Dental anesthetic and delivery injection unit

An improved injection device includes containing a pump and pump control circuitry, a handle with a needle for delivering a therapeutic agent to a patient, a cartridge holding the agent and a tube for delivering the agent from the cartridge to the handle. Preferably the cartridge is disposed in a cartridge holder coupled to the tube. A quick disconnect coupling is provided between the cartridge holder and the housing so that it can be easily mounted thereon. The housing further has a receptacle for receiving and storing the tip of the handle.

BACKGROUND OF THE INVENTION 
This application relates to a dental anesthetic injection delivery system, 
and more particularly to a system for injecting anesthetic safely and 
painlessly during a dental procedure. 
While local anesthetics eliminate the often excruciating pain that would 
otherwise be suffered during dental extractions, drilling and 
reconstructions, the dental patient is still subject to pain from the 
hypodermic injection of the anesthetic into the local dental area of the 
mouth. 
It is well established that more than 50% of adult patients fear injections 
in general, and even a greater number especially fear injections in the 
mouth during a dental procedure. 
Accordingly, it would be desirable to provide a dental anesthetic injection 
delivery system which substantially eliminates the pain suffered by a 
patient during injection of anesthetic into the gum or other dental 
tissue. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with the invention, there is provided a 
dental anesthetic delivery system which renders the nerve bundles which 
supply the teeth and supporting dental structures disabled in the area of 
the injection and in the path of the needle before a pain sensation is 
perceived by the dental patient. The system comprises a drive unit for 
selectively delivering anesthetic through a controllable handpiece unit 
which carries a needle for tissue penetration. Operation of the drive unit 
can be selectively controlled so that different amounts of anesthesia may 
be delivered through the needle, depending upon whether the dental 
practitioner is either penetrating the dental tissue or has otherwise 
reached the location within the mouth tissue where nerve disability is 
required. 
The inventive system uses a linear activator to deliver a controlled amount 
of anesthetic from a cartridge that is locked into the drive unit. The 
drive unit is operated by a foot pedal which is selectively depressed in 
order to deliver anesthetic through the handpiece unit. 
The system of the invention may be used to administer any liquid into the 
dental tissue, and is designed to accommodate a local anesthetic cartridge 
and a variety of needle sizes. The drive unit of the system includes a 
motor and an on-board microprocessor to facilitate control of anesthetic 
flow. The drive unit, utilizing the technology described in U.S. Pat. Nos. 
4,747,824 and 5,180,371, and incorporated herein by reference, enables 
delivery of an anesthetic solution at a constant pressure and volume, 
regardless of variations in tissue resistance. At the delivery end of the 
needle, the system delivers a minute amount of the anesthetic which 
precedes the needle during injection in the dental tissue, creating in 
effect an anesthetic pathway. The combination of an anesthetic pathway and 
controlled pressure and volume (flow rate) results in an effective and 
pain-free injection. 
The handpiece assembly, which carries the delivery needle, enables an 
unprecedented level of visibility and fingertip control. The handpiece 
assembly is linked to the selected anesthetic cartridge by plastic 
microtubing. The drive unit controls the reciprocating movement of a 
plunger which selectively and controllably dispenses anesthetic from the 
anesthetic cartridge and through the handpiece assembly in a predetermined 
flow profile. 
Accordingly, it is an object of the invention to provide a dental 
anesthetic delivery injection system which can be used for most known 
dental injection procedures. 
Yet another object of the invention is to provide a dental anesthetic 
delivery injection system which affords the dentist acute tactile response 
during operation. 
A further object of the invention is to provide a dental anesthetic 
delivery injection system which selectively controls the amount of 
anesthetic being delivered through the dental procedure. 
Yet another object of the invention is to provide a dental anesthetic 
injection delivery system which enhances the precision and accuracy of 
anesthetic delivery. 
Still other objects and advantages of the invention will in part be 
obvious, and will in part be apparent from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Beginning with FIG. 1, a dental anesthetic injection delivery system, made 
in accordance with the invention, is described and generally indicated at 
11. System 11 includes a drive unit, generally indicated at 13, a foot 
pedal 29 cooperatively connected to drive unit 13 by means of air hose 31, 
an anesthetic cartridge holder generally indicated at 17 for selectively 
retaining a cartridge 21 of a desired anesthetic, and a handpiece unit 
generally indicated at 15, cooperatively connected to anesthetic cartridge 
holder 17 by means of a predetermined length of microtubing 19. 
Drive unit 13 comprises a substantially rectangular housing 33 having a 
bottom or base 32, sides 34, top 36 front portion 38, and rear portion 40. 
Housing 33 is defined by two mating and engageable halves 35 and 37, as 
best depicted in FIGS. 1 and 2. Housing 33 further includes a pair of 
lateral hubs 24 disposed on sides 34 of each housing half 35 and 37 along 
the bottom thereof to stabilize the drive unit 13 as it stands on a 
supporting surface. 
Housing 33 of drive unit 13 includes a power switch 152 (shown in FIG. 19) 
along back portion 40 and a reset or aspirate with other controls, is 
selectively pressed in order to operate system 11 in three different modes 
of operation, as described hereinafter. Front portion 38 of housing 33 
further includes a series of cartridge volume indicator lights 61, a power 
indicator light 62, and an aspirate indicator light 64, as shown in FIG. 
1. Preferably lights 61, 62, 64 are LED's. Reset button 63 and lights 61, 
62 and 64 are mounted on printed circuit board 59 installed inside unit 13 
(see FIG. 2). As described in more detail below, power indicator light 62 
is illuminated when the power switch is turned on; aspirate indicator 
light 64 is illuminated when the reset button 63 has been pressed and the 
aspirator function of the system is activated; and volume indicator lights 
61 are selectively illuminated in order to show the volume of anesthetic 
solution remaining in cartridge 21 during operation of system 11. 
Continuing now with FIG. 1, foot pedal 29 is preferably a round air 
bellows-type device and functions as an air-activated control element for 
selectively controlling the operation of system 11. Foot pedal 29 is 
operatively connected to air hose 31, which in turn is coupled to a 
pressure sensor 64 located inside unit 13. In operation of system 11, the 
pressure change from pedal 29 is detected by the sensor 64. Sensor 64 then 
generates a corresponding control signal used to control the flow of the 
anesthetic solution. In the preferred embodiment, as described in more 
detail below, when foot pedal 29 is slightly or partially depressed, 
system 11 operates at a slow speed. When foot pedal 29 is firmly or fully 
depressed, system 11 operates at a fast speed. 
Turning now to FIGS. 3-7, cartridge holder 17 of system 11 is now 
described. Cartridge holder 17 is used for holding anesthetic cartridge 21 
in proper engaged position in drive unit 13 in order to enable controlled 
dispensing of anesthetic solution to handpiece unit 15 for delivery 
therefrom. Cartridge holder 17 comprises an elongated plastic transparent 
cylindrical tube 71 having a forward end 94 and a rear end 96, as best 
shown in FIG. 7. Forward end 94 includes an outwardly projecting delivery 
sleeve 93 and an inwardly projecting protrusion or spike 83, both of which 
serve to define an exit pathway 90 or lumen through end 94 of holder 17. 
Sleeve 93 is engaged to and mates with one end of microtubing 19. Spike 83 
is preferably designed with a surface cut at an angle of about 30 degrees 
and is used to puncture a sealing diaphragm of anesthetic cartridge 21 
when said cartridge 21 is loaded into holder 17, as described below. 
Rear end 96 of cartridge holder 17 includes a pair of opposite radially 
projecting wings 73, as shown in FIGS. 3 and 4. Wings 73 are used to 
engage and form an interference fit between end 96 of cartridge holder 17 
and a receptacle 25 on housing 13. Preferably, receptacle 25 is formed 
along the top portion of housing 13, as best shown in FIGS. 1 and 3. 
Cartridge holder receptacle 25, as best seen in FIGS. 3-5, is formed with a 
generally round opening 75 having a pair of oppositely disposed keyways 
75A that are sized for receiving and accommodating wings 73 of cartridge 
holder 17 (see FIG. 5). Receptacle 25 further includes a pair of tongues 
77 formed on each half 35 and 37 of housing 33 below keyways 75A, as well 
as a pair of corresponding cam members 79 disposed above each tongue 77. 
Each set of corresponding tongues 77 and cam members 79 define a locking 
slot 78 therebetween, as shown in FIG. 4. 
Referring now to FIG. 6A, the forward end of holder 17 is provided with a 
plurality of holes 88. These holes can be used to assist in the removal of 
spent cartridges 21, as discussed below. Between these holes, there are 
provided a plurality of radial ribs 92 disposed inside the holder 17 used 
to stabilize cartridge 17 after said cartridge is fully inserted into the 
holder 17, in the position depicted in FIG. 7. 
Turning once again to FIG. 7, the introduction and use of a typical 
anesthetic cartridge 21 is now described. Anesthetic cartridge 21 
comprises a plastic or glass tube 91 defining an inside storage chamber 22 
containing a desired anesthetic. Tube 91 has a forward portion 92 and a 
rear portion 98. Forward portion 92 is formed with a neck region 89 and an 
extending mouth 88 in which is fitted a diaphragm 85 adapted to be 
maintained in position within mouth 88 by means of a cap 87. Rear portion 
98 has an end wall 85A which acts as a piston to express the anesthetic 
from the cartridge 21, in conjunction with plunger 23. 
In order to load anesthetic cartridge 21 into holder 17, forward portion 92 
of cartridge 21 is inserted through rear end 96 of holder 17 until 
approximately 1/4 inch of cartridge 21 extends below end 96 of holder 17. 
Then, rear portion 98 of cartridge 21 and, more particularly, end wall 
85A, is in contact with plunger 23 of unit 13, which is able to 
selectively pass through receptacle 25 during operation of system 11. Once 
plunger 23 is properly aligned with respect to cartridge 21, end 96 of 
holder 17 is seated within holder receptacle 25, as shown in FIG. 5, such 
that wings 73 are disposed within corresponding keyway 75. 
In order to lock holder 17 within receptacle 25, end 96 of holder 17 is 
rotated one quarter turn in a counter-clockwise direction (see FIG. 6) 
such that each of wings 73 passes through or snaps a locking slot 78 and 
between corresponding tongue 77 and cam member 79. The plunger 23 is 
secured to motor 39 by standard right-handed threads (not shown). 
Therefore, as the receptacle 25 is secured in the counter-clockwise 
direction, it does not cause the plunger 23 to loosen. The distance 
between each tongue 77 and cam member 79 is slightly smaller than the 
thickness of wings 73. As each wing 73 turns and goes through one of the 
slots 78, respective tongue 77 flexes slightly downward. Once wing 73 goes 
through slot 78, tongue 77 snaps back, thereby locking the respective wing 
73 in place. The rotation of holder 17 is terminated when wings 73 hit 
stops 78A. 
Preferably, receptacle 25 with its opening 75, keyways 75A, tongues 77, cam 
members 79 and stops 78A are formed within a domed portion 26 of top 36 as 
shown. The bottom of receptacle 25 is defined by transverse walls 110, 
112. Walls 110, 112 have corresponding holes 114, 116 coaxial with opening 
75 which allow piston 23 to reciprocate in and out of housing 13, as 
described more fully below. 
Significantly, during the loading of anesthetic cartridge 21 into drive 
unit 11, as described above, it is urged forward toward end 94 and as a 
result, spike 83 of holder 17 punctures diaphragm 85 of cartridge 21. This 
action provides a pathway or lumen between inside chamber 22 of cartridge 
21 and exit pathway 90 of holder 17 so that anesthetic may flow through 
microtubing 19 and to handpiece unit 15 (see FIG. 8). Preferably, holder 
17 has an outer surface 18 which is not cylindrical but polygonal, as best 
seen in FIG. 5. Surface 18 may have, for example, eight sides, i.e., it is 
octagonal in cross section. At the rear end 96 of holder 17, surface 18 is 
provided with a plurality of axial ribs 20 extending forwardly, which to 
facilitate engaging and disengaging holder 17 from housing 13. The 
octagonal shape of holder 17 is provided to facilitate the handling of the 
holder by the dentist or dental assistant, especially when he or she is 
wearing gloves. 
Referring now to FIGS. 8-14, handpiece unit 15 is now further described. 
Handpiece unit 15 comprises a co-extruded handle member 101 of a 
substantially elongated design and a needle assembly 103 selectively 
engaged to one end of handle member 101. Handle member 101, as shown in 
FIGS. 9 and 10, is defined by a body 115 and has a forward bulbous end or 
head 105 and a rear end 107. End 105 is formed with an inwardly disposed 
Luer thread 113 and an extending plug 109, both of which are used for 
selectively engaging needle assembly 103, as shown in FIGS. 1 and 8. 
Body 115 of handle member 101 defines a U-shaped elongated pathway or 
trough 113 in which microtubing 19 is selectively seated starting at 
forward end 105 and ending at rear end 107 (see FIGS. 9 and 10). During 
assembly, one end of microtubing 19 is first threaded into plug 109 of end 
105, after which the rest of tubing 19 is press fitted into pathway 113. A 
solvent such as MEK (methylethyl ketone) is then used to permanently bond 
microtubing 19 in place. 
Body 115 of handle member 101 further includes a longitudinal slot 108, 
which cooperates with pathway 113 to enhance a user's ability to grasp 
handle member 101. 
Significantly, handle 101 is formed with a pair of cut-outs 111 adjacent 
forward end 105 in order to define a tapered weak zone in body 115. As a 
result, and as best depicted in FIG. 8, handle 101 may be flexed (with 
some modest effort) and plastically deformed at the location of cut-outs 
111 in order to properly orient needle assembly 103 during operation of 
system 11. 
Turning to a consideration of FIGS. 8A, 15 and 16, needle assembly 103 is 
now further described. Needle assembly 103 includes a needle cover 121 
having a series of longitudinally extending ribs 123 formed along the 
outside surface thereof and a Luer lock needle 102. Needle cover 121 has a 
forward end 125 designed for selective reception by handpiece receptacle 
27 formed along top 36 of drive unit 13, and a rear end for selectively 
engaging with end 105 of handle member 101, thereby covering of needle 
needle 102, which is permanently attached to sleeve 104 (FIG. 8A). Sleeve 
104 is in turn coupled to head 105 by the Luer connection. 
Handpiece receptacle 27 of drive unit 11 is designed to hold needle cover 
121 firmly in place for storage while handpiece 15 is in use, i.e., when 
the cover 121 is removed from the handle member 101. It is also designed 
to store cover 121 with needle assembly 103 captured therein until the 
user is ready to operate system 11. 
Handpiece receptacle 27 is defined by an annular opening 129 having along 
the circumference thereof four outwardly formed arcuate projections 131. 
In order to secure needle cover 121 in receptacle 27, cover 125 is placed 
in opening 129 such that ribs 123 are received within projections 131, as 
best shown in FIG. 17. 
Of course, the size of the needle and its cover may change. If needle cover 
121 is of a smaller size, then its ribs are simply held tightly along the 
ring defined by opening 129 (see FIG. 18). Thus, receptacle 27 is adapted 
to accommodate needle assemblies of various sizes. 
The entire system 11 is powered electrically, with various actions 
occurring under the control of an operator who activates the appropriate 
control functions of the system. To illustrate how this is achieved, FIG. 
19 shows a schematic diagram of the various electrical elements of the 
system. These elements combine to define the control circuit of the 
system. The system includes a microprocessor 150. This microprocessor 150 
receives inputs from the power switch 152, pressure sensor 65, reset 
button 63 and a cartridge sensor 154 used to sense when the cartridge 21 
is in place as discussed above. The microprocessor 150, in response to the 
signals received from these input members, activates lights 61, 62 and 64, 
as well as an audio beeper 160, as discussed below. The microprocessor 150 
further operates a motor 156. The motor activates a driver mechanism 158 
used to reciprocate plunger 23. 
The sequential operation of the subject injection device is illustrated by 
the flow chart of FIG. 20. Starting with step 200, the power for the 
injection device is turned on by activating switch 152. In step 202 the 
power indicator light 62 is turned on, followed by step 204 when a check 
is performed to determine if the reset button 63 is pressed. If not, then 
in step 206 the plunger 23 is retracted into the housing 13 and the device 
waits for a cartridge 21 to be installed. 
In step 210, a check is performed to determine if cartridge 21 has been 
installed via sensor 154 and if foot pedal 29 has been depressed. If both 
these conditions are met then in step 212, lights 61 are activated. 
Initially, since the cartridge 21 is full, all the lights 61 are on. As 
the process of dispensing the anesthetic solution continues, the 
microprocessor keeps a running count of how much of the solution has been 
expressed from the cartridge by the position of the plunger 23 and 
activates the indicator lights 61 to show the remaining volume. 
After step 212, in step 214, the plunger 23 is advanced so that it comes in 
contact with, and in effect, interlocks with, the rear end 85A of 
cartridge 21 in a manner well known in the art of syringes. Therefore, as 
the plunger reciprocates, it moves end 85A, which acts as a piston as 
previously discussed. The plunger continues to advance by a predetermined 
amount at a low injection rate. The plunger is advanced until sufficient 
solution has been expressed from the cartridge 21 to fill the microtube 19 
and needle 102. In this manner, the system is automatically purged of any 
air bubbles. 
In step 216, the pedal 29 is again checked to see if it is still activated. 
More specifically, in step 218, a check is performed to determine if the 
pedal 29 is in its first position. If it is not, then in step 220 a check 
is performed to determine if the pedal 29 is in its second position. If 
the pedal is not in the first or the second position, then it is inactive 
and the system waits for the pedal to be activated again (step 216). 
If in step 218 it is determined that the pedal 29 is in the first position, 
then in step 222, a slow delivery subroutine is initiated. During this 
subroutine, the plunger 29 is advanced slowly, beeper 160 (controlled by 
the microprocessor 150) beeps at a first, relatively low rate, and the 
lights 61 are all blinking at a low rate as well, for example in 
synchronism with the beep. As part of this subroutine in step 224, a check 
is performed to determine if the cartridge 21 is empty yet. If it is not, 
then in step 226, the pedal is checked to see if it has been released. If 
the pedal 29 is not released, then the plunger 23 continues advancing at 
slow rate (step 222). 
If the pedal 29 is released, then in step 227 a check is performed to see 
if the aspirate function has been enabled via batten 63. If it has then 
the LED 64 is turned on and the process continues to step 228. If the 
aspirate mode has not been enabled, then in step 227A the plunger is 
stopped and the process continues to step 216 to wait for the next 
activation of pedal 29. 
In step 228 a check is performed to determine if the slow delivery has been 
activated for a predetermined time duration, for example four seconds. 
After four seconds of slow delivery, during step 228, the reset button 63 
is checked to see, in this case, if the user has requested aspiration. 
Thus, the user can request an aspiration after four seconds of slow 
delivery. If the user has requested aspiration, then the plunger 23 is 
reversed for a predetermined distance, stops for a programmable time 
period (for example, 2 seconds) and then advances back to its original 
position (i.e., to the position of the plunger at the moment when the 
reset button was depressed). Thereafter, the device goes back to step 216 
and waits for the activation of the pedal 29. 
Going back to step 224, the cartridge 21 becomes empty, as determined by 
the microprocessor, then in step 230 all the lights 61 are deactivated and 
a rapid series of beeps is issued by beeper 160. During these beeps, step 
232 requires a check to be performed to determine if the reset button 63 
has been depressed. When the button 63 is depressed, the device returns to 
step 206 where the plunger 23 retracts and the device is ready for a fresh 
cartridge 21. 
An alternate sequence starts with step 220 if the pedal 29 is found to be 
in the second position, causing a fast delivery subroutine is initiated. 
More specifically, in step 234, the plunger is advanced at faster rate, 
and, simultaneously, the lights 61 and the beeper are activated at a 
higher rate than in step 222. For example, at the slower rate, the device 
may deliver anesthetic at 1 drop/sec. (or typically about 0.02 ml/sec.), 
while at the faster rate, the anesthetic may be delivered at 0.04 ml/sec. 
Meanwhile, in step 236, a check is performed to determine if the cartridge 
21 is empty. If it is, then steps 230 and 232 are performed as discussed 
above. But if cartridge 21 is not empty, then in step 238 a test is 
performed to determine if the pedal 29 has been released. If the pedal is 
released, then the device returns to step 216 and waits for a pedal 
activation; however, if the pedal is not released (step 238), then the 
higher speed anesthetic delivery continues. 
Returning to early step 204, if the reset button 63 was depressed at the 
same time as the power button, then a cleaning subroutine is initiated. As 
part of this subroutine, the plunger 23 is fully extended and remains in 
that position so that it can be cleaned and/or replaced. When the reset 
button 63 is depressed (as determined in step 242) the plunger retracts 
(step 206) and normal operation resumes. 
In summary, reset button 63, pedal 29 and switch 152 of drive unit 13 are 
used to define three different modes of operation of system 11. In the 
first mode of operation, when reset button 63 is held down continuously 
for about two seconds, plunger 23 is fully retracted into the inside of 
drive unit 13 ("home" position). 
In the second mode of operation, the reset button is pressed and then foot 
pedal 29 is slightly depressed and held for four audible beeps and then 
removed. This activates the aspirating mode of system 11, as described. 
In the third mode of operation, when reset button 63 is pressed and held 
down, and when switch 152 is activated, the cleaning of system 11 is 
initiated and plunger 23 is fully extended for easy removal in order to 
clean or replace it. 
In order to remove cartridge 21 from drive unit 13 after use, cartridge 
holder 17 is first released from receptacle 25 by rotating end 96 an 
opposite quarter turn. Then, the operator places the tip of a suitable 
pointed instrument into holes 88 (see FIG. 6A) formed along forward end 94 
of cartridge holder 17 in order to push cartridge 17 out of holder 17. 
Now that the elements and components of the subject invention has been 
described in great detail, a typical operation is provided for a better 
understanding of its advantages. The system is initialized when the power 
button is turned on. Once the power is on, the plunger 23 retracts to a 
`home` position and the power light 62 goes on. The dentist or dental 
assistant then inserts a cartridge 21 into the cartridge holder 17 and 
positions the head of the plunger 23 into the bottom of the cartridge 
holder 17 so that this head abuts the piston 85A . The cartridge holder is 
then secured to the housing 13 by pressing it down into receptacle 25 and 
twisting it clockwise by about 90.degree. (see FIGS. 1, 3-6). This motion 
also forces the cartridge holder 17 to slide over the piston 23. This 
motion in turn causes the spike 83 to move downward and break the seal 92, 
thereby opening the cartridge 21. Thus, in one movement, the cartridge 
holder 17 is mounted onto housing 13 and, at the same time, the cartridge 
21 is unsealed. Next, the foot pedal 29 is depressed once, causing the 
piston 23 to advance upward by a predetermined amount thereby forcing the 
anesthetic from cartridge 21 to flood the microtube 19 through the handle 
15 to needle 102 until a small amount squirts out. In this manner, air is 
purged from the microtube 19 and needle 102. After this automatic purging, 
the lights 61 go on, indicating that the cartridge is full and that the 
whole system is now ready for operation. 
In order to start infusing the aesthetic, the dentist first takes the 
handle 15, removes the cap 121 and inserts the needle 102 into the 
appropriate tissue. Advantageously, the cap 121 can be stored in 
receptacle 27. If necessary, the dentist may bend the handle 15 as shown 
in FIG. 8. Dispensing of the anesthetic is started by pushing down and 
holding foot pedal 29 in the first position which causes the plunger 23 to 
move upward at a constant first rate. As long as the foot pedal 29 is held 
in the first position, a steady beep is emitted at about 1 second 
intervals and the level gauge lights 61 blink at the same rate. If the 
dentist wants to dispense the anesthetic at the faster rate, he pushed 
down on the pedal 29 to the second position. The plunger 23 then moves 
faster. This mode of operation is indicated by the lights 61 blinking 
faster, and the beeper 160 emitting sounds at a faster rate as well. To 
stop the anesthetic dispensing, the dentist removes his foot from the 
pedal 29. This action causes the plunger 23 to stop. The lights 61 stop 
blinking and stay on and the beeper 160 stops emitting beeps as well. 
Before going on to other procedures, the dentist installs the cap 121 on 
the needle 102 and then rests the handle 15 over receptacle 27 with the 
cap 121 extending into the receptacle 27. 
As the anesthetic is being dispensed, the position of the plunger 23 is 
contiguously monitored to determine the amount of anesthetic left in 
cartridge 21. When the cartridge 21 becomes empty, the plunger 23 stops, 
the lights 61 go off and a series of rapid beeps are emitted by beeper 160 
to indicate that the cartridge 21 needs to be replaced. 
If aspiration is desired, the dentist first operates the foot pedal in the 
first position for at least four seconds (i.e., four beeps). He then 
activates the reset/aspirate button 63. This action causes light 64 to go 
on and the plunger 23 to reverse its forward movement for a predetermined 
distance thereby causing anesthetic (and, occasionally, body fluids) to be 
withdrawn into the microtube 21. After moving back, the plunger 23 stops 
for two seconds (to allow the dentist to visually inspect the contents of 
the microtube 19 within the handle 15) and then advances back to its 
original position. If the foot pedal 29 is still depressed, the plunger 23 
then continues its forward movement. 
At the end of a procedure, the dentist pushes button 63 for two seconds. 
This action causes the plunger 23 to return to its original position, 
thereby allowing the cartridge 21 to be removed and discarded. 
Preferably, for a new patient, all the disposable elements of the system 
are discarded. This can be accomplished by turning the power button 152 
and reset button 63 on simultaneously. These actions initiate the cleaning 
mode during which the plunger is fully extended so that it can be removed 
and discarded, together with the cartridge holder 17, microtube 19 and 
handle 15. 
The system of the invention, under microprocessor control, delivers precise 
pressure and volume ratios of anesthetic. Even in resilient dental tissue, 
such as the palate and periodontal ligament, the inventive system delivers 
an anesthetic drip which precedes needle entry, effectively creating an 
anesthetic pathway. This combination of an anesthetic pathway and 
controlled flow rate results in a virtually imperceptible injection and 
rapid onset of profound anesthesia, all for the patient's comfort and 
relief. 
In addition, the inventive system affords greater tactile control than 
traditional dental syringe units, and precise needle placement is 
therefore assured. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained, and 
since changes may be made without departing from the spirit and scope of 
the invention, it is understood that the above description is merely 
illustrative and does not otherwise limit the scope of the invention. The 
claims that follow define the inventive scope.