Patent Publication Number: US-2021187202-A1

Title: Injection pen with dial back and last dose control

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. Nonprovisional application Ser. No. 15,669,349, filed on Aug. 4, 2017, which is a division of U.S. Nonprovisional Application Ser. No. 13/261,300, fled on Jul. 27, 2012, which is a U.S. national stage application under 35 U.S.C. § 371 of International Application No. PCT/US10/003059, filed Nov. 30, 2010, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/265,562, filed on Dec. 1, 2009, and Ser. No. 61/351,465, filed on Jun. 4, 2010, each of which is incorporated herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present application relates to multi-dose medication injection pen devices with improved functionality, including improved dial-back of a set dose, and improved last dose control. 
     BACKGROUND OF THE INVENTION 
     Various medication injection pen devices are known in the prior an. These prior art devices sometimes include features for enabling a user to correct a dose that has been set too large, which may be referred to as “dial back”. Another feature that may be provided by some of the prior art devices is the ability to control a last dose of a medication cartridge such that a user cannot set a dose greater than the remaining amount of medication in the cartridge. This feature is referred to as last dose control or last dose management. Both of these features are desired by users of such pen devices; however, the prior art devices do not satisfactorily meet these needs. Many prior art devices may provide one of these features, but not both. Further, many of the prior art devices require additional steps for performing dial back, which are cumbersome and not intuitive to the user. Thus, there is a need in the art to provide improved functionality of dial back and last dose control mechanisms together in a medication injection pen. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Exemplary embodiments of the present invention address at least the above problems and/or disadvantages and provide at least the advantages described below. 
     Accordingly, a first exemplary embodiment of the present invention provides a medication injection pen comprising a housing, for housing a dose set knob, a leadscrew, a driver, a setback member, and a dose stop member. The dose set knob is rotatable with respect to said housing to set a desired injection dose, and comprises at least one internal thread. The leadscrew includes a thread element by which it is advanceable in a first direction via a corresponding thread engagement, said first direction being that which expels medication from a cartridge. The driver is rotationally fixed to said leadscrew for preventing relative rotation therebetween, said driver being rotatable in a first direction to rotate and advance said leadscrew in said first direction. The setback member is rotationally fixed to said driver for preventing relative rotation therebetween. The dose stop member is rotationally fixed to said setback member and comprises an external thread in threaded engagement with said internal thread of said dose set knob, said dose stop member being axially movable relative to said dose set knob when said dose set knob is rotated relative to said setback member, and wherein axial movement of said dose stop member limits the user from setting a dose that is greater than an injectable volume of medication remaining in the cartridge. 
     According to another exemplary embodiment of the present invention a medication injection pen is provided comprising a housing, for housing a dose set knob, a leadscrew, a driver, a setback member, and a dose stop member. The dose set knob is rotatable with respect to said housing to set a desired injection dose. The leadscrew includes a thread element by which it is advanceable in a first direction via a corresponding thread engagement, said first direction being that which expels medication from a cartridge. The driver is rotationally fixed to said leadscrew for preventing relative rotation therebetween, said driver being rotatable in a first direction to rotate and advance said leadscrew in said first direction. The setback member is rotationally fixed to said driver for preventing relative rotation therebetween, and is provided with an external thread thereon. The dose stop member is rotationally fixed to said dose set knob and comprises an internal thread in threaded engagement with said external thread of said setback member, said dose stop member being axially movable relative to said dose set knob when said dose set knob is rotated relative to said setback member, and wherein axial movement of said dose stop member limits a user from setting a dose that is greater than an injectable volume of medication remaining in the cartridge. 
     According to yet another exemplary embodiment of the present invention a medication injection pen is provided comprising a housing, for housing a dose set knob, a leadscrew, a driver, a setback member, and a dose stop member. The dose set knob is rotatable with respect to said housing to set a desired injection dose. The leadscrew includes a thread element by which it is advanceable in a first direction via a corresponding thread engagement, said first direction being that which expels medication from a cartridge. The driver is rotationally fixed to said leadscrew for preventing relative rotation therebetween, said driver being rotatable in a first direction to rotate and advance said leadscrew in said first direction. The setback member is rotationally fixed to said driver for preventing relative rotation therebetween. The dose stop member is rotationally fixed to said dose set knob and comprises an internal thread in threaded engagement with said thread of said leadscrew, said dose stop member being axially movable relative to said dose set knob when said dose set knob is rotated relative to said setback member, and wherein axial movement of said dose stop member limits a user from setting a dose that is greater than an injectable volume of medication remaining in the cartridge. 
     According to yet another exemplary embodiment of the present invention a medication injection pen is provided comprising a housing, for housing a dose set knob, a leadscrew, a driver, a setback member, and a click element. The dose set knob is rotatable with respect to said housing to set a desired injection dose. The leadscrew includes a thread element by which it is advanceable in a first direction via a corresponding thread engagement, said first direction being that which expels medication from a cartridge. The driver is rotationally fixed to said leadscrew for preventing relative rotation therebetween, said driver being rotatable in a first direction to rotate and advance said leadscrew in said first direction. The setback member is rotationally fixed to said driver for preventing relative rotation therebetween. The click element is positioned between said dose set knob and said setback member, said click element comprising a first arm member engaging an internal surface of said dose set knob, and a second arm member engaging an external surface of said setback member, wherein one of the said first and second arms produces an audible signal when said dose set knob is rotated with respect to said housing. 
     According to yet another exemplary embodiment of the present invention a medication injection pen is provided comprising a housing, for housing a dose set knob, a leadscrew, a driver, and a setback member. The dose set knob is rotatable with respect to said housing to set a desired injection dose. The leadscrew includes a thread element by which it is advanceable in a first direction via a corresponding thread engagement, said first direction being that which expels medication from a cartridge. The driver is rotationally fixed to said leadscrew for preventing relative rotation therebetween, said driver being rotatable in a first direction to rotate and advance said leadscrew in said first direction. The setback member is rotationally fixed to said driver for preventing relative rotation therebetween. The housing further comprises a flexible protrusion provided on a surface within said housing, and the dose set knob further comprises a flexible tab element which engages said protrusion to produce an audible signal upon completion of injection of a set dose. 
     Additional objects, advantages and salient features of exemplary embodiments of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other exemplary features and advantages of certain exemplary embodiments of the present invention will become more apparent from the following description of certain exemplary embodiments thereof when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a medication injection pen according to an exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  depict unassembled and assembled cross-sectional views, respectively, of exemplary components provided in a medication injection pen according to a first exemplary embodiment of the present invention; 
         FIG. 2C  depicts an unassembled view of a pen needle usable in exemplary embodiments of the present invention; 
         FIGS. 3A and 3B  depict views of a body provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 4A and 4B  depict views of a dose set knob provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 3A and 5B  depict views of a setback member provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 6A and 6B  depict views of a click element provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIG. 7  depicts a view of a driver and leadscrew arrangement provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 8A and 8B  depict views of a dose stop member provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 9A and 9B  depict views or a leadscrew brake provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 10A and 10B  depict views of an alternative injection coupling mechanism provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 11A-11F  depict views of alternative leadscrew brake and threaded insert embodiments provided in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 12A-12E  depict views of an end-of-injection click mechanism in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIG. 13  depicts a view of a muted injection click mechanism in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 14A and 14B  depict views of an additional mechanism for reducing friction between components in a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 15A and 158  depict exemplary mechanisms for connecting a cartridge to a medication injection pen according to the first exemplary embodiment of the present invention; 
         FIGS. 16A and 16B  depict unassembled and assembled cross-sectional views, respectively, of exemplary components provided in a medication injection pen according to a second exemplary embodiment of the present invention; 
         FIG. 17  depicts a view of a driver and leadscrew arrangement provided in a medication injection pen according to the second exemplary embodiment of the present invention; 
         FIG. 18  depicts a view of a last dose control mechanism provided in a medication injection pen according to the second exemplary embodiment of the present invention; 
         FIGS. 19A and 19B  depict views of alternative last dose control mechanisms provided in a medication injection pen according to the second exemplary embodiment of the present invention; 
         FIG. 20  depicts an assembled cross-sectional view of exemplary components provided in a medication injection pen according to a third exemplary embodiment of the present invention; 
         FIG. 21  depicts a view of a body provided in a medication injection pen according to the third exemplary embodiment of the present invention; 
         FIG. 22  depicts a view of an insert provided in a medication injection pen according to the third exemplary embodiment of the present invention; 
         FIG. 23  depicts a view of a driver provided in a medication injection pen according to the third exemplary embodiment of the present invention; 
         FIG. 24  depicts a view of a dose set knob provided in a medication injection pen according to the third exemplary embodiment of the present invention; and 
         FIG. 25  depicts a view of a setback member provided in a medication injection pen according to the third exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The matters exemplified in this description are provided to assist in a comprehensive understanding of exemplary embodiments of the present disclosure with reference to the accompanying drawing figures. Accordingly, those of ordinary skill in the art will recognize that various changes to and modifications of the exemplary embodiments described herein can be made without departing from the scope and spirit of the claimed invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     With reference to the drawing figures, particularly  FIG. 1 , a medication injection pen is shown and generally designated with the reference numeral  10 . The medication injection pen  10  may be used for the administration of various medications, preferably liquid in nature, including but not limited to insulin and human growth hormone. The term “medication” is used in an illustrative and non-limiting manner to refer to any substance that may be injected into a patient for any purpose. The medication injection pen  10  is provided for administering multiple injections, the dose or volume of which may be set by the user and may vary for each injection. Exemplary embodiments of medication injection pen  10  of the present disclosure may be either disposable or reusable when the supply of medication therein has been exhausted. 
     With reference to  FIGS. 2A and 2B , in an exemplary embodiment, the medication injection pen  10  generally includes a cap  12 , a cartridge holder  14 , a spinner  16 , a body  18 , a dose knob  20 , a setback member  22 , a driver member  24 , a leadscrew  26 , a dose stop member  28 , a bi-directional click element  30 , a leadscrew brake  32 , and a push button  34 . 
     The cartridge holder  14  is formed to accommodate a medication cartridge  36 , which may be of any conventional design. By way of non-limiting example, the cartridge  3  may include an elastomeric septum  38  at a distal end thereof, and an open proximal end  37  which exposes a slidable plunger  40 . A medication is contained within the cartridge  36  between the septum  38  and the plunger  40 . As will be described in more detail below, the spinner  16  is configured to engage the plunger  40  and force a distal movement thereof to expel the medication from the cartridge  36 . The spinner  16  includes an aperture formed to snap fit or otherwise engage a distal end bead portion  27  ( FIG. 7 ) of leadscrew  26  in mounting the spinner  16  onto the leadscrew  26 . A standard pen needle  11  ( FIG. 2C ) is used to administer medication from the medication injection pen  10 . The needle is a double-ended cannula  5  which is threadedly mounted onto threads  42  of the cartridge holder  14 , as is well known in the an. One end of the cannula  5  is exposed for insertion into a patient, while the second end of the cannula is disposed to pierce the septum  38  of the cartridge  36 . After administration of a set dose, the needle  11  may be removed, in which case, the septum  38  may be self-sealing. The cap  12  is formed to releasably mount onto the cartridge holder  14 , such as with a snap fit or other releasable engagement, to limit contamination of the septum  38  and the surrounding portions of the cartridge holder  14 . A resilient holding arm  13  may extend from the cap  12  to provide a holding force for retaining the injection pen  10  in the user&#39;s pocket, purse, or carrying case. One or more windows  15  may also be provided in the cartridge holder  14  to give a visual indication of the medication volume remaining in the cartridge  36 . 
     An exploded perspective view of a pen needle  11  of an exemplary injection pen is shown in  FIG. 2C . The pen needle  11  includes the cover (outer shield)  1 , an inner shield  2 , a needle cannula  5  and a hub  3 . During manufacture, a proximal end of the needle cannula  5  is inserted into a center opening in the distal (patient) end  4  of the hub  3  until a predetermined length of the distal (patient) end of the needle cannula  5  remains extended. The needle cannula  5  is secured by epoxy or adhesive in the distal end  4  of the hub  3 . To protect users from injury and the needle cannula  5  from being damaged, the inner shield  2  covers the exposed portion of the needle cannula  5 . The open proximal end of the inner shield  2  is placed over the exposed portion of the needle cannula  5 . The open proximal end of the cover  1  envelops the inner shield  2 , needle cannula  5 , and hub  3 . The distal end of the cover  1  is closed to prevent contamination and damage to the inner components of the pen needle  11 , and to prevent injury to anyone who may handle it prior to use. When the user is ready to use the pen needle, the hub  3  is screwed onto threads  42  of cartridge holder  14  of the injection pen  10  ( FIGS. 1, 2A and 2B ), and the cover  1  and shield  2  are separately removed from the hub  3 /cannula  5  subassembly by a pulling action. The distal end of the inner shield  2  is closed to protect the user from an accidental needle stick by the needle cannula  5  after the cover  1  is removed. The inner shield  2  is then removed to access the needle cannula  5 . 
     With reference to  FIGS. 3A and 3B , the body  18  is generally cylindrical, and includes a cylindrical partition or wall  30  extending across the interior of the body  18  through which a channel  31  is formed comprising an aperture  52 . Cylindrical wall  50  effectively divides body  18  into two compartments, a first compartment proximal to wall  50  for housing the plurality of dose setting and injecting components, as shown in  FIGS. 2A and 2B , and a second compartment distal of wall  50  for housing the leadscrew brake  32  and connecting to the cartridge holder  14 . The channel  1  comprises internal threads  54  threadedly engaging corresponding threads of the leadscrew  26 . In an exemplary embodiment, the leadscrew  26  is provided with a non-circular cross-section, in which case, the aperture  52  is defined to allow rotational and thereby axial movement of the leadscrew  26  therethrough. A plurality of second threads  56  are provided on the interior of the body IS in the first compartment and threadedly engaged with corresponding threads  62  provided on the dose set knob  20 , as discussed further below. Body IX preferably includes a window  57  enabling the user to view a set dose indicated on the outer surface of the dose set knob  20 . As described further below, a series of angled steps or teeth  55  are provided on the interior of the second compartment of body  18 , circumferentially surrounding the distal end of channel  5 . Teeth  55  are provided as part of a unidirectional coupling with the leadscrew brake  32  to allow the leadscrew  26  to rotate through the channel  51  in only one direction, that which causes the leadscrew to expel medication from cartridge  36 . In an exemplary embodiment, body  18  also includes a circumferential rib or groove  58  onto which the cartridge holder  14  may be mounted with a snap fit. 
     With reference to  FIGS. 4A and 4B , a generally cylindrical dose set knob  20  with open proximal and distal ends is provided with an enlarged proximal portion or handle  60  defining a knob-like feature. Handle  60  may include a plurality of grooves  61  enabling a user to securely grip handle  60  to set a dose of medication for an injection. Dose set knob  20  includes at least one thread element  62  provided on its external surface, preferably near the distal end and threadedly engaging corresponding threads  6  on the interior of body  18 . An injection dose is set by the user by rotating dose set knob  20  in a predetermined direction. Due to the threaded engagement with the body  18 , rotation of the dose set knob  20  translates into axial movement of the dose set knob in the proximal direction extending away from and out of the body  18 . Provided on the outer surface of body  18 , area plurality of dosing indicia (not shown) indicating a set dose to be viewed through window  57  provided on body  18 . 
     A plurality of radially directed ridges  63  are provided circumferentially along the interior surface of dose set knob  20  adjacent to handle  60 . Ridges  63  provide part of a clicking means in conjunction with an externally directed ratchet element  82  provided on click arm  81  ( FIG. 6A, 6B ) of click element  30 . Ridges  63  each comprise a sloped edge and a flat face for allowing relative rotational movement between the dose set knob  20  and click element  30  in only one direction in which the click arm  81  is enabled to slide over ridges  63 , thus providing an audible and tactile signal. Additionally, the proximal edges of ridges  63  define a proximally facing surface having a plurality of teeth  64  disposed thereon. In an exemplary embodiment, teeth  64  are included as part of a clutch mechanism when engaged with corresponding teeth  74  ( FIGS. 5A and 5B ) disposed on setback member  22 . When pressed together during injection, teeth  64  and  74  lock together, thus preventing relative rotation between setback member  22  and dose set knob  20 , as further described below. Additionally, during dose setting, teeth  64  provided on the dose set knob  20  function as a shelf, causing axial movement of the setback member  74  together with the dose set knob  20 , as the dose set knob is rotated and moved axially out of the body  19 . Dose set knob  20  also includes a plurality of longitudinally extending keys or splines  65  provided substantially along the interior surface preferably extending from the open distal end to the enlarged proximal portion  60 . Longitudinal splines  65  engage with corresponding grooves  95  provided on the exterior of dose stop member  28  to prevent relative rotation between the dose set knob  20  and the dose stop member  28 , but to allow relative axial movement therebetween. 
     Setback member  22  comprises a generally cylindrical elongated member as shown in  FIGS. 3A and 5B . Provided near the proximal end of setback member  22  are a plurality of ridges  73  spaced along the external surface thereof. When the exemplary injection pen  10  is assembled, ridges  73  of setback member  22  face ridges  63  provided on the internal surface of dose set knob  20 . Ridges  73  include sloped edges and flat faces for engaging an internally directed ratchet element  84  provided on flexible arm  83  of click element  34 . As similarly described above, ridges  73  enable relative rotational movement between the setback member  22  and click element  30  in only one direction in which the internally directed ratchet element  84  is enabled to slide over ridges  73  providing an audible and tactile signal. The allowed direction of relative rotation between the setback member  22  and click element  30  is in the direction opposite that enabled by similar engagement between the dose set knob  20  and the click element  30 , so that the relative rotation between dose set knob  20  and setback member  22  is bi-directional. 
     Click element  30  is described with reference to  FIGS. 6A and 6B . As shown, click element  30  is a cylindrical tube like element comprising a plurality of radially flexible arms  81  and  83  oppositely disposed from each other. The click element  30  is preferably constructed with a longitudinal dimension similar to the length of the ridge portions  63 ,  73  provided on the dose set knob  20  and setback member  22 , respectively. Flexible arm  81  includes an externally directed ratchet element  82  provided at the free end thereof facing ridges  63  provided on the interior of dose set knob  20 . Flexible arm  83 , on the other band, includes an internally directed ratchet element  84  provided at the free end thereof facing ridges  73  provided on the exterior of setback member  22 . During dose setting, click element  30  is permitted to rotate relative to both the setback member  22  and dose set knob  20 , but in only one direction with respect to each. In other words, during dose setting, click element  30  is rotationally locked to one of the setback member  22  or the dose se knob  20  via flexible arms  83  and  81 , respectively, depending on the direction of relative rotation for either normal setting of a dose or dialing back of the set dose. When the dose set knob  20  is rotated in the direction in which ridges  63  are enabled to slide over the externally directed ratchet element  82  and produce an audible signal, the click element  30  does not move rotationally relative to the setback member  22  since such a movement is prevented by an engagement between click arm  83  and ridges  73 . Conversely, when the dose set knob  20  is rotated in the opposite direction, the externally directed ratchet element  82  engages with one of the ridges  63  causing the click element to rotate together with the dose set knob  20 . In this case, the internally directed ratchet element  84  is now permitted to slide past ridges  73  on the setback member, thereby producing an audible signal. 
     The exemplary construction of click element  30  described above allows relative rotation in both directions between the setback member  22  and the dose set knob  20 . Such a click element is not restricted to the design depicted in  FIGS. 6A and 6B . Any similar element(s) enabling bi-rotational movement between setback member  22  and dose set knob  20 , as described above, may be implemented in this embodiment, as would be evident to one of ordinary skill in the art. 
     As shown in  FIG. 3A , setback member  22  includes an adapter element  71  for snap-fitting with an internal cavity of push button  34 . Push button  34  is of any conventional design, but it is preferred that the snap engagement enables the push button to freely rotate on the adapter element  71 . Alternatively, push button  34  may be unitarily formed with the adapter element  71 . Additionally, as shown in  FIG. 5B , a plurality of longitudinally extending keys or splines  75  are provided along the internal cylindrical surface of setback member  22 . Splines  75  are formed to engage corresponding longitudinal grooves  85  provided on the external surface of driver member  24 , shown in  FIG. 7 , thus preventing relative rotation between the setback member  22  and driver  24 , while allowing relative axial movement therebetween. With reference to  FIG. 7 , the driver  24  includes open proximal and distal ends which provide a passage  86  for leadscrew  26 . In an exemplary embodiment, passage  86  comprises a non-circular cross-section corresponding to the non-circular cross-section of leadscrew  26 , thus preventing relative rotation therebetween. Driver  24  includes a disk  87  formed at the proximal end for snap-engaging with at least one flexible tab  97  provided on the interior surface of dose stop element  29  (FIG. A). With the snap engagement, the driver  24  is fixed axially relative to the dose stop element  28 , yet is able to rotate relative thereto. Driver  24  may also include one or more flexible legs  88  inwardly biased to engage leadscrew  26 . Flexible leg  88  may be provided to reduce any play between the mating cross sections of leadscrew  26  and aperture  86  to improve dose accuracy of the exemplary injection pen  10 . 
     With reference to  FIGS. 8A and 8B , a generally cylindrical dose stop element  28  is provided for enabling last dose control so that a dose cannot be set or dialed up that is greater than the amount of medication remaining in the cartridge  36 , as further discussed below. Dose stop element  28  includes a plurality of longitudinal grooves  95  on the external surface thereof. Grooves  95  engage with corresponding splines  65  provided on the interior of dose set knob  20 , thereby preventing relative rotation therebetween, but allowing relative axial movement. Dose stop element  28  has an open proximal end  91  and open distal end  92 , the distal end  92  preferably comprising a section of reduced diameter. Open distal end  92  defines a threaded opening with threads  93  disposed thereon for threadedly engaging corresponding threads  25  of the leadscrew  26  when assembled. Proximal end  91  defines a cavity housing setback member  22 , driver  24 , and leadscrew  26 . Flexible tabs  97  are provided adjacent to the open distal end  92 , extending into the interior of the dose stop element  28 . A recess  96  or cutout is provided in the external cylindrical wall of the dose stop element defining an area into which flexible tabs  97  are allowed to flex. During assembly, driver  24  is inserted into the open proximal end  91 , upon which disk  87  disposed near the distal end of driver  24  engages flexible tabs  97 , and causes them to flex outwardly into recess  9  until the disk  87  moves past the flexible tabs, at which time the flexible tabs  97  return to their initial positions to provide a blocking surface for the driver  24  preventing relative axial movement therebetween. 
     While the above components are described as comprising specific features for engaging and interconnecting other components of an exemplary injection pen, the above components are not limited to these specific features. For instance, instead of the described mating non-circular cross-sections to prevent relative rotation between leadscrew  26  and driver  24 , one of ordinary skill in the art will appreciate that similar functionality may be provided using a spline/groove engagement for preventing relative rotation therebetween while also allowing relative axial movement. Conversely, the above described spline/groove features may be replaced with non-circular mating arrangements or other known features for preventing relative rotation while allowing relative axial movement therebetween. 
     With reference to  FIGS. 9A and 9B , leadscrew brake  32  comprises a generally cylindrical housing portion  90  provided with a first diameter large enough to surround channel  51 , as shown in  FIG. 9B . Extending in the distal direction, a pair of wall portions  92  is provided defining an aperture  93  with a non-circular cross-section to mate with the non-circular cross section of leadscrew  26 . Due to the mating non-circular aperture  93 , the leadscrew  26  is prevented from rotating with respect to the leadscrew brake. Further, the leadscrew brake  32  comprises a pair of flexible ratchet arms  94  configured to engage the cylindrical ring of ratchet teeth  55  provided on the interior of the body  18  to define a unidirectional coupling therebetween. Ratchet arms  94  are configured to allow rotation of the leadscrew brake  32 , and therefore the leadscrew  26 , in only one direction with respect to the body  18 . The allowed direction is that which causes the leadscrew  26  to rotate through the threaded channel  51  in the distal direction to expel medication. During injection, the leadscrew brake  32  rotates relative to body  18 , and ratchet arms  94  ride over the slanted or ramped portion of teeth  55  to produce an audible clicking signal indicating the injection is being performed. Rotation of the leadscrew brake in the opposite direction causes the free ends of ratchet arms  94  to engage the flat faces of teeth  55 , which resist ratcheting of the ratchet arms  94  and thereby prevent relative rotation in this direction. Due to the unidirectional coupling between leadscrew brake  32  and teeth  55 , an undesired rearward movement of the leadscrew  26  is prevented. 
     Having described exemplary structures, features and interrelationships between particular elements of the exemplary embodiment of medication injection pen  10  herein, the intended functionality of such an exemplary medical pen device will now be described. 
     Following assembly of the exemplary elements as shown in  FIGS. 2A, 2B , and as described above, to set a desired dose, the patient or user first grips and rotates the enlarged proximal end  60  of the dose set knob  20 . The dose set knob  20  is rotated a number of rotations relative to the body  18  until a desired dose is shown through the window  57  on the body  18 . Due to the threaded engagement of thread  62  on the dose set knob  20  with the internal thread  56  of the body, the dose set knob is caused to screw out of the proximal end of the body, carrying the setback member  22  along with it by substantially the same distance. The dose stop member  28  is also caused to rotate together with the dose set knob  20  due to the spline/groove engagement between spline  65  provided on the interior of the dose set knob  20  and groove  95  provided on the exterior of dose stop  28 . Rotation of the dose stop member  28  causes axial movement of the dose stop member with respect to the body  18  in the proximal direction due to the threaded engagement between threads  93  on the dose stop member and threads  25  of the leadscrew  26 . The dose stop element  28 , however, moves a shorter distance axially than the dose set knob  20  due to a difference in the pitch of the thread  25  of the leadscrew  26  and the inner thread  56  of the body  18 . 
     During normal dose setting for increasing a set dose, the leadscrew  26  is prevented from rotating with respect to the body IS in the dose setting direction due to the unidirectional coupling between the leadscrew brake  32  and the teeth  55  disposed on the body  18 . Setback member  22  and driver  24 , which are rotationally fixed to each other due to spline/groove connection  75 / 85 , are therefore also prevented from rotating with respect to body  18  during dose setting, since the driver is rotationally fixed to the leadscrew via the mating of the non-circular cross-section of leadscrew  26  and the non-circular aperture  86  of the driver  24 . Due to the snap it between disk  87  of the driver  24 , and flexible tabs  97  provided on the dose stop member  28 , as the dose stop member screws out of the body in the proximal direction, the driver  24  moves axially by the same distance, but does not rotate. 
     The setback member  22  is interconnected to the dose set knob  20  through the bi-directional click element  30 . During normal dose setting, the dose set knob  20  rotates relative to the click element  30 , and thus an audible signal is provided due to the inner grooves  63  of the dose set knob  20  sliding past the externally directed ratchet element  82  on flexible arm  81 . The externally directed ratchet element  2  tends to slide past ridges  63  in the dose setting direction because the internally directed ratchet element  84  of flexible arm  83  is locked with the ridges  73  provided on the setback member  22 , which is prevented from rotating in this direction due to its engagement with the driver  24  and leadscrew  26 . 
     If the user initially sets a dose larger than desired, the set dose can be “dialed back” or reduced by simply turning the dose set knob  20  in the opposite direction. Rotation of the dose set knob  20  in this reverse direction, which is the direction of injection, would normally cause rotation of the leadscrew  26  and thus axial movement of the leadscrew into the cartridge  36 . During injection, rotation of the leadscrew  26  is effected due to the coupling between teeth  64  on the dose set knob and teeth  74  on the setback member  22 , which is indirectly rotationally fixed to the leadscrew  26 . During dial back, however, the dose set knob  20  and setback member  22  are not coupled via teeth  6474  and the dose set knob  20  rotates in this reverse direction relative to the setback member  22  through the click element  30 . Reverse rotation of the dose set knob  20 , during dial back, now causes the internal ridges  63  on the dose set knob  20  to engage and lock with the externally directed ratchet element  82 , forcing the click element  30  to rotate in this same direction. The internally directed ratchet element  84  is now caused to slide over ridges  63  provided on the setback member  22 , thereby producing an audible signal indicating the dose is being reduced. Ratchet element  84  tends to slide over ridges  73  in this direction since there is less friction provided between ratchet element  84  and ridges  73  than there is between the unidirectional coupling between the leadscrew brake  32  and the body  18 . In other words, the force required to dial back a set dose is not great enough to overcome the friction between the ratchet arms  94  of leadscrew brake  32  and the teeth SS of body  1 . 
     Once a desired dose is set, and the user desires to inject the set dose of medication, the medical injection pen  10  is applied to the skin of the patient to insert the needle cannula  5 . The pen needle  11  is attached to the threaded portion  42  of the cartridge holder  14  prior to or after setting the desired dosage, as a matter of user preference. Once the pen needle  11  has been attached to the cartridge holder  14  and inserted into the patient, the push button  34  is depressed. The axial force applied to the push button  34  by the user causes the teeth  74  on the setback member  22  to engage with the teeth  64  on the dose set knob  20  to mesh and rotationally lock the setback member  22  with the dose set knob  20 , forming an injection coupling. The applied force causes the dose set knob  20  (due to a non self-locking threaded engagement with the body  18  via threads  56  and  62 ) to rotate in the direction opposite that which occurs during normal dose setting. This rotation is now imparted to the setback member  22  and therefore the driver  24  (due to the spline/groove connection  75 ,  85 . Since the driver  24  mates with the non-circular cross-section of the leadscrew  26 , the leadscrew is also caused to rotate relative to body  19 , which translates into axial movement of the leadscrew into the cartridge  36  to expel a dose (due to the threaded engagement between threads  25  on the leadscrew and threads  54  disposed on channel  51  of the body  18 ). Axial movement of the leadscrew in the distal direction urges the spinner  16  against the plunger  40  to expel medication from the cartridge  32 . The injection force is greater than the frictional force in the leadscrew brake  32 , and hence the leadscrew brake allows rotation of the leadscrew  26  in this direction during injection. As the leadscrew brake  32  rotates with the leadscrew  26 , oppositely disposed ratchet arms  94  slide over the teeth  55  disposed on the interior of body  18  to produce a clicking sound as the injection is carried out. 
     The dose administration process described above may be repeated until the medication in the cartridge  36  is spent. Prior to expelling the last dose from the cartridge  36 , it is desired to ensure that the last dose expelled is consistent with the dose set by the user. In other words, the user should not be able to set a dose for an amount greater than the remaining volume of medication in the cartridge  36 . This last dose control is realized when threads  93  disposed on the dose stop element  28  abut against a non-threaded portion of the leadscrew  26  at its proximal end, preventing further rotation of the dose stop member  28  on leadscrew  26 . When this occurs, the indicia on the dose set knob  20 , read through window  57 , indicate the last remaining injectable volume of medication in the cartridge  36 . Once the dose stop member  28  is prevented from rotating further, the dose set knob  20  is also prevented from further rotating in this direction for setting a larger dose, due to the spline/groove engagement  65 / 95  between dose set knob  20  and dose stop member  28 . 
     During dose setting, the dose stop member  28  changes its relative position on the leadscrew  26  based on the number of rotations of the dose set knob  20 . Axial movement of the dose stop member  28  during dose setting is by substantially the same distance as the leadscrew  26  moves into cartridge  36  during injection. The length of axial movement of leadscrew  26 , and therefore the volume of medication to be expelled, is determined in part by the thread pitch of the leadscrew threads  25  and threads  54  of the body, which is substantially the same as the pitch of the threads  93  of the dose stop member  28 . Thus, the relative position of the dose stop member  28  on the leadscrew  26  throughout administration is indicative of the remaining dosage amount in the cartridge  36 . The dose stop member maintains its relative position on the leadscrew  26  during injection due to its spline/groove engagement  6595  with the dose set knob  20 . During injection, the dose sent knob  20 , dose stop member  28 , setback member  22 , driver  24  and leadscrew  26 , are all rotationally locked together. Since the threads  93  of the dose stop member  28  and the threads  54  of the body  18  are of substantially the same pitch, simultaneous rotation of the dose stop member  28  and leadscrew  26  results in the same axial movement. Thus, during injection, the dose stop member  28  does not move axially relative to leadscrew  26 , and therefore maintains its relative position with respect to the leadscrew as determined during the dose setting procedure. After administration of the last dose, if the injection pen  10  is reusable, the cartridge can be replaced, whereas, if the pen is disposable, the entire pen  10  may be disposed of. 
     As will be appreciated by those skilled in the art, various modifications can be made to the above exemplary embodiments without substantially altering the functionality of the injection pen  10 . For example, such modifications may be made to ease the assembly of the various components, reduce the complexity of manufacturing, reduce the number of elements, or provide some additional improved functionality. Some such exemplary modifications are described below. 
     In one alternative embodiment, teeth  64  on the dose set knob  20 , described above as part of an injection coupling with corresponding teeth  74  ( FIGS. 5A and 5B ) disposed on setback member  22 , can generate dose setting click signals in the absence of click element  30 . A spring element or wave washer with similar functionality may be provided to bias the teeth  74  of setback member  22  toward the corresponding teeth  64  provided on the dose set knob  20 , so that they are in constant meshed engagement. The spring force, however, is easily overcome by relative rotation between the dose set knob  20  and setback member  22 , which causes the corresponding teeth  64 / 74  to slip over each other producing an audible and tactile signal. 
     In another exemplary embodiment, an alternate injection coupling is provided between a modified dose set knob  20  (teeth  64  removed) and a modified setback member  22 ′, shown in  FIG. 10 . In this embodiment, the injection coupling described above with respect to teeth  64  on the dose set knob  20  and corresponding teeth  74  on the setback member  22 , is replaced by an extended surface  101  provided near the proximal end of the setback member  22 ′ the extended surface  101  being defined by a larger diameter with respect to ridges  73 . Click element  30  includes a first surface  102  positioned co-axially on and surrounding the extended surface  101 . During the dose setting operation, click element  30  is positioned on setback member  22 ′, such that click arms  81  and  83  are free to flex and slide past ridges  73  and  63 , respectively. In this embodiment, the dose setting and dial back mechanism is unchanged. During injection, however, upon the user applying an injection force to push button  34 , the setback member  22 ′ is pushed into the dose set knob  20  and into click element  30 . As the setback member  22  moves axially toward click element  30 , the extended surface  101  is moved into engagement with click arm  83 , as shown in  FIG. 108 . In this position, click arm  83  is prevented from flexing radially inward to slide past grooves  63  in the dose set knob, thus locking click element  30  to dose set knob  20 . Relative rotation of the dose set knob  20  with respect to the setback member  22 ′ in this direction during dose setting would have enabled the ratchet arm  83  to ride over ridges  63  to reduce a set dose. During injection, however, ratchet arm  83  is now prevented from flexing away from ridges  63 , and thus prevented from sliding over ridges  63 , by the blocking engagement of extended surface  101 . Accordingly, the setback member  22 ′ is now rotationally locked to dose set knob  20  via non-sliding engagement with ratchet arm  83 , thus enabling injection of a set dose, as described above. 
     In another embodiment, the exemplary injection pen  10  is modified to facilitate the manufacture of injection pens providing different dosing needs. For example, an injection pen for administering a first medication may desire liner dosing intervals for more precise dosage control than that of another medication. To utilize the same dose setting and injection functionality of the exemplary injection pen described above, it is desired to be able to provide a plurality of pens meeting the various dosing needs with greater compatibility, so as to reduce the complexity of manufacturing multiple such pens. 
     One such modification is made to the embodiment illustrated in  FIG. 9B  to switch the ratchet arms  94  provided on the leadscrew brake  32  with the ratchet teeth  55  provided on body  18 , as shown in  FIG. 11A . As shown, the body  18 ′ now includes ratchet arms  111  and the leadscrew brake  32 ′ now includes teeth  112 . Engagement between ratchet arms  111  and teeth  112  serves to provide similar unidirectional functionality as described in the previous embodiment. Leadscrew brake  32 ′ in this alternative embodiment facilitates a change in a desired injection click interval necessitated by a desired change in a dosing interval. For example, the injection clicks realized by relative rotation of the leadscrew brake  32  preferably correspond to a dose increment, and are related to the spacing of the teeth  112 . If the dose increment is changed to have a greater or smaller interval, a leadscrew brake with a corresponding spacing of teeth  112  is assembled in the injection pen as shown, as opposed to providing a new body  18  with the desired spacing of teeth  55 , as in the earlier embodiment. The smaller leadscrew brake  32  is easier and less costly to manufacture than the body  18 , and hence it is advantageous to replace leadscrew brake  32  in the modified injection pen as opposed to replacing the body  18 . 
     A further modification, shown in  FIGS. 11B-11F , enables an easier change of the thread pitch of the leadscrew if desired to increase or decrease a dose rate. For example, the threads  25  of the leadscrew  26  may be modified to include a larger pitch, so that the same number of rotations of the leadscrew results in greater axial movement of the leadscrew into cartridge  36  and therefore a larger dose volume. In the previous embodiment, if the threads  25  of the leadscrew  26  are modified, the threads  54  of the body are also modified accordingly. The additional modification shown in  FIG. 11B  provides an insert  114  which replaces the features of the partitioning wall  50  and channel  51  with threads  54  disposed thereon of the exemplary embodiment (see  FIG. 3A ). Insert  114  is a nut-like element with ratchet arms  115  disposed thereon. Insert  114  comprises a wall  118  with an aperture  119  therethrough. Aperture  119  is defined by a cylindrical channel  116  with threads  117  disposed on the interior thereof. Body  18 ″ now includes a shelf or ledge  121  forming a contact surface engaging with a proximal surface of insert  114  to determine axial placement of the insert  114  into body  18 ″. Shelf  121  comprises at least one protrusion member  122  configured to engaged a corresponding recess  120  on the proximal face of insert  114 . Engagement between protrusion  122  and recess  120  prohibits relative rotational movement between insert  114  and body  18 ″ Alternatively, any similar key/groove type structure may be provided to limit relative rotational movement between insert  114  and body  18 ″. 
     Insert  114  may also be provided with an additional molded spring feature  124  to maintain the positioning of insert  114  against shelf  121  in body  18 ″ Molded spring feature  124  also presses against cartridge  36 , as shown in  FIG. 11F , to prevent the cartridge from moving when the needle  5  is inserted into the cartridge septum  38  prior to injection. This feature provides greater accuracy in dose injection and prevents undesired wasting of medication, in prior art injection pens, the cartridge may be allowed to move a slight distance in the proximal direction during this operation, resulting a small waste or “drool” of the medication. 
     In another exemplary embodiment, an end of injection click or signal is provided by a modified dose set knob  20 ′ including a radially flexible leg  128  near the distal end of dose set knob  20 ′ extending in the distal direction. Flexible leg  128  interacts with an angled protrusion  130 , shown in  FIG. 12B , disposed on a proximal surface of partitioning wall  50  of the body  18 . Angled protrusion  130  is preferably fixed to partitioning wall  50  at only one end, which is the end spaced farther away from the internal surface of body  18 . At a zero dose position, when the dose set knob  20 ′ abuts partitioning wall  50 , flexible leg  128  is positioned near protrusion  130 , but not in touching engagement. Upon setting of a desired dose, as the dose set knob  20 ′ is rotated, flexible leg  128  moves between the angled protrusion  130  and an internal surface of body  18 , as shown in  FIG. 12C . Since the angled protrusion  130  is not fixed to the partitioning wall  50  at the end closest to the internal surface of the body  18 , the angled protrusion flexes radially to allow passage of the flexible leg therebetween and reduce the friction for initially overcoming the protrusion during dose setting. Once flexible leg  128  passes behind protrusion  130 , continued rotation of the dose set knob  20 ′ will result in axial movement of the dose set knob away from partitioning wall  50  so that the flexible leg  128  no longer interacts with protrusion  130 . Normal setting of the dose is now performed. 
     As the set dose is injected, dose set knob  20 ′ screws back into body  18  and moves toward partitioning wall  50 . As the injection is nearing its end, flexible leg  128  once again engages protrusion  130  as shown in  FIG. 12E . This time, as flexible leg  128  abuts against protrusion  130 , it is not allowed to pass between the protrusion  130  and the internal surface of body  18 . Now, the flexible leg  128  is caused to flex radially inward to slide past protrusion  130  until it moves past the end of protrusion  130 , at which time flexible leg  128  snaps against the internal surface of body  18  providing an audible and tactile signal. At this point, the set dose is completely delivered and the injection pen is at a zero dose position. 
     In one embodiment, an end of dose click may be provided as a distinct signal distinguishable from the injection clicks provided by the leadscrew brake  32  as discussed above with respect to  FIG. 9 . In another embodiment, however, the injection clicks are muted, and a user senses only the end of injection click provided between the flexible leg  128  and protrusion  130 . One way to mute the injection clicks is to replace ratchet teeth  55  provided on the body  18  with a rubber like ring or brake  134 , as shown in  FIG. 13 . During injection, the leadscrew brake  32  still rotates with respect to body  18 , but in this embodiment ratchet arms  94  slide along the surface of the rubber brake  134  without providing an audible or tactile signal. Rubber brake  134  is fixed to the body  18  using an adhesive or other structure, so that it does not rotate relative to the body  18 , and therefore is still capable of functioning as a unidirectional coupling with leadscrew brake  32 . Ratchet arms  94  of the leadscrew brake  32  are preferably beveled or otherwise configured to grip the rubber brake  134  to prevent relative rotation therebetween, similar to the embodiment discussed in  FIG. 9B . Additionally, one of ordinary skill in the art will appreciate that rubber brake  134  may be modified as similarly discussed in  FIGS. 11A and 11B . 
     In another exemplary embodiment, an element or elements are added to improve the mechanical efficiency of an exemplary injection pen  10 , by eliminating or reducing the friction between elements rotating relative to each other or those moving axially with respect to each other. One particular engagement with undesirable friction is between the push button  34  and the adapter element  71  provided on the setback member. During dose setting and injection, push button  34  preferably rotates freely on adapter element  71 . In an exemplary embodiment, as partly shown in FIG. M, adapter element  71  includes a point  77  provided at the center of the axis of rotation of the setback element  22 . This point  77  contacts push button  34  near its center of rotation. Providing such a contact surface between these elements at or near the center of rotation reduces frictional torque between these elements during relative rotation, and thereby increases efficiency. To further reduce the friction between setback member  22  and push button  34 , one embodiment includes at least one rolling ball (i.e. ball bearing)  140  situated between an internal surface of the push button  34  and a surface of the adapter element  71 , as shown in  FIG. 14A . Rolling balls  140  function to translate sliding friction between engaging elements into a reduced rolling friction. In another embodiment, shown in  FIG. 148 , a pair of magnets  142   a  and  142   b  with the same polarity are provided on adjacent contact surfaces facing each other. For instance, a first magnet  142   a  is provided on the interior of push button  34 , whereas the second magnet  142   b  is provided on a contact surface of adapter element  71  facing the first magnet. Due to the same polarity between magnets  142   a  and  142   b , the resulting repulsion force reduces the contact force between these two surfaces, thus reducing friction therebetween without affecting the push force required for injecting medication. One of ordinary skill in the an will appreciate that the above methods may also be implemented in combination. Further, such methods may be implemented between any two components with a linear or rotational contact surface, to further improve mechanical efficiency. 
     The exemplary embodiments described above may be provided as a reusable or disposable pen. In a disposable implementation, cartridge holder  14  and body  18  are preferably irreversibly assembled. In one embodiment, as described with respect to  FIG. 3A , a circumferential rib provided on the cartridge holder  14  snaps into engagement with a groove  58  on body. In another embodiment, shown in  FIG. 1A , threaded insert  114  may include at least one tab  125  for snap-fitting with a recess  126  provided on cartridge holder  114 . Since threaded insert  114 , as discussed with respect to  FIGS. 11B and 11C , is fixed both axially and rotationally to body  18 , snap-engagement of the cartridge holder  14  to threaded insert  114  prevents relative rotation between the cartridge holder and the body. 
     In a preferred embodiment shown in  FIG. 158 , cartridge holder  14  and body  18  comprise a set of threads  151  and  152  which provide a secure threaded coupling between the cartridge holder  14  and body  18 . Additionally, one of the cartridge holder and the body comprises a snap  154  and the other comprise a recess  156  for engaging snap  154 . The snap/recess engagement is preferably a one-way radial snap. Thus, once the body  18  and the cartridge holder  14  are screwed together, the snap  154  moves into engagement with recess  156  until they snap together providing a secure, irreversible connection, with minimal or no play between the cartridge holder  14  and body IN, thereby increasing accuracy of the dose injection and reducing/eliminating unnecessary waste of medication. 
     In view of the above description, another exemplary embodiment comprising similar components and functionality is shown in  FIGS. 16A and 168 . The components shown in  FIGS. 16A and 168  have similar functionality to those described above, unless noted otherwise, and therefore their detailed description is omitted herein, in this embodiment, body  218  is similar to the body  18 ″ shown in  FIGS. 11B and 11D . A first compartment defined by the interior of body  218 , proximal to wall  250 , houses a dose set knob  220 , a setback member  222 , a dose stop member  228 , a driver  224 , and a leadscrew  226 . The second compartment defined by the interior of body  218 , distal to wall  250 , houses a threaded drive insert  233  and a distal end  225  of driver  224 . Wall  250  is provided with an aperture sized to fit the main cylindrical body of driver  224 , but not the enlarged distal end  225 , as shown in  FIG. 16B , thus axially fixing the driver  224  to the body  218 . 
     In this embodiment, the leadscrew  226  has a circular cross-section, and is rotatably fixed to driver  224  via a key/groove engagement as shown in  FIG. 17 . A proximal end of leadscrew  226  includes keys  217  engaging in longitudinal grooves  223  provided on the interior of driver  224 . Via this key/groove engagement, the leadscrew  226  is rotationally fixed to the driver  224  but is allowed to move axially relative thereto. Driver  224  comprises longitudinal grooves  285  engaging internal keys or splines  275  provided on the interior of setback member  222  to rotationally lock the driver  224  thereto. Driver  224  now includes an enlarged distal end  225  provided with a ring of teeth  255  circumferentially disposed thereon and functioning similarly to the toothed leadscrew brake  32 ′ in  FIGS. 11A and 11B . Distal end  225  comprises part of a unidirectional coupling along with the threaded insert  233 , as similarly discussed above with respect to  FIGS. 9B and 11B . Threaded insert  233  comprises an aperture with threads disposed thereon, which are threadedly coupled to corresponding threads on leadscrew  226 , similar to insert  114  of  FIG. 11C . 
     In this embodiment, last dose control is provided by a modified dose stop member  228 , as shown in  FIG. 18 . Dose stop member  228  comprises a ring link structure provided with a series of threads  290  disposed on the exterior surface thereof and threadedly engaged to threads  291  disposed on the dose set knob  220 . Dose stop member  228  is rotationally fixed to setback member  222  via corresponding ridges  272  provided on the interior surface of dose stop member  228 , which mesh with similar ridges  273  disposed on the setback member, as shown. In this embodiment, as the dose set knob  220  is rotated to set a desired dose or decrease a too-large dose, dose stop member  228  screws into threads  291  disposed on the dose set knob by an amount related to the set dose. During injection, the dose stop member  228  maintains its relative position with respect to threads  291 , since the setback member  228  is rotationally fixed to the dose set knob  220 . Therefore, dose stop member  228  and dose set knob  220  rotate together and there is no relative movement therebetween. Once the dose stop member  228  screws into the end thread of the threads  291 , it is prevented from rotating further, and thus further rotation of the dose set knob to set a larger dose is also prevented. Such an occurrence indicates a final dose of medication remaining in the cartridge. 
     To set a desired dose for injection the user rotates the dose set knob  220 . Audible clicking of the set dose is provided by slipping of teeth  264  on dose set knob  220  with teeth  274  on setback member  222 , as similarly described in the previous embodiment above. Teeth  264  and  274  are held in meshed engagement by a spring element  233  provided within push button  234 . Similar to the previous embodiment, when the user presses push button  234  to inject a dose, the setback member  222  is rotationally locked to the dose set knob  220  via engagement between the teeth  264  and  274 . Setback member  222  now rotates with dose set knob  220 , as the dose set knob screws back into body  21 . Rotation of the setback member  222  translates to driver  224  which rotates the leadscrew  226 . The leadscrew  224  rotates through the fixed threaded drive insert  233  and into the cartridge to expel a dose. As the driver  225  rotates in this direction, the distal ring of teeth  255  provides the injection clicking as teeth  255  slip past ratchet arms disposed on threaded drive insert  233 . 
     In another embodiment, last dose control is similarly provided with a modified dose stop element  328 , as shown in  FIG. 19A . Dose stop element  328  is a half-nut like member with a series of threads disposed on the internal surface thereof, threadedly engaging threads  332  provided on threaded setback member  322 . Dose set knob  320  comprises two longitudinally extending ribs or splines  330 , circumferentially spaced from each other by a distance substantially the same as the length of the dose stop element  328 . Splines  330  engage corresponding edges of dose stop element  328  to rotationally lock the dose stop element to the dose set knob  320 . During setting of a dose, dose stop member  329  screws onto threads  332  of setback member  322 , its relative position indicated the remaining volume of medication in the cartridge. When the dose stop member  329  reaches an end of thread  332  or a fixed stop on either the dose set knob  320  or setback member  322 , dose stop element  328  is prevented from rotating further and thus limiting the dose to that remaining in the cartridge. 
     Yet another embodiment, using a similar principle of operation, is shown in  FIG. 19B . In this embodiment, dose stop element  428  is threaded along its outer surface with threads  427 . The inner surface of dose set knob  420  is provided with at least one thread disposed thereon with a length sufficient to maintain constant engagement with threads  427  of dose stop member  428 . Setback member  422  comprises two longitudinally extending ribs or splines  430 , circumferentially spaced from each other by a distance substantially the same as the length of the dose stop element  428 . Splines  430  engage corresponding edges of dose stop element  428  to rotationally lock the dose stop element to the setback member  422 . The outer surface of the setback member  422  in this embodiment is provided with a substantially smooth surface to enable axial movement of dose stop element  428  thereon. In this embodiment, the last dose volume is that indicated when dose stop element  428  is prevented from moving any farther axially with respect to setback member  422 . Axial movement of dose stop element  428  is prevented when a first edge of element  428  abuts a fixed stop in the dose set knob  420  or on the setback member  422 . 
     In view of the above description, yet another exemplary embodiment of an injection pen comprising similar functionality is shown in  FIG. 20 . As shown in the cross-sectional view, an exemplary injection pen in this embodiment comprises a main body  518 , a dose set knob  320 , a setback member  522 , a driver  524 , a leadscrew  526 , a dose stop member  528 , and an insert  530 . The body  518  is modified as shown in  FIG. 21 . As shown, at least one ratchet arm  555 , attached at one end of a sidewall or the body  18  distal to the partitioning wall  550 , is internally directed and preferably provided with series of ridges or teeth  557  at the free end thereof. Teeth  557  engage with teeth  595  disposed on the outer surface of insert  530 , as shown in  FIG. 22 . Teeth  557  are forced into engagement with teeth  595  on the insert  530  when a cartridge holder is attached to body  18 , due to the cartridge holder engaging protrusion  559  provided on the outer surface of ratchet arm  555 . When the cartridge holder is attached to the body  518 , the insert  530  is prevented from rotating in either direction due to the forced toothed engagement. When the cartridge holder is removed, such as to re-use the injection pen, the ratchet arms  555  are free to ratchet and enable relative rotation between the insert  530  and the body  518 , to reload the leadscrew for subsequent use of the injection pen. The insert  530  comprises an aperture  531  with a non-circular cross-section for mating a similar non-circular cross-section of the leadscrew, to prevent relative rotation therebetween. A plurality of teeth  556  are provided circumferentially along an internal surface of the body  518  proximal to the partitioning wall  550 . Teeth  56  serve to engage a ratchet element  586  provided near a distal end of driver  524 , as shown in  FIG. 23  described further below. 
     As shown in  FIG. 23 , driver  524  comprises an elongated cylindrical member with open distal and proximal ends for allowing passage of the leadscrew  526  therethrough. Driver  524  includes a plurality of splines  583  provided near the proximal end for engaging corresponding grooves on the interior of setback member  522  for rotationally coupling the driver  524  and setback member  522  together. A pair of protrusions  585  is provided near the distal end of driver  524  for snap-engaging with the body  518  behind partitioning wall  550 . This snap-engagement prohibits relative axial movement between the driver  524  and the body  518  while allowing relative rotational movement therebetween. Driver  524  includes at least one thread element  582  provided on the interior surface for threadedly engaging with a corresponding thread of the leadscrew  526 . As discussed further below, it is this thread engagement that forces the leadscrew  526  to move axially in the distal direction to inject a set dose. 
     As shown in  FIG. 24 , the dose set knob  520  is an elongated cylindrical member provided with an outer thread  562  threadedly engaging an internal thread of the dose set knob, similar to the above embodiments. In this embodiment, the dose set knob  520  comprises at least one ratchet arm  564  provided near the distal end thereof, for engaging a plurality of ridges  573  provided on the outer surface of the setback member  522 , as shown in  FIG. 25 . The ratchet arm  564  includes a rounded protrusion  565  for enabling slipping of the ratchet element in both directions over ridges  573  provided on setback member  522 , to provide audible click signals during both normal dose setting and dial-back. Dose set knob  520  also includes a plurality of teeth  563  provided circumferentially along an internal surface of the dose set knob, as shown. During dose setting, teeth  563  are situated in the recess  576  on the outer surface of the setback member, as shown in  FIG. 25 . The teeth  563 , in this embodiment, serve as an injection coupling to rotationally lock the dose set knob  524 ) to the setback member  522 . In another embodiment, the injection coupling may be between a set of engaging teeth provided on the setback member  522  and the dose set knob  520 , as similarly discussed with respect to the first embodiment shown in  FIGS. 4A and 5A  (teeth  64  and  74 ). Dose set knob  520  comprises a plurality of threads  591  provided along the internal surface thereof for threadedly engaging with threads  590  of the dose stop member  528 . 
     Having described exemplary structures, features and interrelationships between the particular elements of  FIGS. 20-25 , the intended functionality of such an exemplary injection pen will now be described. Discussion of particular elements and features similar to the above embodiments, have been omitted herein. 
     To set a desired dose for injection, a user rotates the dose set knob  520  in a first direction. Relative rotation between the dose set knob  520  and the setback member  322  produces a series of dose-setting clicks due to the engagement between ridges  573  and ratchet elements  564 ,  65 . If a too-large dose is set by the user, the user can rotate the dose set knob  320  in a second, opposite direction to dial back the set dose. During dose setting, the dose set knob is free to rotate in both the first and second direction with respect to the setback member  522 . To inject a set dose, the user presses a push button  34 , which pushes the setback member  522  in the distal direction and causes ridges  573  on the setback member to engage teeth  563  provided on the dose set knob. The dose set knob  520  and the setback member  522  are now rotationally fixed with respect to each other. Now, as the dose set knob rotates back into the body  518 , the setback member  522  is also caused to rotate which forces driver  524  to rotate therewith. The leadscrew is prevented from rotating with respect to body  518  due to its mating engagement with the insert  530 , which is rotationally fixed to the body  518  when the cartridge holder is attached to the body  518 , as described above. Since the leadscrew  526  is rotationally fixed, relative rotation between the driver  524  and the leadscrew  526  causes the leadscrew  526  to move axially into the cartridge to inject a set dose, due to its threaded engagement with threads  582  provided on the driver  524 . During injection, as the driver  524  rotates relative to body  518 , ratchet arms  586  provide an injection click signal as they ride over teeth  556  provided on the interior of body  518 . In this embodiment, last dose control is performed similarly to that described above with respect to  FIG. 18 , to prevent a user from setting a dose larger than a remaining volume of medication remaining in the cartridge. 
     While the present invention has been shown and described with reference to particular illustrative embodiments, it is not to be restricted by the exemplary embodiments but only by the appended claims and their equivalents. It is to be appreciated that those skilled in the art can change or modify the exemplary embodiments without departing from the scope and spirit of the present invention.