Patent Publication Number: US-2016220759-A1

Title: Injection Device for Selective Fixed or Variable Dosing

Description:
FIELD OF THE INVENTION 
     The present invention relates to drug delivery devices, e.g. to power assisted drug delivery devices such as automatic injection devices. In particular, the invention relates to dose setting mechanisms for such delivery devices. 
     BACKGROUND OF THE INVENTION 
     Within some therapy areas the tendency of a patient to adhere to the prescribed therapy is dependent on the simplicity of the specific treatment regimen. For example, many people with type 2 diabetes are diagnosed with the disease at a relatively high age where they are less prone to accept a treatment that intervenes too much with their normal way of living. Most of these people do not like constantly being reminded of their disease and, as a consequence, they do not want to be entangled in complex treatment patterns or waste time on learning to operate cumbersome delivery systems. 
     Basically, people with diabetes need to minimise their glucose excursions. Insulin is a well-known glucose lowering agent which must be administered parenterally to be effective in the body. The presently most common way of administering insulin is by subcutaneous injections. Such injections have traditionally been performed using a vial and a syringe, but in recent years so-called injection devices, or injection pens, have gained more and more attention in the marketplace. Many people find these injection devices easier to handle and generally more convenient than the vial and syringe solution. For example, because an injection pen carries a prefilled drug container, or is adapted to receive a prefilled drug container, the user is not required to carry out a separate filling procedure before each injection. 
     Some prior art injection devices suitable for self-injection are adapted to deliver multiple settable doses of drug. The user can set a desired dose by operating a dose setting mechanism and subsequently inject the set dose by operating an injection mechanism. In this case the dose is variable, i.e. the user must set a dose which is suitable in the specific situation each time a dose is to be injected. An example of such an injection device is found in U.S. Pat. No. 5,226,896 (Eli Lilly and Company). 
     Other prior art injection devices are adapted to repeatedly deliver a fixed dose of drug. These devices are typically designed to simplify the administration procedure for people following a therapeutic treatment regimen which involves intermittently injecting the same amount of a drug, and they accordingly offer quick and easy preparation of the fixed dose. An example of such a fixed dose injection device is found in WO 2005/039676 (Eli Lilly and Company). 
     While the above mentioned types of injection devices are quite suitable for delivery of various kinds of drugs according to various treatment plans they do possess obvious drawbacks. For example, in regard to a variable dose injection device each and every dose delivery procedure requires the user to pay particular attention to the dose display during dose setting in order to assure that the correct dose is set before an injection is commenced. In particular when it comes to all mechanical devices striving to be as small and handy as possible the surface useable for displaying the set dose is rather limited and visually impaired users often have trouble reading the physically small dose indicating numerals. In regard to a fixed dose injection device even the most constant treatment regimens may need a dose adjustment at some point, e.g. during a dose titration period, in which case the fixed dose injection device becomes useless and another injection device adapted to deliver a dose of a different size is needed. 
     WO 2009/098299 (Novo Nordisk A/S) discloses a fixed dose injection device which offers an opportunity to change a predetermined fixed dose to a new fixed dose level by rotation of an adjustment ring. This allows a user to change from one predetermined fixed dose to another predetermined fixed dose in a simple manner, useful for example during dose titration. However, the user can only choose between a few predetermined fixed doses selected by the manufacturer, and the feature is therefore only suitable for a limited group of people. 
     In view of the above there is a need for an even more flexible drug delivery device which offers a greater possibility to follow a specific dose regimen, whether it be fixed, varied, or a combination thereof, while still being simple to use and easy to carry about during the day. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to eliminate or reduce at least one drawback of the prior art, or to provide a useful alternative thereto. 
     In particular, it is an object of the invention to provide a drug delivery device which can function selectively as a fixed dose device or as a variable dose device, according to the user&#39;s needs or desires. 
     It is a further object of the invention to provide such a drug delivery device which is safe to use and simple to handle. 
     It is an even further object of the invention to provide a drug delivery device of the above mentioned kind which requires relatively few constructional components, thereby minimising its manufacturing costs. 
     In the disclosure of the present invention, aspects and embodiments will be described which will address one or more of the above objects and/or which will address objects apparent from the following text. 
     A drug delivery device embodying the principles of the present invention comprises a housing, a dose setting mechanism for setting a dose to be delivered from a substance reservoir when the substance reservoir is coupled with the housing, and a dose delivery mechanism, where the dose setting mechanism comprises a dose setting structure operable to set an initial dose and a dose indicating structure, where the dose setting structure and the dose indicating structure are coupled to undergo correlated displacements relative to the housing during setting of the initial dose, respectively during dose delivery, where the correlated displacements during setting of the initial dose are opposite the correlated displacements during dose delivery, where the dose setting structure is moved to a dose prepared position to set the initial dose, the dose prepared position being fixed relative to the housing, and the dose indicating structure is moved to a dose stop position during dose delivery, the dose stop position being fixed relative to the housing, and where the dose indicating structure is selectively displaceable relative to the housing when the dose setting structure is in the dose prepared position to thereby allow adjustment of the initial dose and setting of a final dose. 
     Thus, in one aspect of the invention a drug delivery device is provided comprising:
         a housing,   a dose setting mechanism operable to set a dose to be delivered from a substance reservoir when the substance reservoir is coupled with the housing, and   a dose delivery structure activatable during a dose expelling procedure to cause expelling of a set dose,
 
wherein the dose setting mechanism comprises:
   a dose indicating structure for indicating a size of the set dose, the dose indicating structure being coupled with the dose delivery structure during the dose expelling procedure and moved relative to the housing to a zero dose indicating position, and   a dose setting structure adapted to be moved in a dose preparing direction relative to the housing to a dose prepared position to set a dose of a first size,
 
wherein the zero dose indicating position is fixed with respect to the housing,
 
wherein the dose prepared position is a position along the dose preparing direction which is fixed with respect to the housing,
 
wherein the dose setting structure and the dose indicating structure are coupled and configured to undergo first correlated displacements relative to the housing during movement of the dose indicating structure to the zero dose indicating position and further coupled and configured to undergo second correlated displacements relative to the housing during movement of the dose setting structure to the dose prepared position, the first correlated displacements and the second correlated displacements being mutually reverse, and
 
wherein when the dose setting structure is in the dose prepared position the dose indicating structure is selectively displaceable relative to the housing while the dose setting structure remains stationary in the dose preparing direction to allow adjustment of the dose of the first size and thereby setting of a dose of a second size.
       

     In another aspect of the invention a drug delivery device is provided comprising:
         a housing,   a dose setting structure, and   a dose indicating structure, the dose indicating structure being configured to move from a first dose set position to a zero dose indicating position in response to an operation of a dose activation means to cause a dose delivery structure to deliver a corresponding metered dose from a substance reservoir, the zero dose indicating position being fixed with respect to the housing, and the dose setting structure being configured to move from a dose prepared position to an end-of-dose position in response to a movement of the dose indicating structure from the first dose set position to the zero dose indicating position,
 
wherein the dose setting structure is further movable in a dose preparing direction from the end-of-dose position to the dose prepared position, and the dose indicating structure is further configured to move from the zero dose indicating position to the first dose set position in response to a movement of the dose setting structure from the end-of-dose position to the dose prepared position, and when the dose setting structure is in the dose prepared position the dose indicating structure is selectively repositionable to a second dose set position while the dose setting structure remains stationary in the dose preparing direction, and
 
wherein the dose prepared position is a position along the dose preparing direction which is fixed with respect to the housing.
       

     In a further aspect of the invention a drug delivery device is provided comprising:
         a housing,   a dose setting structure movable in a dose preparing direction relative to the housing to a dose prepared position to define a prepared dose to be delivered from a substance reservoir, the dose prepared position being a position along the dose preparing direction which is fixed with respect to the housing,   a dose indicating structure selectively manipulable when the dose setting structure is in the dose prepared position to adjust the prepared dose and define an adjusted dose to be delivered from the substance reservoir, the dose indicating structure being subsequently movable a first distance or a second distance relative to the housing to a zero dose indicating position in accordance with a delivery of either the prepared dose or the adjusted dose, the zero dose indicating position being fixed with respect to the housing, and   a dose delivery structure responsive to an operation of a dose activation means to be coupled with the dose indicating structure and cause movement of the dose indicating structure to the zero dose indicating position to thereby effect the dose delivery, and to be decoupled from the dose indicating structure thereupon,
 
wherein when the dose indicating structure is being moved to the zero dose indicating position during dose delivery the dose setting structure and the dose indicating structure undergo first correlated displacements relative to the housing, and when the dose setting structure is subsequently being moved to the dose prepared position the dose setting structure and the dose indicating structure undergo second correlated displacements relative to the housing, the first correlated displacements and the second correlated displacements being mutually reverse, and
 
wherein when the dose setting structure is in the dose prepared position displacement of the dose indicating structure relative to the housing is allowed while the dose setting structure remains stationary in the dose preparing direction with respect to the housing.
       

     The substance reservoir, which may be a variable volume reservoir, e.g. comprising a selectively openable and closable drug outlet and a movable wall (such as a conventional cartridge type reservoir comprising a self-sealing septum and a slidable piston), may be nonreleasably coupled with the housing or may be adapted to be coupled with the housing, e.g. via a reservoir support structure configured to hold at least a portion of the substance reservoir, before use of the drug delivery device. Regardless of which, when the substance reservoir is coupled with the housing the drug outlet defines an outlet end of the drug delivery device. 
     The zero dose indicating position is the position which the dose indicating structure takes up when no dose has been set, e.g. following a complete delivery of a dose and before the automatic dose setting resulting from a movement of the dose setting structure to the dose prepared position. In the zero dose indicating position the dose indicating structure signals to the surroundings that no dose is set, e.g. by virtue of being in a certain visually inspectable position relative to the housing. In this certain position the dose indicating structure may e.g. convey a “0” or a like unequivocal symbol. 
     In accordance with the above aspects of the invention a dose setting procedure for the inventive drug delivery device comprises a dose preparation and a selective dose adjustment (at least after the very first dose delivery, as the drug delivery device may be offered by the manufacturer in a pre-use state where the dose setting structure is already in the dose prepared position, while the dose indicating structure is in the zero dose indicating position). Following a first dose delivery the dose preparation is executed by movement of the dose setting structure to the dose prepared position, which is essentially a dose ready position indicating that a set dose is ready for delivery. Movement of the dose setting structure from a first end-of-dose position to the dose prepared position causes a movement of the dose indicating structure which is exactly the reverse of the movement that it underwent during the previous movement of the dose setting structure from the dose prepared position to the first end-of-dose position in connection with the first dose delivery. Thereby, a dose is prepared which corresponds to the dose that was last delivered. This enables a user to use the device as a fixed dose delivery device for a selective number of dose deliveries and thereby avoid having to repeatedly carry out a dose setting procedure that requires scrutiny of small dose indicating numerals. If the dose needs to be changed at a certain point in time the user can easily adjust the prepared dose and set a new dose that corresponds exactly to the dose needed by simple manipulation of the dose indicating structure after dose preparation. Further, the automatic setting of a dose equaling the one that was last delivered provides for an inexpensive mechanical memory in the drug delivery device which allows the user to easily verify the size of the latest administered dose. Such verification is attractive, especially for people who self-administer medication on a regular basis because the administration act itself tends to become a matter of routine, i.e. the procedure is in risk of being carried out without sufficient attention, entailing an increased likelihood of e.g. the dose being confused with other recent administrations. 
     The dose preparing direction may be an axial direction, such as e.g. a longitudinal direction being parallel to a longitudinal axis of the housing, a rotational direction, or a combination of an axial and a rotational direction. In other words, the dose setting structure may be movable to the dose prepared position by translation, rotation, or helical motion, relative to the housing. 
     It is noted that the phrase “the dose setting structure remains stationary in the dose preparing direction with respect to the housing” designates that the dose setting structure neither moves in the dose preparing direction, nor in the direction opposite to the dose preparing direction (relative to the housing). 
     The drug delivery device as defined by the above aspects of the invention can be realised by use of relatively few constructional components and is therefore relatively inexpensive to produce. Further, because the zero dose indicating position and the dose prepared position are well-defined and unchangeable with respect to the housing the dose preparation solution is both accurate and reproducible, as the prepared dose (i.e. the dose of the first size) only depends on the relative position of the dose setting structure and the dose indicating structure at the onset of a dose administration. Also, any accidental, or deliberate, user provoked change of one or both of the zero dose indicating position and the dose prepared position with a resulting compromise of the mechanical memory function is prevented. 
     The dose setting structure and the dose indicating structure may be arranged concentrically along the longitudinal axis, and the dose indicating structure may surround at least a portion of the dose setting structure. This enables the provision of a slender dose setting mechanism which may serve to reduce the overall size of the drug delivery device. 
     Further, the dose delivery structure and the dose setting structure may be arranged concentrically, and the dose setting structure may surround at least a portion of the dose delivery structure. Thereby, the drug delivery device may be realised as a pen-type device having a generally circular cylindrical configuration. Such devices may be preferred by some, as they are of a particularly slender design. 
     The dose preparation (i.e. the setting of the initial dose) may be effected automatically in response to a predetermined user action. The user action may comprise directly contacting the dose setting structure and moving the dose setting structure to a stop, or operating a dose arming structure to abut the dose setting structure, or an intermediate element operatively coupled with the dose setting structure, and move the dose setting structure, or the intermediate element, to a stop. The stop may e.g. be a position defined by an abutment of the dose setting structure or the intermediate element with a wall, or a position defined by an extent of possible movement of a portion of the dose arming structure relative to the housing. 
     In particular embodiments of the invention, the drug delivery device further comprises a cap receiving portion adapted to receive and releasably retain a cap in a position where the cap covers a distal most portion of the drug delivery device, e.g. at least an end portion of the substance reservoir, and the predetermined user action comprises mounting the cap onto the cap receiving portion. In some of these embodiments the cap comprises the dose arming structure, while in others the dose arming structure forms part of the drug delivery device, and the cap is configured to interact with the dose arming structure during mounting onto the cap receiving portion. Either way, a very simple drug delivery device is provided because the cap can be used as a protective cover for the distal portion of the drug delivery device while also being useable to automatically prepare the drug delivery device for delivery of a dose of drug. The dose preparation step is thus in practice carried out unnoticed by the user, as it is integrated into a normal use pattern of the drug delivery device, which already includes a mounting and dismounting of the cap on/from the cap receiving portion between two dose deliveries. 
     When the dose setting structure is in the dose prepared position a user may selectively manipulate the dose indicating structure to adjust the prepared dose, i.e. the user has the option to set a final dose which differs from the last dose delivered, if she/he so desires. Such manipulation may be enabled automatically in response to the dose setting structure reaching the dose prepared position, or manually subsequent to the dose setting structure reaching the dose prepared position, e.g. by release of a lock. 
     The dose indicating structure may comprise dose related indicia usable in a display of the set dose, and the housing may comprise a window through which the dose related indicia are successively viewable, e.g. one indicium at a time. Thereby, the position of the dose indicating structure relative to the housing will be correlated with, and indicative of, the size of the actual set dose. Particularly, the dose indicating structure may be or comprise a scale drum, an odometer, or the like. 
     The drug delivery device may further comprise a user operable dose adjustment structure, e.g. in the form of a dose dial, for displacing the dose indicating structure relative to the housing. In that case the dose adjustment structure is operable to displace the dose indicating structure only when the dose setting structure is in the dose prepared position. This provides for an additional safety measure, as it is thus not possible to manually reposition the dose indicating structure with respect to the housing during a dose delivery procedure (i.e. an operation of the dose activation means leading to an expelling of a set dose from the device), e.g. during a temporary pausing of the dose expelling, and thereby introduce uncertainty as to the dose actually being delivered. 
     The dose adjustment structure may be configured to be decoupled from the dose indicating structure in response to the dose setting structure being moved away from the dose prepared position, and further to be coupled with the dose indicating structure in response to the dose setting structure being brought to the dose prepared position. 
     Alternatively, or additionally, the dose adjustment structure may be configured to be rendered inoperable in response to the dose setting structure being moved away from the dose prepared position, and further to be rendered operable in response to the dose setting structure being brought to the dose prepared position. 
     In some embodiments of the invention the dose adjustment structure comprises a dose dial and the dose indicating structure comprises a scale drum, and the dose dial is rotationally coupled with the scale drum when the dose setting structure is in the dose prepared position and rotationally decoupled from the scale drum when the dose setting structure is moved away from the dose prepared position. 
     In a particular embodiment of the invention the dose setting structure comprises a non-self-locking thread on a first exterior surface portion and a longitudinal groove on a second exterior surface portion bordering on the first exterior surface portion, and the drug delivery device further comprises a rotator serving as an intermediate connecting piece for the dose setting structure and the dose indicating structure. The rotator comprises an exterior longitudinal track, which longitudinal track is adapted for engagement with a protrusion on an interior surface of the dose indicating structure so as to provide a rotational interlocking connection between the rotator and the dose indicating structure. The rotator further comprises an interior projection adapted for engagement with the non-self-locking thread, when the dose setting structure is in the dose prepared position, and for rotational interlocking connection with the longitudinal groove when the dose setting structure is away from the dose prepared position, thereby allowing for a helical displacement of the dose indicating structure in response to a translational displacement of the dose setting structure during dose delivery as well as during dose preparation and for a helical displacement of the dose indicating structure in response to a rotational displacement of the dose setting structure, when the dose setting structure is in the dose prepared position. 
     The dose delivery structure may comprise a piston rod, or a pressure plate, or a like structure capable of applying a force to the substance reservoir. In particular embodiments of the invention the dose delivery structure comprises a piston rod configured to cause displacement of a piston in a cartridge. The dose delivery structure may further comprise a drive member for actuating the piston rod (or the like structure) in response to an operation of the dose activation means. 
     The dose activation means may comprise a user operable dose activation button shiftable between a passive position and an activated position. A shifting of the dose activation button to the activated position may cause movement of the dose indicating structure towards the zero dose indicating position. Further, the dose delivery structure may be configured to be coupled with the dose indicating structure in response to the dose activation button being shifted to the activated position, and to be decoupled from the dose indicating structure in response to the dose activation button being shifted to the passive position. 
     The drug delivery device may be power assisted, e.g. spring assisted, in which case it further comprises a spring means capable of storing and releasing energy for actuation of the dose delivery structure. The spring means may be pre-tensioned and dimensioned to effect an emptying of the substance reservoir without being re-tensioned, or it may be adapted to be tensioned in connection with e.g. a dose setting activity. The spring means may particularly comprise a torque inducing spring, such as e.g. a spiral spring, or a force inducing spring, such as e.g. a compression spring. 
     The spring means may be retained when the dose activation button is in the passive position and released when the dose activation button is in the activated position. Particularly, the spring means may be configured to be released in response to the dose activation button being shifted from the passive position to the activated position and to be detained in response to the dose activation button being shifted from the activated position to the passive position during expelling of the set dose. This enables a pausing of an ongoing dose administration by a shift of the dose activation button to the passive position. 
     The dose activation button may be biased towards the passive position, e.g. by a return spring, whereby a pausing of an ongoing dose administration may be obtained simply by the user terminating the force being applied to the dose activation button. 
     The dose activation button and/or the dose dial may be arranged with respect to the housing (e.g. in the distal half of the housing) so as to become covered by the cap when the cap is mounted onto the cap receiving portion. This will automatically prevent any undesired operation of the dose activation button and/or the dose dial when the drug delivery device is in a non-use state, i.e. without requiring a conscious action by the user. 
     In the present context the term “mutually reverse”, as used in connection with the correlated displacements of the dose setting structure and the dose indicating structure, designates that the individual displacements of the dose setting structure and the dose indicating structure relative to the housing when the dose indicating structure is being moved to the zero dose indicating position during dose delivery, respectively when the dose setting structure is subsequently being moved to the dose prepared position are identical in size but opposite in direction. 
     Further, in the present context the term “proximal” refers to a portion, position or direction opposite or away from the outlet end of the drug delivery device, whereas “distal”, conversely, refers to a portion, position or direction close to or towards the outlet end of the drug delivery device. 
     In the present specification, reference to a certain aspect or a certain embodiment (e.g. “an aspect”, “a first aspect”, “one embodiment”, “an exemplary embodiment”, or the like) signifies that a particular feature, structure, or characteristic described in connection with the respective aspect or embodiment is included in, or inherent of, at least that one aspect or embodiment of the invention, but not necessarily in/of all aspects or embodiments of the invention. It is emphasized, however, that any combination of the various features, structures and/or characteristics described in relation to the invention is encompassed by the invention unless expressly stated herein or clearly contradicted by context. 
     The use of any and all examples, or exemplary language (e.g., such as, etc.), in the text is intended to merely illuminate the invention and does not pose a limitation on the scope of the same, unless otherwise claimed. Further, no language or wording in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following the invention will be further described with references to the drawings, wherein 
         FIGS. 1 a -1 h    illustrate the movement pattern of certain elements of a drug delivery device in an exemplary embodiment of the invention, 
         FIG. 2  is a longitudinal section view of a drug delivery device according to another embodiment of the invention, 
         FIG. 3 a    is a perspective view of a portion of the drug delivery device, 
         FIG. 3 b    shows an enlargement of a section of the drug delivery device shown in  FIG. 3   a,    
         FIG. 4  is a perspective view of a scale drum used in the drug delivery device, 
         FIG. 5  is a perspective view of a dose defining rod used in the drug delivery device, 
         FIGS. 6 a  and 6 b    are different view detailing a rotator used in the drug delivery device, and 
         FIGS. 7-12  are longitudinal section views of the drug delivery device in different states during use. 
         FIG. 13  is a perspective view of a portion of an injection device according to a further embodiment of the invention, 
         FIG. 14  is a perspective view detailing elements of a drive mechanism in the injection device of  FIG. 13 , 
         FIGS. 15-19  are perspective views of the portion of the injection device in different states during use, 
         FIG. 20  is an exploded view of a drug delivery device according to yet a further embodiment of the invention, 
         FIG. 21  is an exploded view of a nut assembly used in the drug delivery device of  FIG. 20 , 
         FIG. 22  is a longitudinal section view of the drug delivery device, 
         FIG. 23  is a close-up section view of a proximal portion of the drug delivery device as delimited by the area Q in  FIG. 22 , 
         FIG. 24  is a perspective longitudinal section view of the nut assembly in a locked state corresponding to the state of the drug delivery device shown in  FIG. 23 , 
         FIG. 25  is a close-up section view of a proximal portion of the drug delivery device showing the nut assembly in an unlocked state, 
         FIG. 26  is a close-up section view of a proximal portion of the drug delivery device showing the nut assembly during drug delivery, and 
         FIG. 27  is a perspective longitudinal section view of the nut assembly in a position corresponding to the one shown in  FIG. 26 . 
     
    
    
     In the figures like structures are mainly identified by like reference numerals. 
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     When in the following relative expressions, such as “upwards” and “downwards”, are used, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only. 
       FIG. 1  is a simplified schematic representation of the principles underlying the present invention.  FIGS. 1 a  through 1 h    illustrate the movement pattern of certain reciprocative drug delivery device elements during dose setting and dose delivery, respectively. The respective movements are to be understood as displacements relative to a base structure of the drug delivery device (not shown in  FIG. 1 ), such as e.g. a drug delivery device housing. In the interest of clarity, the movements are shown as purely axial movements along straight lines, but it is noted that they could just as well be purely rotational movements between angularly offset points of reference, or any combination of axial and rotational movements. 
     As described previously the dose setting procedure to be carried out by a user of a drug delivery device according to the present invention comprises a dose preparation step and an optional dose adjustment step. The dose preparation step will automatically set a dose corresponding to the dose that was last ejected, which will be clear from the below, while the optional dose adjustment step allows a user to change the automatically set dose to a new dose, which then becomes a final set dose. Depending on whether the user has carried out the dose adjustment step or not the final set dose ejected from the drug delivery device in response to an execution of the dose delivery procedure is either the automatically set dose or the new dose. In case the user has made use of the opportunity to change the automatically set dose and the dose ejected thereby differs from the previous dose ejected the next dose preparation step will automatically set a dose corresponding to the new dose. Thereby, if the treatment plan prescribes that the same dose is administered each time the user simply abstains from performing the dose adjustment step and benefits from an effortless automatic dose setting, but she/he has the option to change the dose at any time should the treatment so require. 
     In  FIG. 1 a    a dose preparation element  20 ′ is shown in solid lines at a line P that defines a dose prepared position for the dose preparation element  20 ′. When the dose preparation element  20 ′ is at the line P the drug delivery device is in a “DOSE PREPARED” state in which it is ready to deliver a dose from a drug reservoir (not shown). The dose prepared position of the dose preparation element  20 ′ is an axial position which is fixed with respect to the aforementioned base structure of the drug delivery device. 
     Further, a dose defining element  40 ′ is shown in solid lines in a first extreme position in which it abuts a stop surface M that defines a maximum dose set position for the dose defining element  40 ′. When the dose defining element  40 ′ abuts the stop surface M the maximum settable dose for the drug delivery device is set. Thus, when the dose preparation element  20 ′ and the dose defining element  40 ′ are in the shown positions the drug delivery device is ready to deliver the maximum settable dose. 
       FIG. 1 a    illustrates the movement pattern of the dose preparation element  20 ′ and the dose defining element  40 ′ during dose delivery. The stippled arrows indicate that during dose delivery the dose preparation element  20 ′ and the dose defining element  40 ′ are coupled and undergo correlated movements, whereby the dose defining element  40 ′ moves from the maximum dose set position to a second extreme position in which it abuts a stop surface Z that defines a zero dose indicating position for the dose defining element  40 ′, and the dose preparation element  20 ′ moves from the dose prepared position to a position indicated by a line X that defines a maximum dose delivered position for the dose preparation element  20 ′. 
     A dose delivery mechanism (not shown) is coupled with the dose defining element  40 ′ during movement of the dose defining element  40 ′ from the maximum dose set position to the zero dose indicating position to cause the maximum settable dose to be expelled from the drug reservoir. 
     When the dose defining element  40 ′ abuts the stop surface Z the drug delivery device is in a “DOSE DELIVERED” state in which a set dose has been delivered and no further dose delivery can take place until the dose setting procedure has been carried out. The stop surface Z is fixed with respect to the base structure of the drug delivery device. As will be clear from the following, a movement of the dose defining element  40 ′ to the zero dose indicating position is, however, only accompanied by a movement of the dose preparation element  20 ′ to the maximum dose delivered position when the maximum settable dose is being delivered. 
       FIG. 1 b    illustrates the movement pattern of the dose preparation element  20 ′ and the dose defining element  40 ′ during the subsequent dose preparation. The stippled arrows again indicate that the dose preparation element  20 ′ and the dose defining element  40 ′ are coupled and undergo correlated movements. Thus, when the dose preparation element  20 ′ moves back to the dose prepared position, e.g. as a result of the user performing a simple predetermined action, the dose defining element  40 ′ moves back accordingly to the maximum dose set position. Thereby, the maximum settable dose has automatically been prepared for delivery. During the dose preparation the dose delivery mechanism is decoupled from the dose defining element  40 ′. It is noted that the direction in which the dose preparation element  20 ′ moves towards the dose prepared position is referred to as the dose preparing direction. 
       FIG. 1 c    illustrates the optional dose adjustment which the user may choose to make use of to alter the prepared dose and set a different final dose to be delivered. When, or after, the dose preparation element  20 ′ reaches the dose prepared position, as shown with solid lines, the dose preparation element  20 ′ and the dose defining element  40 ′ become decoupled with respect to movements in the dose preparing direction, allowing the user to reposition the dose defining element  40 ′ relative to the dose preparation element  20 ′ and the base structure of the drug delivery device, while the dose preparation element  20 ′ remains stationary, at least in the dose preparing direction. This repositioning is indicated by the stippled arrow. In  FIG. 1 c    the dose defining element  40 ′ is moved to a position d 1 , which is a dose set position corresponding to a particular dose, “Dose 1”, smaller than the maximum settable dose. “Dose 1” is thus the final set dose which will be delivered in response to the next execution of the dose delivery procedure. In case the user chooses not to reposition the dose defining element  40 ′ relative to the dose preparation element  20 ′ then the prepared maximum settable dose will be delivered in response to the next execution of the dose delivery procedure. 
       FIG. 1 d    illustrates the movement pattern of the dose preparation element  20 ′ and the dose defining element  40 ′ during delivery of “Dose 1”. The dose preparation element  20 ′ and the dose defining element  40 ′ are once again coupled and undergo correlated movements, whereby the dose defining element  40 ′ is brought from position d 1  to the zero dose indicating position and the dose preparation element  20 ′ is brought from the dose prepared position to a position e 1  between the dose prepared position and the maximum dose delivered position. The position e 1  is thus an end-of-dose position for the dose preparation element  20 ′ corresponding to the delivery of “Dose 1”. 
       FIG. 1 e    illustrates the movement pattern of the dose preparation element  20 ′ and the dose defining element  40 ′ during the subsequent dose preparation. Here, because of the operative coupling between the dose preparation element  20 ′ and the dose defining element  40 ′ the reverse movement of the dose preparation element  20 ′ from position e 1  to the dose prepared position, e.g. resulting from a predetermined user action similar or identical to the one previously performed, causes a movement of the dose defining element  40 ′ back to position d 1 . The drug delivery device is thus brought into a “DOSE PREPARED” state in which it is automatically ready to deliver “Dose 1” once more. 
       FIG. 1 f    illustrates another dose adjustment, following the preparation of “Dose 1”. Because the dose preparation element  20 ′ has reached the dose prepared position repositioning of the dose defining element  40 ′ relative to the dose preparation element  20 ′ is possible and the dose defining element  40 ′ is moved to a position d 2 , which corresponds to another particular dose, “Dose 2”, smaller than the maximum settable dose but larger than “Dose 1”. “Dose 2” is now the final set dose which will be delivered in response to the next execution of the dose delivery procedure. 
       FIG. 1 g    illustrates the movement pattern of the dose preparation element  20 ′ and the dose defining element  40 ′ during delivery of “Dose 2”. The two elements are once again coupled and undergo correlated movements, bringing the dose defining element  40 ′ from position d 2  to the zero dose indicating position and the dose preparation element  20 ′ from the dose prepared position to a position e 2  between position e 1  and the maximum dose delivered position. Position e 2  is thus an end-of-dose position for the dose preparation element  20 ′ corresponding to the delivery of “Dose 2”. 
     In  FIG. 1 h   , illustrating the subsequent dose preparation, the dose preparation element  20 ′ is moved to the dose prepared position, e.g. in response to a predetermined user action, causing the dose defining element  40 ′ to move back to position d 2  due to the operative coupling with the dose preparation element  20 ′. Thus, the predetermined user action has this time caused “Dose 2” to be automatically prepared for delivery. Any subsequent execution of the dose delivery procedure without intermediate dose adjustment will therefore cause “Dose 2” to be delivered from the drug delivery device (as long as a dose of that particular size is available for delivery from the drug reservoir). 
     From the foregoing it is clear that every time a dose is being prepared the dose preparation element  20 ′ is brought to the dose prepared position, regardless of from which position it departs, and every time a dose is being delivered the dose defining element  40 ′ is brought to the zero dose indicating position, regardless of from which position it departs. Furthermore, the dose preparation element  20 ′ and the dose defining element  40 ′ are coupled and undergo mutually reverse correlated movements during dose preparation and dose delivery and are decoupled at least with respect to movements in the dose preparing direction when the drug delivery device is in the “DOSE PREPARED” state. It is therefore the position of the dose defining element  40 ′ relative to the base structure of the drug delivery device (e.g. the housing) in the “DOSE PREPARED” state that determines the size of the dose to be delivered. Each set dose matches a unique position of the dose defining element  40 ′ relative to the base structure of the drug delivery device, and each delivered dose matches a unique position of the dose preparation element  20 ′ relative to the base structure of the drug delivery device. This means that every time a dose has been delivered the dose preparation element  20 ′ holds a specific position relative to the base structure of the drug delivery device which depends on the size of the delivered dose, and because the dose preparation element  20 ′ and the dose defining element  40 ′ are coupled and undergo mutually reverse correlated movements during dose preparation and dose delivery every time a dose has been prepared the dose defining element  40 ′ has been brought back to the position from which it last departed. In other words, a movement of the dose preparation element  20 ′ to the dose prepared position will always cause an automatic preparation of a dose which equals the last dose delivered. This automatically prepared dose can then be selectively adjusted by the user to set a new final dose before the dose delivery procedure is carried out. 
     The above described solution is attractive from a user perspective because the device is simple to handle and offers automatic preparation of a dose corresponding to the one that has last been delivered in response to a simple predetermined user action, while it also offers selective manual adjustment of the prepared dose. It is also attractive from a manufacturing perspective because relatively few components are required to provide a highly accurate device offering a high degree of user convenience. 
     The respective stop surfaces M, Z in  FIG. 1  make up axial stops for axial movements of the dose defining element  40 ′. It is noted, however, that within the scope of the invention these stops need not be axial, but may alternatively be rotational or transversal, or they may be a combination of axial, rotational and transversal. 
     Also, the various indicated correlated movements of the dose preparation element  20 ′ and the dose defining element  40 ′ in  FIG. 1  may give the impression that the two elements move exactly the same distance in the same direction when a dose is prepared, respectively when a dose is delivered. This is, however, not necessarily the case. In different implementations of the inventive concept the dose preparation element  20 ′ and the dose defining element  40 ′ may in fact move different distances and/or in different directions during dose preparation and/or dose delivery, as will be clear from the below. The magnitude and direction of displacement of the respective elements are thus inessential to the practice of the invention, as long as the movements are correlated, i.e. as long as one particular movement of the one element (in the dose preparation/dose delivery phase) is always accompanied by one particular movement of the other element, and vice versa. 
       FIG. 2  displays a longitudinal section view of an injection device  1  according to an exemplary embodiment of the invention. The injection device  1 , which is depicted in a pre-use state, has a base structure in the form of a housing  2  to which a drug cartridge  10  is attached. The cartridge  10  is axially fixed with respect to the housing  2  by means of a cartridge holder  14 . The distal end portion of the cartridge  10  is closed by a penetrable self-sealing rubber septum  11  and the proximal end portion is sealed by a slidable rubber piston  12 , the septum  11  and the piston  12  together defining a variable volume chamber  13  within the frame of the elongated cartridge wall. 
     A needle assembly, comprising an injection needle  17  fixedly retained in a needle hub  16 , is mounted on a threaded needle interface  15  of a needle mount  18  such that the back end of the injection needle  17  transpierces the septum  11  and resides in the chamber  13 . 
     A threaded piston rod  60  abuts the piston  12  via a piston rod foot  61  and is adapted to cause the piston  12  to move downwards in the cartridge  10  and force a volume of the substance in the chamber  13  out through the injection needle  17 . The movement of the piston rod  60  is helical and guided by a nut  62  which is integrally formed with the housing  2 . The piston rod  60  is in splined engagement with a piston rod guide  63  which is adapted to impart a rotary motion on the piston rod  60  during dose delivery. The piston rod guide  63  is an elongated cylindrical structure which surrounds a portion of the piston rod  60  and which has a proximal extension  64  that is fitted about an interior sleeve  85  of a spring housing  3  fixedly attached to the housing  2 . The proximal extension  64  carries a couple of flexible arms (not visible) which interact with a ring of jagged teeth  87  arranged interiorly in the spring housing  3  to provide a ratchet mechanism ensuring a unidirectional rotation of the piston rod guide  63  relative to the housing  2 . 
     The piston rod guide  63  further has a toothed exterior belt portion (not visible) at its centre region which is adapted for engagement with an interior toothing (not visible) in an axially displaceable gear  80  during dose delivery, as will be explained below. The gear  80  has a proximal toothed rim portion  81  adapted for engagement with an interior circumferential toothing  86  in the spring housing  3 , except from during dose delivery, and a central toothed rim portion  82 . The gear  80  is axially displaceable by means of a transmission bar  70  which extends axially from the distal portion of the injection device  1 , as is best seen in  FIG. 3 a   , and which includes a coupling rod  72  with a catch portion  73  for engagement with a distal portion of the gear  80 . The transmission bar  70  is biased towards the proximal end of the injection device  1  by a return spring  65 , which means that the proximal toothed rim portion  81  is biased towards engagement with the circumferential toothing  86 . 
     A pre-tensioned spiral spring  50  is arranged in the spring housing  3 . The spring  50  comprises an outer spring end portion which is fixed to the spring housing  3  and an inner spring end portion which is fixed to a rotatable spring shaft  51 . The spring shaft  51  extends axially downwards from the spring  50  and is rotationally locked to a cog wheel  52  which is in turn rotationally coupled with the central toothed rim portion  82  of the gear  80 . 
     A rotator  30  having a proximal toothed head  32  is axially fixed to the housing  2  just distally of the cog wheel  52 . The toothed head  32  is adapted to receive and engage with the central toothed rim portion  82  when the gear  80  is displaced distally against the bias of the return spring, as described below. The rotator  30  further comprises a hollow cylindrical sleeve  31  which accommodates an elongated dose preparation rod  20 . The dose preparation rod  20  has an exterior helical groove  23  which terminates distally in a longitudinal groove  22 . An interior protrusion  33  in the sleeve  31 , which in  FIG. 2  is positioned within the longitudinal groove  22 , is adapted to travel at least a portion of the helical groove  23  during dose setting and dose delivery. The dose preparation rod  20  further comprises a coupling head  21  which in the shown state of the injection device  1  abuts an interior wall  58 . The dose preparation rod  20  is able to move axially between the shown proximal position and a distal position in which the coupling head  21  abuts a radial wall  59 . 
     The rotator  30  is further rotationally locked to a scale drum  40  which carries dose related indicia  41  (see  FIG. 4 ) on an exterior surface and which is structured to undergo helical displacement along an interior thread  28  in the housing  2  when rotated by the rotator  30 . In the shown pre-use state of the injection device  1  the scale drum  40  is in a zero dose indicating position relative to the housing  2  which indicates that no dose is set. Any particular position of the scale drum  40  relative to the housing  2  corresponds to a particular set dose, which is displayed through a window  99 . From the position in  FIG. 2  the scale drum  40  is displaceable helically downwards until a stud  46  on the distal end surface of the scale drum  40  meets a distal stud  92  in the housing  2 , constituting a rotational stop for displacement of the scale drum  40  in the distal direction. This defines a bottom position of the scale drum  40  relative to the housing  2  and corresponds to a maximum settable dose being set. 
     The coupling head  21  is provided with teeth  24  along its circumference (see  FIG. 5 ) which in the shown position of the dose preparation rod  20  are in rotational engagement with an inner toothed portion of a dose dial  55 . The dose dial  55  is adapted to be operated by a user to define a final dose that will be administered during dose delivery. During dose delivery the coupling head  21  is displaced distally whereby the teeth  24  are disengaged from the dose dial  55  and engaged with an interior spline  29  instead, preventing rotation of the dose preparation rod  20  relative to the housing  2 . 
     An injection button  57  is arranged distally of the dose dial  55  and is axially slidable relative to the housing  2  between a proximal passive position and a distal activated position in which a dose delivery is initiated. The injection button  57  is axially locked to a carriage  56  which is configured to bring the dose preparation rod  20  to a dose start position in response to the injection button  57  being slid to the activated position. 
     In the pre-use state shown in  FIG. 2  a cap  4  is mounted on the injection device  1 , covering the distal portion of the cartridge  10 , the needle assembly, the dose dial  55  and the injection button  57 . The cap  4  comprises a cylindrical side wall  5  and an end wall  6 . An interior sleeve  7  is formed on an inner surface of the end wall  6  and provides a cavity  8  for reception of an actuation rod  9 . The actuation rod  9  is axially locked to the interior sleeve  7  and extends proximally through the radial wall  59  to the distal end surface of the coupling head  21 . The cap  4  is received and releasably retained by a cap receiving portion of the housing  2 . 
       FIG. 3 a    is a perspective view of the injection device  1  stripped of the housing  2  and the cap  4  to reveal that the transmission bar  70  is axially coupled at its distal end portion to the injection button  57 . It is further seen that the transmission bar  70  comprises an axially extending leg  71 . The leg  71  is axially coupled with the coupling rod  72  and the axial position of the gear  80  in the housing  2  is therefore determined by the axial position of the injection button  57  in the sense that when the injection button  57  is moved from the passive position to the activated position the transmission bar  70  and the gear  80  are urged distally against the bias of the return spring  65  and when the injection button  57  is released the return spring  65  moves the injection button  57  and thereby the transmission bar  70  and the gear  80  proximally backwards until the injection button  57  returns to the passive position. 
       FIG. 3 b    is an enlargement of the portion of the injection device  1  delimited by the area Q in  FIG. 3 a   . The enlargement further details the various connections between the dose dial  55 , the carriage  56 , the dose preparation rod  20 , and the rotator  30 . In particular, the figure shows the rotational engagement between the dose dial  55  and the dose preparation rod  20 , and the axial engagement between the carriage  56  and the dose preparation rod  20  via the interior wall  58 . Also shown are axial splines  35  on the exterior of the rotator  30  used for rotationally locking the rotator  30  to the scale drum  40 . 
       FIG. 4  is a perspective view of the scale drum  40 , showing some of the dose related indicia  41  printed, embossed or otherwise applied to its peripheral surface. For the sake of clarity, only the numbers  0 ,  24 ,  48 , and  72  are shown in this figure, but the scale drum  40  is preferably configured to also display set doses between these numbers, e.g. in single unit increments. A helical track segment  42  is arranged on the surface for engagement with the interior thread  28 . Finally, the figure shows a longitudinal interior projection  44  adapted for reception in one of the splines  35 . The scale drum  40  has three such projections distributed equidistantly along an interior circumference for reception in corresponding splines on the rotator  30  to ensure a stable rotational connection between the scale drum  40  and the rotator  30 . 
       FIG. 5  is a perspective view of the dose preparation rod  20  which details that the helical groove  23  and the longitudinal groove  22  are connected and furthermore shows the toothed structure of the coupling head  21 . Twenty-four teeth  24  are distributed equidistantly along the circumference of the coupling head  21 , corresponding to the number of possible rotational positions per revolution of the dose preparation rod  20  relative to the housing  2 . The twenty-four teeth  24  allow the dose preparation rod  20  to become rotationally locked relative to the housing  2  by interaction with the interior spline  29  during dose delivery, regardless of its actual rotational position relative to the housing  2 . The coupling head  21  has an annular abutment face  25  which serves as a means for engagement with the interior wall  58 . 
       FIGS. 6 a  and 6 b    are perspective, respectively longitudinal section views further detailing the rotator  30 . As seen, the toothed head  32  holds a number of teeth  34  which are provided for interaction with the central toothed rim portion  82  in a simple gear connection. 
       FIGS. 7-12  are longitudinal section views of the injection device  1  in different states during use. In the following a use sequence of the injection device  1  will be explained with reference to these figures. 
       FIG. 7  shows the injection device  1  in a state just before delivery of the very first dose. The cap  4  has been dismounted from the housing  2  and a dose has been set by rotation of the dose dial  55  about the longitudinal axis defined by the dose preparation rod  20 . When the dose dial  55  is rotated the rotational engagement between the teeth  24  and the inner toothed portion of the dose dial  55  causes a joint rotation of the dose preparation rod  20  which due to the interior protrusion  33  residing in the longitudinal groove  22  causes a joint rotation of the rotator  30 . Because of the engagement between the axial splines  35  and the longitudinal interior projections  44  the scale drum  40  is also forced to rotate, whereby the scale drum  40  is displaced helically downwards in the housing  2  along the interior thread  28 . As the scale drum  40  performs this helical movement relative to the housing  2  the dose indicia  41  sequentially pass by the window  99  to indicate which dose is set. If the user by accident dials too large a dose the direction of rotation of the dose dial  55  is simply reversed, whereby the direction of rotation of the dose preparation rod  20 , the rotator  30 , and the scale drum  40  is similarly reversed and the scale drum  40  moves helically upwards in the housing  2 . The total angular displacement of the dose dial  55  relative to the housing  2  thus correlates with the helical displacement of the scale drum  40  relative to the window  99  and thereby with the actual dose set. The position of the scale drum  40  in  FIG. 7  indicates that a dose corresponding to approximately one third of the maximum settable dose has been set. 
     In  FIG. 8  the injection button  57  is slid forward to its distal activated position and the figure shows the injection device  1  in a dose begin state just before the spring  50  releases energy. The distal movement of the injection button  57  causes the carriage  56  to pull the dose preparation rod  20  distally due to the engagement between the interior wall  58  and the abutment face  25 . The axial movement of the dose preparation rod  20  causes the teeth  24  to disengage from the dose dial  55  and engage with the interior spline  29  instead, thereby locking the coupling head  21  rotationally to the housing  2 . Furthermore, the relative axial motion between the dose preparation rod  20  and the rotator  30  causes the interior protrusion  33  to become positioned at the junction between the longitudinal groove  22  and the helical groove  23 . The distal movement of the injection button  57  simultaneously causes the transmission bar  70  to pull the gear  80  distally, against the biasing force from the return spring  65 , via the coupling rod  72  and the catch portion  73 , whereby the central toothed rim portion  82  firstly moves into additional engagement with the toothed head  32 , and the proximal toothed rim portion  81  subsequently disengages from the circumferential toothing  86  and releases the pre-tensioned spring  50 . 
       FIG. 9  illustrates that as a result thereof the spring  50  unwinds and the inner spring end portion rotates the spring shaft  51 . This rotation is transferred to the toothed head  32  via the central toothed rim portion  82  and as the rotator  30  spins the scale drum  40  is driven upwards along the interior thread  28  towards its zero dose indicating position, while the interior protrusion  33  travels upwards in the helical groove  23  and the dose preparation rod  20  consequently is urged downwards due to its rotational locking engagement with the housing  2 . 
     When a proximal portion of the scale drum  40  rotates into abutment with a proximal stud (not visible) in the housing  2 , the scale drum  40  has returned to the zero dose indicating position and therefore cannot rotate further in that direction relative to the housing  2 . As a consequence, the rotator  30  stops rotating and the dose preparation rod  20  stops the downwards displacement relative to the housing  2  in an axial position which constitutes an end-of-dose position corresponding to the particular dose delivered. The spring  50  is at this point prevented from releasing further energy, and the injection device  1  is in a “DOSE DELIVERED” state. In principle, the above described respective movements of the scale drum  40  and the dose preparation rod  20  correspond to the respective movements of the dose defining element  40 ′ and the dose preparation element  20 ′ sketched in  FIG. 1   d.    
     As long as the spring  50  releases energy the gear  80  rotates due to the interaction between the spring shaft  51  and the central toothed rim portion  82 , and because the gear  80  has been moved distally by the transmission bar  70 , and the interior toothing (not visible) in the gear  80  thereby has moved axially into engagement with the toothed exterior portion (not visible) on the piston rod guide  63 , the rotation of the gear  80  is transferred to the piston rod guide  63  and therefrom to the piston rod  60 , which due to the engagement with the nut  62  is advanced helically in the distal direction. The piston  12  is thereby pushed into the cartridge  10  to reduce the volume of the drug containing chamber  13  and expel an amount of drug through the injection needle  17 . Once the scale drum  40  reaches the zero dose indicating position and the spring  50  is prevented from further unwinding the gear  80  stops rotating and the dose expelling consequently stops. In this position of the scale drum  40  the dose indicia  41  indicate through the window  99  that no dose is prepared for delivery. 
     By release of the injection button  57 , as shown in  FIG. 10 , the transmission bar  70  and, consequently, the gear  80  are moved proximally by the return spring  65 . This causes the central toothed rim portion  82  to disengage from the toothed head  32  and the proximal toothed rim portion  81  to re-engage with the circumferential toothing  86 , thereby securing the spring  50 . The proximal movement of the transmission bar  70  also causes the injection button  57  to return to its proximal passive position, pushing the carriage  56  along whereby the interior wall  58  is forced into abutment with the dose dial  55 . 
     Notably, however, the dose preparation rod  20  is not moved by this action. The dose preparation rod  20  remains in the end of dose position and is thus still rotationally locked with respect to the housing  2 . This means that in the dose delivered state of the injection device  1 , where a new dose has not yet been automatically prepared for delivery, a rotation of the dose dial  55  has no effect on the position of the scale drum  40 , as the two are decoupled. 
     It is further noted that in case the user for some reason wishes to pause a dose delivery, at any time during the expelling of drug from the chamber  13 , she/he simply releases the injection button  57 , whereby the return spring  65  will force the transmission bar  70  and the gear  80  proximally in the housing  2  and cause the proximal toothed rim portion  81  to engage with the circumferential toothing  86  and stop the spring  50  from further unwinding, similar to what is described above. Importantly, as the coupling head  21  becomes disengaged from the dose dial  55  before the spring  50  is released, when the injection button  57  is activated, and does not return when the injection button  57  is released, it is not possible to operate the dose dial  55  to change the dose to be delivered when an injection is paused. Thereby, the user is prevented from adjusting the final set dose in the course of a delivery procedure and thus potentially becoming uncertain of the actual dose set. 
       FIG. 11  shows the injection device  1  in a “DOSE PREPARED” state after remounting of the cap  4 . The remounting motion of the cap  4  relative to the housing  2  causes the actuation rod  9  to exert a proximally directed push force on the coupling head  21  which then causes the dose preparation rod  20  to undergo a reverse, axial displacement back to the dose prepared position along the interior spline  29 . This displacement back to the dose prepared position, where the abutment face  25  abuts the interior wall  58 , forces the interior protrusion  33  to travel downwards in the helical groove  23  and further into the longitudinal groove  22  back to the position it originally held in the longitudinal groove  22  before the injection button  57  was slid to the activated position. While the interior protrusion  33  travels back down in the helical groove  23  the rotator  30  rotates and slaves the scale drum  40 . When the interior protrusion  33  reaches the junction between the helical groove  23  and the longitudinal groove  22  the rotation of the rotator  30  stops, and at that point the scale drum  40  has undergone a helical displacement along the interior thread  28  which is identical in size but opposite in direction to the one it underwent during its movement to the zero dose indicating position, and it has thus been brought back to the same position relative to the housing  2  that it had before the dose delivery was commenced, i.e. the number that can be read through the window  99  equals the dose that was just delivered. 
     The last part of the relative motion between the dose preparation rod  20  and the rotator  30  during the mounting of the cap  4  onto the injection device  1  is purely axial as the interior protrusion  33  is then positioned in the longitudinal groove  22 . In other words, during the last part of the returning displacement of the dose preparation rod  20 , where the coupling head  21  disengages from the interior spline  29  and re-engages with the inner toothed portion of the dose dial  55 , the scale drum  40  remains stationary relative to the housing  2 . 
     The mounting of the cap  4  onto the cap receiving portion of the housing  2  thus causes a) the dose preparation rod  20  to undergo a displacement relative to the housing  2  which is exactly opposite to the displacement it underwent in the course of the dose delivery procedure, and b) the scale drum  40  to undergo a displacement relative to the housing  2  which is exactly opposite to the displacement it underwent in the course of the dose delivery procedure. 
     When the cap  4  is mounted on the cap receiving portion after the very first dose delivery the injection device  1  is not only in a “DOSE PREPARED” state but also in a secured or inactive state, because even though a dose has actually been prepared for delivery the side wall  5  covers the injection button  57  which is thus kept inaccessible for operation. Similarly, when the cap  4  is on the dose dial  55  is inaccessible for operation, and the prepared dose can therefore not be adjusted. Consequently, the injection device  1  may be carried about safely in e.g. a bag or pocket without the user risking an inadvertent adjustment of the prepared dose or an administration of the prepared dose to the cap  4 . 
     When the dose preparation rod  20  is in the dose prepared position it is snap fitted to the rotator  30  and is thereby axially releasably fixed with respect to the housing  2 . It will therefore stay in that position when the user removes the cap  4  before taking the next injection, as illustrated in  FIG. 12 . After removing the cap  4  the user has two options; in case the dose required in connection with the upcoming injection is the same as the one previously delivered no dose setting actions are needed and the injection needle  17  can simply be inserted into the skin and the injection button  57  slid to the activated position. In case the user needs to adjust the dose, up or down, the dose dial  55  is operable to carry out the adjustment, preferably before the injection needle  17  is inserted into the skin. 
     As set out in the above, when taking the injection device  1  into use for the first time the user may initially set a dose to be delivered and then only operate the dose dial  55  again in case the initial dose needs to be changed for a subsequent injection. Alternatively, the injection device  1  may be pre-set by a health care professional, or by the manufacturer, in which case the user is free from setting a dose initially, or entirely. 
     Further, as described in the above, following the very first dose delivery a dose is prepared for delivery automatically in response to the mounting of the cap  4  on the cap receiving portion. It is, however, clear that other means of returning the dose preparation rod  20  to the dose prepared position than the cap  4  may be employed. For example, the actuation rod  9  may be provided as a separate item and may be used by the user to push the coupling head  21  back into abutment with the interior wall  58  independently of the cap  4 . 
       FIG. 13  is a perspective view of a portion of an injection device  100  according to another exemplary embodiment of the invention, specifically of a proximal portion of the injection device  100 , carrying a dose engine. The injection device  100  is in a pre-use state and portions of some elements thereof have been removed from the figure to provide a detailed overview of the construction. 
     The injection device  100  is of the so-called pen injector type and has a tubular housing  102  extending along a longitudinal general axis and accommodating a number of functional components. The housing  102  is coupled with a drug containing cartridge (not shown) in a manner conventionally known in the art, i.a. meaning that the cartridge during use of the injection device  100  is at least axially fixed with respect to the housing  102 . Central to the function of the injection device  100  is an axially extending piston rod  160  which is in threaded engagement with a nut  162  that is both axially and rotationally fixed in the housing  102 . The distal end portion of the piston rod  160  is coupled to a piston (not shown) in the cartridge such that any advancing axial motion of the piston rod  160  is transferred to the piston, essentially for pressurisation of the cartridge, as is also conventionally known in the art. 
     It is noted that all rotational movements described in relation to this embodiment of the invention and referred to as clockwise or counter-clockwise are described as seen from the distal end of the piston rod  160  (i.e. from left to right in  FIG. 13 ). 
     The housing  102  is provided with an interior thread  128  which cooperates with an exterior helical track segment  142  on a scale drum  140 , allowing the scale drum  140  to undergo a well-defined helical motion in the housing  102 . The scale drum  140  carries a plurality of dose indicia  141  for indicating to a user the particular size of a set dose. The dose indicia  141  are successively viewable through a window  199  in the housing  102  when the scale drum  140  travels along the interior thread  128  e.g. from a proximal “zero dose” position to a distal “maximum dose set” position. The proximal “zero dose” position is defined by a proximal stop surface (not visible) providing a rotational stop for proximal motion of the scale drum  140  at the proximal end of the interior thread  128 , whereas the “maximum dose set” position is defined by a distal stop surface (not visible) providing a rotational stop for distal motion of the scale drum  140  at the distal end of the interior thread  128 . 
     The scale drum  140  is rotationally locked to a rotator  130  via a longitudinal interior projection  144  (see  FIG. 16 ) and an axially extending spline  135  on the exterior surface of the rotator  130 . While rotationally interlocking the scale drum  140  and the rotator  130  this splined connection allows relative axial motion between the two. The rotator  130  is at its distal end portion axially locked to a coupling piece  173  which comprises an axially aligned leg  171  with a radially inwardly facing toothed surface  172 . At the proximal end portion of the rotator  130  a push button  157  is arranged, which is axially locked to but rotationally decoupled from the rotator  130 , and the two together serve as an injection button. Further, a sleeve  131  extends axially from an inner end face  103  of the rotator  130 . The sleeve  131  has a toothed inner surface and is configured to be brought into and out of rotational interlocking engagement with a toothed end portion  122  of a dose preparation tube  120  which extends axially within the housing  102 . 
     The dose preparation tube  120  has a threaded end portion  123  opposite the toothed end portion  122 . The threaded end portion  123  interfaces with a drive nut  195  in a non-self-locking thread engagement. The drive nut  195  forms part of an actuation rod  109 , the function of which will be described in detail below. The actuation rod  109 , which is axially displaceable but rotationally fixed with respect to the housing  102 , has a longitudinal extension  196  which ends in an abutment face  197 . The longitudinal extension  196  is transversally offset from a main portion of the actuation rod  109  and is adapted to slide along a cartridge holder (not shown in  FIG. 13 ) both during dose delivery and dose preparation. The cartridge holder is attached to a distal portion of the housing  102  and serves to hold and protect the cartridge in a manner conventionally known in the art. 
     A pre-tensioned compression spring  150  is arranged to act between the inner end face  103  and the actuation rod  109 , constantly biasing the rotator  130  and the push button  157  proximalty, out of the housing  102 , and the actuation rod  109  distally. In the shown pre-use state of the injection device  100  distal motion of the actuation rod  109  is prevented by a lock member  180  abutting a transversal surface  198  of the actuation rod  109 . The lock member  180  is pivotally arranged on the nut  162  but is in  FIG. 13  prevented from pivoting by an edge portion of a button coupling rod  175  which is axially displaceable but rotationally fixed with respect to the housing  102 . The button coupling rod  175  has a toothed straight edge  178 , which is in engagement with a transmission wheel  170 , and a longitudinal extension  176 , which is transversally offset from the toothed straight edge  178  and which ends in an abutment face  177 . The transmission wheel  170  is further in engagement with the toothed surface  172 , such that the coupling piece  173 , the button coupling rod  175 , and the transmission wheel  170  together provide a double rack and pinion drive. 
     In the present situation, given that the actuation rod  109  is prevented from undergoing distal motion in the housing  102  due to the lock member  180 , the bias of the spring  150  on the rotator  130  causes the rotator  130  to exert a pulling force on the coupling piece  173  which then via the double rack and pinion arrangement is converted to a distal movement of the button coupling rod  175 , unless a counter-acting force is applied to the abutment face  177 . Although not shown in  FIG. 13 , in the depicted pre-use state of the injection device  100  a removable protective cap is securely mounted onto a cap receiving portion at the distal end portion of the housing  102  such that a portion of the cap abuts the abutment face  177  and resists the bias conveyed to the longitudinal extension  176 , thereby maintaining the button coupling rod  175  in position. The injection device  100  is thus in fact stably locked in a tensioned state. As will be explained in more detail below once the retaining force on the abutment face  177  is removed the relaxation of the spring  150  will cause the rotator  130  and the push button  157  to translate proximally. A stop surface  136  on the rotator  130  limits the proximal motion of the rotator  130  and the push button  157  relative to the housing  102 . 
       FIG. 14  is a detailed view of the piston rod advancement mechanism as employed in the injection device  100 . A rotatable piston rod guide  163  couples the nut  162  and the dose preparation tube  120  via an inner groove  167  for axial interlocking connection with the nut  162  and an inner groove  168  for axial interlocking connection with the threaded end portion  123 . The piston rod guide  163  has a distal pawl  164 , which in combination with a plurality of circumferentially spaced apart indentations  187  on the nut  162  provide a distal ratchet mechanism, and a proximal pawl  166  which in combination with a plurality of circumferentially spaced apart indentations  126  on the dose preparation tube  120  provide a proximal ratchet mechanism. The distal ratchet mechanism allows clockwise rotation of the piston rod guide  163  relative to the nut  162  but prevents counter-clockwise rotation of the piston rod guide  163 . The proximal ratchet mechanism allows relative rotation between the dose preparation tube  120  and the piston rod guide  163  when the dose preparation tube  120  is rotated counter-clockwise, but prevents relative rotation between the dose preparation tube  120  and the piston rod guide  163  when the dose preparation tube  120  is rotated clockwise. The double ratchet comprised of the distal ratchet mechanism and the proximal ratchet mechanism thus allows the dose preparation tube  120  to drag the piston rod guide  163  along in the clockwise direction and to rotate freely in the counter-clockwise direction while the piston rod guide  163  remains stationary. 
     The piston rod guide  163  further has a radially inwardly directed protrusion (not visible) for engagement with an axial groove  169  on the piston rod  160 . The piston rod  160  and the piston rod guide  163  are thus rotationally interlocked but capable of relative axial motion. 
     The functionality of the dose setting and delivery mechanisms will now be described with reference to  FIGS. 15-19 . When taking the injection device  100  into use the protective cap is firstly removed. This removes the retaining force on the abutment face  177  and allows the spring  150  to expand. The spring  150  thus urges the rotator  130  with the push button  157  proximally until the stop surface  136  abuts the interior end wall of the housing  102 , and the double rack and pinion drive accordingly forces the button coupling rod  175  a distance distally. The end result of this is illustrated by  FIG. 15 . 
     The proximal motion of the rotator  130  also causes the sleeve  131  to disengage from the toothed end portion  122 . The rotator  130  is thus now capable of being rotated without affecting the dose preparation tube  120 . A dose is set by rotation of the rotator  130  relative to the housing  102 . Due to the spline connection between the rotator  130  and the scale drum  140  and the threaded interface between the scale drum  140  and the housing  102  when the rotator  130  is dialed counter-clockwise the scale drum  140  displaces helically downwards in the housing  102  in response, and when the rotator  130  is dialed clockwise the scale drum  140  displaces helically upwards in the housing  102 . In  FIG. 16  the rotator  130  has been dialed to set a dose of “72” units. 
     Dose delivery is executed by depression of the push button  157 , as illustrated in  FIG. 17 . The push button  157  may actually be depressed a certain distance without causing more than a reversed motion of the double rack and pinion drive and a compression of the spring  150 . A discontinuation of the depression force in this instance will simply cause the spring  150  to return the push button  157  to its proximal most position. However, once the button coupling rod  175 , during its proximal displacement, reaches a specific axial position in the housing  102  an end surface  179  passes the fulcrum of the lock member  180  and the lock member  180  will be free to pivot, whereby the pre-tensioned spring  150  will be released and as a result force the actuation rod  109  distally. As the lock member  180  pivots to allow passage of the actuation rod  109  the button coupling rod  175  becomes prevented from distal motion in the housing  102  due to the lock member  180  being prevented from returning to the original position by the actuation rod  109 . At this point if the user releases the pressure on the push button  157  the rotator  130  will consequently be prevented from proximal motion and will thus stay inside the housing  102 . 
     The depression of the push button  157  also leads to a rotational re-engagement of the sleeve  131  and the toothed end portion  122 . This happens before the flipping over of the lock member  180 , such that when the spring  150  is released and the actuation rod  109  is suddenly propelled distally the rotator  130  and the dose preparation tube  120  are rotationally interlocked. Due to the threaded engagement between the drive nut  195  and the threaded end portion  123  the distal movement of the actuation rod  109  causes the dose preparation tube  120  to spin clockwise. 
     The clockwise rotation of the dose preparation tube  120  causes a clockwise rotation of the piston rod guide  163 , due to the above described double ratchet mechanism, and thereby also of the piston rod  160 . The threaded engagement between the piston rod  160  and the nut  162  thus results in a helical advancement of the piston rod  160 , whereby the piston (not shown) is advanced axially in the cartridge (not shown) to expel a volume of drug through an attached injection needle (not shown). The volume expelled is determined by the position of the scale drum  140  in the housing  102  at the time of release of the spring  150  because the clockwise rotation of the dose preparation tube  120  also causes a clockwise rotation of the rotator  130  and thereby of the scale drum  140 , and the rotation of the three continues until the scale drum  140  meets the proximal stop surface which defines the “zero dose” position. This end-of-dose state of the injection device  100  is illustrated in  FIG. 18 . 
     It is noted that as the injection progresses the actuation rod  109  is moved further distally and the axial end position of the abutment face  197  corresponding to the “zero dose” position of the scale drum  140  is uniquely correlated with the distance traveled by the scale drum  140  from its position at release of the spring  150  to the proximal stop surface. 
     In the end-of-dose state of the injection device  100  the push button  157  is prevented from proximal motion and therefore has to stay depressed in the housing  102 . Hence, it is not possible to set a dose at this point. It is common practice when handling injection devices to re-mount the protective cap following an injection. In the course of re-mounting the protective cap onto the cap receiving portion of the injection device  100  a portion of the cap, such as e.g. a segment of the cap rim or a protrusion, abuts the abutment face  197  and pushes the actuation rod  109  proximally with respect to the housing  102 . 
     The resulting proximal movement of the drive nut  195  causes the dose preparation tube  120  to spin counter-clockwise, relative to the housing  102  but also relative to the piston rod guide  163  due to the double ratchet mechanism, so the piston rod  160  is left unaffected. The counter-clockwise rotation of the dose preparation tube  120  causes a corresponding counter-clockwise rotation of the rotator  130  which leads to a downward helical displacement of the scale drum  140 . 
     The proximal movement of the drive nut  195  also causes a compression of the spring  150  which is progressive until the actuation rod  109  reaches the axial position where the transversal surface  198  passes the fulcrum of the lock member  180 . At this position of the actuation rod  109  the lock member  180  is free to pivot and thus no longer functions as a block for distal motion of the button coupling rod  175 . So, as the spring  150  seeks to relax and constantly biases the inner end face  103  in the proximal direction, the rotator  130  is urged proximally, pulling the coupling piece  173 , and the double rack and pinion drive consequently urges the button coupling rod  175  distally, causing the lock member  180  to flip over and abut the transversal surface  198 . The spring  150  will displace the rotator  130  proximally a small distance until the abutment face  177  abuts the protective cap and further distal motion of the button coupling rod  175  thereby is prevented. This corresponds to the state of the injection device  100  shown in  FIG. 19 . In this state the lock member  180  stably prevents distal motion of the actuation rod  109 , as it is prevented from pivoting by the button coupling rod  175 . As long as the protective cap remains mounted on the cap receiving portion a depression of the push button  157  only leads to an additional compression of the spring  150  which has no effect on the secured injection mechanism. At termination of the push force the spring  150  will return to the slightly less compressed state shown in  FIG. 19 . 
     Notably, when the protective cap is re-mounted on the cap receiving portion the actuation rod  109  is returned to the exact same axial position within the housing  102  that it initially assumed before the dose ejection was commenced. Due to the threaded interface between the drive nut  195  and the threaded end portion  123  this means that the dose preparation tube  120  is consequently returned rotationally to the exact same angular position relative to the housing  102  that it initially assumed before the dose ejection was commenced. The dose preparation tube  120  has thus during re-mounting of the protective cap undergone the exact opposite rotation to the one it underwent during the dose delivery, and since the dose preparation tube  120  and the rotator  130  are rotationally interlocked so has the rotator  130 . Consequently, due to the splined connection between the rotator  130  and the scale drum  140  and the threaded connection between the scale drum  140  and the housing  102 , the scale drum  140  has been returned to the position it assumed immediately before the push button  157  was depressed and the spring  150  was released. In other words, by the re-mounting of the protective cap onto the cap receiving portion a setting of the last ejected dose has automatically been performed. 
     In fact, every time the protective cap is mounted onto the cap receiving portion the dose preparation tube  120  will be returned, in the above described manner, to the initial angular position, which can be defined as a dose prepared position within the housing  102 , thereby bringing the injection device  100  in a “DOSE PREPARED” state. 
     When the user dismounts the protective cap before the next injection the rotator  130  and the push button  157  will re-protrude from the housing  102  and the sleeve  131  will disengage from the toothed end portion  122 , as described above in connection with  FIG. 15 . The user can now either choose to simply position the injection device  100  at the desired skin site and press the push button  157  to deliver the same dose as was last delivered, or adjust the dose size by dialing the rotator  130  in the appropriate direction before performing the injection procedure. 
     In case the user chooses to adjust the dose, and thereby set a new dose, the scale drum  140  will change position within the housing  102  and assume a new position corresponding to the new dose viewed through the window  199 . Because the rotator  130  is decoupled from the toothed end portion  122  the repositioning of the scale drum  140  will not affect the dose preparation tube  120 . Only when the push button  157  is subsequently depressed and the sleeve  131  reengages with the toothed end portion  122  the scale drum  140  and the dose preparation tube  120  become coupled to undergo correlated movements relative to the housing  102 , provoked by the spring  150 , as previously described. During these correlated movements the scale drum  140  will again reach the “zero dose” position and abruptly stop further expansion of the spring  150  and distal motion of the actuation rod  109 . When this happens the axial end-of-dose position of the abutment face  197  relative to the housing  102  will be different from its previous end-of-dose position and, consequently, the dose preparation tube  120  will have undergone a different angular displacement than the one it underwent during the previous dose delivery. Nevertheless, when the cap is re-mounted on the cap receiving portion the actuation rod  109  will once again be returned to the same axial position as before, since that axial position is defined by the position of the cap portion abutting the abutment face  197  relative to the housing  102  when the cap is securely mounted. Due to the engagement between the drive nut  195  and the threaded end portion  123  the reversed motion of the actuation rod  109  will lead to a reversed motion of the dose preparation tube  120 , which will again lead to a reversed motion of the scale drum  140 . Thereby, the dose preparation tube  120  is returned to the exact same angular position relative to the housing  102  that it assumed before the dose ejection (the dose prepared position), and the scale drum  140  is returned to the position in which the new dose is viewed through the window  199 . 
       FIG. 20  is an exploded view of a drug delivery device according to yet another exemplary embodiment of the present invention. The drug delivery device is in the form of an automatic injection device  200  which is adapted to deliver set doses of drug from a cartridge  210 . The injection device  200  comprises a proximal housing part  201  and a distal housing part  202  which are connected to form a unitary exterior cabinet of a generally tubular configuration. The distal housing part  202  has a distally extending cartridge holder  214  configured to hold and protect the cartridge  210 , and to which a needle mount  218  for receiving a needle assembly (not shown) is attached, a transversal partition  206 , and a proximally extending hollow spindle  205  serving as a guide structure for a nut assembly  220 . 
     The spindle  205  comprises an exterior helical track  287  which leads into a longitudinal track  289  at a transition point  288 . The helical track  287  is configured to be non-self-locking which means that its pitch enables a helical displacement of a mating nut structure under the influence of a purely axial external force. The transversal partition  206  has a pair of narrow slots  207 , each slot  207  being configured to receive and to allow longitudinal displacement of a respective loading rod  209 . Each loading rod  209  has a distal abutment edge  290  for interaction with a rim  298  of a protective cap  204  when the cap  204  is inserted into a cap receiving portion  208  of the distal housing part  202 , and a proximal catch portion  291  for interaction with the nut assembly  220  in a manner that will be described in more detail in the following. A reset spring  295  is arranged to act between the distal abutment edge  290  and a distally facing portion of the transversal partition  206 . 
     The nut assembly  220  is connected to a nut connector  275  which is an intermediate coupling element in a dose setting mechanism of the injection device  200 . The nut connector  275  has an annular base from which two connecting arms  276  extend proximally. Each connecting arm  276  is provided with a toothing  277  at its free end. The base of the nut connector  275  has a circumferential set of teeth  278  on an interior surface portion and a plurality of protrusions  279  distributed along an exterior surface portion. 
     The toothing  277  on each arm  276  is adapted for rotational interlocking engagement with a mating toothed rim  253  of a dose dial  255 . The dose dial  255  is connected to the proximal housing part  201  via a number of snap locks  254  and is configured to accommodate an injection button  257  in a way as to allow the injection button  257  to move axially up and down under influence of a button spring  265 . The injection button  257  has a distally directed protrusion  259  for interaction with the nut connector  275  in a manner which will be further described in the below. 
     The protrusions  279  are received in respective longitudinal tracks  234  (see  FIG. 23 ) in an interior surface of a scale drum connector  230 , providing for an axially free but rotationally interlocked relation between the nut connector  275  and the scale drum connector  230 . Openings  236  are provided in a transversal portion of the scale drum connector  230  to allow passage of the arms  276 , and a toothed rim  232  is provided at the proximal end. On the exterior surface of the scale drum connector  230  a plurality of splines  235  are distributed which are in engagement with respective splines  244  in a scale drum  240 . 
     The scale drum  240  is a tubular structure having a helically extending groove  242  for threaded engagement with an interior wall portion of the proximal housing part  201  as well as a plurality of dose related numerals (not shown) printed in a helical path on its exterior surface. The scale drum  240  is thus rotationally locked to the scale drum connector  230  and bound to move helically with respect to the proximal housing part  201  in response to a rotation of the scale drum connector  230 . In every possible angular position of the scale drum  240  relative to the proximal housing part  201  at least one dose related numeral will be viewable through a window  299 , allowing the user to easily identify the size of a set dose. 
     A driver  280  for effecting the dose delivery is slidingly arranged in the hollow of the spindle  205 . The driver  280  has an elongated tubular body with a driver head  283  at a proximal end and a toothed transversal surface  281  arranged at a distal position. The driver head  283  has a distally oriented toothed rim  282  for rotational interlocking engagement with the toothed rim  232  during dose delivery, and the toothed transversal surface  281  is adapted to releasably engage with a mating toothing (not visible) on a distally facing portion of the partition  206 , providing for a releasable rotational interlocking connection between the driver  280  and the distal housing portion  202 . 
     The distal most portion of the driver  280  is connected to an inner free end of a spiral spring  250 . An outer free end of the spiral spring  250  is connected to an interior surface of a spring housing  203 , which spring housing  203  is rotationally locked in the distal housing part  202 . 
     The spiral spring  250  is pre-strained and has sufficient capacity to cause a complete emptying of the cartridge  210 . 
     The driver  280  accommodates a piston rod  260  which has a thread  268  for interaction with a guide nut  262  that is fixedly arranged in the distal housing part  202 . The thread  268  is interrupted by a longitudinal track  269  extending along the entire length of the piston rod  260 . This provides for a splined connection between a protrusion (not visible) on an inner surface portion of the driver  280  and the longitudinal track  269 , which ensures that any angular displacement of the driver  280  is passed on to the piston rod  260 . 
     A distal end of the piston rod  260  abuts a piston washer  261  which again abuts a proximal surface portion of an axially displaceable piston  212  arranged in sealing connection with a cartridge wall  219 . A counter-clockwise rotation of the driver  280 , initiated for dose delivery, will thus cause a counter-clockwise rotation of the piston rod  260  which due to the threaded engagement with the guide nut  262  will be advanced helically downwards, pressing the piston washer  261  and the piston  212  into the cartridge  210  for expelling of a volume of drug through an injection needle (not shown) of a mounted needle assembly. 
       FIG. 21  is an exploded view detailing the nut assembly  220 . The nut assembly  220  comprises a primary nut  221 , a lock nut  321 , and a nut spring  296 . The primary nut  221  comprises a transversal spring base  222  which is provided with a round-going set of teeth  226  at its periphery, and a cylindrical portion  223  which has an inwardly projecting helical segment  224  for mating engagement with the helical track  287  and a plurality of circumferentially distributed exterior protuberances  225 . The set of teeth  226  is configured to enable releasable engagement with the set of teeth  278 , to provide for a rotational interlocking connection between the primary nut  221  and the nut connector  275  in a first relative axial position of the two and a rotational decoupling in a second relative axial position of the two. 
     The lock nut  321  comprises an annular base  322 , from which a number of hook members  326  project, and a cylindrical portion  323  which has a plurality of indentations  325  distributed along an inner surface. Each indentation  325  is configured for sliding reception of one protuberance  225  to ensure a rotational interlocking connection between the primary nut  221  and the lock nut  321 . An inwardly directed protrusion  324  on the lock nut  321  is configured for mating engagement with the helical track  287  in one state of the nut assembly  220  and for engagement with the longitudinal track  289  in another state of the nut assembly  220 . The hook members  326  are configured to axially retain an interior edge of the nut connector  275  and thereby prevent relative axial movement, while allowing relative rotation, between the nut connector  275  and the lock nut  321 . The nut spring  296  is arranged to act between the spring base  222  and the annular base  322 , biasing the primary nut  221  and the lock nut  321  axially away from one another. 
       FIG. 22  is a longitudinal section view of the injection device  200  in a dose setting state before a needle assembly has been mounted on the needle mount  218 . The cap  204  is removably mounted on the injection device  200  to cover and protect the cartridge  210 . The cartridge  210  holds a drug substance (not visible) in a variable volume chamber  213  which is bounded by the cartridge wall  219 , the slidable piston  212 , and a penetrable self-sealing septum  211 . 
       FIG. 23  is a close-up view of a proximal portion of the injection device  200  delimited by the area Q in  FIG. 22 . The figure shows details of the various components and their respective connections with other components in the dose setting state of the injection device  200 . In particular, the figure shows the nut assembly  220  in its axially expanded state, being immobilised on the spindle  205 , i.e. where the protrusion  324  is in engagement with the longitudinal track  289 , and where the primary nut  221  and the lock nut  321  are axially spaced apart by the nut spring  296 . This can be termed a top position, or a dose prepared position, of the nut assembly  220 . Notably, after removal of the cap  204  the nut assembly  220  will remain in the top position against the biasing axial force from the reset spring  295  acting on the primary nut  221  via the loading rods  209 , which are axially fixed between the spring base  222  and a lower rim  227 , because the lock nut  321  is prevented from rotating relative to the spindle  205  due to the engagement with the longitudinal track  289 . 
     The injection button  257  is in its non-activated position relative to the proximal housing part  201 , which means that the protrusion  259  rests against a rim  274  at the proximal end of one of the arms  276  without applying any significant force thereto, and the toothing  277  is in engagement with the toothed rim  253 . A rotation of the dose dial  255  will thus cause a rotation of the nut connector  275  which due to the rotational interaction between the protrusions  279  and the longitudinal tracks  234  will cause a rotation of the scale drum connector  230 . The rotation of the scale drum connector  230  is then transferred to the scale drum  240  due to the splined relationship between the two. Hence, when the nut assembly  220  is in the top position any angular displacement of the dose dial  255  will lead to a similar angular displacement of the scale drum  240 . Due to the thread connection between the helically extending groove  242  and the proximal housing part  201  an angular displacement of the dose dial  255  will in fact lead to a combined angular and axial displacement of the scale drum  240 . This displacement of the scale drum  240  relative to the proximal housing part  201  from a zero dose indicating position correlates directly with the size of the set dose which can be read through the window  299 . 
     The injection button  257  is biased towards the non-activated position by the button spring  265  and the driver  280  is accordingly biased towards a proximal position due to the driver head  283  being axially retained by a retainer member  258 . Thus, in the non-activated position of the injection button  257  the toothed transversal surface  281  is held firmly in rotational interlocking connection with the partition  206 , and the spring  250  is thereby safely cocked. 
       FIG. 24  is a perspective section view of a proximal portion of the spindle  205  showing the nut assembly  220  in the top position. Here, it can be seen more clearly that the helical segment  224  is in engagement with the helical track  287 , while the protrusion  324  is in engagement with the longitudinal track  289 , effectively preventing any rotation of the lock nut  321  and any movement at all of the primary nut  221  due to the engagement between the respective protuberances  225  and indentations  325  providing the rotational interlocking connection between the primary nut  221  and the lock nut  321 . Further, it is seen that the relative axial position of the primary nut  221  and the nut connector  275  is such that the set of teeth  226  on the spring base  222  is disengaged from the set of teeth  278  on the interior surface of the base of the nut connector  275 , whereby the nut connector  275  is capable of rotation with respect to the nut assembly  220 . 
       FIG. 25  shows a proximal portion of the injection device  200  and illustrates what happens when the injection button  257  is being depressed. Apart from a compression of the button spring  265  a depression of the injection button  257  entails four major changes to the interrelations between certain components. Firstly, the rim  274  is forced downwards by the protrusion  259  causing the toothing  277  to disengage from the toothed rim  253 . The dose setting mechanism is thereby disabled because of the resulting decoupling of the scale drum  240  from the dose dial  205 . Secondly, the lock nut  321  is forced downwards by the nut connector  275  exerting a pushing force on the annular base  322 . This compresses the nut spring  296  and brings the lock nut  321  to the transition point  288 . Thirdly, the driver head  283  is forced downwards, whereby the toothed rim  282  is brought into engagement with the toothed rim  232  on the scale drum connector  230 , rotationally coupling the driver  280  and the scale drum  240 . Fourthly, the downward movement of the driver  280  brings the toothed transversal surface  281  out of the rotational locking engagement with the partition  206 , and the spring  250  is thus released. 
       FIG. 26  shows the effect of the release of the spring  250 . When the toothed transversal surface  281  is no longer prevented from angular displacement relative to the partition  206  the spring  250  is free to release stored energy for rotation of the driver  280 . This causes a corresponding rotation of the piston rod  260  which due to the threaded engagement with the guide nut  262  is advanced helically through the distal housing part  202 , forcing the piston  212  along the cartridge wall  219 . As the driver head  283  rotates the scale drum connector  230  and the scale drum  240  rotate, which causes a helical upwards movement of the scale drum  240  in the proximal housing part  201 . 
     The spring  250  will release energy and rotate the dose delivery components until the scale drum  240  reaches a physical stop surface in the proximal housing part  201  signifying an end-of-dose state of the injection device  200 . The total angular displacement of the scale drum  240  relative to the proximal housing part  201  during the release of energy from the spring  250  correlates with the delivered amount of drug substance from the chamber  213 , and in the end-of-dose state the scale drum  240  is in a zero dose indicating position. A subsequent removal of the depressing force from the injection button  257  will cause the button spring  265  to return the injection button  257  to the non-activated position, bringing along the driver head  283  and thereby decoupling the toothed rim  282  from the toothed rim  232  and moving the toothed transversal surface  281  back into rotational interlocking engagement with the partition  206 . 
     Returning to the effect of the release of the spring  250 , when the lock nut  321  is positioned at the transition point  288  the set of teeth  278  on the interior surface of the nut connector  275  has moved into engagement with the set of teeth  226  on the circumference of the spring base  222 , and the nut connector  275  is thereby rotationally locked to the primary nut  221 . The spring induced rotation of the driver  280  which is transferred to the scale drum connector  230  is thus also transferred to the nut connector  275 , due to the engagement between the respective protrusions  279  and longitudinal tracks  234 , and to the nut assembly  220 . Accordingly, the protrusion  324  enters into the helical track  287  and the nut assembly  220  as a unit is displaced helically down the spindle  205  since now both the primary nut  221  and the lock nut  321  are in engagement with the helical track  287 . This helical displacement of the nut assembly  220  continues as long as the driver  280  rotates. As the primary nut  221  is thereby moved down towards the partition  206 , bringing along the loading rods  209 , the reset spring  295  is allowed to expand. 
       FIG. 27  shows a perspective section view of the nut assembly  220  in the same position as the one depicted by  FIG. 26 . The figure shows the nut assembly  220  in its axially compressed state, and it is clearly seen that both the helical segment  224  and the protrusion  324  are in engagement with the longitudinal track  287 . 
     Following a completed dose delivery when the user re-mounts the cap  204  onto the injection device  200  the rim  298  of the cap  204  abuts the abutment edge  290  on the respective loading rods  209  and forces the loading rods  209  towards the proximal end of the proximal housing part  201  against the biasing force from the reset spring  295 . Thereby, the respective catch portions  291  act on the spring base  222  and push the primary nut  221  in the same direction, while the reset spring  295  is compressed. Due to the helical segment  224  being in engagement with the helical track  287  and the rotational interlocking connection between the respective protuberances  225  and indentations  325  the entire nut assembly  220 , in its compressed state, travels helically up the spindle  205  towards the transition point  288 . 
     The resulting angular displacement of the primary nut  221  is transferred to the nut connector  275  and further on to the scale drum connector  230  and the scale drum  240 , causing the scale drum  240  to move helically in the proximal housing part  201  away from the zero dose indicating position for automatic setting of a dose which corresponds to the dose that was just delivered. If during the travel of the nut assembly  220  up the spindle  205  the user suddenly chooses to remove the cap  204  the reset spring  295  will expand and drive the nut assembly  220  back down the spindle  205 , causing an opposite angular displacement of the primary nut  221 , the nut connector  275 , the scale drum connector  230 , and the scale drum  240 , which will take the scale drum  240  back to the zero dose indicating position. The reset spring  295  thus serves to ensure that the scale drum  240  is not left in an intermediate position where an arbitrary dose is set. 
     When the lock nut  321  passes the transition point  288  the nut spring  296  expands and forces the protrusion  324  further proximally along the longitudinal track  289  while the primary nut  221  remains in position because the cap  204  is then fully mounted on the injection device  200 . The nut assembly  220  is thereby immobilised on the spindle  205  and the reset spring  295  is securely cocked. Further, the axial displacement of the lock nut  321  causes the nut connector  275  to re-engage with the dose dial  255  and the set of teeth  226  to disengage from the set of teeth  278 , thereby rendering the scale drum  240  accessible for displacement without impact on the nut assembly  220  and thus allowing for manual adjustment of the automatically set dose (compare  FIG. 26  and  FIG. 23 ). 
     The cap  204  and the loading rods  209  are designed such that when the cap  204  is fully mounted on the injection device  200  the nut assembly  220  has been moved to the dose prepared position and the injection device  200  is accordingly in a “DOSE PREPARED” state. This dose prepared position of the nut assembly  220  is maintained until the next dose expelling procedure is commenced, regardless of whether the position of the scale drum  240  is manually adjusted or not. The execution of the dose expelling procedure brings the nut assembly  220  from the dose prepared position to an end-of-dose position on the spindle  205  which uniquely corresponds to the movement of the scale drum  240  from its dose set position to the zero dose indicating position, and thereby to the particular dose expelled. When the scale drum  240  reaches the zero dose indicating position the injection device  200  is in a “DOSE DELIVERED” state. 
     The subsequent re-mounting of the cap  204  thus takes the nut assembly  220  back to the dose prepared position, regardless of from which end-of-dose position it departs, and due to the coupling between the nut assembly  220  and the scale drum  240  during the return movement of the nut assembly  220  the scale drum  240  is forced to undergo a movement which is the reverse of the one it underwent during the previous dose expelling procedure, and it is thereby returned to the exact same dose set position relative to the proximal housing part  201  it had when the nut assembly  220  last departed from the dose prepared position. So, the dose prepared position and the zero dose indicating position are both positions which are fixed with respect to the proximal housing part  201 , whereas the end-of-dose position and the dose set position are both variable and depend on the specific user input.