Injection device for delivering liquid medication and method of assembly - Patents.com

Abstract

The present invention relates to a pre-filled injection device with a non-removable cartridge, wherein the drive mechanism comprises a threaded piston rod that moves helically within a nut fixed to a housing structure. During assembly of the injection device, the nut member can be rotated, thereby advancing the piston rod to a position that eliminates any air gap that would otherwise occur between the piston rod and the plunger inside the cartridge.

Description

[Technical field]

[0001] The present invention relates to a pre-filled injection device for delivering one or preferably more doses of a liquid drug, in particular in such an injection device that the distance between the piston rod that moves the plunger inside the cartridge and the plunger itself is eliminated or at least significantly reduced before the pre-filled injection device is delivered to the end user.

[0002] In a second aspect, the invention relates to a method for assembling a pre-filled injection device having such a zero point adjustment. [Background technology]

[0003] Injection devices are widely known and are used for the treatment of a wide variety of different diseases, in particular within the field of diabetes. A very common type of injection device is the so-called disposable or pre-filled injection device. This type of injection device usually contains a cartridge containing a predetermined amount of liquid medicament to be injected, which cartridge is irreplaceably embedded in the injection device. Once the content contained in the enclosed cartridge has been used, the user therefore discards the entire device. An almost classic pre-filled injection device is the FlexPen® from Novo Nordisk A / S, described in more detail in US Pat. No. 5,399,363. This injection device comprises a housing structure including a cartridge holder in which a cartridge containing 3 ml of liquid medicament is fixedly mounted.

[0004] A number of different tolerances apply when producing such pre-filled injection devices: there are various tolerances in molding the different parts that make up the injection device, there are tolerances in the various click-and-snap connections that permanently connect the individual parts, and there are tolerances in filling the liquid drug into the cartridge.

[0005] As a result of all these tolerances, the assembled injection device is usually delivered to the end user with a particular distance between the plunger inside the cartridge and the piston rod. This distance is called the air gap. Before starting to use the injection device to inject a dose of liquid, the user needs to remove the air gap, which is usually done by setting and expelling a small dose, which causes the piston rod to move forward without actually expelling any liquid from now on. This process is often called the initial priming of the injection device. Thus, the first few empty (from liquid) dose expulsions will cause the piston rod to move to abut the plunger, and liquid will only be expelled when contact is established between the plunger and the piston rod, resulting in the correct expulsion of the set dose size.

[0006] A wide variety of different solutions have been proposed to avoid the process of initially priming the injection device.

[0007] In US Pat. No. 5,399,436 it is proposed to provide a piston rod foot which can be axially slidable relative to the piston rod during assembly of the injection device and which is physically connected to the piston rod at a position where contact between the piston rod foot and the plunger is established during the assembly process.

[0008] The point where the piston rod engages the plunger is often referred to as the zero point, i.e. the point where the air gap is zero, and hereafter the process of obtaining this position is referred to as zero point adjustment.

[0009] US Patent No. 5,399,633 describes a method in which an adjustment member threaded onto a piston rod rotates relative to a housing structure during assembly of the injection, so that the piston rod moves forward into contact with the plunger. When contact is established between the piston rod and the plunger, the adjustment member is physically fixed to the housing structure. Then, when the injection is performed, the adjustment member acts as a conventional nut element to helically advance the rotatable piston rod.

[0010] A similar method is disclosed in US Pat. No. 5,399,633 in which a nut element translates with the piston rod into position and is physically fixed in place to the housing structure.

[0011] What all the above solutions have in common is that they require a physical connection, e.g. welding, during assembly of the injection device, which obviously complicates the assembly of the injection device.

[0012] Recently, a new type of pre-filled injection device has been developed. These new injection devices are constructed to eject a limited predefined number of dose volumes, which are predetermined by the injection device manufacturer and are equal in volume. This new type of injection device is called a "multiple-use fixed-dose injection device." When using such a fixed-dose injection device, the user cannot set and eject small doses, since all dose volumes are predetermined by the fixed-dose device manufacturer. An example of such a multiple-use fixed-dose device is provided in US Pat. No. 5,399,992. Such injection devices are well suited for injecting liquid GLP-1 drugs, which are usually injected in fixed dose volumes, typically administered once a day or once a week.

[0013] Injection devices used to inject insulin are usually provided with a so-called end-of-volume mechanism, which ensures that the user cannot set a dose higher than the injectable content remaining in the injection device. When operating with an insulin injection device, the settable increments are usually very small and the injectable content in the cartridge needs to be very accurate. A standard content of an insulin device is like 3.0 ml. This usually requires that the free length that the piston rod must be able to travel in order to completely expel the injectable content of the cartridge before the end-of-volume mechanism reaches the locking position needs to be very accurate. In the case of such insulin injection devices, there is no option to use any part of the free length of the piston rod for zero adjustment from this point onwards. This is because the free length that the piston rod can travel during ejection is too short to expel the entire injectable content, which would lead to an incorrect operation of the end-of-volume mechanism. However, for multiple-use fixed-dose devices that expel larger doses, the exact free length of the piston rod is less important. For such devices, it is sufficient if the free length is long enough to expel a predefined number of dose volumes while ensuring that no further dose volumes can be expelled. Therefore, for such injection devices, it is acceptable to use part of the free length of the piston rod for the zero-point adjustment. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] International Publication No. 1999 / 38554 [Patent Document 2] International Publication No. 2009 / 095332 [Patent Document 3] European Patent No. 2,968,777 [Patent Document 4] International Publication No. 2017 / 001694 [Patent Document 5] International Publication No. 2018 / 007259 Summary of the Invention

[0015] It is an object of the present invention to provide a pre-filled injection device in which air gaps can be more easily eliminated during assembly of the injection device, preferably without the use of physical connections such as welds or the like.

[0016] Thus, in a first aspect of the present invention there is provided a pre-filled injection device for delivering a liquid medicament, preferably in a dose. The pre-filled injection device comprises: - a housing structure containing a volume of liquid medicament, the liquid medicament being contained in a cartridge that is permanently and irremovably fixed within the housing structure, the liquid medicament being expelled from the cartridge by having a plunger movable to move distally inside the cartridge; a piston rod for advancing a movable plunger during dispensing, the piston rod being provided with an external thread and a longitudinal track structure; a rotatable drive element, which engages with a longitudinal track structure of the piston rod, so that a rotation of the drive element is transferred to a rotation of the piston rod; - a nut element provided with a first thread, the first thread engaging with an external thread of the piston rod such that the piston rod moves helically when rotated relative to the nut element.

[0017] The piston rod is permitted to rotate only in a first rotational direction relative to the housing structure, the permitted rotational direction being a direction that spirally advances the piston rod in the dispensing direction.

[0018] Further, the nut element is rotatably coupled to the housing structure with a connection provided with a unidirectional ratchet interface surface, thereby permitting the nut element to be rotated only in a second rotational direction opposite the first rotational direction.

[0019] From this point onwards, the piston rod can only rotate in the dispensing direction (first rotational direction) and thus not in the second rotational direction which would move the piston rod proximally within the injection device.

[0020] The nut element is rotatably mounted within the housing structure, but is further provided with a unidirectional ratchet interface surface such that the nut element can only be rotated in a second rotational direction opposite the first rotational direction.

[0021] As a result, when the nut element rotates relative to the housing structure in the allowed rotational direction (second direction), the piston rod moves in the distal direction without rotation, which cannot be translated into a rotation of the piston rod because the allowed rotational direction of the nut element (second direction) is relative to the allowed rotational direction of the piston rod (first direction).

[0022] Thus, rotation of the nut element in the allowed direction of rotation (second direction) translates the piston rod forward. The translation of the piston rod can continue until the piston rod, or the piston rod foot, if present, encounters and abuts the plunger inside the cartridge. The nut element cannot then be rotated further. The phrase "encounter and abut" should also be understood in a virtual and calculated interpretation.

[0023] When expelling the fluid agent, the drive element rotates, which in turn rotates the piston rod in a permitted rotational direction (first direction) in which the nut element is prevented from rotating by ratcheting engagement with the housing structure, resulting in helical rotation of the piston rod in the distal direction.

[0024] Thus, the prevention of rotation of the piston rod in one direction and the one-way joint between the nut element and the housing structure ensures that the nut element can be rotated in a permitted direction (the second direction) to minimise and preferably eliminate any air gaps, whilst at the same time ensuring that the nut is self-locking so that it cannot rotate in either direction after contact is made between the piston rod (or foot) and the plunger.

[0025] In one rotational direction (first direction), the nut element is prevented from rotating by a one-way ratchet interface, and in the opposite direction (second direction), the nut element is prevented from rotating due to the engagement between the piston rod (or foot) and the plunger, thus eliminating the need for the nut element to be physically secured to the housing structure.

[0026] The longitudinal track structure of the piston rod may be any geometric form of the piston rod with which the rotatable drive element can engage and transmit rotation, but need not necessarily be a physical track as such; in one embodiment the track structure may be a flange provided on the piston rod, or alternatively may be a flat longitudinal surface such that the piston rod has a non-circular cross-section, e.g. as disclosed in WO 2017 / 001694.

[0027] The above principles can be used in any type of pre-filled injection device in which a threaded piston rod advances inside a cartridge by rotating the piston rod relative to a threaded nut element carried with or otherwise associated with a housing structure.

[0028] It is expressly noted that although many of the examples used throughout this application, and particularly in the more detailed sections of the description, relate to so-called pre-filled, multi-use, fixed dose injection devices, the general teachings of this application are suitable for a wide range of different pre-filled injection devices and are in no way limited to the specific examples. More specifically, the invention claimed in the appended claims is in no way limited to the examples used.

[0029] A broad range of pre-filled injection devices encompassed by the appended claims is also a pre-filled injection device having a dose setting mechanism that allows a user to set different individual dose sizes to be dispensed with each dispense action.

[0030] The unidirectional ratchet interface established in the coupling zone between the nut element and the housing structure preferably comprises one or more flexible ratchet arms that engage with the toothed surface. Either the nut member or the housing structure can carry the flexible ratchet arms, but the respective opposing element, be it the housing structure or the nut member, carries the toothed surface. It is not important which construction element carries the ratchet arms and which construction element carries the toothed surface. What is important is the existence of a unidirectional operable connection between the nut member and the housing structure.

[0031] The ratchet arm preferably extends in a radial direction perpendicular to the longitudinal extension of the injection device, i.e. perpendicular to the central axis "X". The interface between the ratchet arm and the toothed surface is designed such that only one rotational direction is permitted.

[0032] In one embodiment where the piston rod is maintained in a rotatable state, the nut element rotates in the second rotational direction and the piston rod translates in the dispensing direction.

[0033] Preventing the piston rod from rotating in the second rotational direction is preferably achieved by rotatably engaging a rotatable drive element with the piston rod only in the first rotational direction.

[0034] This is accomplished in one embodiment by providing a one-way ratchet between the drive element and the housing structure, the ratchet interface being designed to only allow the drive element to rotate in a first rotational direction, thereby preventing both the drive element and the piston rod from rotating in a second direction.

[0035] Thus, the unidirectional ratchet interface between the nut element and the housing structure allows the nut element to rotate relative to the housing structure in a rotational direction that advances the piston rod in the dispensing direction. Thus, rotation of the nut element is only possible as long as an air gap exists. Once the piston rod (or the piston rod foot) encounters and abuts the plunger inside the cartridge, the nut member cannot rotate in a rotational direction that moves the piston rod distally.

[0036] The housing structure of the injection device comprises several different parts, which are preferably click-fit, glued or welded together to form one closed unit, which also holds the cartridge, so that the cartridge can only be separated from the housing structure by destroying the housing structure. Furthermore, the housing structure comprises at least a housing part that supports the bridge structure from the inside, so that the bridge structure is carried on the inner surface of the housing structure by a number of radial bearings.

[0037] The bridge structure is preferably a tube-like construction fixed to the inner surface of the housing part by means of several radial bearings. The bridge structure including the radial bearings is preferably molded as an integral part of the housing part. The longitudinal open areas provided between these radial bearings can be used to translate axial movements inside the housing structure.

[0038] In one embodiment, a connector element is provided that translates axial motion from the needle shield to the drive element and is operable in these longitudinal openings.

[0039] In a further embodiment, the toothed surface of the ratchet interface is provided on an inner surface of a circular opening in the bridge structure.

[0040] In a still further embodiment, the nut element is provided with threads on its outer surface which preferably engage with threads provided internally in the housing part, preferably in a circular opening in the bridge structure.

[0041] In one embodiment, the threaded interface between the nut element and the housing structure comprises one or more protrusions on one element and one or more helical flanges on the opposing element, the purpose of the threaded interface being to displace the nut element proximally as a result of the nut element rotating relative to the housing structure.

[0042] If the pitch of the first threaded connection between the piston rod and the nut member is high, i.e. the piston rod moves a long distance with each rotation, it is advisable to have a second threaded connection between a threaded protrusion on the nut member and a threaded flange inside the housing part so that the nut member can be screwed helically in the proximal direction relative to the housing structure during rotation of the nut member 11.

[0043] This has the advantage that when the nut member is rotated in the permitted direction, it moves proximally while moving the piston rod distally. Therefore, the pitch of the second threaded connection between the nut member and the housing structure needs to be subtracted from the pitch of the first threaded connection between the piston rod and the nut member to find the effective zero point adjustment pitch.

[0044] The low effective pitch of the zero adjustment makes the process of eliminating the air gap easier to fine tune. Therefore, it is beneficial to have a second threaded connection between the nut member and the housing part when operating with a piston rod with a high pitch, which is the case when a relatively large volume needs to be dispensed with each revolution of the piston rod.

[0045] If the pitch of the first threaded connection between the piston rod and the nut member is low, the second threaded connection is not considered necessary, and therefore the nut member only needs to rotate relative to the housing structure without the ability to move axially.

[0046] The present invention further relates to a method for assembling such a pre-filled injection device, the pre-filled injection device preferably being as defined herein, a nut element coupled to a housing structure of the injection device; a cartridge having a plunger, the cartridge being secured within a housing structure; When the piston rod is rotated in a first rotational direction relative to the nut element, the piston rod can be advanced in an ejection direction; a nut element rotationally coupled to the housing structure by a unidirectional ratchet interface; The method comprises: i) fixing the piston rod against rotation in a second rotational direction opposite the first rotational direction; ii) rotating the nut element relative to the housing structure in a second rotational direction to advance the piston rod in a direction toward the plunger.

[0047] By using this method when assembling a pre-filled injection device, the distance between the piston rod (or piston rod foot) and the plunger inside the cartridge can be easily eliminated or at least significantly reduced during assembly of the injection device.

[0048] Definition: An "injection pen" is an injection device that typically has an oval or elongated shape somewhat resembling a pen for writing. Such pens usually have a tubular cross-section, but can easily have different cross-sections, such as triangular, rectangular, or square, or any variation based on these or other geometric shapes.

[0049] The term "needle cannula" is used to describe the actual conduit that performs the penetration of the skin during injection. The needle cannula is typically made from a metallic material, such as, for example, stainless steel, and is preferably connected to a hub made from a suitable material, such as a polymer. However, the needle cannula may also be made from a polymeric or glass material. For example, the needle cannula, which is mounted within a hub, may be either replaceably or permanently attached to the injection device.

[0050] As used herein, the term "liquid drug" is meant to encompass any drug-containing flowable pharmaceutical product that can be passed through a delivery means such as a hollow needle cannula in a controlled manner, such as a liquid, solution, gel, or fine suspension. Exemplary drugs may include pharmaceutical products such as peptides, proteins (e.g., insulin, insulin analogs, and C-peptide), and hormones, biologically derived active agents or drugs, hormone and gene-based drugs, nutritional formulas, and other substances in solid (formulations) or liquid form.

[0051] "Cartridge" is a term used to describe the primary container that actually contains the liquid drug. Cartridges are usually made of glass, but can also be molded from any suitable polymer. The cartridge or ampoule is preferably sealed at one end by a pierceable membrane, called a "septum", which can be pierced, for example, by the non-patient end of a needle cannula. Such septums are usually self-sealing, meaning that when the needle cannula is removed from the septum, the opening created during piercing is automatically sealed by its inherent elasticity. The opposite end of the cartridge is typically closed by a plunger or piston made of a rubber composition or a suitable polymer. The plunger or piston can be slidably moved inside the cartridge. The space between the pierceable membrane and the movable plunger holds the liquid drug, which is pushed out when the plunger reduces the volume of the space holding the liquid drug.

[0052] Since the cartridge usually has a narrow distal neck portion into which the plunger cannot move, not all of the liquid agent contained inside the cartridge can actually be expelled. Thus, the term "initial amount" or "substantially used" refers to the amount of injectable content contained in the cartridge, and therefore not necessarily the total amount of content. The amount of injectable content in the cartridge must be at least equal to the volume that constitutes the multiple predetermined sized dose volumes to be expelled. In one embodiment, if a multiple use fixed dose injection device is to contain, for example, three fixed doses each having a volume of 0.3 ml, the injectable content of the cartridge must be at least 0.9 ml, and the total volume of the cartridge must be larger to include the volume that cannot be expelled by the narrow neck portion.

[0053] The term "pre-filled" injection device refers to an injection device in which a cartridge containing a liquid agent is permanently embedded within the injection device such that it cannot be removed without permanently destroying the injection device. Once a predetermined amount of liquid agent in the cartridge is used, the user typically discards the entire injection device. Typically, the cartridge, filled by the manufacturer with a specific amount of liquid agent, is secured within a cartridge holder that is later permanently connected within a housing structure such that the cartridge cannot be replaced.

[0054] This is in contrast to "durable" injection devices, which the user can replace with a cartridge containing the liquid medication whenever it becomes empty. Prefilled injection devices are usually sold in packages containing two or more injection devices, whereas durable injection devices are usually sold one at a time. When using prefilled injection devices, the average user may require 50-100 injection devices per year, whereas with durable injection devices, a single injection device may be used for several years, however, the average user will require 50-100 new cartridges per year.

[0055] By "multiple use fixed dose" injection device is meant to define an injection device capable of delivering a predefined plurality (i.e., two or more) of doses that are substantially identical in volume. Thus, the liquid medicament contained within the cartridge is dispensed in several substantially identical dose volumes. In one embodiment, the cartridge can accommodate, for example, 3 ml of liquid medicament, which can be dispensed in six identical doses, each of 0.5 ml. The number of equally sized doses is often between two and eight, and preferably between four and six identical dose volumes. A multiple use fixed dose injection device may either be pre-filled such that the entire injection device is discarded after a predefined number of dose volumes have been dispensed, or it may be a durable injection device that allows the user to replace the cartridge and dispense a new series of equally sized dose volumes from the new cartridge.

[0056] When the term "automatic" is used in conjunction with an injection device, it means that the injection device can perform an injection without the user of the injection device delivering the force required for the injection device to expel the liquid agent during dosing. The force is typically delivered (automatically) by an electric motor drive or a spring drive. The actual spring for the spring drive is tensioned by the user, for example during dose setting, but such springs are usually pre-tensioned with a low force to avoid problems with delivering very small doses. Alternatively, the spring may be fully pre-loaded by the manufacturer with a pre-loaded force sufficient to expel the entire initial content of the liquid agent contained in the cartridge (i.e. the entire injectable content) through several doses. Typically, when performing an injection, the user activates a release mechanism provided either on the surface of the injection device housing or at the proximal end of the injection device to partially release some of the force stored in the spring. Alternatively, the injection device may be shield-triggered, such that the force required to expel the dose is released by the activation of a movable shield.

[0057] The terms "permanently connected" or "permanently embedded" as used in this description are intended to mean that separation of the parts permanently embedded in the housing structure, and in particular the cartridge, requires the use of tools, and that separation of these parts will result in permanent damage to at least one of these parts, with the attendant inability of the injection device to operate.

[0058] All references cited in this specification, including publications, patent applications, and patents, are incorporated by reference in their entirety to the same extent as if each reference was individually and specifically indicated to be incorporated by reference and was set forth in its entirety herein.

[0059] All headings and sub-headings are used herein for convenience only and should not be construed as limiting the invention in any way.

[0060] The use of any examples or exemplary phrases (e.g., "such as") provided herein is intended merely to clarify the invention and does not limit the scope of the invention, unless specifically stated. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0061] Citation and incorporation of patent documents herein is done for convenience only and does not reflect any view of the validity, patentability and / or enforceability of such patent documents.

[0062] This invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law.

[0063] The invention is explained in more detail below in connection with preferred embodiments and with reference to the drawings. [Brief description of the drawings]

[0064] [Figure 1] FIG. 1 shows an exploded view of a spring-powered injection device according to one embodiment of the present invention. [Figure 2A-2B] Figures 2A and 2B show cross-sectional views of the spring-actuated injection device of Figure 1. Figure 2B is a view rotated 90 degrees relative to Figure 2A. [Diagram 3] Figure 3 shows a cross-sectional view of the proximal part of the spring-driven injection device with the piston rod positioned in an initial position, Figure 3 being rotated 90° with respect to Figure 4. [Figure 4] Figure 4 shows a cross-sectional view of the proximal part of the spring-driven injection device with the piston rod positioned in a parked position, the view being rotated 90° with respect to Figure 3; [Figure 5A-5B] Figures 5A and 5B show a more detailed cross-sectional view of the proximal components of a spring-driven injection device, with Figure 5B being a view rotated 90 degrees relative to Figure 5A. [Figure 6A-6B]6A and 6B show perspective views of the drive tube from different angles. [Figure 7A-7B] 7A and 7B show perspective views of the connector from opposite ends. [Figure 8] FIG. 8 shows a perspective view of the piston rod. [Figure 9] FIG. 9 shows a side view of the interface between the drive tube and the bridge structure inside the housing structure. [Figure 10] FIG. 10 shows a cutaway view of the interface between the drive tube and the housing structure. [Figures 11A-11C] 11A-11C show the attachment of a torsion spring to a spring base of a housing structure. [Figure 12] FIG. 12 shows an exploded assembly view of a piston rod and a nut member for zero point adjustment according to the first embodiment. [Figure 13] FIG. 13 shows a diagram of the zero point adjustment in the first embodiment. [Figure 14A-14B] 14A and 14B show perspective views of the zero adjustment nut member from opposite ends. [Figure 15] FIG. 15 shows a perspective view of the housing parts. [Figure 16] FIG. 16 shows the cut-out parts of the housing parts for the zero point adjustment. [Figure 17A-17B] 17A and 17B show two cut-away images of the interface between the nut member and the housing part for the zero point adjustment. [Figures 18A-18C] 18A-18C show cross-sectional views of an alternative zero adjustment. [Figure 19] FIG. 19 shows a nut member for the alternative solution shown in FIGS. 18A-18C. [Figure 20] FIG. 20 shows an alternative piston rod for a second alternative zero adjustment. [Figure 21] FIG. 21 shows a telescopic connection between the piston rod and the piston rod foot for a second alternative zero point adjustment. [Fig. 22A-22B]Figures 22A and 22B show cross-sectional views of a second alternative zero adjustment, with Figure 22B being a view rotated 90 degrees relative to Figure 22A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] The figures are schematic and simplified for clarity, and they only show details essential to an understanding of the invention, while omitting other details. The same reference numbers are used throughout for the same or corresponding parts.

[0066] When the terms "up" and "down", "right" and "left", "horizontal" and "vertical", "clockwise" and "counterclockwise", or similar relative expressions are used in the following, these are merely intended to refer to the attached figures and not to indicate an actual situation of use. The figures shown are schematic and therefore the relative dimensions thereof as well as the configuration of different structures are intended to serve illustrative purposes only.

[0067] In that context, it may be convenient to define that the term "distal end" in the accompanying figures means the end of the injection device which secures the needle cannula and which is pointed towards the user during injection, whereas the term "proximal end" means the opposite end, as shown in Figures 2A and 2B. Distal and proximal mean along an axial orientation extending along the longitudinal axis (X) of the injection device, as also disclosed in Figures 2A and 2B.

[0068] In the following examples, when referring to clockwise and reverse or counterclockwise directions, it is understood that the injection device is viewed from a position distal to the injection device. Thus, a clockwise direction is rotation toward the right side, like the arms of a clock, and a counterclockwise direction is rotation toward the left side.

[0069] The following terms are used throughout the following detailed description to describe the various movements performed in the described injection devices.

[0070] "Translational motion" means strictly linear motion without any rotation.

[0071] "Rotational motion" is any rotational motion about a center, which may be the midpoint of one planar axis or central axis, i.e. having a longitudinal extension.

[0072] By "axial motion" is meant any motion in an axial direction. Such motion may be strictly translational or may include rotational motion, thus becoming "helical motion", since helical motion is meant to be a combination of axial and rotational motion.

[0073] "Telescopic" is meant to encompass situations where a movable element moves out of and / or into a base element. The telescopic movement may be translational or may include rotation, making the telescopic movement helical.

[0074] 1 discloses an exploded view of a spring injection device according to one embodiment of the present invention. In the disclosed embodiment, the injection device is in the shape of a pen, which is also often referred to as an injection pen.

[0075] The liquid agent to be expelled is contained within a cartridge 5, which is generally a hollow glass ampoule sealed at a distal end by a pierceable septum 6 and at a proximal end by a movable plunger 7. The movable plunger 7 is arranged to be moved distally by a piston rod 60. To properly distribute the force from the piston rod 60 to the plunger 7, a piston rod foot 85 may be provided between the piston rod 60 and the plunger 7, as depicted in FIG.

[0076] The cartridge 5 is typically filled with a liquid agent by the manufacturer and permanently and irreplaceably fixed in the housing structure of the injection device, thus making the injection device a pre-filled injection device. The disclosed housing structure comprises a housing part 10, a cartridge holder 20, a spring base 25, and a shield guide 30. However, the housing structure may comprise any number of components or alternatively may be molded as one single housing unit.

[0077] The housing part 10 (also shown in FIG. 15), the cartridge holder 20, the spring base 25 and the shield guide 30 are preferably permanently fixed to each other such that the cartridge 5 is permanently enclosed within the housing structure, thereby constituting the pre-filled injection device. On the proximal side, the housing part 10 is closed by the spring base 25, which is click-fitted into the housing part 10 during assembly of the pre-filled injection device. On the distal side of the housing part 10, the shield guide 30 is also click-fitted into the housing part 10. The cartridge holder 20 is preferably permanently fixed to the housing part 10 by a pair of resilient click arms 21 or alternatively by being integrally molded therewith.

[0078] Although the figure depicts a pair of resilient click arms 21, any number of arms may be provided. Because the examples herein refer to a pen-shaped injection device having a tubular cross section, many of the various protrusions, arms, guide tracks, and other mechanical elements are provided in pairs. However, any random number of many of these attributes may be provided.

[0079] The shield guide 30 guides the telescopically movable shield 40, whose function will be explained later. On the distal side, the shield guide 30 is provided with a peripheral track 31 having an axial opening 32 on its outer surface. This peripheral track 31 guides a radially pointed protrusion 36 located on the inner surface of the protective cap 35, as disclosed in FIG. 2B (and shown in dashed lines in FIG. 1). From this point on, the user needs to rotate the protective cap 35 in a counterclockwise direction (as viewed from a distal position) relative to the shield guide 30 and thus the housing structure before the radially pointed protrusion 36 can be moved axially out through the axial opening 32 and the protective cap 35 can be removed.

[0080] Distally, the cartridge holder 20 is provided with a needle hub 45 which, at least in use, carries a needle cannula 46. Alternatively, a needle magazine with a plurality of needle cannulas can be integrated into the injection device.

[0081] For example, as disclosed in Figures 2A and 2B, needle cannula 46 has a distal tip for penetrating the user's skin during injection and a proximal end 47 that penetrates through the septum 6 of cartridge 5 so that liquid medication can be forced out of cartridge 5, through the lumen of needle cannula 46 and through the user's skin.

[0082] The needle hub 45 is fixed to the cartridge holder 20 by means of an interface surface which is actuated in an actuation process. During this actuation process, the needle hub 45 is moved axially in a proximal direction such that the proximal end 47 of the needle cannula 46 penetrates through the septum 6 of the cartridge 5. Also, in the sequence of moving the needle hub 45, a proximal locking arm 48 provided on the needle hub 45 irreversibly engages and locks to the distal interface surface 22 on the cartridge holder 20, so that the needle hub 45 is then irreversibly locked to the cartridge holder 20.

[0083] The needle hub 45 is preferably moved proximally by rotation of the telescopically movable shield 40, which can move the needle hub 45 proximally via a helical joint surface. Once the actuation process is completed, the locking arm 48 on the needle hub 45 is locked to the cartridge holder 20, and the click arm 43 provided on the telescopically movable shield 40 engages with the housing structure to prevent the user from rotating the telescopically movable shield 40 back to its previous position. The engagement of the click arm 43 is preferably with the axial inner surface of the shield guide 30 fixed to the housing part 10. From this point on, the actuation process can be performed only once.

[0084] After the start-up process is complete, the injection device is ready for use, as disclosed in Figures 2A and 2B, and a user can use the injection device for multiple injections, as described. As will be further described, the injection of the liquid medication is driven by a spring, which in the embodiment of the present disclosure is a torsion spring that delivers a torsional force. However, any type of spring can be used for the injection process.

[0085] The telescopically movable shield 40 carries a cleaning assembly 50, disclosed in further detail in WO2019 / 101670, which keeps the distal end of the needle cannula 46 biologically clean between injections and is secured to the telescopically movable shield 40 by a shield tip 55 that is click-fitted to the telescopically movable shield 40 by an elastic arm 56 engaging the telescopically movable shield 40, such that the cleaning assembly 50 follows all movements of the telescopically movable shield 40, i.e. both rotational, translational and helical movements.

[0086] The cleaning assembly 50 preferably contains a liquid cleaning agent, which in one embodiment may be the same preservative contained in the liquid agent in the cartridge 5. In a preferred embodiment, the cleaning agent is the same preservative-containing medicinal agent liquid contained in the cartridge 5 that is loaded into the cleaning assembly 50 during start-up of the injection device.

[0087] A torsion spring arrangement is provided to move the piston rod 60 distally during dose ejection. The torsion spring arrangement comprises a torsion spring 65, a drive tube 70 and an inner nut member 11, as described, to drive the piston rod 60 distally.

[0088] The torsion spring 65 is, in the embodiment of the present disclosure, a metal spring with wire wound in a helical shape. In the longitudinal direction, the torsion spring 65 can be divided into different zones or areas. In some of these zones, the coil of wire has no or very little distance between the coils, and in other zones, the coils have a much larger longitudinal distance between the coils. These zones are called compression zones 66 (see, for example, FIG. 1). Such compression zones 66 with distance between the coils provide a compression force, so that the torsion spring 65 can exert both a torsional force and a compression force. When the two ends of the torsion spring 65 are compressed towards each other, the torsion spring 65 returns a longitudinally directed force, urging the two ends away from each other.

[0089] The two ends of the torsion spring 65 are bent into hooks. One hook is attached to the housing structure via the spring base 25 at the proximal end of the injection device, and the other hook is attached to the drive tube 70 at the opposite, more distal end of the injection device. Thus, a torsional force can be provided between the housing structure and the drive tube 70, which can be used to rotate the drive tube 70.

[0090] The torsion spring 65 is preferably installed by passing the hooks through the axial openings of the respective parts 25, 70 such that the hooks are captured by the edges of the axial openings, followed by relative rotation of the respective parts 25, 70 and the torsion spring 65. The torsion spring 65 preferably first engages the drive tube 70, which then engages the spring base 25 in the assembly process. Both the drive tube and the spring base, in one embodiment, may be provided with snap protrusions as described.

[0091] A piston rod 60, disclosed in detail in FIG. 8, is provided with an external thread 61 on an outer surface, and further provided with a longitudinal track structure 62 that is open at a distal end but terminates in a stop surface 63 at a proximal end. The longitudinal track structure 62 has a free length referred to as "L". The free length "L" is the length measured from the engagement of the piston rod 60 with an inwardly pointed projection 75 on the drive tube 70 to the stop surface 63 at the proximal end of the track structure 62, as best seen and described in FIGS. 3 and 5A. Thus, the free length "L" is an expression for the axial length that the piston rod 60 may travel distally before the stop surface 63 engages with the inwardly pointed projection 75. Thus, as also seen in FIG. 3, the free length "L" is less than the actual length of the track structure 62.

[0092] Longitudinal track structure 62 refers to any type of structure provided in or on piston rod 60 that can define a longitudinal free length "L." For example, it may be any type of track, groove, or similar recess.

[0093] The internal nut member 11 is fixed to the housing structure both rotationally and axially. In one embodiment, the nut member 11 is an integral part of the housing part 10. Alternatively, the nut member 11 may be a separate part that is fixed to the housing part 10 during assembly of the injection device, for example by gluing or welding. The nut member 11 is provided with an internal thread 12 on its inner surface that engages with an external thread 61 on the piston rod 60, so that the piston rod 60 moves in a helical manner when it rotates relative to the housing structure.

[0094] As disclosed in Figures 12-17A and 17B, in a further embodiment, a nut member 11 can be utilized to eliminate air gaps during assembly of the injection device as described.

[0095] The longitudinal track structure 62 on the piston rod 60 is engaged by an inwardly pointed protrusion 75 provided on the inner surface of the drive tube 70, so that every time the drive tube 70 rotates, the piston rod 60 simultaneously rotates and therefore spirals distally within the internal threads 12 of the nut member 11. The inwardly pointed protrusions 75 disclosed in Figures 6A and 6B are preferably provided in pairs, but may be provided in singular or any random number.

[0096] The torsion spring 65 is contained between the housing structure and the drive tube 70, so that the torque stored in the torsion spring 65 can rotate the drive tube 70 relative to the housing structure. In the disclosed embodiment, the torsion spring 65 engages the drive tube 70 at its distal end and the spring base 25 at its proximal end. The torsion spring 65 is in tension during the manufacture of the injection device, i.e. during the assembly of the injection device, so that a relatively high torque is stored in the torsion spring 65 when the injection device is delivered to the user. The torque stored in the unused delivery state of the injection device is preferably sufficient to eject the entire initial content of the cartridge 5, which means that the torque is sufficient to drive the piston rod 60, and thus the plunger 7, towards or near the distal end of the cartridge 5. In a preferred embodiment, such a multi-use fixed dose injection device will have a torsion spring 65 that is in a tensioned state and ready to eject approximately two to eight predetermined and equally sized dose volumes, so that the user does not need to tension the torsion spring 65 during each of these two to eight injections.

[0097] 6A and 6B has a first helical form 71 at its distal end. That is, the distal end of the drive tube 70 is made from a sleeve that tapers off circumferentially. The first helical form 71 extends axially and terminates in a first axial drive flange 72.

[0098] Additionally, the distal most part of the drive tube 70 has an outer surface that is radially offset relative to the remaining parts of the drive tube 70. This radial indentation in the outer surface of the drive tube 70 defines a second axial drive flange 78 that is parallel to the first axial drive flange 72 but rotationally offset 180°, as best seen in FIGURE 6A. These two axial flanges 72, 78 define rotational stops for the drive tube 70, as will be described.

[0099] Until it meets the second axial drive flange 78, the radial recess has a helical configuration that forms a helical surface that abuts against a similar helical surface provided inside the housing structure. This helical surface has the same configuration as the housing helix 16, but is longitudinally offset in the proximal direction, as best seen in FIG. 17B. The interface between these two surfaces has the same effect as the interface between the first helical configuration 71 and the housing helix 16, as will be explained. By having two such helical interfaces, the operation of the injection device is more stable.

[0100] Additionally, two outwardly pointed protrusions 73, 74 are provided on the outer surface of the drive tube 70. In the disclosed embodiment, these two protrusions 73, 74 are offset 180° relative to one another and are also offset longitudinally by a distance.

[0101] As will be explained, one or more inwardly pointed protrusions 75 on the inside of drive tube 70 engage longitudinal track formation 62 in piston rod 60. On an outer surface, drive tube 70 is provided with a helical flange 76, the use of which will be explained later.

[0102] The housing part 10 of the housing structure is internally molded with an internal bridge structure 15 having an axial opening through which the piston rod 60 can move. On its inner surface, the bridge structure 15 guides and supports the distal part of the drive tube 70. This guide is depicted in FIG. 6A, where the outline of the bridge structure 15 is shown in dashed lines. The bridge structure 15, which also carries the nut member 11, can in one embodiment be molded separately and attached to the housing part 10. In either case, the bridge part 15 contacts the housing part 10 only through a radial bearing part 19, which provides limited angular space so that an axial opening exists around the bridge part 15. This can be best seen in FIGS. 17A and 17B.

[0103] A first helical form 71 at the distal end of the drive tube 70 also extends axially and engages with a similar helical form 16 (see, e.g., FIG. 10 ) provided in a bridge structure 15 inside the housing part 10 of the housing structure (hereafter referred to as housing helical form 16). This housing helical form 16 as first helical form 71 tapers off circumferentially and is a sleeve that terminates in a first axial housing flange 17 that can engage a first axial drive flange 72 of the drive tube 70.

[0104] FIG. 9 discloses the interface between the drive tube 70 and the bridge structure 15 located inside the housing part 10. That is, the housing part 10 is visually cut away. FIG. 10 discloses the interface between the drive tube 70 and the housing structure including the bridge structure 15. In the view of FIG. 10, the housing structure, not the drive tube 70, is cut radially along line "A" in FIG. 5A and viewed from a distal position. Thus, the cut is made through the first axial housing flange 17. The housing part 10 is further visually cut open in a longitudinal plane according to centerline "X".

[0105] The internal bridge structure 15 is further provided with a second axial housing flange 18, as seen in Figures 9 and 17B, which can abut against the second axial drive flange 78. Thus, the rotational engagement between the drive tube 70 and the housing structure is defined by the abutment of the first axial drive flange 72 with the first axial housing flange 17, in addition to the abutment of the second axial drive flange 78 with the second axial housing flange 18, as best seen in Figures 9 and 10. All four flanges 72, 17; 78, 18 are preferably parallel to each other and to the central longitudinal axis "X" of the injection device. Furthermore, as will be explained, the axial lengths ("dl" shown in Figures 6A and 6B) of these four flanges 72, 17; 78, 18 are the same.

[0106] As the torsion spring 65 constantly exerts a torsional force on the drive tube 70, the drive tube 70 will rotate (in the example embodiment) in a counterclockwise direction when viewed from the distal end of the injection device. However, the engagement between the first axial drive flange 72 and the first axial housing flange 17, and the engagement between the second axial drive flange 78 and the second axial housing flange 18, prevent the drive tube 70 from rotating relative to the housing structure.

[0107] Additionally, several ratchet arms 77 provided on the drive tube 70 engage with a tooting 26 inside the spring base 25, so that the drive tube 70 can only rotate in one direction, which in the disclosed embodiment is counterclockwise when viewing the injection device from a distal position, as illustrated, for example, in FIG.

[0108] The telescopically movable shield 40 is rotatable relative to the housing structure and can be rotated between a locked position and an unlocked position. As can be seen in FIG. 1, the telescopically movable shield 40 is provided on its outer surface with a helical structure 41 terminating in radial ends 44a, b positioned at an angular distance apart such that they together define an axial opening. On the inner surface of the housing part 10, an inwardly pointed protrusion is provided which can slide through the axial opening of the helical structure 41 when the telescopically movable shield 40 is rotated to the unlocked position. In any other position, this inwardly pointed protrusion will abut against the helical structure 41 when an attempt is made to translate the telescopically movable shield 40 in the proximal direction, which henceforth defines the locked position.

[0109] The helical structure 41 forces the needle shield 40 to move in a helical manner as it rotates, and thus allows the needle shield 40 to be moved to a position where the distal tip of the needle cannula 46 is located just outside the cleaning assembly 50 when the needle shield 40 is unlocked.

[0110] In the locked position, the telescopically movable shield 40 is prevented from moving in a translational direction, whereas in the unlocked position, the telescopically movable shield 40 can move in a translational direction. In this context, translation is meant to define axial movement along the central axis "X" without any rotation.

[0111] The housing part 10 is provided with a pair of longitudinal windows 13. These longitudinal windows 13 align with similar windows 23 provided in the cartridge holder 20 so that the user can visually inspect the content of the cartridge 5. A telescopically movable shield 40, radially sandwiched between the housing part 10 and the cartridge holder 20, is rotatable between a locked position and an unlocked position and is provided with a further set of windows 49. These windows 49 align with the other windows 13, 23 so that the user can only see the content of the cartridge 5 when the telescopically movable shield 40 is rotated to its unlocked position. When the telescopically movable shield 40 is in the locked position, the solid parts of the telescopically movable shield 40 prevent the user from visually seeing the cartridge 5. This rotation of the set of windows 49 in the telescopically movable shield 40 therefore also indicates when the injection device is ready for injection.

[0112] In one embodiment, a pair of longitudinal windows 13 provided in the housing part 10 may be provided with a scale indicating the number of doses of the injection device. In the embodiment of Figures 1 and 15, this scale shows four sections, each representing one of the predetermined dose volumes. Thus, the user can visually see the physical position of the plunger 7 within the section of the window 13 and can subsequently see how many doses have been taken and how many remain in the cartridge 5.

[0113] The telescopically movable shield 40 is also used to release the torque stored in the torsion spring 65 when moved translationally in the proximal direction, thereby expelling a predetermined dose volume. During an injection, the user presses the shield tip 55, and hence the telescopically movable shield 40, against the skin, which causes the telescopically movable shield 40 to move in the proximal direction.

[0114] To transmit the translational movement from the telescopically movable shield 40 to the drive tube 70, a connector element 80 is provided as disclosed in Figures 7A and 7B. This connector element 80 is guided in translational direction, i.e. without any rotation, with respect to the housing part 10 and is provided on its inner surface with two inwardly pointed projections 81, 82, which projections 81, 82 are also offset both rotationally and axially.

[0115] One of the two inwardly pointed protrusions 81, 82 (designated as "81") is not directly visible in the enclosed view, but is shown with a puncture line in Figure 7B. The two protrusions 81, 82 are offset 180° relative to each other in the disclosed embodiment.

[0116] Both the telescopically movable shield 40 and the connector element 80 are provided with hooks 42, 83. When the telescopically movable shield 40 is rotated, the two hooks 42 on the telescopically movable shield 40 can be brought into engagement with two hooks 83 provided on the connector element 80.

[0117] Hooks 83 on connector element 80 are provided distally on a pair of axially extending pieces 84. These axially extending pieces 84 enable connector part 80 to encircle bridge piece 15 of housing part 10 and to operate through axial openings between radial bearings 19 in the connection between bridge piece 15 and housing part 10, as best seen in Figures 2A, 5A and 17A.

[0118] Spring installation As disclosed in Figures 2A and 2B, the torsion spring 65 is located between the drive tube 70 and the spring base 25 such that the torsion spring 65 can rotate the drive tube 70 relative to the spring base 25, which is part of the housing structure.

[0119] In one embodiment, disclosed in Figures 11A-11C, a torsion spring 65 is provided with a hook 67 at an end of the torsion spring 65. For purposes of illustrating the attachment of the spring, Figures 11A-11C show only the proximal end of the torsion spring 65, and also show only a portion of an alternative spring base 25. Although Figures 11A-11C disclose only one end of the torsion spring 65, it will be apparent that both ends may be provided with such hooks 67 and attached in the same manner.

[0120] To load the torsion spring 65, first one hook 67 is passed translationally through the axial passage 26, for example, in the spring base 25. This translational motion is indicated by arrow "A" in FIG. 11A, which indicates that the motion between the spring base 25 and the torsion spring 65 is relative, i.e., one or both of the elements may move translationally.

[0121] As disclosed in FIG. 11B, when hook 67 is passed axially through axial passage 26, torsion spring 65 and spring base 25 rotate relative to one another so that hook 67 catches on top of ledge 27 formed in spring base 25, as shown in FIG. 11C.

[0122] A radial snap protrusion 28 is located on the spring base 25 within the axial passage 26 to irreversibly secure the torsion spring 65 to the spring base 25 .

[0123] When the torsion spring 65 and the spring base 25 rotate relative to one another, the hook 67 of the torsion spring 65 passes over this radial snap protrusion 28 and thus becomes irreversibly locked to the spring base 25, as shown in FIG. 11C.

[0124] The radial snap protrusion 28 has two sides provided in the direction of rotation. The side that first encounters the hook 67 during rotation has a sloped surface 29a to make it easier for the hook 67 of the torsion spring 65 to slide over the radial snap protrusion 28. The opposite side of the snap protrusion 28 is preferably provided with a steeper surface 29b to prevent the hook 67 of the torsion spring 65 from being able to rotate in the opposite direction after it has been installed.

[0125] In one embodiment, the inclined surface 29a is angled such that the hooks 67 of the torsion spring 65 cannot pass through the radial snap protrusions 28 without being forced through by an assembly tool. In such an embodiment, it is not sufficient to rotate the spring base 25 and the torsion spring 65 relative to one another. This is particularly the case where the torsion spring 65 has open windings such that sufficient torque cannot be transmitted to the proximal end carrying the hooks 67 when the torsion spring 65 is rotated. In such cases, it is necessary to use an assembly tool that grips the torsion spring 65 at the proximal end and forces the hooks 67 past the radial protrusions.

[0126] The assembly tool, in one embodiment, may be a support element that enters the axial passage when the torsion spring 65 is in the position disclosed in FIG. 11B and abuts the rear hook 67, thereby urging the hook 67 rotationally onto the radial snap protrusion 28 and into the position disclosed in FIG. 11C.

[0127] Although radial snap protrusions 28 are disclosed in connection with spring base 25, such radial snap protrusions 28 may also be provided on stationary drive tube 70 to secure the other end of torsion spring 65. Hereinafter, snap protrusions 28 may be provided either on spring base 25, or on drive tube 70, or on both elements.

[0128] In one embodiment, the torsion spring 65 is first attached to either the spring base 25 or the drive tube 70 purely by rotational motion and, for example, by use of a tool. This forms a pre-assembled unit comprising either the spring base 25 or the drive tube 70 and the torsion spring 65. Because the torsion spring 65 is irreversibly attached due to the radial snap protrusions 28, this pre-assembled unit can be moved around in the assembly process without the torsion spring 65 becoming separated from either the spring base 25 or the drive tube 70.

[0129] At a later stage during the assembly process, torsion spring 65 can likewise be attached to spring base 25 or other components of drive tube 70 by rotating this component and torsion spring 65 relative to one another.

[0130] Preferably, the pre-assembled unit consists of a torsion spring 65 and a drive tube 70. During the pre-assembly process, the torsion spring 65 is irreversibly attached to the drive tube 70 as described above. When this pre-assembled unit is positioned inside the housing part 10, the spring base 25 rotates to engage a proximal hook 67 of the torsion spring 65 and is axially secured to the housing part 10 by engaging a pair of flexible connecting arms 9 (best seen in Figures 15 and 16) provided on the housing part 10. In one embodiment, radial snap protrusions 28 are provided only on the drive tube 70 and not on the spring base 25.

[0131] injection When the telescopically movable shield 40 is rotated to the unlocked position, the user presses the distal shield tip 55 of the telescopically movable shield 40 against the skin, thereby expelling a predetermined dose volume by causing the telescopically movable shield 40 to translate in the proximal direction, which translational motion is transferred to a similar translational motion of the connector element 80.

[0132] 7A and 7B, is guided translationally relative to the housing part 10 during dispensing, with two inwardly pointed protrusions 81, 82 abutting outwardly pointed protrusions 73, 74 on the outer surface of the drive tube 70, which therefore also moves translationally together with the connector element 80. A compression zone 66 on the torsion spring 65 allows the drive tube 70 to move translationally in the proximal direction, and compression of the torsion spring 65 applies an additional axial force to the drive tube 70, urging the drive tube 70 in the distal direction.

[0133] Translational movement of drive tube 70 in the proximal direction causes first and second axial drive flanges 72, 78 on drive tube 70 to slide along first and second axial housing flanges 17, 18, respectively, of housing part 10. At the same time, inwardly pointed protrusions 75 on drive tube 70 slide an axial distance within longitudinal track formation 62 on piston rod 60.

[0134] Hereinafter, the size of a predetermined dose volume prepared by this translational movement of the drive tube 70 is related to the longitudinal distance traveled by the drive tube 70, i.e. the axial length of engagement between the first axial drive flange 72 and the first axial housing flange 17, and the axial length of engagement between the second axial drive flange 78 and the second axial housing flange 18, as well as the pitch of the threaded connection 12, 61 between the piston rod 60 and the nut member 11. The translational distance traveled by the drive tube 70 when a predetermined dose volume is prepared is referred to as "dl" (working distance).

[0135] As the first and second axial drive flanges 72, 78 translate out of engagement with the first and second axial housing flanges 17, 18, the torque stored in the torsion spring 65 forces the drive tube 70 to rotate, causing the helical feature 71 on the drive tube 70 to rotate down the housing helical feature 16 inside the housing part 10 until the first and second axial drive flanges 72, 78 again abut the first and second axial housing flanges 17, 18. The helical motion may be supported by additional helical mating surfaces as previously described. This rotation is 360° in the disclosed embodiment. That is, the drive tube 70 rotates one complete revolution each time it translates proximally the working distance "dl". From this point on, the piston rod 60 also rotates 360° and therefore moves a distal distance determined by the pitch of the threads 61 on the piston rod 60 and the pitch 12 of the engagement nut member 11 .

[0136] Thus, each predetermined dose volume is prepared by translating the drive sleeve 70 proximally a working distance "dl" and is expelled by rotating the drive sleeve 70 distally and returning it to its initial position.

[0137] A shield spring 90 in the form of a helically coiled compression spring is provided between the connector element 80 and the housing structure, preferably between the connector element 80 and the spring base 25, and applies a compressive force to the connector element 80 when the connector element 80 translates proximally during dose preparation. Compression of the shield spring 90 biases the connector element 80 in a distal direction.

[0138] 6A and 6B, drive tube 70 is provided with a helical flange 76 on an outer surface which engages inwardly pointed projections 81, 82 on the inside of connector element 80 when torsion spring 65 begins to rotate drive tube 70. This engagement between inwardly pointed projections 81, 82 and helical flange 76 supports the helical guide of drive tube 70.

[0139] The helical flange 76 on the drive tube 70 is provided with two axial openings "d" (FIGS. 6A and 6B) through which the inwardly pointed projections 81, 82 inside the connector element 80 can slide in a translational direction when the openings "d" are rotationally aligned with the inwardly pointed projections 81, 82. This alignment occurs when the first and second axial drive flanges 72, 78 on the drive tube 70 are about to abut again against the first and second axial housing flanges 17, 18 inside the housing part 10, which is when the predetermined dose volume has been expelled. Thus, the shield spring 90 will push the connector element 80 and the telescopically movable shield 40 in a distal direction when the predetermined dose size has been expelled, i.e., after the drive tube 70 has rotated (in this embodiment) 360° to reach its initial position.

[0140] Also, in this state, the inwardly pointed protrusions 81, 82 will be aligned with the outwardly pointed protrusions 73, 74 so that the next subsequent dose volume can be released by repeating the procedures described herein.

[0141] When the telescopically movable shield 40 is moved back to the initial position, the cleaning assembly 50 carried by the telescopically movable shield 40 is returned to the initial position with the distal tip of the needle cannula 46 positioned inside the cleaning chamber 50 .

[0142] During movement of the telescopically movable shield 40 in the distal direction, the helical structure 41 on the telescopically movable shield 40 abuts against a similar helical path 33 provided inside the housing structure and preferably on the inner surface of the shield guide 30, thereby forcing the telescopically movable shield 40 to rotate to the locked position as the telescopically movable shield 40 moves back to its initial position.

[0143] Contents sold out Thus, a predetermined dose volume is prepared by moving the drive tube 70 proximally by pressing the telescopically movable shield 40 against the user's skin. When the drive tube 70 moves a working distance "dl" in the proximal direction, the first helical feature 71 on the drive tube 70 reaches a release position and the axial drive flanges 72, 78 are released from the axial housing flanges 17, 18. In this release position, the drive tube 70 can rotate under the influence of the torque stored in the torsion spring 65. As will be explained, the drive tube 70 moves helically in the distal direction during rotation. During this rotation, the drive tube 70 also rotates the piston rod 60 due to the engagement between the longitudinal track structure 62 of the piston rod 60 and the inwardly pointed protrusion 75 inside the drive tube 70. The piston rod 60 moves helically in the distal direction during rotation because the piston rod 60 is threaded (61, 12) into a nut member 11 fixed within the housing structure.

[0144] Each time the drive tube 70 moves a working distance "dl" in the proximal direction and is released, the piston rod 60 is forced to rotate, in this embodiment, 360°, i.e., one complete revolution, and therefore move forward an axial distance determined by the pitch of the threads between the piston rod 60 and the nut member 11. When the remaining distance between the inwardly pointed projection 75 on the drive tube 70 and the stop surface 63 on the piston rod 60 becomes less than the length "dl" of the axial flanges 72, 78, 17, 18, it is not possible to move the drive tube 70 to the released position to release an additional fixed dose.

[0145] When the injection device is delivered to a user, the stop surface 63 on the piston rod 60 is located at the proximal end of the injection device, as disclosed in Fig. 3. However, for each ejection of a predetermined dose size, the piston rod 60 moves distally until the stop surface 63 on the piston rod 60 is in a position that does not allow the drive tube 70 to move the full working distance "dl" in the proximal direction. When this occurs, it is not possible to move the drive tube 70 to the release position, and therefore no further predetermined dose sizes can be selected, thereby preventing the user from expelling further predetermined doses.

[0146] In other words, if the remaining portion of the free length "L" of the raceway structure 62 of the piston rod 60 is shorter than the working distance "dl", it is not possible to move the axial drive flanges 72, 78 out of engagement with the axial housing flanges 17, 18 and therefore release further fixed dose volume.

[0147] Finally, the drive tube 70 translates a working distance "dl" in the proximal direction each time the user prepares one of the predetermined dose volumes, and rotates back to the initial position when the prepared predetermined dose volume is expelled. In this rotational movement, the drive tube 70 preferably rotates approximately 360°. If the time accumulated while the drive tube 70 moves the working distance "dl" and the distance accumulated while the piston rod 60 moves distally remain less than the length "dl" of the free length "L" of the track structure 62 of the piston rod 60 available, the stop surface 63 on the piston rod 60 prevents the dose tube 70 from moving the full fixed dose setting (i.e., the full working distance "dl") in the proximal direction, and therefore prevents the user from being able to select the full predetermined dose size.

[0148] In different embodiments, the first helical form 71 and the housing helical form 16 may be divided into two or more surfaces such that two or more axial flange abutments (72, 18; 78; 17) are provided. In such cases, the possible rotation of the drive tube 70 and the piston rod 60 for each translational movement may differ from 360°. For example, if twice as many axial flange abutments were provided, the rotation would be 180°, such that the piston rod 60 rotates half a full rotation for each dose release.

[0149] Example of end of content In one embodiment, the free length "L" of the track structure 62 may be, for example, 43 mm. That is, the translation distance between the engagement of the inwardly pointed protrusion 75 of the drive tube 70 with the piston rod 60 and the stop surface 63 within the piston rod 60 is set at the factory to 43 mm.

[0150] To release one of the fixed doses, the drive tube 70 moves a working distance "dl" in the proximal direction. In the example, "dl" may be 5 mm. Once the drive tube 70 has moved the working distance "dl" = 5 mm in the proximal direction, the torsion spring 65 rotates the drive tube 70 one complete revolution (i.e., 360°) back to its initial position. During this rotation, the piston rod 60 is also forced to rotate the same number of degrees, i.e., 360°. Depending on the pitch of the threaded connection between the piston rod 60 and the nut member 11, the piston rod 60 moves a given axial distance in the distal direction for each complete revolution. The pitch may, for example, be such that the distance traveled by the piston rod 60 is, for example, 10 mm for each full revolution (360°) of the piston rod 60. This means that once four fixed doses have been released (i.e. the drive tube 70 has moved the working distance "dl" four times), the piston rod 60 has moved 40 mm in the distal direction before reaching the stop face 63, leaving only a 3 mm free length "L" of the track structure 62, and that the drive tube 70 requires an axial movement of "dl" = 5 mm to release a further fixed dose volume, so that although there is a 3 mm free length "L" of the track structure 62 remaining, it is no longer possible to release a further fixed dose volume.

[0151] Zero point adjustment In one embodiment of the present invention, primarily disclosed in Figures 12-17A and 17B, the nut member 11 may be a separate element that is fixed to the housing part 10 of the housing structure during assembly of the injection device. In such an embodiment, the nut member 11 may be fixed in the housing structure without the use of physical attachment means such as gluing or welding. By using a dedicated assembly, in such an embodiment, the nut member 11 may also be used to completely eliminate, or at least substantially minimize, any air gaps resulting from different tolerances in the assembly process. Eliminating such air gaps is often also referred to as zero point adjustment. The zero point means the point where the piston rod 60 (or the piston rod foot 85) abuts against the plunger 7 inside the cartridge 5. If such abutment is achieved during the manufacture of the injection device, the user does not need to perform an initial priming of the injection device before expelling the first dose volume.

[0152] A nut member 11 for this purpose is disclosed in Figures 14A and 14B and comprises an internal thread 12 which engages with an external thread 61 on the piston rod 60 and two external thread projections 95 provided proximally on the nut member 11. These two angled thread projections 95 together form an external thread on the nut member 11. However, the external thread may be made either from one or more flanges or any number of external thread projections 95.

[0153] The nut member 11 is also provided with a number of ratchet arms 96 on its outer surface, the use of which is described below. In the disclosed embodiment, two ratchet arms 96 are disclosed distally on the nut member 11, although any suitable number may be provided.

[0154] A bridge structure 15 inside the housing part 10, as disclosed for example in Figure 16, which supports the nut member 11, is in this embodiment provided with axial teeth 97 which allow the nut member 11 to rotate in only one direction, the permitted direction being clockwise, meaning that the ratchet arm 96 and teeth 97 join in such a way that rotation in the counterclockwise direction is prevented.

[0155] The bridge structure 15 inside the housing part 10 is further provided with internal threads 98 oriented such that the nut member 11 is helically threaded in a proximal direction upon permitted clockwise rotation.

[0156] During assembly of the injection device, one important objective is to eliminate the distance, the so-called air gap, that exists between the piston rod 60 and the plunger 7 inside the cartridge 5. When the piston rod foot 85 is attached to the piston rod 60 as disclosed in Fig. 13, the objective is to eliminate the physical distance between the distal face of the piston rod foot 85 and the proximal face of the plunger 7, so that they abut when the injection device is delivered to the end user in an unused state.

[0157] As the nut member 11 rotates relative to the housing structure during final assembly, the piston rod 60 advances distally until the piston rod 60 or piston rod plunger 85 abuts the plunger 7 inside the cartridge 5 .

[0158] Rotation of the nut member 11 is preferably performed by using a special tool on the production line that can engage with the nut member 11 and transmit rotation to the nut member 11. In one preferred embodiment, the piston rod 60 is initially located at the engagement with the nut member 11, located in the bridge structure 15 of the housing part 15. Electronic computerized equipment is then used to detect the position of the plunger 7 in the cartridge 5 used for that particular injection device. If the position of the plunger 7 and the position of the piston rod 60 (or the piston rod foot 85) are measured and known, the computer can determine how much the nut member 11 needs to be rotated to abut the piston rod foot 85 or the piston rod 60 of the particular injection device with the plunger 7 when the injection device is assembled.

[0159] Therefore, the position of the proximal-most end of the piston rod 60 or piston rod foot 85 is fine-tuned by rotating the nut member 11 at its unidirectional interface with the bridge structure 15. What is important here is that it is possible to rotate the nut member 11 in a rotational direction that advances the piston rod 60 (or piston rod foot 85) into contact with the plunger 7.

[0160] The piston rod 60 is further provided with an axial track structure 62 which is engaged by an inwardly pointed protrusion 75 on the drive tube 70, which in turn is provided with several ratchet arms 77 which engage with the inner tooting 26 of the spring base 25 to form a unidirectional ratchet interface, so that the drive tube 70 can only rotate in one direction, which in the disclosed embodiment is counterclockwise when the injection device is viewed from a distal position. These ratchet arms 77 therefore prevent the piston rod 60 from rotating in a clockwise direction.

[0161] From this point on, the engagement between the piston rod 60 and the drive tube 70 prevents the piston rod 60 from rotating in a clockwise direction. Thus, when the nut member 11 is rotated in a clockwise direction, this rotation is transmitted to a translation of the piston rod 60 in the distal direction, since the piston rod 60 cannot follow the clockwise rotation of the nut member 11.

[0162] As the dose is expelled, the drive tube 70 and piston rod 60 rotate in a counterclockwise direction. Because the nut member 11 is prevented from rotating in the counterclockwise direction by the unidirectional ratchet interface surfaces 96, 97 between the nut member 11 and the housing part 10 (via the bridge structure 15), the nut member 11 does not rotate and therefore supports the helical motion of the piston rod 60 in the distal direction.

[0163] To eliminate the air gap between the piston rod 11 (or piston rod foot 85) and the plunger 7 inside the cartridge 5, the nut member 11 rotates relative to the housing structure in a clockwise direction which translates the piston rod 60 in a distal direction.

[0164] If the piston rod 11 (or the piston rod foot 85) abuts the plunger 7, the nut member 11 cannot be rotated further in the clockwise direction. However, in one embodiment, this is done by electronically measuring the position of the piston rod foot 85 before final assembly, so that it is in the correct position when assembled with the cartridge holder part 20.

[0165] The unidirectional interface surfaces 96, 97 between the nut member 11 and the housing part 10 prevent the nut member 11 from being rotated in a counterclockwise direction (when viewed from a distal position).

[0166] A result of the above is that the nut member 11 is self-locking with respect to the housing structure and there is no need to physically fasten the nut member 11 to the housing structure. There is no need to weld or glue the nut member 11 to the housing structure as described hereafter in the prior art.

[0167] By self-locking, it is meant in this specification that the nut member 11 cannot rotate in a clockwise direction because the piston rod 60 (or foot 85) abuts the plunger 7, and that the nut member 11 cannot rotate in a counterclockwise direction because of the unidirectional ratchet interface surfaces 96, 97.

[0168] When the piston rod 60 rotates in a counterclockwise direction to expel the prepared dose volume, the nut member 11 is unable to follow this rotation due to the similarly unidirectional ratchet interface surfaces 96, 97, and when the nut member 11 rotates in a clockwise direction to eliminate the air gap during assembly, the piston rod 60 is prevented from following this rotation by its interface with the drive tube 70 (62, 75) and the engagement between the drive tube 70 and the housing structure (77, 26).

[0169] If the pitch of the first threaded connection 61, 12 between the piston rod 60 and the nut member 11 is high, i.e. the piston rod 60 moves a long distance with each rotation, it is advisable to have a second threaded connection 95, 98 between a threaded protrusion 95 on the nut member 11 and a threaded flange 98 inside the housing part 10 so that the nut member 11 can be screwed helically in the proximal direction relative to the housing structure during rotation of the nut member 11.

[0170] As best seen in Fig. 17B, the projection 95 that constitutes the external threads on the nut member 11 is secured internally within the bridge structure 15 of the housing part 10 behind the threaded flange 98 for proximal movement when the nut member 11 rotates relative to the housing structure. Fig. 16 further depicts the threaded flange 98 as having an axial opening internally within the bridge structure 15 that allows the threaded projection 95 on the nut member 11 to proximally engage behind the threaded flange 98.

[0171] This means that when the nut member 11 rotates in a clockwise direction, it moves proximally while moving the piston rod 60 distally. Therefore, the pitch of the second threaded connection 95, 98 between the nut member 11 and the housing structure needs to be subtracted from the pitch of the first threaded connection 61, 12 between the piston rod 60 and the nut member 11 to find the effective zero point adjustment pitch.

[0172] The low effective pitch of the zero adjustment makes the process of eliminating the air gap easier to fine tune. It is therefore beneficial to have a second threaded connection 95, 98 between the nut member 11 and the housing part 10 when operating with a piston rod 60 with a high pitch, which is the case when a relatively large volume needs to be dispensed with each revolution of the piston rod 60.

[0173] If the pitch of the first threaded connection 61, 12 between the piston rod 60 and the nut member 11 is low, the second threaded connection 95, 98 is not considered necessary and therefore the nut member 11 only needs to rotate in one plane relative to the housing structure without the ability to move axially.

[0174] Alternative Zero Adjustment An alternative nut member for zero adjustment is disclosed in Figures 18A-19B. This alternative nut member is assigned the reference numeral 111, with the various elements added in this embodiment being preceded by a "1". The remaining components of this embodiment are numbered with the same numbers as used in the previous embodiment.

[0175] A nut element 111 is provided on an inner surface with an internal thread 112 and on an outer surface with a pair of resilient arms 113. In this embodiment, only two resilient arms 113 are disclosed, but any number of resilient arms 113 may be provided.

[0176] The axial opening in the bridge structure 15 of the housing structure guiding the nut element 111 is in this embodiment provided with at least one, preferably two, axially extending grooves 115 which guide the resilient arms 113 in a translational direction. Henceforth, the engagement between the grooves 115 and the flexible arms 113 ensures that the nut element 111 can only slide in a translational direction, i.e. without rotating relative to the housing structure.

[0177] The groove 115 is provided with a sloped bottom surface 116 (see FIG. 18B) that slopes radially outward in the distal direction, which has the effect that the resilient arms 113 are exposed to an increasing radial force when moved in the proximal direction, and are therefore urged distally with an increasing force as the nut element 111 is moved further proximally.

[0178] As can be seen, the flexible arms 113 are preferably radially inclined so that they follow an angle relative to the central axis that corresponds to the angle of the inclined bottom surface 116 .

[0179] To assemble the injection device, the nut element 111 and the piston rod 60 are first preassembled by rotating the nut element 111 and the piston rod 60 relative to each other such that the nut element 111 is screwed onto the piston rod 60. The preassembled piston rod 60 and nut element 111 are then placed inside an opening in the bridge structure 15 as disclosed in FIG. 18A.

[0180] When the injection device is fully assembled, the piston rod 60 may directly abut the plunger 7 inside the cartridge 5, or a piston rod foot 85 may be provided between the piston rod 60 and the plunger 5, such that there is abutment between the piston rod foot 85 and the plunger 7, as disclosed in FIG. 18C.

[0181] In one embodiment, the piston rod foot 85 can be connected to the piston rod 60 either simultaneously before the piston rod 60 is preassembled with the nut element 111, or after the piston rod 60 is preassembled with the nut element 111, as shown in FIG. 18B.

[0182] The piston rod foot 85, in one embodiment, may be click-fit to the piston rod 60 with a bearing-like connection such that the piston rod foot 85 can rotate relative to the piston rod 60. In another embodiment, the piston rod foot 85 is a separate or untethered element positioned between the piston rod 60 and the plunger 7. Alternatively, the piston rod foot 85 may be rotationally connected to the piston rod 60 so as to rotate with the piston rod 60.

[0183] In the latter embodiment, the piston rod foot 85 may house an electronic sensor capable of registering the number of rotations of the piston rod 60 relative to the cartridge 5, and hence the housing structure, to determine the volume expelled.

[0184] When the piston rod 60 and the nut element 111 are pre-assembled with or without the piston rod foot 85, the nut element 111 slides axially so that the flexible arm 113 engages with a groove 115 provided in the opening of the bridge section 15 of the housing structure.

[0185] In the final step of assembly, the cartridge 5 is placed inside the cartridge holder 20 and the cartridge holder 20 moves proximally together with the cartridge 5 so that the plunger 7 inside the cartridge 5 contacts the piston rod 60 (or foot 85) and the cartridge holder 20 clicks into the housing part 10 of the housing structure.

[0186] Thus, the plunger 7 inside the cartridge 5 abuts against the piston rod 60 (or foot 85), which forces the nut element 111 to slide translationally in the proximal direction. The resilience of the flexible arm 113 automatically urges the nut element 111, and thus the piston rod 60, in the distal direction when the flexible arm 113 presses against the inclined bottom surface 116 of the groove 99, thereby maintaining contact between the plunger 7 and the piston rod 60 (or foot 85). Whenever contact between the plunger 7 and the piston rod 60 (or foot 85) is obtained and maintained, a laser beam (indicated by "L" in FIG. 18C) is directed through an opening 14 in the housing structure onto the outer surface of the bridge structure 15. As disclosed in FIG. 18C, there can be two such openings 14 or more, if necessary.

[0187] The bridge structure 15 is preferably molded from a polymer that is more transparent to laser light than the polymer from which the flexible arms 113 of the nut element 111 are molded, so that the energy of the laser beam is converted into heat on the contact surface area between the bridge structure 15 and the flexible arms 113, i.e., the inner surface of the bridge structure 15.

[0188] The injection device according to this embodiment therefore has a nut element 111 that can operate between two different states: a first state in which the nut element 111 is axially movable and in which it is preferred that the nut element 111 moves slightly in the proximal direction due to impact with the plunger 7 in the cartridge 5. During this axial movement in the proximal direction, the elasticity of the nut element 111 causes it to bounce back in the distal direction, thereby maintaining physical contact with the plunger 7.

[0189] At the position where physical contact between the plunger 7 and the piston rod 60 (or foot 85) is achieved, the nut element 111 is welded or otherwise connected to the housing structure, which henceforth defines the second state of the nut element 111.

[0190] In this second state, the nut element 111 is axially fixed to the housing structure, after which the piston rod 60 moves helically when rotated relative to the nut element 111 and the housing structure.

[0191] The positioning of the pre-assembled nut element 111 and piston rod 60 can alternatively be determined electronically so that welding can take place before the cartridge is actually positioned.

[0192] Second Alternative Zero Adjustment 20 to 22B disclose an alternative solution in which a telescopic element 100 is arranged between the piston rod foot 85 and the piston rod 60.

[0193] An alternative piston rod 60 is disclosed in Figure 20 and is provided with two claws 105 on its distal side that can grip around the telescoping element 100. Although only two such claws 105 are disclosed, any number of claws 105 can be provided.

[0194] A piston rod 60 is provided with external threads 61 which engage internal threads 12 provided in the nut member 11, which in this alternative embodiment is preferably an integral part of the housing structure. The piston rod 60 is further provided with a longitudinal track structure 62 which is engaged by a drive element 70.

[0195] The telescopic element 100, shown in more detail in Fig. 21, is provided with a circular rib 101 on the distal side, behind which a click arm 86 provided on the piston rod foot 85 engages such that the telescopic element 100 can be axially fixed to the piston rod foot 85. This click-fit connection allows the piston rod foot 85 to rotate relative to the telescopic element 100.

[0196] The telescopic element 100 is further provided with several outwardly pointed surfaces 102 on which the claws 105 can abut, as disclosed in Figures 22A and 22B. Hereafter, the claws 105 are suitable to slide on these outwardly pointed surfaces 102 so that the telescopic element 100 can slide relative to the piston rod 60.

[0197] For this purpose, the telescopic element 100 is slidably arranged in the axial opening 106 of the piston rod 60 so that the piston rod foot 85, together with the telescopic element 100, can slide axially relative to the piston rod 60.

[0198] As disclosed in Figures 22A and 22B, when the piston rod foot 85 (connected to the telescopic element 100) slides into contact with the plunger 7 inside the cartridge 5, the telescopic element 100 is welded to the piston rod 60 by directing a laser beam "L" through the opening 14 in the housing structure and onto the claw 105 of the piston rod 60, thereby welding the claw 105 to the outwardly pointed surface 102 of the telescopic element 100.

[0199] The telescopic element 100 is preferably square shaped such that there are four outer surfaces 102, and the axial opening 106 is also preferably square shaped such that the telescopic element 100 can only translate relative to the piston rod 60 in the first state.

[0200] Although certain preferred embodiments have been set forth above, it is emphasized that the present invention is not limited thereto but may be embodied in other ways within the scope of the subject matter defined in the following claims.

Claims

1. 1. A pre-filled injection device for delivering a liquid medication, comprising: a housing structure in which a cartridge (5) having a movable plunger (7) is permanently secured; a piston rod (60) for advancing said movable plunger (7) during dispensing, said piston rod (60) having an external thread (61) and a longitudinal track formation (62); a rotatable drive element (70) that engages with the longitudinal track structure (63) of the piston rod (60) so that rotation of the drive element (70) is transferred to rotation of the piston rod (60); a nut element (11) having a first thread (12) that engages with the external thread (61) of the piston rod (60) so that the piston rod (60) moves helically when rotated relative to the nut element (11) during dispensing; the piston rod (60) is only permitted to rotate relative to the nut element (11) in a first rotational direction, the first permitted rotational direction being a direction that advances the piston rod (60) helically in a dispensing direction; The injection device is characterized in that the nut element (11) is rotatably connected to the housing structure by a connection portion having unidirectional ratchet interface surfaces (96, 97) such that the nut element (11) can be rotated only in a second rotational direction opposite to the first rotational direction.

2. 2. The injection device of claim 1, wherein the unidirectional ratchet interface (96, 97) rotatably connecting the nut element (11) to the housing structure comprises one or more flexible ratchet arms (96) that engage with a toothed surface (97).

3. 3. An injection device according to claim 2, wherein the nut element (11) comprises the one or more flexible ratchet arms (96).

4. An injection device according to claim 2 or 3, wherein the housing structure comprises the toothed surface (97).

5. An injection device according to any one of claims 1 to 4, wherein when the piston rod (60) is maintained in a non-rotatable state, rotation of the nut element (11) in the second rotational direction translates the piston rod (60) in the dispensing direction.

6. An injection device according to any one of the preceding claims, wherein the rotatable drive element (70) is rotatable only in the first rotational direction.

7. An injection device according to any one of the preceding claims, wherein the nut element (11) is provided on its outer surface with threads (95) which engage with threads (98) provided internally within the housing structure.

8. An injection device according to any one of claims 1 to 7, wherein the housing structure comprises at least a housing part (10) which internally supports a bridge structure (15) which is molded on an inner surface of the housing part (10).

9. 9. An injection device according to claim 8, wherein the bridge structure (15) is carried on an inner surface of the housing structure by a plurality of radial bearings (19).

10. An injection device according to claim 9 when dependent on claim 2, wherein the toothed surface (97) is provided on an inner surface of a circular opening in the bridge structure (15).

11. A method for eliminating the distance between a piston rod (60) and a plunger (7) when assembling a pre-filled injection device, comprising: A nut element (11) is connected to the housing structure of the injection device, A cartridge (5) having a plunger (7) is fixedly mounted within the housing structure; When the piston rod (60) rotates in a first rotational direction relative to the nut element (11), the piston rod (60) can be advanced helically in a discharge direction; the nut element (11) is rotationally coupled to the housing structure by unidirectional ratchet interface surfaces (96, 97) such that the nut element (11) can be rotated in a second rotational direction opposite to the first rotational direction; The method further comprising: iii) fixing the piston rod (60) against rotation in the second rotational direction; iv) rotating the nut element (11) relative to the housing structure in the second rotational direction and translating the piston rod (60) towards the plunger (7) opposite to the ejection direction.

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