# Patent

## Title
Shield lock for spring driven injection device

## Abstract

The present invention relates to an injection device for automatic torsion spring driven injection of a liquid drug. The injection device is of the type wherein a telescopically movable needle shield (20) activates the release of the torque of the torsion spring to drive the injection when the needle shield is moved from its extended position to a retracted position. The injection device is provided with a needle interface (30) carrying the injection needle, which needle interface is provided with means (32) preventing axial movement of the shield when no injection needle is mounted. Further, other means are provided to restrain the needle shield in the retracted position when the injection device is its “zero” position such that a user has access to the injection needle in order to exchange the injection needle.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 35 U.S.C. §371 National Stage application of International Application PCT/EP2013/063249 (published as WO 2014/001318), filed Jun. 25, 2013, which claimed priority of European Patent Application 12174289.4, filed Jun. 29, 2012; this application claims priority under 35 U.S.C. §119 of U.S. Provisional Application 61/667,069; filed Jul. 2, 2012.


THE TECHNICAL FIELD OF THE INVENTION
The invention relates to a spring driven injection device for injection of multiple set doses. In particular, the invention relates to a spring driven injection device of the type where the injection needle is shielded during injection and where the axial movement of the needle shield releases the injection of the set dose under influence of the spring which is preferably, but not exclusively, a torsion spring.
DESCRIPTION OF RELATED ART
An automatic torsion spring driven injection device is disclosed in EP 338,806. In one embodiment, the injection needle is covered by a telescopically movable needle shield. When the shield is pressed against the skin of a user, a compression spring drives the body of the pen forward such that the tip of the injection needle penetrates the skin of the user, and a torsion spring is released to perform an injection of the liquid drug contained in the injection device. However, exchanging the injection needle is troublesome as the user needs to remove the needle shield in order to gain access to the injection needle.
A different spring driven pen-shaped injection device having a shielded injection needle is known from U.S. Pat. No. 7,112,187. The injection device disclosed is an automatic spring-driven injection in which an actuation spring provided inside the housing thrusts the piston rod forward during injection. An important characteristic of such automatic injection device is that no element move out from the injection device during dose setting. Thus, the injection device has a constant length during operation. This particular injection device has a mode selector which is rotated to select one out of three different modes. In one mode the shield is locked and in a different mode the shield is unlocked. In the unlocked position, the shield can be moved axially between an extended and a retracted position. In the retracted position a user has access to the distal end of the injection device and is thus able to attach or remove an injection needle. Further, the mode selector can be rotated to an injection position, in which position the set dose is released when the shield is moved to its retracted position during injection.
For automatic spring driven injection devices for multiple injections of set doses in which the triggering of the injection is made by the backward movement of the needle shield a particular challenge is present. In order to exchange the injection needle, the needle shield needs to be axially removed from the hub of the injection needle such that the user can rotate or twist the hub in order to couple or decouple the injection needle. However, when performing the injection, the needle shield must trigger the release of the dose when only the distal part of the needle cannula is penetrated into the body.
In EP 338,806 this is solved by simply removing the needle shield during changing of the injection needle whereas in U.S. Pat. No. 7,112,187 it is done by a complicated mechanism involving a mode selector. However, in both examples it is difficult to explain the user how to handle the injection device as the user has to perform many different steps and remember which steps in the sequence of executing the injection he or she has fulfilled.
A further manual injection device in which the cartridge is disconnected from the injection needle by axial movement of the cartridge is described in WO 2011/051366.
DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a spring driven injection device for multiple automatic injections of set doses which are very simple to handle and which do not require any explanation to the user but wherein the working of the injection device is self-explanatory.
It is particularly an object of the present invention to provide a shield triggered automatic injection device which facilitates easy exchange of the injection needle without the user unintentionally pushing on the needle shield and thereby activating the injection process.
It is specifically an object to provide a mechanism which secures and restrains the needle shield in its retracted position.
The invention is defined the claims.
Accordingly, in one embodiment of the present invention, an injection device for a spring driven injection of a liquid drug is provided, which in a first embodiment comprises:

    
    
        A housing storing a cartridge containing the liquid drug to be injected.
        A needle interface to which an injection needle is mountable,
        A needle shield for covering the injection needle during use,
        Spring means for urging the needle shield in the distal direction, and

wherein, the needle shield is slidable relatively to the needle interface and which needle interface is provided with a number of flexible arms preventing axial movement of the needle shield when no injection needle is mounted on the needle interface and which arms are activated when an injection needle is mounted on the needle interface thereby allowing the needle shield to move axially relatively to the needle interface.

    
    


In this way, the needle shield can only be urged by the spring to slide relatively to the needle interface when an injection needle has been mounted onto the needle holder.
When no injection needle is mounted, the needle shield is secured in its proximal retracted position thus allowing a user to change the injection needle, and when an injection needle is mounted and the flexible arms are activated i.e. moved to a position allowing the needle shield to pass in its sliding movement towards its distal position.
The arms, which can be any number including one, are preferably moved into alignment with the needle interface by the hub of the injection needle when mounted onto the needle interface.
The needle interface is preferably provided with means engaging similar means in the interior of a needle hub. These means can be any known means such as a luer, a thread or a bayonet coupling or any combination thereof.
The arms are preferably moulded together with the needle interface as flexible polymer arms which has a build-in resiliency and flexes in an outward direction radial to the longitudinal direction of the injection device such that the arms are pressed inwardly by the hub of the injection needle when mounted and flexes to the initial stopping position once the hub of the injection needle is removed after use.
All though the term “arms” are used throughout the description it is clear that what is meant is any means similar to arms that provides the effect of blocking axial movement of the shield when no injection needle is mounted and which means are radially moved to an aligned position by mounting of the injection needle thus allowing the shield to slide freely.
However, in a further embodiment of the invention, the needle shield is prevented from axial movement by engagement between the needle shield and the scale drum. In this further aspect, the spring driven delivery device comprises:

    
    
        A housing storing a cartridge containing the liquid drug to be injected.
        A scale drum threadedly engaged with the housing to perform a helical movement away from a zero position during setting of a dose.
        A needle shield for covering the injection needle during use, which needle shield can slide axially in relation to the housing,
        Spring means for urging the needle shield in the distal direction, and

wherein the scale drum engages and locks the needle shield when in the zero position.

    
    


By locking is meant that the needle shield is prevented from moving in the distal direction under influence of the spring means urging the needle shield distally.
Whenever the scale drum is in its zero position, i.e. the position in which no dose has been set, and the cipher “0” (or a similar indicia) appears in the window or display of the injection device, engagement means on the scale drum arrests the needle shield and secures it from axial movement.
When a user dials a dose and the scale drum moves away from its zero position, these engagement means releases and set the needle shield free to move axially under influence of the spring whereby the needle shield is slidable into a position in which it covers the injection needle.
The engagement means are preferably hooks and indentations provided respectfully on the scale drum and on the needle shield.
During injection the user presses the needle shield towards the skin to activate the injection. Once the scale drum has returned to its initial zero position the injection is over and the user removes the needle shield from the skin.
The distance between the hook and the indentation are preferably such that they only engage when the shield has moved a distance in the distal direction to make sure that they only hook and lock when the shield is fully removed from the skin of the user.
When pressing the shield proximally with the scale drum located in its zero position, the indentation on the shield passes on the inside on the hook. To facilitate this, the hook is made flexible in a radial direction by providing a cut-out in the proximity of the hook.
A further embodiment combines the two previous embodiments and thus requires the user to both mount an injection needle to move the flexible arms away from the needle shield and to set a dose to release the needle shield. Only when both these two actions have been performed, will the needle shield slide into its extended position covering the distal end of the injection needle.
DEFINITIONS
An “injection pen” is typically an injection apparatus having an oblong or elongated shape somewhat like a pen for writing. Although such pens usually have a tubular cross-section, they could easily have a different cross-section such as triangular, rectangular or square or any variation around these geometries.
The term “Needle Cannula” is used to describe the actual conduit performing the penetration of the skin during injection. A needle cannula is usually made from a metallic material such as e.g. stainless steel and connected to a hub to form a complete injection needle also often referred to as a “needle assembly” or simply an “injection needle” A needle cannula could however also be made from a polymeric material or a glass material. The hub also carries the connecting means for connecting the needle assembly to an injection apparatus and is usually moulded from a suitable thermoplastic material. The “connection means” could as examples be a luer coupling, a bayonet coupling, a threaded connection or any combination thereof e.g. a combination as described in EP 1,536,854.
Needle assemblies specially designed for pen injections systems are defined in ISO standard No. 11608, part 2, and are often referred to as “pen needles”. Pen needles have a front-end for penetrating into the skin of the user and a back-end for penetrating into the cartridge containing the drug.
As used herein, the term “drug” is meant to encompass any drug-containing flowable medicine capable of being passed through a delivery means such as a hollow needle in a controlled manner, such as a liquid, solution, gel or fine suspension. Representative drugs includes pharmaceuticals such as peptides, proteins (e.g. insulin, insulin analogues and C-peptide), and hormones, biologically derived or active agents, hormonal and gene based agents, nutritional formulas and other substances in both solid (dispensed) or liquid form.
“Scale drum” is meant to be a cylinder shaped element carrying indicia indicating the size of the selected dose to the user of the injection pen. The cylinder shaped element making up the scale drum can be either solid or hollow. “Indicia” is meant to incorporate any kind of printing or otherwise provided symbols e.g. engraved or adhered symbols. These symbols are preferably, but not exclusively, Arabian numbers from “0” to “9”. In a traditional injection pen configuration the indicia is viewable through a window provided in the housing. When reference is made to a “zero position” of the scale drum, this does not necessarily mean that the number “0” is present, however it merely refers to the position of the scale drum in which no dose has been set.
“Cartridge” is the term used to describe the container containing the drug. Cartridges are usually made from glass but could also be moulded from any suitable polymer. A cartridge or ampoule is preferably sealed at one end by a pierceable membrane referred to as the “septum” which can be pierced e.g. by the back-end of an injection needle. The opposite end is typically closed by a plunger or piston made from rubber or a suitable polymer. The plunger or piston can be slidable moved inside the cartridge. The space between the pierceable membrane and the movable plunger holds the drug which is pressed out as the plunger decreased the volume of the space holding the drug. However, any kind of container—rigid or flexible—can be used to contain the drug.
Since a cartridge usually has a narrower neck portion into which the rubber plunger cannot be moved, not all of the drug contained inside the cartridge can be expelled. The term “initial quantum” therefore refers to the initial quantum of the injectable content. The term “remaining content” in the same way refers to the remaining injectable content.
Using the term “Automatic” in conjunction with injection device means that, the injection device is able to perform the injection without requiring the user of the injection device to manually deliver the force needed to expel the drug. The force is typically delivered by an electric motor or by a spring as herein described which spring is strained by the user during dose setting. Such springs are usually prestrained in order to avoid problems of delivering very small doses. Alternatively, the spring can be preloaded by the manufacturer with a preload sufficient to empty the drug cartridge though a number of doses. Typically the user activates a latch or a button on the injection device to release the force accumulated in the spring when carrying out the injection.
All references, including publications, patent applications, and patents, cited herein are incorporated by reference in their entirety and to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
All headings and sub-headings are used herein for convenience only and should not be constructed as limiting the invention in any way.
The use of any and all examples, or exemplary language (e.g. such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
The citation and incorporation of patent documents herein is done for convenience only and does not reflect any view of the validity, patentability, and/or enforceability of such patent documents.
This invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:
FIG. 1 show a perspective view of the injection device prior to use.
FIG. 2 show a perspective view of the injection device with the cap removed.
FIG. 3 show a perspective view of the injection device with the injection needle mounted.
FIG. 4 show a perspective view of the injection device with the needle shield in its extended position.
FIG. 5 show a perspective view of the injection device during injection.
FIG. 6 show a perspective view of the injection following injection.
FIGS. 7.1-7.8 show a schematic view of the various sequences of performing an injection using the injection device according to the invention.
FIG. 8 show a cross sectional view of the injection device according to a first example.
FIG. 9 show an exploded view of the injection device in FIG. 8.
FIG. 10 show a detailed cross sectional view of the engagement between the needle shield and the scale drum.
FIG. 11 show a detailed cross sectional view of the needle shield released from the scale drum.
FIG. 12 show a detailed cross sectional view of the connecting means of the needle holder without an injection needle attached.
FIG. 13 show a detailed cross sectional view of the connecting means of the needle holder with an injection needle attached.
FIG. 14 show a detailed cross sectional view of the activation mechanism during injection.
FIG. 15 show a different detailed cross sectional view of the activation mechanism.
FIG. 16 show a detailed cross sectional view of the activation mechanism upon activation.
FIG. 17 show a detailed cross sectional view of the proximal end of the injection device.
FIG. 18 show a perspective view of the mechanism releasing the needle holder following injection.
FIG. 19 show a cross sectional view of the injection device according to a second example.
FIG. 20 show an exploded view of the injection device in FIG. 19.
FIG. 21 show a cross sectional view of the injection mechanism according to the FIGS. 19-20 in the non-dosing position.
FIG. 22 show a cross sectional view of the injection mechanism according to the FIGS. 19-20 when releasing a set dose.
FIG. 23 show a side view of the interior of the injection device of the FIGS. 19-20 with the needle holder locked to the needle shield.
FIG. 24 show a perspective view of FIG. 23.
FIG. 24A show a perspective view of the inside of scale drum.
FIG. 25 show a side view of the interior of the injection device of the FIGS. 19-20 with the needle holder released from the needle shield and the needle shield locked to the scale drum.
FIG. 26 show a perspective view of FIG. 25.



The figures are schematic and simplified for clarity, and they just show details, which are essential to the understanding of the invention, while other details are left out. Throughout, the same reference numerals are used for identical or corresponding parts.
DETAILED DESCRIPTION OF EMBODIMENT
When in the following terms as “upper” and “lower”, “right” and “left”, “horizontal” and “vertical”, “clockwise” and “counter clockwise” or similar relative expressions are used, these only refer to the appended figures and not to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as there relative dimensions are intended to serve illustrative purposes only.
In that context it may be convenient to define that the term “distal end” in the appended figures is meant to refer to the end of the injection device which usually carries the injection needle whereas the term “proximal end” is meant to refer to the opposite end pointing away from the injection needle and carrying the dose dial button as depictured in the FIGS. 1 to 6.
FIG. 1 to FIG. 7 discloses a torsion spring driven injection device during its different stages. The features and the working modes disclosed in the FIGS. 1 to 7 is common for both examples.
When delivered to a user, the injection device 1 has a cap 2 secured to the distal end of the housing 3 as disclosed in FIG. 1. Further, the injection device 1 has a dose setting button 4 at its proximal end and a window 5 provided in the housing 3 through which the user can visually inspect the size of the dose being set by rotating the dose setting button 4.
FIG. 2 discloses the injection device 1 with the cap 2 removed. In this mode, the user can inspect the drug contained in the injection device through the inspection opening 6. The shield 20 is in its retracted position and the user has full access to the connecting means 31 provided distally on the needle holder 30.
In this mode the axial movement of the shield 20 in the distal direction is hindered by two flexible arms 32 provided on the needle holder 30 and shown in details on FIG. 12-13. These flexible arms 32 are provided in conjunction with the connecting means 31 such that once an injection needle 100 is connected to the connection means 31, this injection needle 100 pushes the flexible arms 32 inwardly to allow passage of the axial movable shield 20 in the distal direction.
In FIG. 3, the user has attached an injection needle 100 to the connecting means 31. The injection needle 100 is a conventional pen needle 100 (see FIG. 12) comprising a hub 102 to which a metallic needle cannula 101 is secured. The needle cannula 101 has a distal end 103 for penetrating the skin of a user and a proximal end 104 for entering into a cartridge 105 contained in the injection device 1.
When a user dials a dose by rotating the dose setting button 4 (indicated by the arrow S in FIG. 4), the shield 20 is automatically moved into its extended position as disclosed in FIG. 4. In this extended position the shield 20 visually covers the distal end 103 of the needle cannula 101, at least when the injection device 1 is viewed radially i.e. when vied from the side.
An injection is hereafter performed simply by pressing the distal end of the shield 20 softly against the skin of the user. This is indicated with the arrow I in FIG. 5. The distal part 103 of the needle cannula 101 penetrates through the skin of the user, and the shield 20 when moving into its retracted position automatically activates the injection of the set dose as will be explained later.
Following injection, when the distal end 103 of the needle cannula 101 is removed from the skin of the user as disclosed in FIG. 6, the shield 20 is maintained in its retracted position and the needle holder 30 moves axially into its extended position making it possible for the user to exchange the injection needle 100.
The various sequences of an injection are schematically disclosed in FIG. 7. The details will be further explained in the following.
In FIG. 7.1, the cap 2 has been removed and the injection needle 100 has been connected to the connection means 31 of the needle holder 30. No dose has been dialed as can be seen in the window 5.
In FIG. 7.2 the user dials a dose, and the shield 20 is moved forward to cover the distal end 103 of the injection needle 101.
In FIG. 7.3 the shield 20 is pressed against the skin S of a user.
In FIG. 7.4, the distal end 103 of the needle cannula 101 has penetrated through the skin S of the user and the needle holder 30 and the shield 20 locks to each other and moves axially together as further disclosed in FIG. 15.
In FIG. 7.5, the proximal end 104 of the needle cannula 101 has penetrated through the septum 106 of the cartridge 105. The injection itself is being executed in this position, indicated by the number “0” appearing in the window 5. Once the scale drum in returned to zero it locks the shield to the housing (via the scale drum).
In FIG. 7.6 the needle holder 30 is forced forward which decouples the proximal end 104 of the needle cannula 101 from the cartridge 105.
In FIG. 7.7 this decoupling is fulfilled and in FIG. 7.8 the needle holder 30 locks to the housing 3.
FIRST EXAMPLE
The operation of the injection device 1 according to a first example will be explained in details in conjunction with the FIGS. 8 to 18.
The mechanics of the injection device 1 is contained in an outer housing 3, which is preferably made from two parts, a distal cartridge holder 3a which is permanently secured to a proximal housing part 3b to form one housing 3. An intermediate part 7 carrying a partition 8 is provided between the cartridge holder 3a and the proximal housing part 3b. Alternatively the housing 3 can be formed as one unitary unit.
As identified before, the distal end of the housing 3 carries a shield 20 and a needle holder 30 whereas the proximal end of the housing 3 carries the dose setting button 4.
The distal cartridge holder part 3a further stores the cartridge 105 which is a conventional cartridge 105 having a septum 106 at its distal end and an axially movable plunger 107 slidable provided at its proximal end. By moving the plunger 107 in the distal direction, the volume of the area between the septum 106 and the plunger 107 is reduced with the volume being pressed out through the needle cannula 101 of the injection needle 100.
The cartridge 105 is axially locked to the housing 3. The proximal end 109 of the cartridge 105 abuts the partition 8 as disclosed in FIG. 10-11 and the shoulders 108 of the cartridge 105 abut inwardly pointing protrusions 9 internally in the housing 3 such that the cartridge 105 cannot slide axially in relation to the housing 3. In order to obtain the tolerances of the cartridge a number of distally pointing fingers 12 can be provided on the partition 8. These fingers 12 preferably has a sloping surface to press against the proximal end 109 of the cartridge 105 as disclosed in FIG. 11. The inwardly pointing protrusions 9 securing the cartridge 105 distally can be provided at a distal end of the opening 6, they could however be provided wherever needed. Alternatively the cartridge 105 can be moulded to the housing 3.
In order to move the plunger 107 forward a drive mechanism is provided which mechanism comprises a threaded piston rod 40 which at its distal end presses against the plunger 107 preferably with a washer 43 provided between the piston rod 40 and the plunger 107.
The intermediate housing part 7 with its internal partition 8 is inrotatable secured to the housing 3 preferably between the two housing parts 3a, 3b. The intermediate housing part 7 could alternatively be moulded as an integral part of the housing 3. The outside thread 41 of the piston rod 40 engages an internal thread 10 provided centrally in the intermediate housing part 7 such that whenever the piston rod 40 is rotated it moves axially in relation to the intermediate housing part 7 a distance determined by the number of revolutions of the piston rod 40 and the pitch of the threads 10, 41.
The piston rod 40 is further provided with an axial stretching track 42 which is engaged by a piston rod guide 50 such that whenever the piston rod guide 50 rotate, the piston rod 40 rotates simultaneously and is screwed forward in the thread connection 10/41.
The needle holder 30 is as disclosed in FIG. 12-13 provided with a number of flexible arms 32 which are pressed outwardly by the inherent force of the flexible arms 32. In this outwardly pointing position, the flexible arms 32 abuts the shield 20 in a pair of grooves 21 provided on a flange 22 of the shield 20 such that the shield 20 is prevented from moving distally relatively to the needle mount 30, as indicated in FIG. 2. In this position a user can mount the injection needle 100 to the connection means 31. When the injection needle 100 is mounted onto the connecting means 31, the flexible arms 32 will bend and be brought into alignment with the outside surface of the connecting means 31 of the needle holder 30 and the shield 20 can slide freely relatively to the needle mount 30 as depictured in FIG. 13.
In order to visually indicate the size of the dose being set by rotating the dose setting button 4 a scale drum 60 is provided. The scale drum 60 is provided with an external thread 61 engaging a similar thread provided internally in the housing 3 as seen in FIG. 10.
The dose setting button 4 engages the drive tube 70 at its proximal end via a ratchet mechanism which is described in details in the not yet published PCT/EP 2013/055403 to Novo Nordisk A/S, which is hereby incorporated by reference. This ratchet mechanism as disclosed in FIG. 17 comprises a spring base 80 which is permanently secured to the housing 3 and has an internal toothing engaging the ratchet arms 86 of the ratchet element 85.
The ratchet element 85 is further provided with an internal toothing 87 engaging a similar toothing 71 externally provided on the drive tube 70 such whenever a user rotates the dose setting button 4 to set a dose, the dose setting button 4 rotates the ratchet element 85 and together with it the drive tube 70.
The dose setting button 4 engages directly with the ratchet element 85 which rotates together with the dose setting button 4 both when setting a dose and when lowering the set dose by rotating the dose setting button 4 in the opposite direction. The dose setting button 4 further has an internally provided protrusion which is able to move the ratchet arm 86 out of engagement with the internal toothing of the spring base 80 when the dose is being lowered.
A torsion spring A is provided between the drive tube 70 and the spring base 80, which torsion spring A is strained whenever the drive tube 70 is rotated in the dose setting direction. The spring base 80 could alternatively be a part of the housing 3 in which case the torsion spring A would be encompassed between the housing 3 and the drive tube 70.
Besides the torsion spring A delivering the torsional force to perform the injection, two other springs are provided. A helical spring B applying an axial force is provided between a flange 28 on the shield 20 and the internal partition 8 of the intermediate part 7 of the housing 3 urging the shield 20 in the distal direction and a second axially working helical spring C is provided between a flange 33 provided distally on the needle mount 30 and the housing 3 urging the needle mount 30 in the distal direction. The helical spring C can rest against the distal side of the internal protrusions 9 inside the housing part 3a securing the cartridge 105 as disclosed in FIG. 8.
The drive tube 70 is on an outside surface provided with an axial groove 72 being engaged by a corresponding raised bar 62 internally in the scale drum 60, such that the scale drum 60 follows rotation of the drive tube 70 and can move axially in relation to the drive tube 70. Since the scale drum 60 is threaded to the housing 3 it performs a helical movement whenever rotated. Externally the scale drum 60 is provided with a series of indicia indicating the size of the dose which indicia can be viewed through the window 5 in the housing 3.
As disclosed in FIGS. 10-11, the distal end the scale drum 60 is internally provided with hooks 63 engaging similar indentations 23 provided proximally on a pair of identical arms 24 a,b provided proximally on the shield 20.
The shield 20 is urged in the distal direction by the helical spring B which is encompassed between the partition 8 of the intermediate housing part 7 and the shield 20 thus applying a distally orientated force on the shield 20. The shield 20 is further provided with an axial surface 25 which is guided by a similar axial surface 11 provided along the inspection opening 6 inside the housing 3 such that the shield 20 is guided solely axially without the possibility of rotating relatively to the housing 3.
Whenever the scale drum 60 is in its most distal position, the hooks 63 will engage the indentations 23 on the shield 20 and thus prevent the helical spring B form urging the shield 20 in the distal direction (FIG. 10), however when the scale drum 60 is rotated to set a dose as disclosed in FIG. 11, the indentations 23 are released from the hooks 63 and the shield 20 travels in the distal direction under the force applied by the helical spring B. Thus, when a user dials a dose as depictured in FIG. 4 (arrow D), the shield 20 is released to move in the distal direction to cover the distal end 103 of the needle cannula 101.
End-of-Content
An end of content mechanism disclosed in FIG. 17 comprises an EOC tube 45 and an EOC ring 55. The EOC ring 55 carries an outside thread 56 which is threaded inside the EOC tube 45 and is axially guided on the drive tube 70 by having an internal protrusion 57 guided in an axial track 73 on the drive tube 70.
Further the EOC tube 45 has an internal flange 49 which is captured by hooks 75 provided on the drive tube 70 allowing the EOC tube 45 to slide a short distance axially in relation to the drive tube 70.
Whenever a dose is set, the drive tube 70 is rotated and the EOC tube 45 is held inrotatable by having teeth 46 engaging similar teeth inside the spring base 80. The EOC ring 55 is thereby dialed up the EOC tube 45 a distance which relates to the size of the set dose. During expelling of the dose, the drive tube 70 is moved axially in relation to the EOC tube 45 as will be explained later, the result being that the distal toothing 47 on the EOC tube engages a similar toothing in the drive tube 70 such that the drive tube 70 and the EOC tube 45 rotates simultaneously during dosing whereby the EOC ring 55 remains in its relative position. The position of the EOC ring 55 in the internal thread of the EOC tube 45 is therefore an expression of the accumulated set doses.
The helical length of the internal thread of the EOC tube is made such that the EOC ring 55 reaches the end of the internal thread of EOC tube 45 when the cartridge 105 is empty, or at least empty for its usable content. At this point the EOC ring 55 abuts the end of the internal thread and prevents further rotation of the drive tube 70, thus no further dose can be set.
The EOC tube 45 can further be provided with teeth 48 engaging similar teeth in the spring base 80 providing dose clicks as the EOC tube 45 rotates with the drive tube 70 during dosing.
Dose Setting
When a user sets a dose by rotating the dose dial button 4 as indicated in FIG. 4, the rotation of the dose setting button 4 causes the ratchet element 85 to also rotate. This rotation is transferred to the drive tube 70 via the toothing 71, 87.
As the drive tube 70 is rotated the torsion spring A encompassed between the spring base 80 and the drive tube 70 is strained. The torsion thereby being built up in the torsion spring A is held by the ratchet arms 86 engaging the internal toothing of the spring base 80. The ratchet arms 86 can be actively released in order to dial down the size of the set dose.
As the drive tube 70 is rotated, the scale drum 60 rotate and move helically thus indicating the size of the set dose in the window 5.
As the scale drum 60 starts to rotate, the hooks 63 of the scale drum 60 moves out of the engagement with the indentations 23 of the shield 20 which is then free to move axially under the influence of the helical spring B. The shield 20 thus moves to a position covering the distal end 103 of the needle cannula 101 as depictured in FIG. 4.
Once the user has set the size of the dose to be injected, the injection device 1 is set and ready to perform an injection.
Activation
In order to release the torque applied to the torsion spring A and thereby to perform an injection, the shield 20 is pressed against the skin of the user as indicated by the arrow I in FIG. 5 which will trigger the injection as explained below.
The needle holder 30 is provided with an identical set of flexible arms 34 a,b. These arms 34 a,b are blocked in the axial direction by an internal flange 13 provided inside the housing 3 as disclosed in FIG. 14-16.
Further, the shield 20 is provided with two identical arms 24 a,b carrying the indentations 23. These arms 24 a, b are each further provided with a protrusion 26 as best seen in FIG. 16. As the shield 20 travels in the proximal direction, this protrusion 26 presses the flexible arms 34 a,b inwardly such that the flexible arms 34 a,b, can slide under the internal flange 13 of the housing 3 as in FIG. 14 and thus allow the needle shield 30 to slide axially.
The needle holder 30 is further provided with a second pair of identical arms 35 a,b. These arms 35 a,b carries proximally an extension 36.
The arms 24 a,b of the shield 20 each carry a radial protrusion 27 which peripherally follows the outside surface of the cartridge 105. As the shield 20 is moved axially in the proximal direction, the radial protrusions 27 abuts the extension 36 and further axial movement of the shield 20 will thus force the needle holder 30 to move along with the shield 10 in the proximal direction as the arms 34a,b in this position is able to escape under the flange 13.
As both the shield 20 and the needle holder 30 slides in the proximal direction, the proximal end 104 of the needle cannula 101 penetrates through the septum 106 of the cartridge 105 since the cartridge 105 which proximally rest against the partition 8 of the intermediate part 7 is prevented from axial movement.
Dose Release
Once the proximal end 104 of the needle cannula 101 has penetrated through the septum 106 of the cartridge 105 and the distal end 102 has penetrated through the skin of the user, the set dose is released in the following manner.
The axial track of the piston rod 40 is engaged by the piston rod guide 50 as disclosed in FIG. 16. The piston rod guide 50 is further provided with an external toothing 51 (see e.g. FIG. 11) engaging a similar toothing 74 internally in the drive tube 70. The drive tube 70 and the piston rod guide 50 can slide axially relatively such they can shift between a position in which the toothing 74 of the drive tube 70 engages with the toothing 51 of the piston rod guide 50 such that the piston rod guide 50 follows the rotation of the drive tube 70 and a position in which the drive tube 70 and the piston rod guide 50 is disengaged.
Distally, the piston rod guide 50 is engaged by the activator 90.
The activator 90 has a central part 91 which engages the piston rod guide 50 such that the activator 90 can move the piston rod guide 50 axially while the piston rod guide 50 can rotate relatively to the activator 90. Distally, the activator is provided with two identical legs 92 a,b. 
During activation as the needle holder 30 is moved in the proximal direction by the shield 20, the proximal end of the arms 34 a,b abuts the arms 92 a,b of the activator 90 and slides the activator 90 in the proximal direction.
This axial movement of the activator 90 pushes the piston rod guide 50 into engagement with the drive tube 70.
The central part 91 of the activator 90 is provided proximally from this wall partition 8 and the legs 92 a,b, extend through openings in the partition 8. The partition 8 is further provided with a toothing 14 (FIG. 8) which engages a similar toothing 53 internally in the piston rod guide 50 such that the piston rod guide 50 is prevented from rotation relatively to the partition 8 (and the housing 3) as long as the piston rod guide 50 axially abuts the partition 8.
As the arms 34 a,b axially moves the piston rod guide 50 out of engagement with the toothing 14 of the partition 8 and into engagement with the drive tube 70 it also moves the drive tube 70 slightly in the proximal direction. This axial movement of the drive tube 70 moves the proximal toothing 71 of the drive tube 70 out of its engagement with the internal toothing 87 of the ratchet 85 as shown in FIG. 17. As the ratchet element 85 prevents the torque of the torsion spring A from being released, the torsion spring A is now able to rotate the drive tube 70 which rotates with it the piston rod guide 50 and thereby the piston rod 40 to perform an ejection of the liquid drug.
After Dosing
Due to the engagement between the axial groove 72 of the drive tube 70 and the raised bar 62 of the scale drum 60, the scale drum 60 rotates back to its zero position during injection. As the scale drum 60 returns to its zero position as shown in FIG. 18, the raised bar 62 engages an axial surface 93 provided externally on the activator 90. The impact of the scale drum 60 with the activator 90 makes the activator 90 rotate an angle. This rotation makes the arms 92 a,b of the activator 90 move under a peripheral extension 36e provided peripheral on the extension 36 which lifts the extension 36 and thereby the proximal end of the arms 35 a,b over the radial protrusion 27. In order to enhance this lifting, both the arms 92 a,b and the extension 36e are preferably provided with an inclined surface as depictured in FIG. 18. The helical spring C urging an axial force on the needle holder 30 now pushes the needle holder 30 in the distal direction such that the proximal end 104 of the needle cannula 101 is moved out of its engagement with the septum 106 of the cartridge 105. When setting a dose the activator 90 is rotated back to its initial position.
The needle shield 20 is in the zero position locked to the scale drum 60 due to the engagement 63/23 and is thus hindered from moving axially. The result being as shown in FIG. 6, that the needle shield 20 remains in its retracted position and the needle holder 30 moves into its most distal position thereby making it possible for the user to exchange the injection needle 100.
SECOND EXAMPLE
A second example which essentially works the same way as the first example is disclosed in the FIGS. 19-26.
Whenever possible the individual elements of the second example are numbered with the same number as in the first example, however, having the number “1” or “10” in front. The same apply for elements performing the same or similar activity.
As in FIG. 9 the second example is disclosed in an exploded view in FIG. 20.
As disclosed the housing 1003 is made up from three housing parts; a distal cartridge holder part 1003a holding a cartridge 1105, a proximal housing part 1003b which at its most proximal end is provided with a dose setting button 1004 and an intermediate housing part 1007 connecting the cartridge part 1003a and the proximal housing part 1003b. The cartridge holder part 1003a is further, in the non-use situation, covered by a removable cap 1002.
The intermediate housing part 1007 is further provided with a distal extension 115 which will be explained later and a proximal extension 116. The proximal extension 116 has at its most proximal end a pointer which will appear in the scale window 1005 when the housing 1003 is assembled (best seen in FIG. 21-22). Further, the proximal extension 116 can carry an inwardly pointing thread segment 117 for engaging the external thread 161 on the scale drum 160.
The proximal housing part 1003a is provided with two opposite located openings 1006 (one of which is depictured in FIG. 20) into which two frames 111 are press fitted. One or both of these frames 111 are internally provided with a holding mechanism 1009 for holding the neck part 1108 of the cartridge 1105. The proximal end 1109 of the cartridge 1105 is secured against the partition 1008 of the intermediate part 1007 such that the cartridge 1105 is fixed in the housing 1003.
Movable relatively to the housing 1003 is the shield 120 which cover the injection needle during injection and the needle holder 130 having connecting means 131 for securing an injection needle to the needle holder 130. The needle holder 130 is further provided with a flexible arm 132 preventing proximal movement of the shield 120 relatively to the needle holder 130 when no injection needle is mounted on the needle holder 130.
Further, a torsion spring A supplies the torque for performing an injection whereas a compression spring B urges the shield 120 in the distal direction and another compression spring C urges the needle holder 130 in the distal direction.
The torsion spring A is encompassed between the spring base 180 and the drive tube 170. The spring base 180 is permanently fixed to the proximal housing part 1003b but could alternatively be moulded as an integral part of the housing 1003.
The compression spring B is encompassed between a flange (or similar) 128 on the shield 120 and the internal partition 1008 of the intermediate housing part 1007. The flange 128 could alternatively be a number of knobs supporting the spring B.
The compression spring C is made from a suitable polymer and is moulded as an integral part of the needle holder 130 and encompassed between the needle holder 130 and the partition 1008 of the intermediate housing part 1007. The partition 1008 can e.g. be provided with a hole 114 for securing the moulded compression spring C.
The intermediate housing part 1007 further has an internal thread 110 through which the piston rod 140 is screwed forward when rotated.
A scale drum 160 for showing the size of the set dose is via an outside thread 161 threaded to the proximal housing part 1003b or to the threaded segment 117 of the proximal extension 116 of the intermediate housing part 1007 (or it can be threaded to both as a security measure) such that the scale drum 160 moves helically when rotated. Internally the scale drum 160 is provided with a raised bar 162 axially guided in a corresponding axial groove or the like 172 provided externally on the drive tube 170. The raised bar 162 and axial groove 172 engagement could of cause be vice versa in respect of the parts.
The scale drum 160 is further provided with a hook 163 which holds the shield 120 in its retraced position when the scale drum 160 is in its most distal position i.e. when no dose is set.
The proximally provided dose setting button 1004 is internally rotatable connected to the ratchet element 185 such that rotation of the dose setting button 104 is transformed into rotation of the ratchet element 185. The ratchet element 185 is urged in the distal direction by a compression spring D provided between the proximal end of the ratchet element 185 and the dose setting button 1004. This spring D is preferably moulded integrally with the ratchet element 185 as depictured in FIG. 19. The dose setting button 1004 is further rotatable connected to the housing 1003 such that it can rotate relative to the proximal housing part 1003b but not move axially.
The ratchet element 185 is further provided with a ratchet arm 186 which engages an internally toothing 181 provided internally in the spring base 180. The engagement of the ratchet arm 186 with the internal toothing 181 prevents the ratchet element 185 from counter rotating. However, the dose setting button 1004 is internally provided with a protrusion 1010 (FIG. 19) which can move the ratchet arm 186 inwardly when the dose setting button 1004 is rotated oppositely to lower the set dose. In this way the ratchet element 185 can rotate step by step in the opposite direction during dial-down of the dose and under influence of the torsion spring A.
Dose Setting
When setting a dose, as depictured in FIG. 21, the user simply rotates the dose setting button 1004 which in turn rotates the ratchet element 185. Rotation of the ratchet element 185 is transferred to a rotation of the drive tube 170 as the inner toothed surface 174 of the drive tube 170 is in engagement with the outer tooting 187 provided on the ratchet element 185. The rotation of the drive tube 170 strains the torsion spring A. The torque build up in the torsion spring A is held by the engagement of the ratchet arm 186 with the internal toothing 181 of the spring base 180.
The scale drum 160 rotates together with the drive tube 170 such that the user can view the set dose through the window 105 provided in the proximal housing part 103b. 
Once the scale drum 160 is rotated away from its “zero” position the hook 163 is rotated out of its engagement with the indentation 123 (see FIG. 23-26) provided on the shield 120 which is then urged distally by the compression spring B.
The needle holder 130 is further urged forward by the compression spring C such that the back-end of the injection needle is maintained outside the septum 1106 of the cartridge 1105 when not injecting.
Injecting
In order to inject the set dose, the distal end of the shield 120 is pressed against the skin of the user.
When the user starts to press the shield 120 against the skin, the front-end 103 of the injection needle 100 penetrates through the skin while the back-end 104 of the injection needle is out of contact with the septum 1106 of the cartridge 1105 as the needle holder 130 is in its distal position. In this position, the needle holder 130 is prevented from moving proximally by the engagement of the protrusion 134a carried on the flexible arm 134 against an inwardly pointing protrusion 113 (see FIG. 20) provided on the frame 111.
However, once the front-end 103 of the injection needle 100 is fully inserted into the skin as depictured in FIG. 7.4, the shield 120 will force the protrusion 134a out of its engagement with the inwardly pointing protrusions 113 of the frame 111 since the vertical protrusion 134b of the flexible arm 134 is guided in a track 126 in the shield 120. The shape of this track 126 moves the protrusion 134a (via 134b) out of its engagement with the inwardly pointing protrusion 113 on the frame 111 where after the shield 120 and the needle holder 130 move axially together.
The shield 120 and needle holder 130 moves together since the protrusion 136 provided on the arm 135 locks to the hook 127 proximally provided on the shield 120. Axial movement of the shield 120 is thus transferred to the needle holder 130. This is best seen in FIG. 23-24 which depictures the situation occurring during dose expelling and just before the scale drum 160 reaches its zero position. The hook 127 engages the protrusion 136 thus transferring axial movement of the shield 120 to axial movement of the needle holder 130.
The arm 135 carrying the protrusion 136 is guided into position by the curved wall 129 leading up to the hook 127. Oppositely the arm 135 is supported by the curved extension 115 on the intermediate housing part 1007.
When the shield 120 is fully retracted, as depictured in FIG. 7.5, the back-end 104 of the injection needle 100 has penetrated through the septum 1106 and into the cartridge 1105 and the release of the dose will be activated.
Dose Release
Proximally on the needled holder 130 one or more release arms 137 are provided. These release arms 137 extent parallel with the flexible arm 135. When the shield 120 and the needle holder 130 is in their proximal position, the release arms 137 moves the clutch 190 as depictured in FIG. 22. This brings the external toothing 191 on the clutch 190 into engagement with a similar toothing 174 provided internally in the drive tube 170 such that the drive tube 170 and the clutch 190 rotate together.
During dose setting, the ratchet element 185 rotates the drive tube 170 via the engagement of the externally provided toothing 187 engaging the toothing 174 internally in the drive tube 170, see FIG. 21.
Further, during dose setting, the ratchet element 185 rotate together with the dose setting button 1004, but during dosing both the dose setting button 1004 and the ratchet element 185 remains non-rotatable. When the clutch 190 is moved proximally by the arms 137, the ratchet element 185 is also moved proximally against the bias of a proximally moulded spring arm D provided proximally on the ratchet element 185 and resting against an inside surface of the dose setting button 1004.
This proximal movement of the ratchet element 185 releases the coupling between the toothing 174 of the drive tube 170 and the toothing 187 provided on the ratchet element 185 such that the drive tube 170 is free to rotate under influence of the torque of the torsion spring A.
The rotation of the drive tube 170 is transferred to a rotation of the clutch 190 by the coupling between the internal toothing 174 on the drive tube 170 and the toothing 191 externally and proximally on the clutch 190.
I.e. when the clutch 190 slides proximally (moves from the position in FIG. 21 to the position of FIG. 22) the toothing 174 of the drive tube 170 releases the engagement with the toothing 187 of the ratchet element 185 and couples to the toothing 191 of the clutch 190.
A further toothing 171 is also provided internally in drive tube 170, which toothing 171 operates against click arms 188 provided externally on the ratchet element 185 to provide dose clicks during injection i.e. when the drive tube 170 rotate relatively to the ratchet element 185.
Please note that in the FIGS. 21-22 part of the clutch 190 is not visible due to the cross sectional view. However, the clutch 190 is fully depictured in FIG. 19.
Further, the rotation of the clutch 190 is transferred to a rotation of the piston rod guide 150 as the toothed outer surface 151 of the piston rod guide 150 is in engagement with the internal toothing 194 of the clutch 190 when the clutch 190 is moved proximally during dosing as disclosed in FIG. 22.
As seen in FIG. 21-22 this internal toothing 194 of the clutch 190 is in engagement with a toothing 118 on the intermediate housing member 1007 when the injection device is not activated.
The rotation of the piston rod guide 150 is transferred to a rotation of the piston rod 140 as the piston rod guide 150 engages a longitudinal track in the piston rod 140.
Pressure Relief
The pressure relief mechanism is similar to the one disclosed in EP 12-188471 by Novo Nordisk A/S and serves the purpose of allowing axial movement of the rubber plunger 1107 of the cartridge 1105 and thus also of the piston rod 140. Such axial movement of the rubber plunger occurs e.g. as a result of temperature variations.
The pressure relief mechanism comprises of the clutch 190, the piston rod guide 150, a click element 165 and a leaf spring E.
If the liquid drug inside the cartridge 1105 expands due to increasing temperatures, the piston rod 140 will be forced proximally by the rubber plunger 1107 inside the cartridge 1105, which will push the piston rod foot 143 and thus the piston rod 140 proximally. This will generate a rotation of the piston rod 140 as the piston rod 140 is threaded to the thread 1008 of the intermediate part 1007.
This will force the piston rod guide 150 to rotate as the piston rod guide 150 is keyed to the piston rod 140.
The click element 165 is externally provided with a plurality of click fingers 166 which operates in a toothing 195 provided internally in the clutch 190. This toothing 195 is adapted to prevent rotation of the click element 165 in one direction and adapted to have reluctance to rotation in the opposite direction (due to the inherent outwardly flexibility of the click fingers 166).
The direction having the reluctance is the one being used when the piston rod 140 move proximally as the temperature rises.
The leaf spring E is encompassed between the piston rod guide 150 and the click element 155 such that one leg of the leaf spring D is attached to the piston rod guide 150 and the other leg is attached to the click element 165 thus a torque will be introduced in the leaf spring E when the piston rod guide 150 and the click element 165 rotate relative to each other independently of the direction of this rotation.
When the piston rod 140 move proximately and the piston rod guide 150 rotates, the leaf spring E is tighten until the rotational reluctance of the click element 165 is overcome where after the click fingers 166 will move to the subsequent teeth of the toothing 195. The result being that the piston rod guide 150 can perform an unlimited rotation in the expanding direction.
When the temperature decreases and the rubber plunger move distally the torque introduced in the leaf spring E will rotate the piston rod guide 150 since the toothing 195 prevents the click element 165 from rotation in this opposite direction. The result being that the piston rod 140 is rotated forward.
End-of-Content
The End-of-Content mechanism is a so-called non-axial movable cycloid End-of-Content mechanism which is disclosed in details in EP 13-153628 by Novo Nordisk A/S.
The End-of-Content mechanism comprises an End-of-Content ring 155 which internally rides on an outside surface of the clutch 190 and externally is connected to a toothed ring 175 provided inside drive tube 170 such that the End-of-Content ring 155 rotate when the drive tube 170 is rotated relatively to the clutch 190.
During dose setting the drive tube 170 is rotated and the clutch 190 is static thus the End-on-Content ring 155 is rotated. During dosing the drive tube 170 and the clutch 190 rotate together thus maintaining the End-of-Content ring 155 in the same relative position.
Due to the cycloid gearing the End-of-Content ring 155 is rotated a greater angle for each angular rotation of the drive tube 170 thereby counting the rotational movement of the drive tube 170. The total allowable angular movement of the End-of-Content ring 155 is predetermined such that the End-of-Content ring 155 encounters a stop just before the injectable content of the cartridge 1105 has been set. Once the End-of-Content ring 155 reaches its stop, the drive tube 170 cannot be rotated further thus no further dose can be set.
The End-of-Content mechanism thereby counting the accumulated set and ejected doses and stopping further dose setting when this accumulated value equals the initial injectable amount of liquid drug in the cartridge 1105.
After Dosing
Following dose release the user removes the shield 120 from the skin as disclosed in FIG. 7.8.
This makes the needle holder 130 move distally under the influence of the compression spring C as depictured in FIG. 23-24 such that the back-end 104 of the attached pen-needle 100 is pulled out of the septum 1106 of the cartridge 1105.
As the scale drum 160 approaches its zero position as depictured in FIG. 23-24, the surface 164 on the scale drum 160 encounters the protrusion 136 and pushes it out of its engagement with the hook 127 such that the needle holder 130 can move distally independently of the shield 120.
To make sure that the needle holder 130 do not return until the shield 120 has been fully removed from the skin of the user, the protrusion 136 is hindered by axial movement by the supporting surface 115 until the shield 120 has moved a little distance in the distal direction (the distance is indicated by the arrow Z in FIG. 23). The axial movement being the axial distance Z between hook 163 and indentation 123 seen in FIG. 23. Only when the shield 120 has moved to the hooked position (FIG. 25) is the distance to the supporting surface 115 sufficient to allow the protrusion 136 to be fully released where after the needle holder 130 returns to its extended position.
Also following injection, the hook 163 provided on the scale drum 160 once again engages the indentation 123 provided proximately on the shield 120 thus preventing further axial movement of the shield 120 as depictured in FIG. 24-25.
This mechanism is further disclosed in European patent application No.: EP 13-170422 by Novo Nordisk A/S.
Re-Setting
When setting a dose by rotating the dose setting button 1004, the shield 120 is urged forward by the spring B. However, if the user for some reason regrets this and wants to retract the shield 120 this can be done by counter rotating the dose setting button 1004 until the scale drum 160 reaches its zero position and then press the shield 120 in the proximal direction.
The axial movement of the shield 120 with the scale drum 160 in its zero position however results in the proximal side surface of the indentation 123 striking against the distal side surface of the hook 163 of the scale drum 160. Visually this would be as in FIG. 25, however with the indentation 123 being on the left side of the hook 163.
In order for the indentation 123 to pass the hook 163 and obtain an engagement, the hook 163 has to have a certain degree of flexibility in a radial direction (i.e. perpendicular to the axial direction of the scale drum 160) such that the hook 163 can flex outwardly as the indentation 123 passes on the back-side of the hook 163.
In order to obtain such flexibility, the scale drum 160 is provided with a cut-out 167 (see FIG. 24), which cut-out 167 is located in the proximity of the hook 163.
FIG. 24A discloses the area of the scale drum 160 around the hook 163 seen from the backside. The hook 163 is provided with a slanted distal face 168 better allowing the inclined proximal surface of the indentation 123 to pass behind (i.e. on the inward side) the hook 163 in its axial movement.
Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject matter defined in the following claims.

## Claims

1. A spring driven drug delivery device for delivering set doses of a liquid drug comprising:
a housing fixating a cartridge containing the liquid drug to be injected,
a rotatable dose setting button at a proximal end for dialing the size of the dose;
a scale drum threadedly engaged with the housing to perform a helical movement away from a zero position during setting of a dose,
a window provided in the housing through which a user can visually inspect the size of the dose being set by rotating the dose setting button and in which window the cipher “0” or indicia appears whenever the scale drum is in its zero position,
a needle shield for covering the injection needle during use, which needle shield under influence of a spring structure is axially slidable in relation to the housing, and
wherein the scale drum is provided with engaging structure which engages and locks the needle shield from axial movement when in the zero position
and wherein when a dose is dialed the scale drum moves away from its zero position whereby the engagement structure releases and sets the needle shield free to move axially under influence of the spring structure whereby the needle shield is slidable into a position in which the needle shield covers the injection needle.

2. A spring driven drug delivery device according to claim 1, wherein the scale drum is provided with at least one hook engaging a similar indentation provided proximally at the needle shield.

3. A spring driven drug delivery device according to claim 2, wherein the hook engages the indentation once the scale drum has returned to its zero position and the shield has been released.

4. A spring driven drug delivery device according to claim 2, wherein the scale drum is provided with a cut-out.