Piston rod drive mechanism

Abstract

The present invention relates to a piston rod displacement mechanism for a pen injection device, the mechanism comprising: a tubular housing (2) extending along a reference axis and having an inner housing surface (3) with a first engagement structure (4); a nut member (5) secured within the tubular housing (2); a first non-self-locking screw (18) having a first thread direction and a first thread pitch and engaging with the nut member (5); and a second non-self-locking screw (17) superimposed on the first non-self-locking screw (18). ) comprises a piston rod (15) having a second thread direction and a second thread pitch, and a dose dial sleeve (20) operable to advance the piston rod (15) within the nut member (5), the dose dial sleeve (20) comprising an outer sleeve surface (21) having a second engagement structure (24) slidingly engaging the first engagement structure (4), and an inner sleeve surface (23) having a third engagement structure (29) engaging the second non-self-locking screw (17). One of the first engagement structure (4) and the second engagement structure (24) comprises a closed-circuit raceway configuration, and the other of the first engagement structure and the second engagement structure comprises a raceway follower, the closed-circuit raceway configuration comprising a helical raceway portion (24h) having a helical raceway direction equal to the second thread direction and a helical raceway pitch equal to the second thread pitch, and an axial raceway portion (24a) extending between a distal raceway end (24d) and a proximal raceway end (24p) and connected to the helical raceway portion (24h) at the distal raceway end (24d).

Description

[Technical field]

[0001] The present invention relates to a medication injection device adapted to deliver the same predetermined dose size with each injection session. [Background technology]

[0002] Parenteral drug administration performed using traditional vial and syringe systems is increasingly being replaced by administration using pen injection devices or pre-filled syringes. Such devices are particularly convenient in that they allow the user to make dosed injections from a pre-filled drug reservoir without first having to manually transfer a specific dose from one reservoir (the vial) to another (the syringe).

[0003] Mainly, two types of pen injection devices are available: durable pen injectors capable of delivering one or more doses of a drug from a pre-filled drug cartridge that can be loaded into the device before use and replaced after depletion, and disposable pen injectors capable of delivering one or more doses of a drug from a pre-filled, non-replaceable drug cartridge. Each of these types of pen injection devices is or in principle can be realized in various subtypes, such as, for example, single-shot devices adapted to deliver only one dose of a predetermined or selectable size from the drug cartridge, multi-shot devices capable of delivering multiple doses from the drug cartridge, manual devices where the user provides the force required for injection, automatic devices with a built-in energy source that can be released during injection, fixed-dose devices adapted to deliver multiple doses of the same size, variable-dose devices providing the delivery of multiple doses that can be set by the user each time from a range of possible dose sizes, etc.

[0004] As the label suggests, a durable pen injector is intended to be used over a significant period of time during which multiple medication cartridges are depleted and replaced, whereas a disposable pen injector is intended to be used until its dedicated medication cartridge is depleted and then the entire injection device is discarded.

[0005] Pen injection devices are conventionally used with a matching pen needle assembly, which provides access to the subcutaneous compartment and serves as a means for administering medication to the subcutaneous compartment. Regardless of the type of pen injection device, it is recommended that the pen needle assembly be used only once to minimize the risk of skin infection and lacerations.

[0006] Pre-filled syringes are usually supplied with a needle and contain a volume of medication that is administered one time, after which both the syringe and needle are discarded.

[0007] Disposable injection devices may be considered as particularly convenient for the user, requiring fewer handling steps and little or no maintenance, but have an equally high environmental impact since they are disposed of after only one or a few doses. In this regard, multi-shot injection devices suffer from the need to be able to reliably deliver multiple doses over time, and thus involve relatively complex injection mechanisms with a relatively large number of parts.

[0008] US Patent No. 5,399,633 discloses a disposable injection device adapted to deliver multiple doses of a fixed dose size. The device has a simple user interface in which a dose is automatically prepared for injection by a cycle of attaching and detaching a protective cap, and is automatically ejected by a spring when a button is touched. However, although the device is simple, it contains many different components made from at least four different types of materials: plastic, metal, rubber, and glass. Therefore, when such devices are collected for recycling, they need to be disassembled in pieces to separate the individual materials. Considering the large number of disposable injection devices typically present in various therapeutic segments, this material separation would require an automatic setup of considerable complexity to be performed properly.

[0009] US Patent No. 5,999,336 discloses another disposable injection device adapted to deliver multiple doses of a fixed dose size. The device is fully manual in that both the dose preparation and dose injection actions require dedicated user manipulation of the tubular sleeve, including rotating the tubular sleeve during dose preparation and pushing the tubular sleeve down towards the device housing during dose injection. Thus, while the device can be realized without any metal parts, the disclosed piston rod drive mechanism requires an additional tubular sleeve, increasing the complexity of the construction as well as the total number of components of the construction. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] WO 2009 / 092807 (Novo Nordisk A / S) [Patent Document 2] WO 2009 / 080775 (Novo Nordisk A / S) Summary of the Invention

[0011] It is an object of the present invention to eliminate or reduce at least one disadvantage of the prior art, or to provide a useful alternative to the prior art solutions.

[0012] In particular, it is an object of the present invention to provide a disposable, multi-shot pen injection device that has a comparable low environmental impact.

[0013] It is a further object of the present invention to provide a simple piston rod drive mechanism for a pen injection device consisting of very few parts.

[0014] It is a further object of the present invention to provide such an apparatus and mechanism which is robust and reliable.

[0015] In the present disclosure, aspects and embodiments will be described that address one or more of the above objectives and / or that address objectives that are apparent from the following passages.

[0016] In one aspect, the present invention provides a dose delivery mechanism as defined in claim 1.

[0017] Thus, a piston rod displacement mechanism or piston rod drive mechanism for a pen injection device is provided, the piston rod displacement mechanism comprising a tubular housing extending along a reference axis, a nut member fixed within the tubular housing, a piston rod having a first non-self-locking screw with a first thread direction and a first thread pitch and engaging the nut member, and a second non-self-locking screw superimposed on the first non-self-locking screw, the second non-self-locking screw having a second thread direction and a second thread pitch, and a dose dial sleeve operable to advance the piston rod within the nut member.

[0018] The tubular housing includes an inner housing surface having a first engagement structure, and the dose dial sleeve includes an outer sleeve surface having a second engagement structure slidingly engaging the first engagement structure, and an inner sleeve surface having a third engagement structure engaging the second non-self-locking screw.

[0019] One of the first and second engagement structures includes a closed circuit raceway arrangement, and the other of the first and second engagement structures includes a raceway follower. The closed circuit raceway arrangement further includes a helical raceway portion having a helical raceway direction equal to the second thread direction and a helical raceway pitch equal to the second thread pitch, and an axial raceway portion extending between a distal raceway end and a proximal raceway end and connected to the helical raceway portion at the distal raceway end. The dose dial sleeve is thereby operable to advance the piston rod a predetermined distance within the nut member by a proximal helical movement about the piston rod relative to the tubular housing and a subsequent distal translational movement along a reference axis relative to the tubular housing.

[0020] Thus, by pivoting the dose dial sleeve relative to the tubular housing, the track follower advances through the helical track section until it contacts a rotation stop that defines the transition from the helical track section to the axial track section at the distal track end. Because the helical track direction is equal to the second thread direction and the helical track pitch is equal to the second thread pitch, the third engagement structure advances the second non-self-locking screw, and thereby the dose dial sleeve moves helically about the piston rod as the track follower advances through the helical track section. However, the piston rod remains stationary relative to the tubular housing due to the engagement between the first non-self-locking screw and the nut member. As a result, the dose dial sleeve undergoes a proximal helical displacement relative to the tubular housing in preparation for the final displacement of the piston rod.

[0021] The dose dial sleeve is then forced distally relative to the tubular housing, causing the track follower to advance through the axial track section until it contacts an axial stop at the proximal track end that defines the transition from the axial track section to the helical track section or the second helical track section. The translational movement of the dose dial sleeve results in a distal helical displacement of the piston rod relative to the tubular housing, as the piston rod includes oppositely directed and overlapping threads in the form of a first non-self-locking thread and a second non-self-locking thread. The translational movement of the dose dial sleeve also results in relative axial movement between the piston rod and the dose dial sleeve, and therefore the extent of this distal helical displacement of the piston rod, i.e., the advancement of the piston rod, is determined by the ratio of the first thread pitch to the second thread pitch.

[0022] The ratio of the first thread pitch to the second thread pitch is selected by the manufacturer, so that the piston rod displacement mechanism is constructed to provide a predetermined axial displacement of the piston rod relative to the tubular housing for each cycle that the track follower advances through the helical track portion and the axial track portion. This provides a very simple and reliable piston rod displacement mechanism for a pen injection device, made from only three components: the tubular housing, the dose dial sleeve, and the piston rod. Given the small number of parts and the fact that all of the parts can be formed using the same material, e.g., recyclable plastic, this solution presents an environmentally attractive alternative to prior art disposable injection device mechanisms.

[0023] In an exemplary embodiment of the invention, the helical track portion and the axial track portion are further connected at the proximal track end, whereby the closed circuit track configuration comprises a single helical track portion and a single axial track portion. To effect a predetermined axial displacement of the piston rod, the dose dial sleeve is accordingly first rotated 360° until the track follower abuts the rotation stop at the distal track end, and then pushed towards the housing until the track follower abuts the axial stop at the proximal track end.

[0024] In another embodiment of the invention, the closed circuit track arrangement comprises a second helical track section connected to the axial track section at a proximal track end, and a second axial track section extending between a second distal track end and a second proximal track end connected to the second helical track section at the second distal track end and connected to the helical track section at a second proximal track end, whereby a predetermined axial displacement of the piston rod is effected twice for one complete revolution of the dose dial sleeve.

[0025] One of the internal housing surface and the external sleeve surface may comprise corrugations, i.e. a number of alternating axially extending ridges and grooves, and the other of the internal housing surface and the external sleeve surface may comprise flexibly supported radial protrusions configured to ride over the corrugations during helical movement of the dose dial sleeve relative to the tubular housing, whereby as the dose dial sleeve rotates about the reference axis relative to the tubular housing, the radial protrusions successively snap over the axially extending ridges to generate auditory and tactile feedback to the user in the form of clicks and impacts signalling that the piston rod displacement mechanism is ready to advance the piston rod.

[0026] The corrugated portion may include a state-shift ridge, and the radial protrusion may be configured to pass the state-shift ridge and enter the axially extending dose groove in response to the track follower reaching the distal track end during proximal helical movement of the dose dial sleeve relative to the tubular housing. The state-shift ridge may be configured to prevent reverse movement of the radial protrusion, thereby preventing return movement of the track follower along the helical track portion.

[0027] Concomitantly, reverse rotation of the dose dial until the trajectory follower reaches the distal trajectory end may cause the user to regret the priming action that was initiated, as the radial protrusion rides over the corrugated portion in the opposite direction while the trajectory follower advances back over the helical trajectory portion. However, once the radial protrusion passes the state-shift ridge, the user can no longer regret the priming action, and the piston rod displacement mechanism is irreversibly primed to displace the piston rod by depression of the dose dial.

[0028] The state-shifting ridge may be steeper than the other ridges and the dose groove may be deeper than the other grooves in the corrugated portion such that when the radial projection passes over the state-shifting ridge, a distinct and louder clicking sound is emitted indicating that the priming action is complete.

[0029] The dose groove may comprise a plurality of axially spaced rises through which the radial protrusion passes during distal translational movement of the dose dial sleeve relative to the tubular housing. This will provide audible and tactile feedback to the user signaling that piston rod displacement is continuing. The rises may be configured to be provided for unidirectional passage of the radial protrusion, thereby preventing the user from suddenly reversing the direction of movement of the dose dial sleeve within the tubular housing during a piston rod displacement action.

[0030] The dose groove may include a distal end configured to engage the radial protrusion and thereby limit proximal displacement of the dose dial sleeve relative to the tubular housing, which will prevent inadvertent separation of the dose dial sleeve from the tubular housing when the track follower is positioned at the distal track end and ready to advance the axial track portion.

[0031] One of the inner housing surface and the outer sleeve surface can include a flexible snap arm and the other of the inner housing surface and the outer sleeve surface can include a snap geometry configured to snap onto the snap geometry in response to the track follower reaching a proximal track end during distal translation of the dose dial sleeve relative to the tubular housing, thereby emitting a distinct clicking sound to provide feedback to the user that the piston rod displacement action is complete.

[0032] The flexible snap arms and snap geometry may be configured to prevent proximal translation of the dose dial sleeve relative to the tubular housing when the track follower is at the proximal track end. This will prevent return movement of the track follower along the axial track portion after a completed piston rod displacement, ensuring that the only way to continue operating the piston rod displacement mechanism is by pivoting the dose dial sleeve about the reference axis to direct the track follower distally within the helical track portion in preparation for the next piston rod displacement action.

[0033] The tubular housing may include a plurality of axially distributed windows that allow visual inspection of the internal housing portion. The plurality of axially distributed windows may be axially offset in a pairwise manner by a distance corresponding to a predetermined axial displacement exhibited by the piston rod relative to the tubular housing when the track follower advances the axial track portion from the distal track end to the proximal track end during distal translation of the dose dial sleeve relative to the tubular housing. The plurality of axially distributed windows may then be arranged such that the distal end portion of the piston rod shifts distally from one window to a neighboring window during a piston rod displacement action. This allows the user to keep track of the number of times the piston rod displacement mechanism has been actuated and the number of times the piston rod displacement mechanism may be further actuated.

[0034] In another aspect, the present invention provides a pen injection device comprising a piston rod displacement mechanism as described above.

[0035] The pen injection device may comprise a drug cartridge holding a volume of drug substance within the cartridge body between a pierceable self-sealing septum and a slidable piston, where the piston rod may abut the slidable piston or an intermediate component connected to the slidable piston, such as, for example, a piston washer. The pen injection device may further comprise or be adapted to receive a pen needle assembly comprising an injection needle having a front portion for insertion into a user's skin and a rear portion for insertion into the drug cartridge through the pierceable self-sealing septum.

[0036] In a pen injection device having a received pen needle assembly, axial displacement of the piston rod results in equal axial displacement of a slidable piston, which expels a correlated volume of drug substance through the injection needle. Thus, the pen injection device is configured to deliver multiple doses of a predetermined dose size.

[0037] The volume of drug substance, together with the closed circuit track configuration, the first thread pitch, and the second thread pitch, then determine the number of doses of the predetermined dose size that the pen injection device provides. In an exemplary embodiment of the present invention, the tubular housing comprises a plurality of axially distributed windows, which are axially offset in a pairwise manner by a distance corresponding to the predetermined axial displacement exhibited by the piston rod during the distal translation of the dose dial sleeve. The number of axially distributed windows is equal to the number of doses of the predetermined dose size that can be initially discharged by the pen injection device, and is arranged such that the distal end portion of the piston rod shifts distally from one window to the neighboring window during the piston rod displacement action. This allows the user to keep track of the number of remaining dischargeable doses in a simple manner by counting the number of windows that the piston rod has not yet seen.

[0038] For the avoidance of any doubt, in this context, the term "injection device" refers to an apparatus suitable for injecting a fluid medium into the body of a subject, for example with the aid of an attachable needle device, and the term "medicament" refers to a medium used in the treatment, prevention, or diagnosis of a condition, i.e. including a medium that has a therapeutic or metabolic effect in the body. Furthermore, the terms "distal" and "proximal" refer to a position on or a direction along a drug delivery device, drug reservoir, or needle unit, with "distal" referring to the drug exit end and "proximal" referring to the end opposite to the drug exit end.

[0039] Reference herein to a particular aspect or embodiment (e.g., "one aspect," "first aspect," "one embodiment," "exemplary embodiment," etc.) indicates that the particular feature, structure, or characteristic described in connection with each aspect or embodiment is included or inherent in at least that aspect or embodiment of the invention, but not necessarily in or inherent in all aspects or embodiments of the invention. However, it is emphasized that any combination of the various features, structures, and / or characteristics described in connection with the invention is encompassed by the invention, unless expressly stated herein or clearly contradicted by context.

[0040] The use of any and all examples or exemplary language (such as, for example, etc.) in the text is intended merely to better clarify the invention and does not limit the scope of the invention unless otherwise claimed. Moreover, no language or phraseology in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0041] In the following the invention will be further explained with reference to the drawings. [Brief description of the drawings]

[0042] [Figure 1]FIG. 1 is an exploded view of an injection device according to an exemplary embodiment of the present invention, but without a drug container. [Diagram 2] FIG. 2 is a longitudinal perspective cross-sectional view of the housing of the injection device. [Diagram 3] FIG. 3 is a perspective view of a combined dose preparation and injection button of the injection device. [Figure 4] FIG. 4 is a longitudinal perspective cross-sectional view of a combined dose preparation and injection button. [Diagram 5] FIG. 5 is a longitudinal perspective cross-sectional view of the injection device in an assembled state (still without the drug container). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In the drawings, like structures are primarily identified with like reference numbers.

[0044] When / if relative expressions such as "up" and "down", "left" and "right", "horizontal" and "vertical", "clockwise" and "counterclockwise" are used below, they refer to the attached figures and do not necessarily refer to actual usage situations. The figures shown are schematic representations, so that the configuration of different structures, as well as their relative dimensions, are intended to serve illustrative purposes only.

[0045] FIG. 1 is an exploded view of an injection device 1 according to an exemplary embodiment of the invention. It is noted that the figure does not show a drug container carrying the drug to be administered by the injection device 1, nor a needle assembly adapted for use with the injection device 1 to deliver the drug to a desired injection site. The drug container and the needle assembly may each in principle be one of several different possible types. However, in the case of the present embodiment, it is particularly envisaged that they are each of the type conventionally used with pen injection devices, i.e. cartridge-type containers, having a generally cylindrical container body each including a necked-down outlet end portion sealed by a pierceable self-sealing septum and an axially opposite portion sealed by a slidable piston, a needle unit including a needle hub in which the injection needle is fixedly mounted, and a skirt portion provided with means for releasably attaching to the injection device 1.

[0046] The injection device 1 comprises a housing 2 extending along a longitudinal axis and a cartridge holder 30 in axial extension. The cartridge holder 30 has a pair of opposed flanges 31 adapted to snap-fit ​​into the housing 2 and a needle mount 32 for receiving the above-mentioned injection needle assembly. A shoulder portion 33 connects the flanges 31 and the needle mount 32 and serves to hold the above-mentioned drug container. Four windows 9 are arranged in an axial row in the distal half of the housing 2 to allow inspection of the contents of the drug container.

[0047] The housing 2 together with the piston rod 15 and the dose dial 20 form part of the dose ejection mechanism of the injection device 1. The piston rod 15 is double-threaded in that it has two counter-wound non-self-locking threads superimposed on one another, a dial connection thread 17 and a nut connection thread 18. The piston rod 15 is adapted to advance helically relative to the housing 2 during dose ejection, thereby advancing a slidable piston within the generally cylindrical container body, in a manner that will be described in more detail below. To that end, a leading end 16 of the piston rod 15 is configured to abut against the slidable piston or a piston washer connected to the slidable piston.

[0048] The dose dial 20 has a tubular dial body 21 with a proximal dial head 22 for easy user operation. The dial head 22 has an end face 27 prepared to receive, for example, a user's thumb. A portion of the outer surface of the dial body 21 has an axial corrugation 28 that is interrupted distally by a track arrangement 24. The track arrangement 24 extends circumferentially around the dial body 21 and comprises a helical dose preparation track segment 24h and an axial dosage track segment 24a. The two track segments are connected distally at a distal joint 24d and proximally at a proximal joint 24p. In particular, the direction and pitch of the helical dose preparation track segment 24h correspond to the direction and pitch of the dial connection screw 17.

[0049] FIG. 2 is a longitudinal perspective cross-sectional view of the interior of the housing 2. A nut member 5 having a nut thread 8 is fixed within the housing 2 and divides the interior into a proximal space 6 and a distal space 7. The proximal space 6 is configured to accommodate a piston rod 15 and a dose dial 20 and is bounded by an internal housing surface 3 from which a knob 4 projects radially. A proximal housing snap 11 is provided on a flexible arm formed by removing surrounding housing material. The proximal housing snap 11 has a proximally facing angled notch 11p. The distal space 7 is configured to accommodate a drug container held in place by a cartridge holder 30 whose flange 31 extends into the distal opening 10 of the housing 2 and is laser welded to the housing 2.

[0050] Figure 3 is a perspective view of the dose dial 20 from a different angle than that of Figure 1. In particular, Figure 3 shows the axial groove 25 formed in the axial corrugations 28 and the distal step 26 formed in the distal end of the dial body 21. The axial groove 25, which terminates distally in a distal groove end 25d, comprises five transverse ribs 25t spaced axially equidistant from one another. Figure 4 is a longitudinal perspective cross-sectional view of the dose dial 20 showing the internal dial surface 23 with a thread segment 29 formed on the distal portion.

[0051] 5 is a longitudinal perspective cross-sectional view of the housing 2, the piston rod 15, the dose dial 20 and the cartridge holder 30 when assembled. It is clear that the housing 2 also has a distal housing snap 12 located just proximal to the distal step 26 in the depicted state of the injection device 1. The distal housing snap 12 is formed with a proximal inclined surface 12p and a distal steepened surface 12d, which respectively lift the distal housing snap 12 past the distal step 26 during distal translation of the dose dial 20 relative to the housing 2 and prevent subsequent proximal translation of the dose dial 20 relative to the housing 2 by preventing return passage of the distal step 26.

[0052] When assembled, the knob 4 is slidably accommodated within the track arrangement 24, the nut connection screw 18 engages the nut screw 8 and the dial connection screw 17 engages the threaded segment 29. This provides a very simple and inexpensive piston rod displacement mechanism based on only three components. In this way, the entire injection device 1, potentially apart from the drug container, can be manufactured using only recyclable plastics and, given the small number of components required to expel a drug dose from the drug container, the injection device 1 can be realised with a relatively low environmental impact.

[0053] In the following, the mode of operation of the injection device 1 will be explained with reference to the figures.

[0054] The trajectory arrangement 24 forms a circumferentially closed loop or circuit on the outer surface of the dial body 21. This closed loop consists of the above-mentioned helical dose-ready trajectory segment 24h and the axial dose-dispensing trajectory segment 24a. In the assembled state of the injection device 1 shown in FIG. 5, the dial body 21 is fully occupied in the proximal space 6, and only the dial head 22 extends proximally beyond the housing 2. This corresponds to the non-ready state of the injection device 1, in which no dose is ready to be expelled from the drug container (not shown). In this state, the knob 4 is positioned at the proximal articulation 24p, which constitutes the beginning of the helical dose-ready trajectory segment 24h as well as the end of the axial dose-dispensing trajectory segment 24a.

[0055] To expel a dose of medication from the injection device 1, the user first grasps the housing 2 and pivots the dial head 22 clockwise (from a proximal perspective) relative to the housing 2. The clockwise rotation of the dial head 22 rotates the dial body 21 clockwise, which causes the knob 4 to begin to advance along the helical dose ready trajectory segment 24h and the thread segment 29 to begin to advance along the dial connection screw 17. The piston rod 15 remains stationary due to engagement with the nut member 5. As the dose dial 20 is thereby displaced both angularly and axially relative to the housing 2, the proximal housing snap 11 overcomes the axial corrugations 28 and generates a clicking sound with each passage of a ridge, which is auditory feedback to the user that a dose is ready.

[0056] As long as the knob 4 has not yet reached the distal articulation 24d, which constitutes the end of the helical dose preparation trajectory segment 24h and the beginning of the axial dose administration trajectory segment 24a, the user may regret his / her initiated action and rotate the dial head 22 counterclockwise, which will cause the knob 4 to return to the proximal articulation 24p, causing the proximal housing snap 11 to ride back over the axial corrugation 28. Thus, the dose dial 20 returns to the position shown in FIG.

[0057] However, once the dose dial 20 has rotated 360° about its longitudinal axis and the knob 4 reaches the distal articulation 24d, the proximal housing snap 11 passes the non-return ridge of the axial corrugations 28 and enters the axial groove 25. At this point, the user can no longer reverse the direction of rotation of the dial head 22 and the dose is prepared for delivery. The non-return ridge and axial groove 25 are configured such that upon passage of the proximal housing snap 11, a distinct and louder clicking sound is emitted, signaling to the user that the dose preparation action is complete.

[0058] After passing the non-return ridge, the proximal housing snap 11 is positioned at the distal groove end 25d which is configured to engage the proximal housing snap 11 and thereby prevent proximal translational movement of the dose dial 20 relative to the housing 2. This ensures that the user cannot accidentally pull the dose dial 20 out of the housing 2.

[0059] Now, with the knob 4 positioned at the distal articulation 24d, the injection device 1 is ready to expel a dose of drug from the drug container. After inserting the injection needle of the attached needle unit (not shown) through the skin at the desired injection site, the user simply pushes the dose dial 20 towards the housing 2 by applying an axial force to the end face 27. The knob 4 thereby starts to advance along the axial dosage trajectory segment 24a, and the resulting translational displacement of the dial body 21 causes the piston rod 15 to rotate within the nut member 5 due to the engagement between the thread segment 29 and the dial connection screw 17. The engagement between the nut screw 8 and the nut connection screw 18 results in a helical distal displacement of the piston rod 15, thereby leading to its advancement through the nut member 5.

[0060] When the knob 4 reaches the proximal articulation 24p, the piston rod has been displaced a certain axial distance that depends on the pitch ratio of the dial connection thread 17 to the nut connection thread 18. In the present case, the pitch of the former is equal to the pitch of the latter, so that the axial displacement of the piston rod 15 is half the axial displacement of the dose dial 20. Since the axial displacement of the piston rod 15 determines the axial displacement of the slidable piston in the drug container, the length of the axial dose trajectory segment 24a effectively determines the size of the expelled dose. Thus, the injection device 1 is designed to expel the same predetermined amount of drug with each dose expelling action.

[0061] Due to the non-self-locking threads joining between the dial connection thread 17 and the thread segment 29, and between the nut connection thread 18 and the nut thread 8, respectively, a very low axial force is required to advance the piston rod 15 through the nut member 5, which is attractive from a user's point of view. Furthermore, since the piston rod 15 can only advance when the knob 4 advances through the axial dosing trajectory segment 24a, it is not possible to intentionally or accidentally inject a dose larger than what the injection device is designed to deliver. The closed circuit trajectory configuration 24 provides a very simple user interface whereby each dose can be administered by simply pivoting the dial head 22 as far as possible (360 degrees) and then depressing the dial head 22 towards the housing 2 until it meets a stop. The dose dial 20 returns to its initial position within the housing 2 after each dose ejection action.

[0062] In connection with dose delivery, during translational movement of the dose dial 20 into the housing 2, as the knob 4 advances along the axial dose trajectory segment 24a from the distal articulation 24d to the proximal articulation 24p, the proximal housing snap 11 successively passes the lateral rib 25t with the proximal facing inclined notch 11p, thereby generating a clicking sound that serves to audibly and tactilely verify that the injection is continuing. When the knob 4 reaches the proximal articulation 24p, the inclined surface 12p slides over the distal step 26, thereby temporarily storing elastic energy within the distal housing snap 12 and deflecting it radially away from the rest position. When the steep surface 12d passes the distal step 26, the elastic energy is released and the distal housing snap 12 returns to the rest position, releasing a distinct and louder clicking sound that signals to the user that the dose ejection action is complete.

[0063] Here, the steep surface 12d interfaces with the distal step 26, preventing proximal translational movement of the dose dial 20 relative to the housing 2. This ensures that the user cannot force the knob 4 to advance in the opposite direction of the axial dose trajectory segment 24a. Thus, the only way to continue using the injection device 1 is to again pivot the dial head 22 clockwise relative to the housing 2, which will bring the knob 4 out of the proximal articulation 24p and into the helical dose preparation trajectory segment 24h in preparation for delivering the next dose.

[0064] The injection device 1 of this embodiment is configured to deliver four predetermined doses of a particular dose size from a medicine container. Thus, the four windows 9 are arranged such that the slidable piston is visible in the most proximal window after delivery of the first dose, in the adjacent window after delivery of the second dose, etc. This allows the user to easily see how many doses remain to be administered using the injection device 1.

Claims

1. 1. A piston rod displacement mechanism for a pen injection device, comprising: - a tubular housing (2) extending along a reference axis and comprising an inner housing surface (3) having a first engagement structure (4); - a nut member (5) fixed in said tubular housing (2); a piston rod (15) comprising a first non-self-locking screw (18) having a first thread direction and a first thread pitch and engaging with said nut member (5), and a second non-self-locking screw (17) superimposed on said first non-self-locking screw (18), said second non-self-locking screw (17) having a second thread direction and a second thread pitch; a dose dial sleeve (20) operable to advance said piston rod (15) within said nut member (5), said dose dial sleeve (20) comprising an outer sleeve surface (21) having a second engagement structure (24) slidingly engaging said first engagement structure (4), and an inner sleeve surface (23) having a third engagement structure (29) engaging said second non-self-locking screw (17); one of the first engagement structure (4) and the second engagement structure (24) comprises a closed circuit track arrangement and the other of the first engagement structure and the second engagement structure comprises a track follower; a piston rod displacement mechanism, the closed circuit raceway arrangement comprising a helical raceway portion (24h) having a helical raceway direction equal to the second thread direction and a helical raceway pitch equal to the second thread pitch, and an axial raceway portion (24a) extending between a distal raceway end (24d) and a proximal raceway end (24p) and connected to the helical raceway portion (24h) at the distal raceway end (24d), whereby the dose dial sleeve (20) is operable to advance the piston rod (15) a predetermined distance within the nut member (5) by a proximal helical movement about the piston rod (15) relative to the tubular housing (2) and a subsequent distal translational movement along the reference axis relative to the tubular housing (2).

2. 2. The piston rod displacement mechanism of claim 1, wherein one of the inner housing surface (3) and the outer sleeve surface (21) comprises a corrugated portion (28), and the other of the inner housing surface (3) and the outer sleeve surface (21) comprises a flexibly supported radial protrusion (11) configured to overcome the corrugated portion (28) during helical movement of the dose dial sleeve (20) relative to the tubular housing (2).

3. 3. The piston rod displacement mechanism of claim 2, wherein the corrugated portion (28) comprises a state-shift ridge, the flexibly supported radial protrusion (11) being configured to pass through the state-shift ridge and enter the axially extending dose groove (25) in response to the track follower reaching the distal track end (24d) during proximal helical movement of the dose dial sleeve (20) relative to the tubular housing (2), the state-shift ridge being configured to prevent reverse movement of the flexibly supported radial protrusion (11), thereby preventing return movement of the track follower along the helical track portion (24h).

4. A piston rod displacement mechanism as described in claim 3, wherein the axially extending dose groove (25) has a plurality of axially spaced raised portions (25t), and during distal translation of the dose dial sleeve (20) relative to the tubular housing (2), the flexibly supported radial protrusion (11) passes through the raised portions (25t).

5. A piston rod displacement mechanism as described in claim 3 or 4, wherein the axially extending dose groove (25) has a distal groove end (25d) configured to engage with the flexibly supported radial protrusion (11) and thereby limit the proximal displacement of the dose dial sleeve (20) relative to the tubular housing (2).

6. 6. The piston rod displacement mechanism according to claim 1, wherein one of the inner housing surface (3) and the outer sleeve surface (21) comprises a flexible snap arm (12) and the other of the inner housing surface (3) and the outer sleeve surface (21) comprises a snap geometric shape (26), the flexible snap arm (12) being configured to snap onto the snap geometric shape (26) in response to the track follower reaching the proximal track end (24p) during distal translation of the dose dial sleeve (20) relative to the tubular housing (2).

7. 7. The piston rod displacement mechanism of claim 6, wherein the flexible snap arm (12) and the snap geometry (26) are configured to prevent proximal translational movement of the dose dial sleeve (20) relative to the tubular housing (2) when the track follower is at the proximal track end (25p).

8. A piston rod displacement mechanism according to any one of claims 1 to 7, wherein the tubular housing (2) comprises a plurality of axially distributed windows (9), the plurality of axially distributed windows (9) being axially offset in a paired manner by a distance corresponding to the axial distance advanced by the piston rod (15) when the track follower advances along the axial track portion (24a) from the distal track end (25d) to the proximal track end (25p) during distal translational movement of the dose dial sleeve (20) relative to the tubular housing (2).

9. 9. A piston rod displacement mechanism according to any one of claims 1 to 8, wherein the axial raceway portion (24a) is further connected to the helical raceway portion (24h) at the proximal raceway end (24p), whereby the closed circuit raceway arrangement comprises a single helical raceway portion and a single axial raceway portion.

10. A pen injection device (1) comprising a piston rod displacement mechanism according to any one of claims 1 to 9.

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