Abstract:
An autoinjector including: a housing, in which can be mounted a syringe ( 28 ) having a plunger, a barrel and needle ( 19 ) at one end thereof through which medicament can be delivered to an injection site,—a syringe support means ( 30 ) capable of causing said syringe to move along an axial path with respect to said housing; biasing means capable of biasing said syringe support means so that said needle is normally inside said housing; and engagement means ( 180 ) selectively moveable by a user from a first position in which rearward movement of the syringe along said axial path is substantially prevented when said syringe is in an extended position where at least part of said needle is out of the housing, to a second position in which rearward movement of the syringe along said axial path relative to said housing is possible and said biasing means causes said needle to retract inside said housing; wherein said housing includes an outer housing ( 16 ) and said autoinjector further comprises a sleeve ( 18 ) intermediate the syringe support means and said outer housing that is capable of moving the engagement means between said first and second positions by rotating said sleeve about a longitudinal axis of said autoinjector between a first rotational position and a second rotational position; and wherein the sleeve is moveable between said first and second rotational positions by moving said outer housing with respect to the sleeve in a substantially axial direction.

Description:
[0001]    This invention relates to the field of autoinjectors for delivering a dose of medicament and, in particular, the invention relates to autoinjectors having a mechanism to control the retraction of the needle from an injection site. 
       BACKGROUND 
       [0002]    An autoinjector is an automatic injection device designed to facilitate automated delivery of a dose of medicament to a patient through a hypodermic needle, the injection usually being administered by the patient themselves. An autoinjector works, for example, by delivering an injection automatically upon actuation by the patient pressing a button, moving a lever or part of a housing etc. This is in contrast to a conventional manual syringe where a plunger must be manually depressed into a barrel containing medicament in order to effect the injection. The terms “autoinjector” and “injection device” are used interchangeably in the following description. 
         [0003]    Examples of autoinjectors are described in WO2003/099358 (Seedlings Life Science Ventures LLC) and WO01/93926 (Mayo Foundation for Medical Education and Research). These are both generally flat devices which are of small size to encourage users to carry the device with them for ready access. GB2396298 (PA Consulting Services Ltd) is an example of a more conventionally-shaped elongate autoinjector, but of relatively complex internal construction. 
         [0004]    All three of the above prior art devices have a custom designed medicament chamber therein rather than being built around a standard pre-filled syringe presentation. The custom medicament chamber, although allowing for a compact overall size for the device, means that the device as whole must be subjected to more rigorous regulatory control as compared with a device containing a standard pre-filled syringe presentation which will have already obtained regulatory approval. 
         [0005]    In general, an autoinjector includes a needle which is located within the housing of the device. Upon activation of a force-generating source, a portion of the needle extends out of the housing and penetrates the outer layer of skin to deliver medicament. In some known autoinjectors, after activation, a needle cover or needle shield moves forward to conceal the needle after use. In GB2396298, the needle automatically retracts back into the housing by means of a biasing spring. 
         [0006]    An improved autoinjector is described in our international patent application, published under number WO 2005/070481. This device requires that the needle is moved axially so that it can appear beyond the end of the nozzle for the duration of the injection, after which the needle retracts automatically, so that it is never in sight of the user. The device also requires that the plunger is moved axially so that medicament is ejected. The overall complexity of the autoinjector is significantly reduced by both of these requirements being effected by one component, namely an inner housing, and the device has the significant advantage that it can be built around a conventional or standard syringe presentation. 
         [0007]    The injection device of WO 2005/070481 is designed to be used in conjunction with a standard drug presentation e.g. a pre-filled syringe comprising a needle, a barrel pre-filled with medicament and a plunger. The plunger may include a separately-provided plunger rod. As mentioned above, there is a significant commercial advantage in being able to use a standard pre-filled syringe, which will have been subjected to numerous clinical trials, drug stability studies and regulatory approval. Any modification to the standard syringe may require further trials and approval, adding delay and expense. 
         [0008]    An improved autoinjector device is described in WO 2009/127861 (The Medical House Limited) wherein the needle does not automatically retract back into the device immediately after a dose of medicament has been delivered. Instead, in the device of WO 2009/127861, the user selectively actuates the retraction of the needle when desired. This may be by removing the device from the injection site, or by actuating some other trigger means. It is known that, in some cases, automatic retraction is undesirable as excessively quick retraction of the needle after delivery may cause the medicament to escape from the puncture hole created by the needle in the tissue. This is especially true if the liquid has not dispersed into the tissue and is in a “pressurised bolus” at the point of delivery which is a known problem when injecting adrenaline, for example. The device of WO 2009/127861 therefore mitigates these problems by allowing the retraction of the needle to be manually controlled or at least delayed. 
         [0009]    It is an object of the present invention to provide an alternative and/or improved solution to the above-mentioned known problems associated with automatic retraction autoinjector devices. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0010]    In accordance with the present invention there is provided an autoinjector including
       a housing, in which can be mounted a syringe having a plunger, a barrel and needle at one end thereof through which medicament can be delivered to an injection site;   a syringe support means capable of causing said syringe to move along an axial path with respect to said housing;   biasing means capable of biasing said syringe support means so that said needle is normally inside said housing,   engagement means selectively moveable by a user from a first position in which rearward movement of the syringe along said axial path is substantially prevented when said syringe is in an extended position where at least part of said needle is out of the housing, to a second position in which rearward movement of the syringe along said axial path relative to said housing is possible and said biasing means causes said needle to retract inside said housing;   wherein said housing includes an outer housing and said autoinjector further comprises   a sleeve intermediate the syringe support means and said outer housing that is capable of moving the engagement means between said first and second positions by rotating said sleeve about a longitudinal axis of said autoinjector between a first rotational position and a second rotational position;   wherein the sleeve is moveable between said first and second rotational positions by moving said outer housing with respect to the sleeve in a substantially axial direction.       
 
         [0018]    Therefore, the present invention facilitates controlled retraction of the needle by movement of the outer housing in an intuitively substantially axial direction. The rotational component, i.e. the sleeve, is within the outer housing, so the user does not feel or see any rotating components. Thus, the autoinjector of the present invention may appear to the user to be substantially identical to the autoinjector of WO 2005/070481, whilst affording the user control over the retraction of the needle. 
         [0019]    In a preferable embodiment, said housing further includes
       an inner housing, at least part of which is positionable, in use, intermediate the outer housing and the syringe support means; and   a rear housing intermediate said outer housing and said inner housing, and wherein said autoinjector further comprises   an energy source in communication with said inner housing, the inner housing being moveable by the energy source between three positions, namely   a first position in which the inner housing is in communication with the barrel such that, in use, the plunger and barrel are movable axially so as to move at least part of said needle out of the outer housing;   a second position in which the inner housing is in communication with the plunger but not the barrel such that, in use, said plunger is movable axially into said barrel so as to expel medicament through the needle; and   a third position in which the inner housing is in communication with neither the plunger nor the barrel such that, in use, the plunger and barrel are able to retract in order to retract the needle into the outer housing.       
 
         [0026]    In this preferable embodiment, the needle penetration, medicament delivery and needle retraction are all controlled by a single component (i.e. the inner housing), making the operation of the device particularly reliable and efficient. In addition to the inner housing being out of communication with the plunger and barrel, the blocking means must also be in the second position for needle retraction to take place. 
         [0027]    Preferably, said engagement means is selectively moveable by a user from said first position to said second position by the release or reduction of a previously-applied forward force. Further preferably, said engagement means is selectively moveable by a user from said first position to said second position by the release or reduction of the previously-applied forward force after delivery of a partial, but not necessarily complete, dose of medicament. Said engagement means preferably forms part of said sleeve. Therefore, in this preferable embodiment, the needle can be retracted before an entire dose of medicament has been delivered. 
         [0028]    In said first position said blocking means preferably engage with the syringe support means to substantially prevent rearward movement of the syringe along said axial path. Preferably, said sleeve comprises biasing means for urging the sleeve towards the second rotational position. 
         [0029]    Preferably, said housing further comprises a front housing, said front housing being disposed at least partially within the outer housing at a forward end of the autoinjector; and wherein engagement between said syringe support means and said front housing restricts movement of the syringe support means to axial movement relative to the front housing. 
         [0030]    Preferably, said front housing has a plurality of axial slots and the syringe support means includes projections that engage with said axial slots to restrict said relative movement between the syringe support means and the front housing to relative axial movement. 
         [0031]    Said sleeve preferably comprises a plurality of legs projecting axially forwardly, wherein said legs each have a foot projecting radially inwardly. 
         [0032]    Said syringe support means preferably comprises a plurality of tabs projecting radially outwardly and said tabs are axially aligned with said feet when said sleeve is in said second rotational position. 
         [0033]    Preferably, forward axial movement of the syringe support means relative to said sleeve is substantially prevented by abutment between said tabs and said feet when said sleeve is in said second rotational position and said tabs are disposed axially rearwards of said feet. 
         [0034]    Said tabs preferably comprise guide ribs that extend in both an axially rearward direction and a circumferential direction from said tabs, where, during assembly of the autoinjector, said guide ribs are capable of abutting said feet and rotating said sleeve away from said second rotational position so that said feet can move from a position axially rearward of said tabs to a position axially forward of said tabs. Preferably said sleeve is capable of rotating back to said second rotational position when said feet are positioned axially forward of said tabs. Said feet are preferably generally wedge-shaped. 
         [0035]    Preferably, said engagement means include blocking means which comprise said feet and a plurality of flexible beams are each connected to said syringe support means at a proximal end and extend both radially outwardly and axially rearwardly to a distal end, where said beams are axially aligned with said feet when said sleeve is in said first rotational position, such that in use, as the syringe support means moves axially forward relative to the sleeve, said beams are capable of contacting said feet and deflecting radially inwardly until said distal ends are axially forwards of said feet and when positioned axially forwards of said feet said beams are capable of flexing radially outwards to a position where said distal ends are axially aligned with or radially beyond said feet. 
         [0036]    Said inner housing preferably comprises a set of rear tags and a set of forward tags where said rear tags are capable of communicating with the plunger and said forward tags are capable of communicating with the barrel. Said rear tags and said forward tags are preferably radially flexible. 
         [0037]    Preferably, said sleeve comprises a plurality of longitudinal ribs on an inner surface, and an outer surface of each rear tag of said inner housing has a radially extending abutment element where said abutment elements are axially aligned with said longitudinal ribs when said sleeve is in said first rotational position. Said abutment elements are preferably tapered in a circumferential direction. Said engagement means preferably include the rear tags, wherein abutment between said longitudinal ribs and said abutment elements preferably causes said rear tags to flex radially inwardly into communication with said plunger when said sleeve is in said first rotational position. Said communication between said rear tags and said plunger is preferably radial gripping of the rear tags on the plunger, or, additionally or alternatively, the communication is axial blocking of the plunger by the rear tags, where the rear tags are axially aligned with and axially rearward of the plunger. In either case, said longitudinal ribs are preferably tapered at a rear end in an axial direction. 
         [0038]    Preferably, said rear housing comprises apertures and said front tags are capable of flexing radially outwardly into said apertures into said second position where said inner housing is not in communication with said barrel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Preferred embodiments of the present invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which: 
           [0040]      FIGS. 1A and 1B  show mutually orthogonal cross-sectional views of an autoinjector device according to the present invention; 
           [0041]      FIGS. 2A and 2B  show side views of the autoinjector corresponding to  FIGS. 1A and 1B  respectively, where the outer packaging and outer housing have been removed for clarity; 
           [0042]      FIG. 3  shows a perspective view of the rotating sleeve; and 
           [0043]      FIGS. 4 and 5  show side views of the autoinjector of  FIGS. 1 and 2 , where certain components have been removed to show the syringe holder. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Throughout the following description, reference to a “forward” direction means the direction which is towards the patient when the injection device is in use. The “forward” end of the injection device is the end nearest the patient&#39;s skin when the device is in use. Similarly, reference to a “rearward” direction means the direction which is away from the patient and the “rearward” end of the device is the end furthest from the patient&#39;s skin when the injection device is in use. 
         [0045]    An exemplary embodiment  10  of an autoinjector according to the present invention is shown  FIGS. 1A and 1B . The autoinjector  10  contains a syringe  28  having a needle  19  and may be actuated by a user to automatically deliver a dose of medicament to them self or another patient. 
         [0046]    The autoinjector  10  is initially stored in an outer packaging  12  and a rear screw cap  14  screwed thereto via a screw thread  14   b . The rear screw cap  14  surrounds a rear end cap  15  and has radial apertures  14   a  that radial projections  15   a  of the rear end cap  15  are located in. The rear end cap  15  is secured to an outer housing  16  of the autoinjector  10  via engagement between circumferential ribs  15   b  of the rear end cap  15  and complementary grooves  16   a  of the outer housing  16 . When the ribs  15   b  are engaged with the grooves  16   a , part of the rear end cap  15  is radially constrained between the outer housing  16  and the outer packaging  12 , and the rear end cap  15  cannot disengage from the outer housing  16 . 
         [0047]    To remove the autoinjector  10  from the outer packaging  12 , the user unscrews the rear screw cap  14  relative to the outer packaging  12 . Because the rear end cap  15  is connected to the rear screw cap  14  and the rear end cap  15  is prevented from disengaging from the outer housing  16 , the unscrewing of the rear screw cap  14  from the outer packaging  12  causes the outer packaging  12  to move axially forwardly relative to the rear screw cap  14  and the outer housing  16 . Internal axial ribs  12   a  on the outer packaging  12  are guided in axial channels  24   a  (see  FIGS. 2A and 2B ) of a front housing  24  of the autoinjector  10  as the outer packaging  12  is unscrewed from the rear screw cap  14 . The axial channels  24   a  prevent rotation of the axial ribs  12   a  (and therefore outer packaging  12 ) relative to the front housing  24  as the outer packaging  12  is unscrewed from the rear screw cap  14 . 
         [0048]    The forward end of the outer packaging  12  is attached to a needle sheath remover  13  which extends axially rearwardly from the front of the device. The needle sheath remover  13  has barbed legs  13   a  that are adapted to engage a needle sheath  17  into which the needle  19  of the device is staked in. As the outer packaging  12  moves axially forwardly relative to the outer housing  16  and front housing  24 , the barbed legs  13   a  of the needle sheath remover  13  are drawn radially inwardly due to contact with an inward boss  24   b  of the front housing  24 . As the barbed legs  13   a  are drawn radially inwardly, they bite into the needle sheath  17  and engage it such that further axial movement of the needle sheath remover  13  causes the needle sheath  17  to be axially withdrawn from the needle  19 . Since the outer packaging  12  is prevented from rotating relative to the front housing  24  and outer housing  16  when being unscrewed from the rear screw cap  14 , substantially no torque is transferred to the needle sheath remover  13 , needle sheath  17  and, importantly, the needle  19 , thereby minimising the risk of damage to the needle  19 . 
         [0049]    The syringe  28  does, however, experience a forward axial force as the needle sheath  17  is removed (since the sheath  17  is fitted over a front portion of the syringe  28 ). This tends to pull the internal components of the device (which are spring loaded) axially forwards and could result in damage if not controlled. In the present invention, this problem is mitigated by preventing the syringe  28  from moving axially forwards when the needle sheath  17  is removed. The syringe  28  is disposed in a syringe holder (syringe support means)  30  which supports the syringe  28  at a forward end and has a pair of tabs  31  projecting radially outwardly. The device further includes a rotating sleeve  18 , intermediate the outer housing  16  and a rear housing  20  of the device, which is shown in more detail in  FIGS. 2A ,  2 B and  3 . The rotating sleeve  18  has a pair of radially inwardly projecting feet  18   a  which are each disposed at the end of an axially extending leg  18   b  of the rotating sleeve  18 . As described below, the feet  18   a  act as blocking means which prevent the premature retraction of the needle  19 . When assembled, the tabs  31  of the syringe holder  30  are axially aligned with the feet  18   a  of the rotating sleeve  18  so that forward axial movement of the syringe holder  30  relative to the rotating sleeve  18  is prevented which substantially prevents the syringe  18  moving axially forwards when the needle sheath  17  is removed. Consequently, the risk of damaging the internal components of the device when the needle sheath  17  is removed is significantly reduced. 
         [0050]    Once the rearmost part of the outer packaging  12  moves axially past the front most part of the rear end cap  15 , the ribs  15   b  can disengage from the grooves  16   a  (i.e. flex radially outwardly as they ride out of the grooves) and the rear screw cap  14  and rear end cap  15  can be removed from the device. Once the rear end cap  15  has been removed, the safety cap  26  can also be removed. The safety cap  26  has an axially forwardly projecting pin  26   a  that, when assembled, is located in an aperture  20   a  of a rear housing  20  of the device. In this position, the pin  26   a  also passes between radially flexible legs  16   b  of the outer housing  16 , and prevents the legs from flexing radially inwardly thereby preventing them from passing through the aperture  20   a  of the rear housing. Due to this restriction, when the safety cap  26  is in place, the device cannot be actuated. Once the safety cap  26  is removed, the flexible legs  16   b  of the outer housing  16  can flex radially inwardly and are able to pass through the aperture  20   a  of the rear housing  20  thereby permitting axial movement of the outer housing  16  relative to the rear housing  20 , which is necessary for actuation of the device. A hook  16   c  projecting axially forwardly from the rear of the outer housing  16  restricts rearward axial movement of the outer housing  16  relative to the rear housing  20  and prevents the two components from separating. 
         [0051]    Turning to  FIGS. 2A and 2B , the rotating sleeve  18  is shown to have a sloped cut-out  18   c  in its rear end. The rotating sleeve  18  has a second identical sloped cut-out  18  diametrically opposite the one shown in the drawings. A pair of internal axially extending ribs  16   c  (see  FIG. 1B ) of the outer housing  16  are disposed in a pair of slots  20   b  of the rear housing and, prior to actuation of the device, are each in axial alignment with a shallow portion  18   c ′ of one of the sloped cut-outs  18   c . The rotating sleeve  18  also has a circumferentially flexible beam  18   d  extending in both an axial and a circumferential direction. Prior to actuation of the device, the beam  18   d  is in a relaxed state and the frontmost end of the beam  18   d  is in contact with a radial abutment  20   c  on the outer surface of the rear housing  20 . The feet  18   a  of the rotating sleeve  18  are each disposed in a circumferential slot  20   d  in the rear housing  20 . 
         [0052]    To actuate the device, the user places the front end of the device against the injection site and pushes the outer housing  16  axially forwards towards the injection site. With the safety cap  26  removed, the outer housing  16  can move axially forwards relative to the rear housing  20  and as it does so, the ribs  16   c  of the outer housing  16  move axially along the slots  20   b  in the rear housing  20  and contact the shallow portion  18   c ′ of the sloped cut-outs  18   c . Continued axial forward movement of the outer housing  16  causes the ribs  16   c  to slide down the sloped cut-outs  18   c  towards deep portions  18   c ″. However, given that the outer housing  16  is rotatably fixed with respect to the rear housing  20  due to engagement of the ribs  16   c  in the slots  20   b , the contact between the ribs  16   c  and the sloped cut-outs causes the rotating sleeve  18  to rotate relative to the outer housing  16  and rear housing  18  from an initial (“second rotational”) position to a fully rotated (“first rotational”) position. In the fully rotated position, the beam  18   d  is flexed and stressed against abutment  20   c  which tends to bias the rotating sleeve  18  back to its initial position (although rotation back to the initial position is prevented by interference with the ribs  16   c  of the outer housing  16 ). When rotated, the feet  18   a  of the rotating sleeve  18  are out of axial alignment with the tabs  31  of the syringe holder  30  thereby permitting forward axial movement of the syringe holder  30  (and therefore, syringe  28 ) relative to the rear housing  20 . Additionally, in this fully rotated position, internal axial ribs  18   e  of the rotating sleeve  18  are in axial alignment with radial abutment elements on rear legs (or tags)  22   b  of an inner housing  22  of the device, which is disposed within the rear housing  20 . The forwardly moving ribs  16   c  also contact and move front legs (or tags)  22   a  of the inner housing  22  radially inwardly. In moving radially inwardly, the front legs  22   a  of the inner housing  22  move into axial alignment with the barrel of the syringe  28  and unlatch the inner housing  22  from the rear housing  20 . 
         [0053]    Unlatched, the inner housing  22  is free to move axially forwards under the influence of the drive spring (not shown). Given that the front legs  22   a  of the inner housing  22  are in axial alignment with the barrel of the syringe  28 , continued forward movement of the inner housing  22  (under the influence of the drive spring—not shown) causes the syringe  28  and plunger rod  21  to move axially forwardly with no relative movement therebetween. This action drives the needle  19  out of the front of the device and into the injection site without expelling any medicament. As the inner housing  22  moves axially forwards, rear legs  22   b  of the inner housing  22  are drawn radially inwards (into contact with a plunger rod  21  of the syringe  28 ) by interference between the ribs  18   e  of the rotating sleeve  18  and the radially outwardly extending abutment elements on the rear legs  22   b . At or around this time, the front legs  22   a  of the inner housing  22  become radially aligned with slots  20   d  in the rear housing  20  and flex radially outwardly (due to their inherent bias towards that position) out of engagement with the barrel of the syringe  28 . Further forward axial movement of the inner housing  22  causes the rear legs  22   b  of the inner housing  22  to move the plunger rod  21  axially forwardly relative to the barrel of the syringe  28  and expel medicament from the needle  19  into the injection site. 
         [0054]    As shown in  FIGS. 4 and 5 , the syringe holder  30  includes a pair of flexible wings (or beams)  32  that are axially rearward of tabs  31  and circumferentially offset therefrom. Each flexible wing  32  extends both axially rearwardly and radially outwardly, when in a relaxed state. When the rotating sleeve  18  is rotated to its fully rotated position, the feet  18   a  of the rotating sleeve are each in axial alignment with one of the flexible wings  32  of the syringe holder  30 . Guide arms  33  on the syringe holder  30  locate in slots  24   c  (see  FIG. 1  B) of the front housing  24  to only permit axial relative displacement between the syringe holder  30  and front housing  24 . Additionally,  FIGS. 4 and 5  show the tabs  31  in greater detail which each extend circumferentially on the surface of the syringe holder  30 . Each tab  31  also has a “guide rib” sloped section  34 , which extends both circumferentially and axially rearwardly on the surface of the syringe holder  30 . The shape of the tabs  31  and guide ribs  34  permits efficient assembly of the device, whereby the feet  18   a  of the rotating sleeve  18  are able to ride down the guide ribs  34  during installation onto the rear housing  20 . Given that the beam  18   d  becomes stressed against the abutment  20   c  on the rear housing  20  as the feet  18   a  ride along the guide ribs  34 , the rotating sleeve  18  immediately rotates back to a relaxed position once the feet  18   a  are axially forwards of the tabs  31  so that the feet  18   a  are in axial alignment with the tabs  31 . 
         [0055]    As the inner housing  22  moves the syringe holder  30  axially forwards, the flexible wings  32  contact the feet  18   a  of the syringe holder  30  and flex radially inwardly. Further forward movement of the syringe holder  30  relative to the rotating sleeve  18  causes the flexible wings  32  to move axially forwards of the feet  18   a  and out of radial alignment with them, so that the wings  32  can flex radially outwardly once more. In this position, the syringe holder  30  is prevented from moving axially rearwardly relative to the rotating sleeve  18  (and therefore the rear housing  20 ) by axial abutment between the flexible wings  32  and the feet  18   a  of the rotating sleeve. It is this abutment that prevents automatic retraction of the syringe  28  occurring (under the influence of a retraction spring—not shown). 
         [0056]    The inner housing  22  will continue to move the plunger rod  21  axially forwardly relative to the syringe  28  (expelling medicament from the needle  19  all the while), until the front legs  22   a , which are moving along slots  20   d  of the rear housing  20 , reach the forward ends of the slots  20   d . This arrests movement of the inner housing  22  and therefore arrests further forward movement of the plunger rod  21  relative to the syringe  28  caused by the force of the drive spring. However, although the device has ceased to deliver the medicament, the needle  19  is still protruding from the front of the device, and the return spring (not shown) will begin to act rearwardly on the syringe  28 . Since the inner housing  22 , biased forwardly by the drive spring, is stationary, any rearward movement of the syringe  28  would cause some further medicament to exit the needle  19 , which is not desirable. In the embodiment of the invention shown in the drawings, the axial engagement between the wings  32  of the syringe holder  30  and the feet  18   a  of the rotating sleeve  18  prevent rearward movement of the syringe  28  and prevent any further medicament from being expelled through the needle  19 . This embodiment has the advantage that the device can deliver a partial dose of medicament, i.e. not discharge the entire contents of the syringe  28  (such as a discharge 0.3 ml from a total volume of 1 ml). The length of the slots  20   d  in the rear housing  20  can determine the volume of the delivered dose by determining the axial position that forward axial movement of the inner housing is arrested at. The axial engagement (axial “blocking”) of the wings  32  and feet  18   a  and the radial engagement (radial “gripping”) of the rear legs  22   b  of the inner housing  22  on the plunger rod  21  both serve to prevent retraction of the needle after the dose (full or partial) has been delivered. 
         [0057]    In alternative embodiments, it may be desirable to allow the stopper on the front end of the plunger rod  21  to reach the forward end of the barrel of the syringe  28  and deliver the entire dose of medicament (unlike the embodiment described above with reference to the drawings). In these embodiments, the forward axial movement of the inner housing  22  will be arrested when the stopper “bottoms out” at the front end of the barrel of the syringe  28 . Therefore, once movement of the inner housing  22  has arrested, the forward axial force from the drive spring continues to act on the syringe  28  and syringe holder  30  through the inner housing  22  and plunger rod  21 . Consequently, the return spring is unable to move the syringe holder  30  and syringe  28  rearwards (since the forward force from the drive spring is greater in magnitude than the rearward force from the retraction spring). Regardless of rearward movement of the syringe  28 , there is no risk of continued expulsion of medicament from the needle  19  since the entire dose has already been delivered. It follows that in these embodiments, the wings  32  and feet  18   a  are not necessarily essential to the operation of the invention, although they may still be employed to provide additional engagement means for preventing retraction of the needle  19 . Absent the wings  32  and feet  18   a , the engagement means are provided solely by the engagement of the rear legs  22   b  of the inner housing  22  on the plunger rod  21 . The rear legs  22   b  may engage the plunger rod  21  by radially gripping it, as shown in the drawings, or by axially blocking it, by moving radially to a point in axial alignment and axially rear of the plunger rod  21 . 
         [0058]    When retraction is desired, the user removes the forward pressure on the outer housing  16  thus allowing the outer housing  16  to move rearwardly relative to the rear housing  20  to its initial position, under the influence of a spring or other biasing means (not shown). As the outer housing  16  moves axially rearwards relative to the rear housing  20 , the ribs  16   c  of the outer housing  16  no longer force the rotating sleeve  18  to its fully rotated position, thus allowing the rotating sleeve  18  to rotate back towards its initial position under the biasing influence of the stressed beam  18   d . Once the rotating sleeve  18  has rotated back to its initial position, the feet  18   a  of the rotating sleeve  18  are no longer axially aligned with the flexible wings  32  of the syringe holder  30 . Additionally, the ribs  16   c  of the outer housing  16  move out of axial alignment with the rear legs  22   b  of the inner housing  22 . Thus, when the rotating sleeve moves back to its initial position, the syringe holder  30  is able to move axially rearwards relative to the rear housing  20  (retracting the needle  29  from the injection site as it does so), and the rear legs  22   b  of the inner housing are able to relax, moving radially outward, out of engagement with the plunger rod  21 . 
         [0059]    Therefore the action of releasing the forward pressure on the outer housing  16  allows retraction of the needle  19  and halts delivery of medicament, if delivery of the intended dose has not been completed. Consequently, the present invention provides a reliable and efficient means for controlling the retraction of a needle after or during delivery of medicament without the risk of medicament exiting the needle during retraction. 
         [0060]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0061]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0062]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.