11 is a side elevational view, partially in cross-section, of the apparatus in accordance with the present invention reflecting movement of the plunger such that fluidic material is discharged from the barrel of the syringe; 
     FIG. 12 is a side elevational view of the cross-section of FIG. 11, shown in greater detail; 
     FIG. 13 is a graphical diagram showing a piston displacement v. time curve typical of existing pneumatic injectors; 
     FIG. 14 is a graphical diagram showing the piston displacement v. time curve of the apparatus in accordance with the present invention; 
     FIG. 15 is a graphical diagram of the pulse train and wave form of the pressurized air used with the apparatus in accordance with the present invention; 
     FIG. 16 is a schematic diagram of a pneumatic power supply that can be used to provide pneumatic pressure to apparatus constructed according to the present invention; 
     FIG. 17 is a side elevational view, shown partially in cross-section, of a digital fluid delivery and aspiration apparatus with a mechanical de-amplifier, constructed according to an alternative embodiment of the present invention; 
     FIG. 18 is a side elevational view, partially in cross-section, of a digital fluid delivery and aspiration apparatus with a mechanical de-amplifier, constructed according to another alternative embodiment of the present invention, this apparatus capable of both irrigation and aspiration; 
     FIG. 19 is an enlarged detail view of the shaft of the plunger used in accordance with the embodiment of the invention in FIG. 18 taken generally from the circle  19  in FIG. 18; 
     FIG. 20 is a cross-sectional view of the shaft taken generally along the line  20 — 20  in FIG. 19; 
     FIG. 21 is a side elevational view, showing additional detail of the ratcheting mechanism used in accordance with the embodiment of the invention shown in FIG. 18; and 
     FIG. 22 is a side elevational view, shown partially in cross-section, of a digital fluid delivery and aspiration apparatus with a mechanical de-amplifier, constructed according to yet another alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIG. 1, a digital fluid delivery and aspiration apparatus  20  includes a syringe  22  and a housing  24  with a plunger  25  inserted through both the syringe  22  and the housing  24 . While the following description is directed mainly toward the delivery apparatus embodiment of the invention, it should be understood by one skilled in the art that the aspiration apparatus embodiment is constructed to provide digital control of the plunger  25  in the opposite direction as the fluid delivery apparatus embodiment. Furthermore, as described below, the aspiration apparatus embodiment and the fluid delivery apparatus embodiment may be combined in one apparatus. 
     As shown in FIG. 2, the syringe  22  is similar to a standard syringe in that it has a hollow barrel  26  having a needle receiving end  28  and a plunger receiving end  30 . The plunger receiving end  30  is attached to the housing  24  and receives the plunger  25  while a needle (not shown) may be attached to the needle receiving end  28 . The plunger  25  can travel the length of the barrel  26  to either force fluidic material  32  out of the barrel  26  or draw fluidic material  32  into the barrel  26  depending on the direction of the motion of the plunger  25 . To accomplish this, the plunger  25  is substantially radially sealed against the interior of the barrel  26 . In a preferred embodiment, this is accomplished by use of a fluidic material engaging head  34  that radially seals the plunger  25  against the interior of the barrel  26 . The fluidic material engaging head  34  forces the fluidic material  32  out of the needle receiving end  28  of the barrel  26  when the plunger  25  moves through the barrel  26  from the plunger receiving end  30  toward the needle receiving end  28 . (FIG.  11 ). The fluidic material engaging head  34  creates a vacuum that draws fluidic material  32  into the barrel  26  when the plunger  25  moves through the barrel from the needle receiving end  28  toward the plunger receiving end  30 . (FIG.  7 ). 
     The plunger  25  includes a shaft  36  having the fluidic material engaging head  34  on one end and a plunger control  38  on the opposite end. The shaft  36  is provided with a number of teeth  44  on its periphery and running along most of its length. As shown in cross-section in FIGS. 5A and 10, in this embodiment, the shaft  36  has two toothed faces  40 , which are shown with curved outer surfaces in this embodiment, and two toothless faces  42 , shown with flat surfaces, along its length. The teeth  44  on the toothed or curved faces  40  are oriented axially toward the plunger control  38  end of the shaft  36 . The plunger  25  can rotate about its longitudinal axis within the barrel  26  and the housing  24 . (FIG.  9 ). Such rotation allows either the curved faces  40  having teeth  44  (FIGS. 9 and 10) or the toothless or flat faces  40  (FIGS. 5 and 5A) of the shaft  36  to be oriented toward a ratcheting mechanism  46  in the housing  24 . 
     When the toothed faces  40  are oriented toward a ratcheting mechanism  46  disposed in the housing  24  (FIGS.  9  and  10 ), the interaction of the teeth  44  and ratcheting mechanism  46  provides stepwise, in a sense digital, control of the travel of the plunger  25  from the plunger receiving end  30  of the barrel  26  to the needle receiving end  28  of the barrel  26 . The ratcheting mechanism  46  provides the stepwise control only in this direction because the plunger  25  is prevented from sliding toward the plunger receiving end  30  of the barrel  26  by a pair of pawls  47  integrally formed with a leaf spring  48  in the housing  24  engaging the teeth  44 , one pawl on each side of the shaft  36 , thus preventing fluidic material  32  from being reintroduced to the barrel  26  after it has been forced out through the needle receiving end  28 . Of course, the orientation of the ratcheting parts, including teeth  44  and ratcheting mechanism  46 , is reversed for the aspiration apparatus embodiment of the present invention. 
     As illustrated in FIG. 7, when the flat faces  42  are oriented toward the ratcheting mechanism  46  (FIG.  5 A), the plunger  25  is free to slide within the barrel  26  and the housing  24  either toward or away from the needle receiving end  28  of the barrel  26  as manipulated by the operator using the plunger control  38 . This orientation is most useful in the fluid delivery apparatus embodiment when loading the barrel  26  with fluidic material  32  for delivery by moving the plunger  25  within the barrel  26  from the needle receiving end  28  toward the plunger receiving end  30 . And in the aspiration apparatus embodiment, this orientation is most useful when emptying the barrel  26  of aspirated material  32  by moving the plunger  25  within the barrel  26  from the plunger receiving end  30  toward the needle receiving end  28 . 
     The housing  24  includes a base  50  and a cap  52  (FIG.  2 ). The syringe  22  attaches to the base  50  at a base plunger hole  54  where the syringe  22  receives the plunger  25  after the plunger  25  has been inserted through the cap  52  and the base  50 . The leaf spring  48  is positioned within the base  50  so that the pawls  47  engage the teeth  44  on the curved faces  40  of the shaft  36  such that the plunger  25  cannot be withdrawn from the barrel  26  without rotating the shaft  36  to a position where the flat faces  42  are oriented toward the pawls, so that the teeth are not engaged. When the flat faces  42  are so oriented, the plunger  25  can then move freely within the length of the barrel  26 . Because it is advantageous to have a visual indication of whether the plunger  25  can move freely within the barrel  26 , a plunger orientation indicator  56  is positioned on top of the cap  52 , which rotates with the plunger. Indicator  56  includes a pointed portion  57  capable of indicating whether the curved faces  40  (FIG. 4) or the flat faces  42  (FIG. 8) are oriented toward the pawls  47 . 
     Also positioned within the housing  24  is the ratcheting mechanism  46  which engages the teeth  44  to provide digital control of the plunger  25 . Mechanical linkage  58  which moves the ratcheting mechanism  46  and a fluid power actuator  59 , including a pneumatic piston  60 , which drives the mechanical linkage  58  are also positioned within the housing  24 . 
     The ratcheting mechanism  46  may be any combination of springs, levers, or other assemblies capable of digitally moving the plunger  25  within the barrel  26  toward the needle receiving end  28 . In the preferred embodiment, the ratcheting mechanism  46  comprises a spring guide  62  and a ratchet spring  64  integrally formed with a pair of pawls  65 , similar to the pawls  47  and leaf spring  48 , in the housing  24 . The spring guide  62  positions the ratchet spring  64  around the shaft  36  of the plunger  25  such that the pawls  65  engage the teeth  44  on the curved faces  40  when the plunger  25  is oriented such that the pawls  47  also engage the teeth  44 . Thus, as the spring guide  62  and ratchet spring  64  are caused by the mechanical linkage  58  to move toward the needle receiving end  28  of the barrel  26 , pawls  65  engage the teeth  44  and cause the plunger  25  to move toward the needle receiving end  28  of the barrel  26 . When, however, the mechanical linkage  58  causes the spring guide  62  and ratchet spring  64  to move toward the plunger receiving end  30  of the barrel  26 , the pawls  47  retain the plunger  25  in its new position and pawls  65  slide to engage another set of the teeth  44  to be positioned to repeat the digital movement of the plunger  25 . The operation is performed in the opposite direction when the invention is configured to aspirate fluids. 
     The mechanical linkage  58  connects the ratcheting mechanism  46  to the fluid power actuator  59 . The mechanical linkage  58  may be a combination of levers, pivot points, and other assemblies capable of allowing the use of lower pressure air to power the piston  60 , capable of permitting more precise control of the rate of fluid delivery by reducing the speed and acceleration of the plunger&#39;s  25  movement, and capable of decreasing the influence of friction on the control of the plunger  25 . In the preferred embodiment, the mechanical linkage  58  comprises a lever arm  66  connected to the base  50 , spring guide  62 , and piston  60  at three pivot points  68  (fulcrum),  69  (load), and  70  (effort) respectively. Pivot pins  68 A,  69 A, and  70 A pivotably connect the lever arm  66  to the three pivot points  68 ,  69 , and  70  respectively. In this embodiment, the lever arm  66  is a second class lever with the fulcrum  68  at the edge of the base  50 , the load  69  at the spring guide  62 , and the effort  70  at the piston  60 . Thus, because the piston  60  (effort) is much further from the base pivot point  68  (fulcrum) than the spring guide  62  (load), the force needed to move the spring guide  62  is substantially reduced. Thus, when the piston  60  moves a distance, the ratcheting mechanism  46  moves a related distance and when a force moves the piston  60 , a related force is provided to move the ratcheting mechanism  46 . In the preferred embodiment, for every five units of displacement of the piston  60 , the ratcheting mechanism  46  moves one unit of length and the force provided to move the ratcheting mechanism  46  is approximately five times more than the force required to move the piston  60 , thereby enabling the use of much lower pneumatic pressure to drive the motion of the piston  60 . 
     The piston  60  is positioned in a chamber  72  formed within the housing  24  and is biased, such as by a spring  73 , toward the end of the chamber  72  formed by the cap  52 . Compressed air  74  from a pneumatic air supply  75  (FIG. 16) may be introduced into the chamber  72  through an air tube  76  to a hollow tubing barb  77  inserted into an air hole  78  in the cap  52  leading into the chamber  72 . The introduction of compressed air  74  into the chamber  72  forces piston  60  to move against the bias of the spring  73  causing the mechanical linkage  58  to move the ratcheting mechanism  46 . This, in turn, moves the plunger  25  one unit toward the needle receiving end  28  of the barrel  26  and expels fluidic material  32 . Piston  60  may be axially sealed within the chamber  72  by an o-ring  78  surrounding the piston. Of course, the biased position of the piston  60  is reversed in the aspiration apparatus embodiment of the present invention. 
     One significant advantage of the present invention over the prior art is that the mechanical linkage  58  used to move the ratcheting mechanism  46  permits the digital fluid delivery and aspiration apparatus  20  to operate at much lower pressure. Thus, the pneumatic air supply  75  need only deliver around 10 psi of air pressure whereas existing systems typically require around 40 psi. The amount of pressure of the compressed air  74  that the apparatus  20  requires depends on many variables and the pressure levels of the compressed air  74  described herein, while the preferred levels, are not the only pressure levels with which the present invention may be practiced. 
     The pneumatic air supply  75  (shown schematically in FIG. 16) may be similar to the ones disclosed in U.S. Pat. Nos. 5,354,268 and 5,520,652 or any of the many similar devices capable of providing the required air waveform. Electric power is provided to the pneumatic air supply  75  via a cable  83 . A compressor  85  generates compressed air or, more broadly, pneumatic power and provides the pneumatic power to the digital fluid delivery and aspiration apparatus  20  through the air tube  76 . The pneumatic air supply  75  forms pressure pulses (FIG. 15) at a rate and strength determined by the operator. The operator can control the frequency, amplitude, and shape (e.g., square, sinusoidal, triangular) of the waveform by adjusting controls  84  of the pneumatic air supply  75 . The number or amount of pulses delivered to the apparatus  20  may be controlled using a foot pedal  86  or other control. 
     Several benefits, including reduced damage to tissue surrounding an injection site, arise from the use of an improved waveform  88  (FIG. 15) from the pneumatic air supply  75 . The waveform  88  has a small, initial jump to a low pressure  89  to overcome friction and stiction in the apparatus  20 . Beyond the friction and stiction level, the pressure is increased linearly along slope  90  and results in a substantially constant speed movement of the plunger  25  toward the needle receiving end  28  of the barrel  26  because of the linear increase in the counter force being provided by the spring compression. This constant speed of the plunger  25  substantially decreases the amount of tissue damage that may occur near the injection site. When the pressure reaches a maximum level  92 , approximately 10 psi, the delivery of pneumatic power is ended, quickly stopping the motion of the plunger  25 . 
     Unlike the prior art devices which have a piston displacement v. time curve substantially as shown in FIG. 13, the present invention (FIG. 14) has only a small jump  100  as frictional forces are overcome and then has a substantially linear acceleration to the point of maximum displacement  102 . While in the prior art devices the initial jump to overcome system friction may be as much as 75% of the total motion of the plunger, the small jump  100  of the plunger in the present invention is a substantially minimal part of the total plunger motion. This reduced initial jump provides a significant decrease in the jetting of fluid and reduces the risk of tissue damage near the surgical site. 
     In operation as a fluid delivery apparatus, the surgeon using the digital fluid delivery and aspiration apparatus  20  connects the housing  24  to the pneumatic air supply  75  by attaching the air tube  76  to the tubing barb  75  leading into the chamber  72 . The surgeon then positions the plunger  25  within the barrel  26  such that the fluidic material engaging head  34  is at the needle receiving end  28  of the barrel  26  and orients the flat faces  42  of the shaft  36  toward pawls  47  and  65  so that the plunger  25  may be freely moved along the length of the barrel  26  by manipulating the manual plunger control  38 . The surgeon then draws fluidic material  32  into the barrel  26  by drawing the fluidic material engaging head  34  toward the plunger receiving end  30  of the barrel  26  until the desired amount of fluidic material  32  is in the barrel  26  of the syringe  22 . When the syringe  22  is properly filled with fluidic material  32 , the operator then orients the curved faces  40  of the shaft  36  to engage the pawls  47  and  65 . The digital injector  20  is now ready to deliver the fluidic material  32  into the patient&#39;s tissue. 
     Referring now to FIGS. 11 and 12, when the syringe  22  is positioned to direct the fluidic material  32  to the proper location, the operator activates the pneumatic air supply  75  which provides compressed air  74  to the digital injector  20  in a pulse train substantially as shown in FIG.  15 . As the pressure of the compressed air  74  is brought to maximum pressure  92 , the piston  60  within the chamber  72  in the housing  24  is forced to move against the bias of the spring  73 . This movement of the piston  60  causes the mechanical linkage  58 , lever arm  66  in the preferred embodiment, to move the pawls  65  a discrete distance. The pawls  65  engage the teeth  44  on the shaft  36  of the plunger  25 . Thus, the movement of the pawls  65  causes the plunger  25  to move a discrete distance toward the needle receiving end  28  of the barrel  26  thereby forcing fluidic material  32  out of the syringe  22 . When the pneumatic air supply  71  reduces the pressure of the compressed air  74 , the piston  60  is returned to its original position by the bias of the spring  73 . As the piston  60  returns, a corresponding movement of the mechanical linkage  58  and the ratcheting mechanism  46  occurs. The shaft  36  is held in place by pawls  47  engaging the teeth  44 . This prevents fluidic material  32  or other substances from being drawn into the syringe  22  as the pawls  65  ratchet over the teeth  44  to their new position. The cycle is complete and the digital fluid delivery and aspiration apparatus  20  is prepared to receive the next pulse of compressed air  74  from the pneumatic air supply  75  and to deliver the next discrete dose of fluidic material  32  to the injection site. With each cycling of the pressure of compressed air  74  delivered to the apparatus  20 , a discrete amount of fluidic material  32  is delivered to the injection site. 
     The operation of the apparatus  20  as an aspiration apparatus is very similar, although opposite, to its operation as a fluid delivery apparatus. In the aspiration apparatus embodiment, each cycling of the pressure of compressed air  74  causes a discrete amount of fluidic material  32  to be drawn into the barrel  26  of the syringe  22 . As should be evident to one of skill in the art, the operation of ratcheting mechanism  46  is opposite of that of the fluid delivery embodiment of the invention. 
     FIG. 17 shows one alternative embodiment of a digital fluid delivery and aspiration apparatus  20  in accordance with the present invention. In this embodiment, a second fluid power actuator  149  is used instead of the spring  73  to bias the piston  60  toward the end of the chamber  72  formed by the cap  52 . The second fluid power actuator  149  is formed by a second chamber  152  located in the base  50  of the housing  24 , and includes a second piston  150 . The second chamber  152  should be dimensioned such that the second piston  150  substantially sealably slides within the second chamber  152 . The sealing interface may be effectuated by using second o-rings  154  around the second piston  150 . The second piston  150  is biased toward the cap  52  by pneumatic or hydraulic pressure introduced into the second chamber  152  through a second air tube  156  and second hollow tubing barb  158 . Of course, the pneumatic air supply  75  used with this embodiment of the invention must be able to control the pressure of the pneumatic or hydraulic fluid provided to the second chamber  152 . 
     FIGS. 18-21 show another alternative embodiment of a digital delivery and aspiration apparatus  20  in accordance with the present invention. In this embodiment, both fluid delivery (“irrigation”) and aspiration activities may be performed by the same device. As with the embodiment shown in FIG. 17, the piston  60  is preferably biased toward the cap  52  of the housing  24  using a second fluid power actuator  149 , including a second piston  150  located in the base  50  of the housing  24 , although a spring  73  may again be used to provide the necessary bias. Again, the pneumatic air supply  75  used with this embodiment must be able to control the pressure of the pneumatic or hydraulic fluid provided to the second chamber  152  if a second piston  150  is used to bias the piston  60 . As shown in FIG.  18  and in detail in FIG. 21, in this embodiment, a second ratchet spring  160 , with integrated pawls  161 , and second leaf spring  162 , with integrated pawls  163 , are used. Furthermore, the shaft  36  of the plunger  25  used in this embodiment has upward teeth  164  positioned side-by-side with downward teeth  166  (FIGS. 19 and 20) on its periphery and along part of the length of the shaft. The shaft  36  can then be rotated such that the upward teeth  164  engage the pawls  65  and  47 , or such that the downward teeth  166  engage the pawls  161  and  163 . When the shaft  36  is rotated such that the upward teeth  164  engage the pawls  65  and  47 , operation of the apparatus  20  provides fluid delivery to the surgical site when the mechanical linkage  58  moves the ratcheting mechanism  46  as described above. When the shaft  36  is rotated such that the downward teeth  166  engage the pawls  161  and  163 , operation of the apparatus  20  provides aspiration of the surgical site when the mechanical linkage  58  moves the ratcheting mechanism  46  as described above. The aspiration function is achieved because as the second ratchet spring  160  is caused by the mechanical linkage  58  to move toward the needle receiving end  28  of the barrel  26 , the pawls  161  ratchet over the downward teeth  166 . And as the mechanical linkage  58  causes the second ratchet spring  160  to move away from the needle receiving end  28  of the barrel  26 , the second ratchet spring  160  engages the downward teeth  166  to cause the plunger  25  to move away from the needle receiving end  28  of the barrel  26 . In this positioning of the shaft  36 , the plunger  25  is prevented from moving toward the needle receiving end  28  of the barrel  26  by the pawls  163 . Thus, digital control of the plunger  25  is achieved in either direction along the barrel  26 . Of course, the barrel  26  must be filled and emptied of fluid by orienting the shaft  36  so that either the upward teeth  164  or the downward teeth  166  engage the shaft  36  when the mechanical linkage  58  is caused to move by activating the pneumatic air supply  75 . The barrel  26  may also be manually filled by having smooth faces on the shaft  36  that do not engage any of the pawls  65 ,  47 ,  161  or  163 . 
     In another embodiment of a digital fluid delivery and aspiration apparatus  20  in accordance with the present invention, shown in FIG. 22, the seal between the piston  60  and the chamber  72  is achieved by a rolling diaphragm  175  rather than an o-ring  78 . In this embodiment, rolling diaphragm  175  unrolls as the piston  60  moves toward the base  50  end of the chamber  72  and rolls-up as the piston  60  moves toward the cap  52  end of the chamber  72 . The rolling diaphragm  175  may be used in conjunction with any of the embodiments of the apparatus  20  disclosed herein or with any other embodiments that may be designed. A rolling diaphragm  175  may also be used to seal the second piston  150  within the second chamber  152 . Of course, other means for sealing the engagement between the piston  60  and the chamber  72  and the second piston  150  and the second chamber  152  may also be used. 
     The various parts and components of the present invention may be made from a wide variety of materials. The materials are preferably corrosion resistant and autoclavable. Such materials include stainless steel, aluminum, glass, and polysulfone, or other plastics. Also, it should be recognized by one of skill in the art of the invention that while a pneumatically driven piston is the preferred embodiment, hydraulically driven pistons may be used. 
     As illustrated by the foregoing description and shown in the FIGS., the present invention is more suitable as a pneumatically controlled fluid delivery and aspiration apparatus than are existing devices. The present invention overcomes the limitations and disadvantages of existing devices by utilizing an effective design of a digital fluid delivery and aspiration apparatus with mechanical de-amplification that can operate at much lower pressure than existing devices, substantially reduces the amount of jetting of fluidic material from the open end of the syringe, and allows more precise control of the rate and volume of flow of fluidic material to and from the syringe. 
     Although the invention has been herein shown and described in what is perceived to be the most practical and preferred embodiment, it is to be understood that the invention is not intended to be limited to the specific embodiments set forth above. Rather, it is recognized that modifications may be made by one of skill in the art of the invention without departing from the spirit or intent of the invention and, therefore, the invention is to be taken as including all reasonable equivalents to the subject matter of the appended claims.