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.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. § 371 of International Application No. PCT/GB2010/051257, titled IMPROVED AUTOINJECTOR, filed Jul. 30, 2010, which claims priority to Great Britain Patent Application No. 0913385.1, filed Jul. 31, 2009, both of which are hereby incorporated by reference in their entireties.

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

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.

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.

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.

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.

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.

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.

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.

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

In accordance with the present invention there is provided an autoinjector includinga 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 comprisesa 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.

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.

In a preferable embodiment, said housing further includesan inner housing, at least part of which is positionable, in use, intermediate the outer housing and the syringe support means; anda rear housing intermediate said outer housing and said inner housing, and wherein said autoinjector further comprisesan energy source in communication with said inner housing, the inner housing being moveable by the energy source between three positions, namelya 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; anda 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.

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.

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.

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.

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.

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.

Said sleeve preferably comprises a plurality of legs projecting axially forwardly, wherein said legs each have a foot projecting radially inwardly.

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.

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.

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.

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.

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.

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.

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.

DETAILED DESCRIPTION

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'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's skin when the injection device is in use.

An exemplary embodiment10of an autoinjector according to the present invention is shownFIGS. 1A and 1B. The autoinjector10contains a syringe28having a needle19and may be actuated by a user to automatically deliver a dose of medicament to them self or another patient.

The autoinjector10is initially stored in an outer packaging12and a rear screw cap14screwed thereto via a screw thread14b. The rear screw cap14surrounds a rear end cap15and has radial apertures14athat radial projections15aof the rear end cap15are located in. The rear end cap15is secured to an outer housing16of the autoinjector10via engagement between circumferential ribs15bof the rear end cap15and complementary grooves16aof the outer housing16. When the ribs15bare engaged with the grooves16a, part of the rear end cap15is radially constrained between the outer housing16and the outer packaging12, and the rear end cap15cannot disengage from the outer housing16.

To remove the autoinjector10from the outer packaging12, the user unscrews the rear screw cap14relative to the outer packaging12. Because the rear end cap15is connected to the rear screw cap14and the rear end cap15is prevented from disengaging from the outer housing16, the unscrewing of the rear screw cap14from the outer packaging12causes the outer packaging12to move axially forwardly relative to the rear screw cap14and the outer housing16. Internal axial ribs12aon the outer packaging12are guided in axial channels24a(seeFIGS. 2A and 2B) of a front housing24of the autoinjector10as the outer packaging12is unscrewed from the rear screw cap14. The axial channels24aprevent rotation of the axial ribs12a(and therefore outer packaging12) relative to the front housing24as the outer packaging12is unscrewed from the rear screw cap14.

The forward end of the outer packaging12is attached to a needle sheath remover13which extends axially rearwardly from the front of the device. The needle sheath remover13has barbed legs13athat are adapted to engage a needle sheath17into which the needle19of the device is staked in. As the outer packaging12moves axially forwardly relative to the outer housing16and front housing24, the barbed legs13aof the needle sheath remover13are drawn radially inwardly due to contact with an inward boss24bof the front housing24. As the barbed legs13aare drawn radially inwardly, they bite into the needle sheath17and engage it such that further axial movement of the needle sheath remover13causes the needle sheath17to be axially withdrawn from the needle19. Since the outer packaging12is prevented from rotating relative to the front housing24and outer housing16when being unscrewed from the rear screw cap14, substantially no torque is transferred to the needle sheath remover13, needle sheath17and, importantly, the needle19, thereby minimising the risk of damage to the needle19.

The syringe28does, however, experience a forward axial force as the needle sheath17is removed (since the sheath17is fitted over a front portion of the syringe28). 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 syringe28from moving axially forwards when the needle sheath17is removed. The syringe28is disposed in a syringe holder (syringe support means)30which supports the syringe28at a forward end and has a pair of tabs31projecting radially outwardly. The device further includes a rotating sleeve18, intermediate the outer housing16and a rear housing20of the device, which is shown in more detail inFIGS. 2A, 2B and 3. The rotating sleeve18has a pair of radially inwardly projecting feet18awhich are each disposed at the end of an axially extending leg18bof the rotating sleeve18. As described below, the feet18aact as blocking means which prevent the premature retraction of the needle19. When assembled, the tabs31of the syringe holder30are axially aligned with the feet18aof the rotating sleeve18so that forward axial movement of the syringe holder30relative to the rotating sleeve18is prevented which substantially prevents the syringe18moving axially forwards when the needle sheath17is removed. Consequently, the risk of damaging the internal components of the device when the needle sheath17is removed is significantly reduced.

Once the rearmost part of the outer packaging12moves axially past the front most part of the rear end cap15, the ribs15bcan disengage from the grooves16a(i.e. flex radially outwardly as they ride out of the grooves) and the rear screw cap14and rear end cap15can be removed from the device. Once the rear end cap15has been removed, the safety cap26can also be removed. The safety cap26has an axially forwardly projecting pin26athat, when assembled, is located in an aperture20aof a rear housing20of the device. In this position, the pin26aalso passes between radially flexible legs16bof the outer housing16, and prevents the legs from flexing radially inwardly thereby preventing them from passing through the aperture20aof the rear housing. Due to this restriction, when the safety cap26is in place, the device cannot be actuated. Once the safety cap26is removed, the flexible legs16bof the outer housing16can flex radially inwardly and are able to pass through the aperture20aof the rear housing20thereby permitting axial movement of the outer housing16relative to the rear housing20, which is necessary for actuation of the device. A hook16cprojecting axially forwardly from the rear of the outer housing16restricts rearward axial movement of the outer housing16relative to the rear housing20and prevents the two components from separating.

Turning toFIGS. 2A and 2B, the rotating sleeve18is shown to have a sloped cut-out18cin its rear end. The rotating sleeve18has a second identical sloped cut-out18diametrically opposite the one shown in the drawings. A pair of internal axially extending ribs16c(seeFIG. 1B) of the outer housing16are disposed in a pair of slots20bof the rear housing and, prior to actuation of the device, are each in axial alignment with a shallow portion18c′ of one of the sloped cut-outs18c. The rotating sleeve18also has a circumferentially flexible beam18dextending in both an axial and a circumferential direction. Prior to actuation of the device, the beam18dis in a relaxed state and the frontmost end of the beam18dis in contact with a radial abutment20con the outer surface of the rear housing20. The feet18aof the rotating sleeve18are each disposed in a circumferential slot20din the rear housing20.

To actuate the device, the user places the front end of the device against the injection site and pushes the outer housing16axially forwards towards the injection site. With the safety cap26removed, the outer housing16can move axially forwards relative to the rear housing20and as it does so, the ribs16cof the outer housing16move axially along the slots20bin the rear housing20and contact the shallow portion18c′ of the sloped cut-outs18c. Continued axial forward movement of the outer housing16causes the ribs16cto slide down the sloped cut-outs18ctowards deep portions18c″. However, given that the outer housing16is rotatably fixed with respect to the rear housing20due to engagement of the ribs16cin the slots20b, the contact between the ribs16cand the sloped cut-outs causes the rotating sleeve18to rotate relative to the outer housing16and rear housing18from an initial (“second rotational”) position to a fully rotated (“first rotational”) position. In the fully rotated position, the beam18dis flexed and stressed against abutment20cwhich tends to bias the rotating sleeve18back to its initial position (although rotation back to the initial position is prevented by interference with the ribs16cof the outer housing16). When rotated, the feet18aof the rotating sleeve18are out of axial alignment with the tabs31of the syringe holder30thereby permitting forward axial movement of the syringe holder30(and therefore, syringe28) relative to the rear housing20. Additionally, in this fully rotated position, internal axial ribs18eof the rotating sleeve18are in axial alignment with radial abutment elements on rear legs (or tags)22bof an inner housing22of the device, which is disposed within the rear housing20. The forwardly moving ribs16calso contact and move front legs (or tags)22aof the inner housing22radially inwardly. In moving radially inwardly, the front legs22aof the inner housing22move into axial alignment with the barrel of the syringe28and unlatch the inner housing22from the rear housing20.

Unlatched, the inner housing22is free to move axially forwards under the influence of the drive spring (not shown). Given that the front legs22aof the inner housing22are in axial alignment with the barrel of the syringe28, continued forward movement of the inner housing22(under the influence of the drive spring—not shown) causes the syringe28and plunger rod21to move axially forwardly with no relative movement therebetween. This action drives the needle19out of the front of the device and into the injection site without expelling any medicament. As the inner housing22moves axially forwards, rear legs22bof the inner housing22are drawn radially inwards (into contact with a plunger rod21of the syringe28) by interference between the ribs18eof the rotating sleeve18and the radially outwardly extending abutment elements on the rear legs22b. At or around this time, the front legs22aof the inner housing22become radially aligned with slots20din the rear housing20and flex radially outwardly (due to their inherent bias towards that position) out of engagement with the barrel of the syringe28. Further forward axial movement of the inner housing22causes the rear legs22bof the inner housing22to move the plunger rod21axially forwardly relative to the barrel of the syringe28and expel medicament from the needle19into the injection site.

As shown inFIGS. 4 and 5, the syringe holder30includes a pair of flexible wings (or beams)32that are axially rearward of tabs31and circumferentially offset therefrom. Each flexible wing32extends both axially rearwardly and radially outwardly, when in a relaxed state. When the rotating sleeve18is rotated to its fully rotated position, the feet18aof the rotating sleeve are each in axial alignment with one of the flexible wings32of the syringe holder30. Guide arms33on the syringe holder30locate in slots24c(seeFIG. 1B) of the front housing24to only permit axial relative displacement between the syringe holder30and front housing24. Additionally,FIGS. 4 and 5show the tabs31in greater detail which each extend circumferentially on the surface of the syringe holder30. Each tab31also has a “guide rib” sloped section34, which extends both circumferentially and axially rearwardly on the surface of the syringe holder30. The shape of the tabs31and guide ribs34permits efficient assembly of the device, whereby the feet18aof the rotating sleeve18are able to ride down the guide ribs34during installation onto the rear housing20. Given that the beam18dbecomes stressed against the abutment20con the rear housing20as the feet18aride along the guide ribs34, the rotating sleeve18immediately rotates back to a relaxed position once the feet18aare axially forwards of the tabs31so that the feet18aare in axial alignment with the tabs31.

As the inner housing22moves the syringe holder30axially forwards, the flexible wings32contact the feet18aof the syringe holder30and flex radially inwardly. Further forward movement of the syringe holder30relative to the rotating sleeve18causes the flexible wings32to move axially forwards of the feet18aand out of radial alignment with them, so that the wings32can flex radially outwardly once more. In this position, the syringe holder30is prevented from moving axially rearwardly relative to the rotating sleeve18(and therefore the rear housing20) by axial abutment between the flexible wings32and the feet18aof the rotating sleeve. It is this abutment that prevents automatic retraction of the syringe28occurring (under the influence of a retraction spring—not shown).

The inner housing22will continue to move the plunger rod21axially forwardly relative to the syringe28(expelling medicament from the needle19all the while), until the front legs22a, which are moving along slots20dof the rear housing20, reach the forward ends of the slots20d. This arrests movement of the inner housing22and therefore arrests further forward movement of the plunger rod21relative to the syringe28caused by the force of the drive spring. However, although the device has ceased to deliver the medicament, the needle19is still protruding from the front of the device, and the return spring (not shown) will begin to act rearwardly on the syringe28. Since the inner housing22, biased forwardly by the drive spring, is stationary, any rearward movement of the syringe28would cause some further medicament to exit the needle19, which is not desirable. In the embodiment of the invention shown in the drawings, the axial engagement between the wings32of the syringe holder30and the feet18aof the rotating sleeve18prevent rearward movement of the syringe28and prevent any further medicament from being expelled through the needle19. This embodiment has the advantage that the device can deliver a partial dose of medicament, i.e. not discharge the entire contents of the syringe28(such as a discharge 0.3 ml from a total volume of 1 ml). The length of the slots20din the rear housing20can 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 wings32and feet18aand the radial engagement (radial “gripping”) of the rear legs22bof the inner housing22on the plunger rod21both serve to prevent retraction of the needle after the dose (full or partial) has been delivered.

In alternative embodiments, it may be desirable to allow the stopper on the front end of the plunger rod21to reach the forward end of the barrel of the syringe28and 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 housing22will be arrested when the stopper “bottoms out” at the front end of the barrel of the syringe28. Therefore, once movement of the inner housing22has arrested, the forward axial force from the drive spring continues to act on the syringe28and syringe holder30through the inner housing22and plunger rod21. Consequently, the return spring is unable to move the syringe holder30and syringe28rearwards (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 syringe28, there is no risk of continued expulsion of medicament from the needle19since the entire dose has already been delivered. It follows that in these embodiments, the wings32and feet18aare 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 needle19. Absent the wings32and feet18a, the engagement means are provided solely by the engagement of the rear legs22bof the inner housing22on the plunger rod21. The rear legs22bmay engage the plunger rod21by 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 rod21.

When retraction is desired, the user removes the forward pressure on the outer housing16thus allowing the outer housing16to move rearwardly relative to the rear housing20to its initial position, under the influence of a spring or other biasing means (not shown). As the outer housing16moves axially rearwards relative to the rear housing20, the ribs16cof the outer housing16no longer force the rotating sleeve18to its fully rotated position, thus allowing the rotating sleeve18to rotate back towards its initial position under the biasing influence of the stressed beam18d. Once the rotating sleeve18has rotated back to its initial position, the feet18aof the rotating sleeve18are no longer axially aligned with the flexible wings32of the syringe holder30. Additionally, the ribs16cof the outer housing16move out of axial alignment with the rear legs22bof the inner housing22. Thus, when the rotating sleeve moves back to its initial position, the syringe holder30is able to move axially rearwards relative to the rear housing20(retracting the needle29from the injection site as it does so), and the rear legs22bof the inner housing are able to relax, moving radially outward, out of engagement with the plunger rod21.

Therefore the action of releasing the forward pressure on the outer housing16allows retraction of the needle19and 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.