Abstract:
An injection device includes a housing, a receptacle for receiving a medicine cartridge and configured for attachment to a proximal end of the housing, and a leadscrew having an end for engaging with a bung of a cartridge. The leadscrew has a first thread extending axially along an outer surface thereof and at least one axially extending guide track. The injection device includes a rotatable drive mechanism for engaging with the first thread or guide track, and a guiding element for engaging with the other of the first thread or guide track, the guiding element being non-rotatable with respect to the housing during an injection operation, whereby rotation of the drive mechanism causes the leadscrew to move axially in a proximal direction in order to deliver a dose from the cartridge. The injection device further includes a stop member located proximal to the rotatable drive mechanism and the guiding element.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to injection devices for delivering a dose of medicine to a user from a cartridge containing medicine, and more particularly to injection devices having a mechanism for accurately controlling the plunger position. 
       BACKGROUND 
       [0002]    Injection devices are routinely used in the medical field to deliver a measured dose of medicine to a user. Due to their user-friendly design, they can be safely used by patients for self-administration, although in some circumstances they may be used by trained medical personnel. 
         [0003]    A typical injection device comprises a relatively large number of parts. For example,  FIGS. 1 to 4  illustrate schematically an OWEN MUMFORD AUTOPEN® device suitable for use with prefilled medicine cartridges, i.e. a device type commonly referred to as a “pen injector”. A related description is provided in WO/2007/063342.  FIG. 1  shows a side profile of the pen injector.  FIG. 2  shows the pen injector in a fully reset configuration.  FIG. 3  shows the pen injector in a fully discharged configuration.  FIG. 4  provides an expanded view of a central section of the device. 
         [0004]    With reference to  FIGS. 1 to 4 , the known pen injector comprises a housing  1  to which a receptacle  2  is connected. The receptacle  2  is arranged to receive the medicine cartridge  3 . The pen injector has a cover  6  for protecting the receptacle  2  when not in use. The housing  1  is provided with a trigger  7  for actuating the device, and a dose knob  8  for selecting a dose. The housing  1  contains a torsion spring  9 . The torsion spring  9  is located coaxially within the housing  1  and is arranged to provide a drive force for ejecting medicine from the cartridge  3 . 
         [0005]    A generally cylindrical ratchet drive shaft  10  extends through the centre of the torsion spring  9 . An enlarged end portion  10   a  of the shaft  10 , located at a proximal end, and has three sprung legs formed around its periphery, the legs being spaced equally around the shaft  10 . At the outermost end of each leg, a tooth is provided. These teeth engage with a rack (not shown) formed around the inner surface of a drive gear  11 , which sits within the housing  1  at a fixed axial position. The drive gear  11  has a second toothed rack  11   a  formed around a lower outer surface portion. The rack  11   a  sits within a correspondingly sized rack  12   a  formed on an inner surface of a retaining ring  12 . The ring  12  is formed integrally with the trigger  7 , with the trigger  7  being slidably mounted within a slot formed in the housing  1 . A spring  13  urges the trigger  7  in a distal direction, maintaining the racks  11   a  and  12   a  in locking engagement in the absence of a user applied force. 
         [0006]    A leadscrew  14  has a screw thread formed along the length of its outer surface. The leadscrew  14  is located within the ratchet drive shaft  10 , and engages a complimentary screw thread formed on the inner surface of the drive gear  11 . An end portion of the leadscrew projects from the ratchet drive shaft  10  and has a leadscrew cap  15  secured thereto. The cap  15  is rotationally attached to the leadscrew  14 , such that it can be rotated relative to the leadscrew  14 . The leadscrew  14  has axial guide tracks  16  extending along its outer surface and which engage with a locking bush  17  via splines  18  on the locking bush  17 . The locking bush  17  is slidably held within a mid-body compartment  19  which itself is secured to the end of the housing  1  via a pair of complimentary screw threads. The locking bush  17  has a serrated edge  20  running along an end for engaging with mating features of the housing  1 . 
         [0007]    Consider now the operation of the pen injector. A user sets a dose by rotating the dose knob  8  in a clockwise direction. As the dose knob  8  is rotated, the distal end of the torsion spring  9  rotates with it, accumulating energy in the spring. Engagement of sprung fingers (not shown) at a distal end of the ratchet drive shaft  10  with a rack formed on the inner surface of the dose selector  8  also causes the ratchet drive shaft  10  to rotate. At a proximal end of the ratchet drive shaft  10 , the teeth of the sprung legs “click” around the rack (not shown) formed around the inner surface of the drive gear. The engagement of the teeth with the rack at the proximal end of the ratchet drive shaft  10  prevents the spring  9  unwinding after each click. Each click corresponds to a predefined angular rotation of the spring and therefore to a predefined ejection dose. It will be readily appreciated that, during the dose setting action, the drive gear  11  is not rotated so no axial movement of the leadscrew  14  is induced. No medication is therefore ejected from the cartridge during the dose setting operation (or indeed air introduced due to back filling). 
         [0008]    Once a dosage is set, the user can apply a force to the trigger  7  in the proximal direction. This disengages the rack  12   a  of the trigger  7  from the rack  11   a  of the drive gear  11 . This frees the drive gear  11  and the torsion spring  9  to rotate. As the drive gear  11  rotates about the leadscrew  14 , the leadscrew  14  is driven through the drive gear  11  causing the leadscrew cap  15  to push the bung of the cartridge  3  through the cartridge body, expelling medication from the cartridge  3  through an attached syringe. 
         [0009]    The known pen injector is able to deliver several doses from the same medication filled cartridge  3 . During delivery of each successive dose, the leadscrew  14  is advanced further forward into the medication filled cartridge  3 . This continues until an enlarged end  21  of the leadscrew  14  engages with a distal end of the drive gear  11 , at which point the leadscrew  14  has reached the end of its travel.  FIG. 3  shows the known pen injector in this position. 
         [0010]    During the injection process, the leadscrew  14  must be prevented from rotating relative to the housing  1 . However, following the removal of the spent cartridge, it must be possible to push the leadscrew  14  back into the housing  1  to a starting position, this operation requiring rotation of the leadscrew within the housing. This is achieved by means of the locking bush  17 . 
         [0011]    Following removal of the receptacle from the housing, a spring  23  urges the locking bush  17  in the proximal direction relative to the housing, disengaging the locking bush from the housing and allowing it to rotate relative thereto. Once a new cartridge has been loaded into the receptacle  2 , the user attaches the receptacle  3  onto the end of the housing  1 . On coupling of the receptacle  2  to the housing  1 , the leadscrew cap  15  abuts the bung in the end of the cartridge  3 . The user pushes the receptacle  2  towards the housing  1 , causing the leadscrew  14  to rotate within the drive gear  11  and move back into the housing  1 . When the screw thread on the receptacle  2  engages with that on the housing  1 , the user screws the two parts together, with the leadscrew  14  continuing to rotate and move into the housing  1  during this process. 
         [0012]    On final coupling with the receptacle  2 , the receptacle  2  and cartridge  3  engage with a cartridge compression cup  22 , which compresses the spring  23  and transmits the loading onto the locking bush  17 . The locking bush  11  re-engages with the housing, preventing the locking bush  17  from further rotation and also locking the leadscrew against rotation by means of the engagement of the splines  18  with the axial guide tracks  16 . This ensures that the leadscrew  14  can move forward when subjected to rotation following release and rotation of the drive gear  11 . 
         [0013]    Medication filled cartridges are generally tubular in shape, although, due to structural considerations, have a slightly tapered portion towards the needle receiving end. If the bung is allowed to be pushed into this tapered end region there is a danger a) that the cartridge  3  may fracture and b) that an unreliable dose will be delivered (as the dosing mechanism assumes a uniform cartridge cross-section). Ideally, the device is configured such that the cap  15  cannot move beyond a point at which the bung is about to enter the tapered end portion of the cartridge  3 . This is achieved by configuring the device such that the enlarged end  21  of the leadscrew  14  engages with a distal end of the drive gear  11  to stop the leadscrew, and therefore the bung, at the correct position relative to the cartridge. Of course, the configuration must take account of manufacturing tolerances, meaning that, in practice, the end  21  must be stopped at some distance distal from the “theoretical” stopping point, i.e. the device must be designed to provide a “buffer zone”. 
         [0014]    The buffer zone that must be incorporated into the device will depend upon the number of connected components. In the case of the device of  FIGS. 1 to 4 , the tolerance with which the stopping point of the end  21  can be defined depends upon the sum of the individual tolerances of the housing  1 , receptacle  2 , drive gear  11 , mid-body compartment  19 , and locking bush  17 . This might be as much as 0.6 mm, meaning that on average the end of the bung will stop 0.6 mm distal from the desired stopping point in the cartridge. As a result, on average, a significant amount of medicine will remain in the cartridge  3  after a final injection from the cartridge. This has significant cost implications. 
       SUMMARY 
       [0015]    It is an object of the present invention to reduce waste of medicine when using an injection device. This object is achieved by reducing the cumulative impact of manufacturing tolerances on the final stopping point of the bung within the medicine cartridge. 
         [0016]    According to a first aspect of the present invention there is provided an injection device comprising a housing, a receptacle for receiving a medicine cartridge and configured for attachment to a proximal end of the housing, and a leadscrew having an end for engaging with a bung of a cartridge. The leadscrew has a first thread extending axially along an outer surface thereof and has at least one axially extending guide track. The injection device further comprises a rotatable drive mechanism for engaging with said first thread or said guide track and a guiding element for engaging with the other of said first thread or said guide track, the guiding element being non-rotatable with respect to the housing at least during an injection operation, whereby rotation of the drive mechanism causes said leadscrew to move axially in a proximal direction in order to deliver a dose from the cartridge. The injection device further comprises a stop member located proximal to said rotatable drive mechanism and said guiding element. The stop member is engaged with the or each guide track of said leadscrew to allow relative axial movement of the leadscrew and the stop member while causing the two components to rotate together, said stop member and a profile of the or each guide track being configured to define an end stop for the leadscrew in the proximal direction. 
         [0017]    The present invention addresses problems associated with the cumulative impact of manufacturing tolerances on the final stopping point of a bung within a medicine cartridge. This is achieved by defining an end stop proximal to the rotatable drive member. As a result, the impact of manufacturing tolerance associated with the rotatable drive member on the final stopping point of the bung is greatly reduced. This allows a smaller buffer zone to be defined and, as a result, less medicine will remain in the cartridge after a final injection from the cartridge. 
         [0018]    As an option the profile may comprise a step change in the depth of the or each guidetrack at an intermediate position, where the or each guide track has a first depth x in a proximal region of the leadscrew and a second depth y in a distal region, where x is greater than y. 
         [0019]    As an option the stop member comprises at least one stop spline, where the or each stop spline is configured to engage with the or each guide track. 
         [0020]    As an option the or each stop spline of the stop member extends a distance greater than y into the or each guide track. 
         [0021]    In a first embodiment of the present invention the rotatable drive mechanism comprises a drive gear having a second thread provided on an inner surface thereof, the leadscrew and the drive gear being rotatably coupled by means of said first and second threads, and said guiding element is a locking bush located proximal to said drive gear and comprising one or more splines for engaging with the or each guide track of the leadscrew to allow relative axial movement of the leadscrew and the locking bush while preventing relative rotational movement. The locking bush is configured to allow rotation of the leadscrew when the receptacle is disconnected from the housing and to prevent rotation when the receptacle is connected to the housing. As an option the or each spline of the locking bush extends a distance less than y into the or each guide track. 
         [0022]    As an option the first embodiment further comprises a cartridge compression cup, which is slidably mounted within the housing and is engagable by the receptacle during attachment of the receptacle to move said locking bush from a position in which it is rotatable with respect to the housing to a position in which it is non-rotatable, and wherein said stop member is located within said compression cup. 
         [0023]    In a second embodiment of the present invention the rotatable drive mechanism comprises a drive collar comprising one or more splines for engaging with the or each guide track such that the drive collar and leadscrew rotate together, and the guiding element is a nut, said nut having a second thread on an inner surface thereof and the leadscrew and the nut being rotatably coupled by means of said first and second threads. 
         [0024]    As an option the second embodiment further comprises a cartridge compression cup. The cartridge compression cup is slidably mounted within the housing and is engagable by the receptacle during attachment of the receptacle to move said nut from a position in which the nut is rotatable with respect to the housing to a position in which the nut is non-rotatable, and wherein said stop member is located within said compression cup. As an option the stop member is rotatable within said compression cup. As a further option the or each drive spline of the drive collar extends a distance less than y into the or each guide track. 
         [0025]    As an option a proximal end position of the stop member is defined by abutment of the stop member with an attached cartridge. 
         [0026]    As an option a torsion spring is coupled to the rotatable drive mechanism in order to rotate the drive mechanism within the housing. 
         [0027]    As an option the stop member is a stop collar. 
         [0028]    According to a second aspect of the present invention there is provided an injection device comprising a housing, a receptacle for receiving a medicine cartridge and configured for attachment to a proximal end of the housing, and a leadscrew having an end for engaging with a bung of a cartridge, the leadscrew having a first thread extending axially along an outer surface thereof and having at least one axially extending guide track. The injection device further comprises a drive gear having a second thread on an inner surface thereof, the leadscrew and the drive gear being rotatably coupled by means of said first and second threads, and a drive mechanism for causing relative rotation of said leadscrew and said drive gear in order to deliver a dose from the cartridge. The injection device further comprises a locking bush located proximal to said drive gear and being engaged with the or each guide track of the leadscrew to allow relative axial movement of the leadscrew and the locking bush while preventing relative rotational movement, the locking bush being configured to allow rotation of the leadscrew when the receptacle is disconnected from the housing and to prevent rotation when the receptacle is connected to the housing. The injection device further comprises a stop member located proximal to said locking bush and being engaged with the or each guide track of said leadscrew to allow relative axial movement of the leadscrew and the stop member while allowing the two components to rotate together, said stop collar and a profile of the or each guide track being configured to define an end stop for the leadscrew in the proximal direction. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]      FIG. 1  illustrates schematically an external view of a prior art pen injector; 
           [0030]      FIG. 2  illustrates schematically a cross-sectional view of the prior art pen injector of  FIG. 1  in a fully reset configuration; 
           [0031]      FIG. 3  illustrates schematically a cross-sectional view of the prior art pen injector of  FIG. 1  in a fully discharged configuration; 
           [0032]      FIG. 4  illustrates an enlarged portion of the prior art pen injector in the configuration of  FIG. 3 ; 
           [0033]      FIG. 5  illustrates schematically an axial cross-section through a modified leadscrew; 
           [0034]      FIG. 6  illustrates schematically a cross-section through an improved pen injector in a fully reset configuration; 
           [0035]      FIG. 7  illustrates schematically a portion of the improved pen injector in the configuration of  FIG. 6 ; 
           [0036]      FIG. 8  illustrates schematically a portion of the improved pen injector in the configuration of  FIG. 6 ; 
           [0037]      FIG. 9  illustrates schematically a cross-section through an improved pen injector in a fully discharged configuration; 
           [0038]      FIG. 10  illustrates schematically a portion of the improved pen injector in the configuration of  FIG. 9 ; 
           [0039]      FIG. 11  illustrates a portion of the improved pen injector in the configuration of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    An injection device of the pen injector type will now be described with respect to  FIGS. 5 to 11 . This device aims to provide better control over the stopping position of a plunger within the device, and hence of the bung within the cartridge, and thus reduce wasted medicine. The pen injector is described as an improvement vis-à-vis the device described in WO/2007/063342, and reference should be made to that document. However, the principles of the improvement can be applied to modifications of that device, and to pen injectors having other configurations. Reference below to a “proximal” end of the device refers to the end closest to the needle end, whilst reference to a “distal” end refers to the end furthest from the needle end. Proximal and distal directions are similarly defined. 
         [0041]    With reference to  FIGS. 5 to 11 , an improved pen injector is illustrated, using like reference numerals to identify components common with the prior art device of FIGS.  1  to  4 . The improved pen injector has a modified leadscrew  24 , shown in  FIG. 5 . The leadscrew  24  features a step profile  25  in each of the axial guide tracks  26 . The profile changes the depth of the track from a first depth x in a proximal region of the track (i.e. the region closest to the needle position) to a second depth y in a distal region (i.e. a region further from the needle position), where x is greater than y. As with the prior art device of  FIGS. 1 to 4 , the axial guide tracks  26  engage with a locking bush  27  via splines  28  of the locking bush  27 . The splines  28  are configured to extend into the guide tracks to a depth just less than y such that the splines do not interfere with movement of the leadscrew  24  in the axial direction. As in the prior art device, the locking bush  27  acts to prevent rotation of the leadscrew  24  when the receptacle  2  and cartridge  3  are attached to the housing, whilst allowing rotation of the leadscrew  24  following removal of the receptacle  2  and prior to its complete reattachment. 
         [0042]    The improved pen injector comprises a stop collar  29 . The stop collar  29  has two stop splines  30  that engage with the axial guide tracks  26  in the leadscrew  24 . The stop splines  30  project into the tracks  26  to a depth that is just less than x and greater than y. As such, the stop splines  30  allow the leadscrew  24  to move axially up to the point at which the stop splines  30  engage with the step profiles  25  in the guide tracks  26 , at which point further axial movement of the leadscrew  24  in the proximal direction is blocked. 
         [0043]    The stop collar  29  may be integrally formed with, or sit within, the cartridge compression cup  22 . The stop collar  29  may define a seat to receive the cap  15  when in a fully reset configuration. This can be seen more clearly in  FIG. 8 . When the receptacle  2  is fully connected to the housing  1 , the distal end of the cartridge  3  abuts the stop collar  29 , restricting axially movement of the stop collar  29 . 
         [0044]    Consider now the operation of the improved pen injector. A user sets a dose by rotating the dose knob  8  in a clockwise direction. Once a dose is set and the needle inserted into the user&#39;s skin, the user can apply a force to the trigger  7  in the proximal direction in order to inject the medicine. This disengages the rack  12   a  of the trigger  7  from the rack  11   a  of the drive gear  11 , in turn freeing the drive gear  11  to rotate under the influence of the torsion spring  9 . As the drive gear  11  rotates about the leadscrew  24 , the leadscrew  24  is driven through the drive gear  11  causing the leadscrew cap  15  to push the bung of the cartridge  3  through the cartridge body, expelling medication from the cartridge  3  through an attached needle. 
         [0045]    As with the known pen injector described in the background, the improved pen injector is able to deliver several doses from the same medication filled cartridge  3 . During delivery of each successive dose, the leadscrew  24  is advanced further forward into the medication filled cartridge  3 . This continues until the step profiles  25  meet the stop splines  30  of the stop collar  29 . The stop collar  29 , abutting the distal end of the cartridge, prevents further axial movement of the leadscrew  24  in the proximal direction, thereby defining the limit of movement of the leadscrew and its end cap  15 . This end point is illustrated in  FIGS. 9 and 11  in which it can be seen that the step profiles  25  in the axial guide tracks  26  abut the stop splines  30 . 
         [0046]    With reference again to  FIGS. 1 to 4 , and as has already been described, the limit of axial movement of the leadscrew in the proximal direction in the case of the prior art pen injector is defined by the point at which the enlarged end  21  of the leadscrew abuts the drive gear  11 . By introducing the stop collar  29  between the drive gear  11  and the cap  15  of the leadscrew  24 , the point at which the leadscrew stops with respect to the cartridge is more precisely defined. This is because manufacturing tolerances of, for example, the drive gear  11  and locking bush  27  no longer influence the stopping point of the leadscrew  24 . It is therefore possible to reduce the axial length of the buffer zone required to prevent the bung from entering the tapered region of the cartridge. On average, and in comparison with the prior art pen injector, less medicine will be wasted. 
         [0047]    While the improved pen-type injection device described refers to a leadscrew having two coaxial guide tracks, it will be understood by a person skilled in the art that any number of tracks and corresponding splines may be used, including one. While the  FIGS. 5, 6, 9, 10 and 11  show the leadscrew  24  having an enlarged end at the distal end, it will be understood that this enlarged end is not required to prevent further axial motion of the leadscrew, as in the prior art. 
         [0048]    While described above in the context of a device having a leadscrew that is rotationally fixed during delivery of a medicine, the present invention may also be applied to devices having leadscrews that rotate relative to the housing during delivery of the medicine. In such a device a drive collar and a nut may replace the drive gear  11  and the locking bush  27  of the pen injector described above. The drive collar is coupled to the torsion spring and the trigger in the same way that the drive gear of the pen injector described above couples to these components. The drive collar has one or more drive splines for engaging with the guide tracks of the leadscrew. The nut is arranged such that it can rotationally fixed when a receptacle with cartridge is attached to the housing, e.g. by means of the cartridge pressing the compression cup and thus the nut against locking features formed on an interior surface of the housing. The thread on the inner surface of the nut engages with the leadscrew thread. A reset mechanism is provided in order to allow the leadscrew to be rotated and moved axially back into the housing after injection of a final dose and removal of a spent cartridge. 
         [0049]    The stop collar again sits within the compression cup. However, in contrast to the embodiment described with reference to  FIGS. 4 to 11 , the stop collar is free to rotate within the compression cup. This is necessary in order that the stop collar can rotate with the leadscrew during drug delivery, with the compression cup remaining fixed relative to the housing. 
         [0050]    During operation, a user may select a dose, thus storing energy in the torsion spring. When the user releases the trigger, the drive collar is rotated as the torsion spring unwinds. Due to the coupling between the drive collar and the leadscrew via the drive splines and guide tracks, the leadscrew also rotates with the drive collar. The interaction of the leadscrew and the rotationally fixed nut via the complementary screw threads causes the rotating leadscrew to move axially through the rotationally fixed nut, and thereby drive medicine out of the cartridge. 
         [0051]    The drive splines are configured to extend into the guide tracks to a depth just less than y such that the drive splines do not interfere with movement of the leadscrew in the axial direction. As a result, the step profiles formed in the guide tracks are free to move past the drive splines, but are stopped by the stop splines of the stop collar, which project into the track to a depth greater than y. The proximal limit of movement of the stop collar occurs when the stop collar abuts and is pressed against the end of the cartridge. 
         [0052]    It will be appreciated by the person of skill in the art that further modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, rather than the depth of the track(s) in the leadscrew having a step profile, a step profile may be provided in the width of the track(s), with the splines of the stop collar and of the locking bush having different widths. It will also be noted that the splines of the locking bush and the stop collar need not have the specific configurations described with reference to the figures. For example, a spline may be any projection that interacts appropriately with the guide track.