Patent Publication Number: US-9895496-B2

Title: Syringe with needle guard injection device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 14/547,495, filed Nov. 19, 2014, which is a continuation of application Ser. No. 13/585,767, filed Aug. 14, 2012, now U.S. Pat. No. 8,900,200, which is a continuation of application Ser. No. 12/938,252, filed Nov. 2, 2010, now U.S. Pat. No. 8,241,255, which is a continuation of application Ser. No. 11/028,902, filed Jan. 3, 2005, now U.S. Pat. No. 7,824,379, which is a continuation of application Ser. No. 11/113,537, filed Mar. 27, 2002, now U.S. Pat. No. 6,976,976, which are fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to injection devices for administering therapeutic agents to patients, particularly to safety devices for syringes, and more particularly to syringe devices that include a needle guard slidably coupled to a syringe for covering a needle of the syringe after use. 
     BACKGROUND 
     Medication is often dispensed using a medicine cartridge, such as a syringe, having a barrel with a needle extending from one end and a plunger slidably inserted into the other end. Such cartridges are often referred to as “pre-filled syringes” because they may contain a specific dosage or volume of medication when they are initially provided, as compared to conventional syringes that are furnished empty and filled by the user before making an injection. 
     Alternatively, a medicine cartridge may be used, such as an ampoule or vial, that includes a penetrable seal instead of a needle on one end of the barrel, and/or a piston rather than a plunger on the other end. Such medicine cartridges are generally inserted into an adapter that includes a hollow body adapted to hold the cartridge, a plunger to engage and move the piston in the cartridge, and/or a double-ended needle to penetrate the seal and communicate with the interior of the barrel. 
     Because of the risk of communicable diseases, a number of syringes and adapters have been developed that are intended to prevent accidental needle sticks and/or inadvertent reuse of a syringe. Retractable needle devices have been suggested for this purpose that include a cartridge that allows a needle of the cartridge to be withdrawn into the barrel after medication is dispensed from it. For example, U.S. Pat. No. 4,973,316 issued to Dysarz discloses a syringe including a barrel having a needle assembly that is slidable within the barrel between an exposed position such that a needle on the assembly extends from the barrel and a retracted position wherein the needle assembly is withdrawn into the barrel. The needle assembly is initially locked in the exposed position, but may be disengaged upon depression of the plunger, whereupon a spring biases the assembly towards the retracted position, thereby withdrawing the needle into the barrel. 
     Alternatively, syringe holders have been suggested that include a body within which a conventional syringe or cartridge may be received, and a shield that is manually slidable with respect to the body to cover the needle. For example, U.S. Pat. No. 6,030,366, issued to Mitchell, which is assigned to the assignee of the present application, discloses a self-shielding guard that includes a body having an open proximal end for inserting a syringe into a cavity within the body, and a distal end with an opening through which a needle on the syringe may extend once received in the body. A shield is slidable over the body between retracted and extended positions to expose and cover the needle, respectively. With the shield in the retracted position and the needle exposed, an injection may be made, and then the shield may be manually advanced to the extended position. In the extended position, cooperating detents and detent pockets on the body and shield substantially permanently lock together, thereby preventing reuse of the needle, reducing the risk of accidental needle sticks, and/or facilitating disposal of the syringe. 
     As an alternative to requiring manual extension of a shield to cover a needle, spring-loaded devices have also been suggested. These devices often include a body and slidable shield, similar to the manual devices described above, but also may include a spring element to bias the shield to advance and cover the needle. An actuator, such as a button or lever, may be activated by the user to release the shield, thereby allowing the spring element to advance the shield to cover the needle. For example, U.S. Pat. No. 5,695,475 issued to Best, Jr. et al. and U.S. Pat. No. 4,923,447 issued to Morgan disclose spring-loaded syringe devices that include inner and outer sliding sleeves that include a button slidable in a longitudinal slot to selectively expose and cover a needle on the devices. A spring in the devices biases one of the sleeves to extend and cover the needle, but this bias may be manually overcome to expose the needle. Thus, these devices may not lock the extending sleeve in a covered position, and therefore may risk accidental needle exposure and/or reuse of the needle. In addition, although these devices are spring-driven, their shields may not extend unless they are manually activated by using a button, and therefore are not truly “passive,” but require an affirmative decision by a user to activate their safety feature. 
     Accordingly, an injection device that includes a needle shield for covering a needle after use would be useful. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to injection devices incorporating both a syringe, such as a pre-filled syringe, or a medicine cartridge, and a guard that covers a needle of the syringe after a medication in the syringe is injected into a patient. The present invention is also directed to methods for making and using such injection devices. 
     In accordance with one aspect of the present invention, an injection device is provided that includes a syringe having a proximal end, a distal end, and a plunger extending from the proximal end. The injection device also includes a guard having a proximal end, a distal end, and a cavity therebetween. The guard is slidably mounted to the syringe and includes a latch member that extends proximally from the guard. A needle may extend from the distal end of the syringe. The guard is biased from a first or retracted position wherein the needle of the syringe is exposed, towards a second or extended position wherein the guard covers the needle. The guard may be biased by a spring element located in the distal portion of the guard and/or coupled to the syringe. 
     First cooperating detents on the syringe and the guard act to retain the guard in the first position. The first cooperating detents may include a ledge on the proximal end of the syringe and a catch on the latch member extending from the guard. Here, the ledge and detent engage one another to retain the guard in the first position. As the plunger is advanced within the syringe, the plunger may engage the latch member and release the first cooperating detents, whereupon the guard may slide towards the second position. Second cooperating detents may be provided on the syringe and guard to retain the guard in the second position. The second cooperating detents may include a detent on the guard and a corresponding detent pocket on the syringe. 
     Optionally, the guard may also include a window or slot that extends axially along the guard. When the slot is present on the guard, the syringe may include a stop tab configured to travel within the slot. The stop tab and slot may be configured to limit the proximal and distal movement of the syringe relative to the guard. For example, the stop tab may abut the distal edge of the slot or window when the guard is in the first position to prevent further distal movement of the syringe (or proximal movement of the guard). Similarly, the stop tab may abut the proximal edge of the slot when the guard is in the second position to prevent further proximal movement of the syringe (or distal movement of the guard). In addition, the guard may also include a set of finger flanges, grips, or protrusions usable for controlling the speed of advancement or sliding of the guard between the first and second positions. 
     In accordance with another aspect of the present invention, an injection device includes a guard having a proximal end and a distal end. The guard may be configured for slidably engaging a syringe, and may be biased between a first or retracted position and a second or extended position relative to the syringe, e.g., by a spring element. The injection device may also include a syringe having a proximal end, a distal end including a needle, and a plunger that extends from the proximal end. The needle may be coupled to the distal end of the syringe, i.e., molded to the distal end or a hub may be provided on the distal end of the syringe to which a needle may be attached, e.g., by threads or a luer lock. A latch member extends proximally from the guard that is detachably coupled with the syringe, thereby retaining the guard in the first position. The latch member is engageable by the plunger as the plunger is depressed for decoupling the latch member from the syringe. When the latch member is decoupled, the guard may be advanced from the first position towards the second position. 
     In a preferred embodiment, the latch member may include an intermediate portion that is engageable with a flange on the syringe, and a proximal portion that is engageable with the plunger as the plunger is advanced into the syringe. When the plunger engages the proximal portion of the latch member, the intermediate portion of the latch member may be deflected and released from the flange of the syringe. Additionally, cooperating detents on the guard and the syringe may secure the guard once the guard is advanced to the second position. 
     In accordance with yet another aspect of the present invention, a method is provided for delivering a therapeutic agent using an injection device. The injection device may include a guard having a proximal end, a distal end, and a latch member extending from the proximal end. The injection device may also include a syringe received in the guard including a needle extending from a distal end thereof, and a plunger extending from a proximal end thereof. The guard may be biased from a first position, wherein the needle is exposed, towards a second position, wherein the needle is covered by the guard. 
     Initially, the latch member may retain the guard in the first position. The needle may be inserted into a patient&#39;s skin while the user holds a portion of the injection device. A distal force may be applied to the plunger to inject the medication into the patient until the plunger contacts the latch member. The plunger may be depressed further to deflect the latch member, whereupon the guard may be released from the first position. The plunger may then be depressed yet further to complete delivery of the therapeutic agent. The user may then withdraw the needle from the patient&#39;s skin, and release the plunger, whereupon the syringe may retract proximally with respect to the guard until the needle is disposed within the guard. When the guard attains the second position, cooperating detents on the guard and the syringe may engage one another to lock the guard in the second position and/or prevent subsequent distal movement of the syringe relative to the guard. 
     Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are side views of a preferred embodiment of an injection device, in accordance with the present invention. 
         FIGS. 2A and 2B  are side views of a syringe for use in the injection device of  FIGS. 1A and 1B . 
         FIG. 2C  is a perspective view of the syringe of  FIGS. 2A and 2B . 
         FIGS. 3A and 3B  are side views of a guard for use in the injection device of  FIGS. 1A and 1B . 
         FIG. 3C  is a perspective views of the guard of  FIGS. 3A and 3B . 
         FIGS. 3D and 3E  are end views of the guard of  FIGS. 3A and 3B , taken from distal and proximal ends of the guard, respectively. 
         FIG. 4A  is a cross-sectional side view of the injection device of  FIGS. 1A and 1B , with the guard in a retracted position relative to the syringe. 
         FIG. 4B  is a detail of the proximal end of the injection device of  FIG. 4A . 
         FIG. 4C  is a detail of the distal end of the injection device of  FIG. 4A . 
         FIGS. 5A-5C  illustrate an alternative spring element for use with an injection device, in accordance with the present invention. 
         FIGS. 6A-6B  illustrate another alternative spring element suitable for use with an injection device, in accordance with the present invention. 
         FIGS. 7A-7B  illustrate yet another spring element that may be used with an injection device, in accordance with the present invention. 
         FIGS. 8A-8C  illustrate one method of using an injection device of the present invention to perform an injection. 
         FIGS. 9A-9C  illustrate another method of using an injection device of the present invention to perform an injection. 
     
    
    
     DETAILED DESCRIPTION 
     Turning to  FIGS. 1A-4C , a preferred embodiment of an injection device  500  is shown for administering a medication, e.g., a vaccine, drug, or other therapeutic or diagnostic agent, to a patient. The device  500  includes a syringe  550  including a needle  556 , and a guard  502  slidably attached to the syringe  550 . The guard  502  is preferably biased from a first position wherein the needle  556  is exposed (shown in  FIG. 1A ) towards a second position wherein the guard  502  covers the needle  556  (shown in  FIG. 1B ). 
     As best seen in  FIGS. 2A-2C , the syringe  550  includes a barrel  552 , a distal end or hub  554  from which the needle  556  extends, and a plunger  553  inserted into a proximal end  522  of the barrel  552 . In one embodiment, the needle  556  is attached to the hub  554  during manufacturing, and the device  500  is provided to a user with the needle  556  pre-attached. In another embodiment, the hub  554  includes a connector, e.g., a thread or luer lock adaptor. Thus, the device  500  may be provided to a user without a pre-attached needle  556 , and the user may attach a desired needle  556  to the hub  554  before use. 
     Although a substantially square cross-section is shown, the barrel  552  may have a substantially cylindrical cross-section, rectangular cross-section, or other suitably shaped cross-section. As illustrated, one or more stop tabs  538  may be provided on the barrel  552 . The stop tabs  538  extend transversely outwardly, e.g., on opposite sides of the barrel  552 . The plunger  553  may include a thumb pad  551  on its proximal end to move the plunger  553  axially within the barrel  552 . The thumb pad  551  may have a diameter or other cross-section that is larger than a diameter of the barrel  552  of the syringe  550 . In addition, or alternatively, the plunger  553  may include an intermediate tab (not shown) located distally to the thumb pad  551 . 
     In the illustrated embodiment, one or more sets of ledges  540  may be provided, e.g., molded, on or near the proximal end  522  and on one or more side walls of the syringe  550 . Preferably, each ledge  540  has a sloping distal edge  540   a  and a substantially blunt proximal edge  540   b  defining a detent pocket adjacent the proximal edge  540   b.    
     In addition, the syringe  550  may include one or more sets of detent pockets  542 ,  543  adjacent the ledges  540 . A proximal set of detent pockets  543  may have substantially blunt proximal edge  543   b  and sloping or substantially blunt distal edges  543   a.  A distal set of detent pockets  542  may be provided at a location on the syringe  550  distal to the ledges  540  and/or adjacent the distal end  554  of the syringe  550 . Similar to the proximal detent pockets  543 , the distal detent pockets  542  may have substantially blunt proximal edges  542   b  and have sloping or substantially blunt distal edges  542   a.    
     Turning to  FIGS. 3A-3E , the guard  502  is a tubular member having an inner cross-section configured to slidably fit on the syringe  550 . For example, the inner cross-section of the guard  502  may be substantially rectangular, square, or cylindrical in cross-section, depending upon the cross-section of the syringe  550 . The guard  502  includes one or more side walls  561   a ,  561   b,  an open proximal end  562 , and an open distal end  563  defining a passage therebetween. Assembly tabs  572  with sloping or ramped interior surfaces  573  may be molded into and extend proximally from the side walls  561   a.  Optionally, at least one side wall  561   a,  e.g., opposing walls  561   a,  may include an elongate opening or slot  564  therethrough for receiving the stop tabs  538  and/or for facilitating observation of the syringe  550  received in the guard  502 . A pair of finger grips  515  may be located on the guard  502 , e.g., on opposite side walls  561   b.    
     One or more latch members or fingers  574  extend proximally from the guard  502 , e.g., molded to the side wall(s)  561   b.  Each latch member  574  includes an inwardly disposed catch or tab  576  located on an intermediate portion of the latch member  574  between the assembly tab  572  and a proximal tip  578  of the latch member  574 . The catches  576  and the ledges  540  may provide a set of cooperating detents for retaining the guard  502  in the first position. Alternatively, in embodiments where the syringe  550  lacks the ledges  540  (not shown), the catches  576  may engage with a flange on the proximal end  522  of the syringe  550 , or may engage the proximal end  522  itself, thereby defining a proximal set of cooperating detents that retain the guard  502  in the first position. Preferably, the latch members  574  are substantially flexible or semi-rigid, and may be curved such that the intermediate portions are disposed radially outwardly further than the proximal tips  578  when the latch members  574  are in a relaxed, unstressed state. Thus, if the proximal tips  578  are directed distally, the latch members  574  may partially bend or bow, thereby directing the intermediate portions, and consequently the catches  576 , transversely outwardly. 
     A plurality of detent arms  570  and a plurality of detents  571  are provided on the guard  502 , preferably molded directly to the side walls  561   b.  The detents  571  preferably have shapes corresponding substantially to the shapes of the distal detent pockets  542  in the syringe  550 . For example, distal edges  571   a  of the detents  571  may be ramped to facilitate slidable engagement with the proximal detent pockets  543  in the syringe  550 . Proximal edges  571   b  of the detents  571  are substantially blunt for positively engaging the proximal edges  542   b  of the distal detent pockets  542  and locking the guard  502  in the second position. Together, the detents  571  and distal detent pockets  542  at least partially define a second set of cooperating detents for securing the guard  502  in the second position. 
     As best seen in  FIGS. 1B, 4A, and 4C , the injection device  500  may include a spring element  504  for biasing the guard  502  towards the second position. The syringe  550  may include a substantially blunt distal surface  525  that is proximal from the hub  554 , and the guard  502  may include a substantially blunt inner collar  567 . The spring  502  may be positioned concentrically within the guard  502  such that respective ends of the spring  504  engage the distal surface  525  and the inner collar  567 . Thus, the guard  502  and syringe  550  together at least partially define an annular space within which the compression spring  504  may be compressed when the guard  502  is in the first position. Alternatively, other configurations may be provided for the spring element  504 , such as those shown in  FIGS. 5A-7B , and described further below. 
     The injection device  500  is pre-assembled such that the guard  502  is disposed in the first position, as illustrated in  FIG. 1A . In the first position, the catches  576  on the latch members  574  may engage the ledges  540  of the syringe  550  to retain the guard  502  in the first position. Specifically, the catches  576  on the latch members  574  engage the proximal edges  540   b  of the ledges  540  on the syringe  550 , as seen in  FIG. 1A , thereby retaining the guard  502  in the first position. In embodiments of the injection device  500  that lack the ledges  540 , the catches  576  may directly engage the proximal end  522  of the syringe  550  or a flange (not shown) on the syringe  550 . Optionally, in the first position, stop tabs  538  on the syringe  550  may abut the distal edge  568  of slots or windows  564  in the guard  502 , as shown in  FIG. 1A . Additional information on assembling the guard  502  and the syringe  550 , may be found in co-pending application Ser. No. 09/724,657, filed Nov. 28, 2000, the disclosure of which is expressly incorporated herein by reference. 
     Once assembled with the guard  502  in the first position, the injection device  500  may be used to inject medication from the barrel  552  into a patient (not shown). A needle cover (not shown) may be removed from the needle  556 , and the needle  556  inserted into the patient. Medication may then be delivered by directing the plunger  553  distally with the user&#39;s thumb until the thumb pad  551  contacts the tips  578  of the latch members  574 . While using the injection device  500 , the user preferably places a finger on each finger grip  515  and the thumb on the thumb pad  551 . As the plunger  553  is depressed further, the thumb pad  551  may cause the latch members  574  to compress axially, thereby deflecting the intermediate portions radially outwardly until the catches  576  are disengaged from the ledges  540  on the syringe  550 . 
     With the ledges  540  and catches  576  disengaged, the spring element  504  automatically biases the guard  502  distally towards the second position, i.e., to advance the distal end  563  of the guard  502  over the needle  556 . The syringe  550  may be maintained substantially stationary relative to the patient&#39;s skin, and the plunger  553  may be depressed further, if necessary, to complete the injection. When the catches  576  are disengaged, the guard  502  may advance until it contacts the patient&#39;s skin, and the needle  556  may be withdrawn from the patient, allowing the guard  502  to extend entirely over the needle  556 . As the guard  502  advances, the latch members  574  may slide distally along the syringe  550 . 
     When the needle  556  is withdrawn from the patient, the guard  502  may advance fully to the second position such that the guard  502  passes over and substantially covers the needle  556 . In the second position, shown in  FIG. 4A , the detents  571  of the guard  502  enter and substantially engage the distal detent pockets  542  of the syringe  550 , thereby preventing subsequent proximal movement of the guard  502 . Stop tabs  538  on the syringe may also abut the proximal edges  566  of the slots  564 , thereby preventing further distal movement of the guard  502 . Thus, the guard  502  may be substantially permanently locked in the second or extended position, thereby preventing reuse of the injection device  500  and/or facilitating safe disposal of the injection device  500 . 
     In the second position, the spring element  504  may remain slightly compressed to provide a distal force that may ensure that the guard  502  is fully advanced and the detents  571  are received in the distal detent pockets  542 . 
     In further alternatives, additional elements may be provided on the syringe  550  and/or guard  502  to control the speed of advancement of the guard  502  from its initial activation in the retracted position, but allow free advancement as the guard  502  approaches the extended position. For example, a frictional material (not shown) may be applied to one or more contact surfaces between the syringe  550  and guard  502  that may increase the frictional resistance in the retracted position. An adhesive tape or viscous material (not shown) may be provided that peels off of, is cut or torn, or otherwise separates from the syringe  550  and/or guard  502  after partial advancement of the guard  502 . One or more “speed bumps” or similar protruding structures (not shown) may be molded into or otherwise provided on the syringe  550  and/or guard  502  that may be contacted when the guard  502  is in or close to the retracted position. A feature on one of the syringe  550  or guard  502  may travel through a corresponding tortuous feature on the other of the syringe  550  or guard  502  to slow the guard  502  down initially. In a further alternative, a spring or cushioned pad (not shown) may be provided on the distal end of the guard  502  that may soften the force with which the guard  502  contacts the patient if the guard  502  is allowed to advance. 
     In the embodiment described above, the spring element  504  is a helical compression spring. Alternatively, one or more springs (not shown) may be provided, e.g., in one or more corners of the guard  502 . For example, springs may be provided in diagonally opposite corners of the guard  502  in order to minimize the risk of subjecting the syringe  550  to torque as it advances from the retracted towards the extended position as may occur if only a single off-center spring is used. In another alternative, one or more helical extension springs  504   a  may be provided that have respective ends that are secured to the syringe  550  and the guard  502 , as is shown schematically in  FIGS. 5A-5C . The extension spring  504   a  may be provided between the guard  502  and the syringe  550 . The extension spring  504   a  may be under tension when the guard  502  is disposed in the retracted position, thereby biasing the guard  502  towards the extended position. 
     Other alternative spring elements may also be provided that may be coupled to the guard  502  and syringe  550 .  FIGS. 6A and 6B  illustrate an embodiment of the present invention having a coil spring element  504   b  that has one end mounted around a hub  584   a  on the guard  502  (attachment point of hub  584   a  to the guard  502  not shown) and the other end secured to the syringe  550 . The coil spring element  504   b  may unwind as the guard  502  is retracted to the retracted position, as in  FIG. 6A , but is biased to wind back around the hub  584   a  to advance the guard  502  towards the extended position, which is shown in  FIG. 6B . In another alternative, illustrated in  FIGS. 7A and 7B , a leaf spring element  504   c  may be provided that has ends fixed to one of the guard  502  and the syringe  550  for distally biasing a hub  584   b  coupled to the other of the syringe  550  and the guard  502  (attached point of hub  584   b  to body  502  not shown). The guard  502  may be retracted, thereby deflecting the leaf spring element  504   c,  as in  FIG. 7A , but when the guard  502  is released, the hub  584   b  and consequently the guard  502  may be advanced to the extended position as the leaf spring element  504   c  attempts to return to its relaxed, undeflected state, as in  FIG. 7B . 
     In addition, if desired, any of the spring elements  504 - 504   c  may have a nonlinear spring rate, preferably having a spring rate when the guard  502  is disposed in the retracted position that is substantially more or less than its spring rate when the guard  502  approaches the extended position. A nonlinear spring rate may be useful to provide an injection device  500  that is “softer” upon activation, i.e., such that the spring element ( 504 - 504   c ) may apply a substantially lower force to the guard  502  when it is initially released from the retracted position than when it approaches the final extended position. For example, the thickness of the spring material may be varied along its length, different materials may be used for different portions of the spring, and the like. This “softer” spring may minimize the force with which the guard  502  contacts a patient being injecting using the injection device  500 , and thereby reduce the discomfort or anxiety that may be experienced by the patient. For example, the guard  502  may be activated with a softer force, such that the guard  502  more gently contacts the patient, but when the needle is withdrawn from the patient, a stronger force may be applied to the guard  502  to ensure that it advances completely to the extended position. 
     Turning to  FIGS. 8A-8C , another method is shown for performing an injection using an injection device  500 . Unlike the previous embodiments, which generally discuss a guard advancing over a syringe, this method involves holding the guard  502  such that the syringe  550  may be automatically and/or controllably directed proximally to retract the needle  556  into the guard  502 . The guard  502  and syringe  550  are slidably attached to each other, similar to the previous embodiment. The guard  502  includes a spring element  504 , similar to the previous embodiments, for biasing the syringe  550  towards a second position wherein the guard  502  substantially covers the needle  556 . 
     As best seen in  FIG. 8A , a needle cover (not shown) may be removed from the needle  556 , and the needle  556  may be inserted into a patient&#39;s skin  700 . A user may grasp finger grips  515  or otherwise grip an outer surface of the guard  502 . Thus, the guard  502  may be grasped to hold the injection device  500  substantially stationary with respect to the skin  700  during the injection. 
     To deliver medication from the syringe  550 , the user may grip the finger grips  515  and apply a distal force to the plunger  553 , preferably until thumb pad  551  engages latch members  574  extending from the guard  502 . As the plunger  553  is depressed further, the thumb pad  551  may cause the latch members  574  to compress axially and thereby deflect radially outwardly until the catches  576  (not shown in  FIGS. 8A-8C ) are disengaged from the ledges  540  (also not shown) of the syringe  550 , or are disengaged from the proximal end  522  if the syringe  550  does not include the ledges  540 . With the catches  576  disengaged, the spring element  504  may automatically bias the syringe  550  towards the second or retracted position, shown in  FIG. 8C . 
     Once the ledges  540  or the proximal end  522  are disengaged from the catches  576 , the user may gradually remove the distal force applied to the plunger  553 , allowing a controlled retraction of the syringe  550  to the retracted position. As the syringe  550  retracts to the retracted position, the guard  502  may remain substantially stationary with respect to the patient&#39;s skin  700 , and the needle  556  may be withdrawn from the patient and into the guard  502 , as shown in  FIGS. 8B and 8C . Once in the fully retracted position, cooperating detents  571 ,  542  on the guard  502  and syringe  550  engage one another to substantially and permanently lock the guard  502  over the needle  556 . Thus, the user may hold the finger grips  515  to position the injection device  500  a predetermined distance way from the patient&#39;s skin  700 , thereby avoiding any sudden contact between the guard  502  and the patient&#39;s skin  700  when the ledges  540 , or proximal end  522 , of the syringe  550  and the catches  576  are disengaged. 
     Alternatively, as shown in  FIGS. 9A-9C , the user may hold the injection device  500  using finger grips  515 , insert the needle  556  into the patient&#39;s skin  700 , and then inject medication by applying a distal force to the plunger  553  until the thumb pad  551  engages a latch member  574  extending from the guard  502 . As described above, further depression of the plunger  553  deflects the latch member  574  to disengage the catches  576 , and release the guard  502 . The guard  502  may then automatically advance into contact with the patient&#39;s skin  700 , as shown in  FIG. 9B . The plunger  553  may be further advanced to complete the injection. Preferably, because the spring element  504  is not fully expanded, the syringe  550  remains biased to retract with respect to the guard  502  towards the retracted position. At this point, the user may gradually remove the distal force applied to the plunger  553 , allowing controlled retraction of the syringe  550  to the retracted position, as shown in  FIG. 9C . Once the syringe  550  retracts to the retracted position, the cooperating detents  571 ,  542  may engage one another to substantially permanently lock the guard  502  over the needle  556 , similar to the embodiment discussed above. 
     While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.