fig1 a ) and 1 b ) are respective central axial cross - sectional views of an injection device 1 in accordance with a first embodiment of the invention wherein the depicted cross - sectional views are made at an angular separation of 90 degrees by rotation of the injection device 1 about a central longitudinal axis 3 . the injection device 1 is illustrated in a prepared or loaded state ready to deliver a dose of liquid drug to a user or patient by self - administration . the injection device 1 comprises a tubular housing 20 , a cartridge 85 holding a volume of liquid drug and an injection button 5 protruding axially from the tubular housing 5 . an injection needle ( not shown ) is attached to a distal portion of the cartridge 85 for subcutaneous injection of a predetermined dose of liquid drug in accordance with the user &# 39 ; s setting of a dose size . a toothed elongate piston rod 30 is rigidly attached or coupled to a movable piston 70 via a piston foot 65 . the movable piston 70 is arranged within an interior volume of the cartridge 85 . consequently , advancing the toothed piston rod 30 a predetermined distance axially in distal direction will cause a corresponding axial displacement of the movable piston 70 and cause a dose of the liquid drug to be expelled via the injection needle . a dose setting structure is responsive to mounting of a removable cap 80 to place the injection device 1 in the prepared or loaded state with a dose of liquid drug of a first size . the dose setting structure comprises an intermediate element in the form of a pusher 50 configured for engagement with the removable cap 80 and axially displaceable by mounting of the removable cap 80 . the pusher 50 is configured for engaging a sliding element 35 and axially displacing the sliding element 35 in proximal direction , i . e . towards the injection button 5 . the displacement of the sliding element 35 leads to a loading or preparation of the injection device 1 with a dose of liquid drug of the first size as explained in further detail below . the sliding element 35 comprises teeth arranged on an inner surface and configured to engaging mating teeth of the toothed elongate piston rod 30 . the mating teeth of the sliding element 35 and toothed elongate piston rod 30 are configured to solely allow unidirectional displacement of the sliding element 35 relative to the toothed elongate piston rod 30 or piston rod . only proximal displacement of the sliding element 35 relative to the piston rod 30 is allowed . consequently , the mating teeth of piston rod 30 and the sliding element 35 are brought into operative engagement when the latter moves in an opposite direction , i . e . a distal direction towards the second or distal position defined by an adjustable shelf 60 . the sliding element 35 is coupled to a helical compression spring 25 co - axially arranged around a tubular neck or portion of the sliding element 35 . the compression spring 25 is torsionally pre - tensioned and compressible by proximal displacement of the sliding element 35 during a loading sequence or operation of the injection device 1 . the loading sequence therefore causes potential energy or spring force to be stored in the compression spring 25 for release in connection with forward firing or advancement of the piston rod 30 and movable piston 70 during injection of the set dose of liquid drug . one end portion of the compression spring 25 engages the sliding element 35 and an opposing end portion engages a spring base 15 rigidly attached to the housing 20 . the injection device 1 furthermore comprises user operable dose adjustment structure comprising dose dial 55 and configured to , in the prepared or load state , increase or decrease the dose of the first size to set a dose of a second size . the dose dial 55 is configured to adjust a position , relative to the housing 20 , of an axially translatable distal clamping structure in form of the adjustable shelf 60 so as to vary the dose size in accordance with the user &# 39 ; s adjustment of the dose dial 55 as explained in further detail below . the user operable dose adjustment structure additionally comprises a clutch mechanism configured to decouple the dose setting structure from the injection structure in the prepared state . the clutch mechanism comprises the pusher 50 which is configured to selectively engage or disengage a toothed nut 40 operatively coupled to the piston rod 30 as as explained in further detail below in connection with fig2 b )- 2 f ). the injection button 5 is configured to noticeably project from the housing 1 in the prepared state of the injection device 1 as depicted on fig1 a ) and 1 b ) to indicate a current state of the injection device to the user or patient . by depression of the injection button 5 a firing sequence is initiated where the sliding element 35 is released from a a proximal clamping structure and the piston rod 30 advanced from a first or proximal position relative to the housing 1 to the second or distal position in an unprepared or unloaded state of the injection device 1 . an axial distance between the first and second positions accordingly corresponds to the delivery of the dose of the second size . the clutch mechanism comprises a toothed nut 40 , a nut spring 45 and a toothed inner peripheral surface of the pusher 50 . the clutch mechanism is configured to decouple the dose setting structure from the injection structure in the prepared state of the injection device so as to allow dose adjustment in the prepared state by actuation of the dose dial 55 without advancing the toothed piston rod 30 and the moveable piston 70 and spillage of liquid drug as explained in further detail below . fig2 a ) illustrates a first step of the loading and firing sequence where the injection device 1 depicted on fig1 is loaded or prepared . in connection with the first step , loading or preparation is initiated by the user by twisting the replaceable cap 80 onto the injection device 1 following a helical trajectory as indicated by arrow 72 . fig2 b ) illustrates the second step of the loading and firing sequence where the injection device 1 is loaded or prepared . the pusher 50 is the first portion of the dose setting structure to move in response to the mounting of the removable cap 80 . as previously explained , the piston rod 30 can only move in one direction , distally , relative to the housing 20 of the injection device . this effect is created by a pair of one way snaps mounted in the housing 20 and engaging the teeth on piston rod 30 . the pusher 50 is rotationally locked to the housing 20 . the pusher 50 is axially displaced by the twisting operation of the removable cap 80 , but the toothed nut 40 , which is rotatably mounted on the piston rod 30 , stands still due to an inner thread ( not shown ) that engages a mating thread on the piston rod 30 . the disengagement between the toothed nut 40 and the pusher 50 allows the toothed nut 40 to rotate as it is pushed proximally / upwards by the pusher 50 and the nut spring 45 . the toothed nut 40 will start to rotate due to the threaded non - locking interface with the mating teeth on the piston rod 30 . the teeth of the toothed nut 40 are arranged around a circumferential outer perimeter of the toothed nut 40 . the teeth of the pusher 50 , which are arranged on an inner tubular surface of the pusher 50 as explained above , are forced to disengage the mating teeth on the toothed nut 40 by this translation due to an axially directed biasing force supplied by the nut spring 45 . the toothed nut 40 can now rotate freely about the piston rod 30 . in effect , the dose setting structure has been decoupled from the injection structure because the piston rod 30 is no longer operatively coupled to the sliding element 35 . fig2 c ) illustrates the third step of the loading and firing sequence where the injection device 1 is undergoing loading or preparation . in this step , the helical compression spring 25 is compressed and loaded with axial spring force and a torque . the axial force is later on used to supply dose delivery force or energy during a user initiated firing or dose delivery sequence as explained below . the torque is obtained by torsionally pre - tensioning the helical compression spring 25 and using this torque to radially rotate the sliding element 35 into engagement with a proximal clamping structure at a first or proximal position of the piston rod 30 . the helical twisting of the removable cap 80 is configured to axially translate the pusher 50 and the sliding element 35 to a first position guided by an axial slot ( not shown ) in a tubular wall section of the housing 20 . at the first position , a circumferentially and essentially horizontally extending slot or channel 32 in the tubular wall section guides rotary movement of the sliding element 35 about a longitudinal housing axis 3 ( refer to fig1 a )). the combination of the axial slot and the circumferentially extending slot 32 forms an l - shaped slot in the housing 20 . the toothed nut 40 is free to rotate in the non self -. locking thread engaging the piston rod 30 when the pusher 50 and the sliding element 35 translate . fig2 d ) illustrates the fourth step of the loading and firing sequence where the injection device 1 is loaded or prepared . the sliding element 35 will rotate because of the freedom in the housing 20 and the torque generated by the pretensioned helical compression spring 25 . furthermore , the injection button 5 is rotated and axially translated , in response to the axial displacement and rotation of the sliding element 35 , from an unloaded or unprepared state indicated by its non - protruding placement inside the housing 20 of injection device to a loaded or prepared state indicated by the protruding placement depicted in fig2 d ). consequently , after completion of step 4 , the injection device is rendered in the prepared state with the removable cap 80 mounted on the injection device . the sliding element 35 rests in the circumferentially extending slot 32 in the housing 20 with the sliding element 35 decoupled from the pusher 50 . it is now possible to adjust the axial position of the adjustable shelf 60 which effectively defines the second position or end - step of the sliding element 35 . since the first position of the sliding element , as defined by the circumferentially extending slot 32 , remains fixed , the predetermined axial distance which the sliding element 35 and piston rod 30 travels during the dose delivery is varied . this leads in turn to the desired adjustment of the size of the initially set dose . fig2 e ) illustrates the fifth step of the loading and firing sequence where the injection device 1 is fired or unloaded . the sliding element 35 rests in the circumferentially extending slot 32 in the housing 20 when the removable cap 80 is removed by the user as explained above . furthermore , the pusher 50 is configured to translate a small distance axially and engage with the toothed nut 40 so as to rotationally lock to , or engage , the toothed nut 40 by virtue of the mating sets of teeth arranged on the pusher 50 and the toothed nut 40 as explained above . the engagement can be made in a manner where the interacting teeth make an incremental rotation of the toothed nut 40 to compensate for possible tolerances caused by small variations in the user &# 39 ; s mounting process of the replaceable cap 80 . this will improve the dose accuracy . when the injection button 5 is depressed as indicated by the arrow adjacent to the button 5 , the sliding element 35 will also be forced to rotate due to a helical spiralling movement of the injection button 5 under engagement with an end surface of the sliding element 35 . the rotary movement of the sliding element 35 is guided by the circumferentially extending slot 32 and continues until the sliding element 35 reaches the axial slot in the housing 20 . fig2 f ) illustrates the sixth step of the loading and firing sequence where the injection device 1 is fired or unloaded . when the sliding element 35 reaches the axial slot in the housing 20 , the sliding element 35 is translated axially in distal direction because of the axial force generated by the compressed helical compression spring 25 . the toothed nut 40 will translate axially in a corresponding manner because of the locked engagement with the pusher 50 . the toothed nut 40 , which is coupled to the piston rod 30 by the threaded interface , will subsequently advance the piston rod 30 and the movable piston 70 inside the cartridge 85 to make a dosing in accordance with the user selected dose size . the depicted end - of - dose or second position of the piston 70 , and corresponding end position of the piston rod 30 , is defined or set by the adjustable shelf 60 operating as the distal clamping structure or end - stop . the adjustable shelf 60 interrupts any further distal advancement of the piston 70 once the adjustable shelf 60 engages the toothed nut 40 . fig3 a ) and 3 b ) depict the user operable dose adjustment structure in respective cross - sectional views and illustrate functionality of the dose adjustment structure of the injection device 1 depicted on fig1 . fig3 a ) illustrates a current state of the injection device 1 after completion of step 4 above , i . e . the current state is the prepared state where the dose already has been set to a first size . this first size will correspond to a previously injected dose size . the user is now able to adjust the dose of the first size to set a dose of a second size in accordance with his / hers current condition by axially moving the adjustable shelf 60 either distally or proximally . the adjustable shelf 60 is translatable inside the housing 20 . the dose dial 55 is configured for rotation about the housing 20 but is unable to move axially or translate relative to the housing 20 . the dose dial 55 comprises an internal thread 62 which mates to a corresponding circumferential end structure of the adjustable shelf 60 as illustrated . the adjustable shelf 60 is accordingly forced to move axially in response to rotation of the dose dial 55 . since the sliding element 35 is rests safely on the proximal clamping structure ( the circumferentially extending slot 32 ), the axial position of the adjustable shelf 60 can be safely adjusted without inducing any corresponding displacement of the piston rod 30 and movable piston 70 . therefore , without causing any spillage of the liquid drug . on the other hand , the adjustment of the axial position of the adjustable shelf 60 leads to the desired dose size adjustment because any positional change alters the axial distance of travel of the piston rod 30 and piston movable piston 70 during the firing sequence or dose delivery . fig4 a ) is a central cross - sectional view of a user operable dose adjustment structure of an injection device 400 in accordance with a second embodiment of the invention . the injection device 400 has many features in common with the above - described first embodiment of the injection device . however , the dose adjustment structure of the first embodiment utilized axial movement of a distal clamping structure ( the adjustable shelf 60 ) to adjust the desired dose size in the prepared state of the injection device 1 . the present injection device 400 utilizes axial movement of a proximal clamping structure ( for example an adjustable shelf ) to adjust the dose size in the prepared state of the injection device 400 . a distal clamping structure or end - stop remains fixed . furthermore , the present injection device 400 utilizes a different type of clutch mechanism to decouple a dose setting structure from an injection structure in the prepared state of the injection device where the clutch mechanism is formed integrally with a toothed piston rod 430 and a sliding element 435 . fig4 a ) shows the injection device 400 in the prepared state ready to deliver a dose of liquid drug to a user or patient by self - administration when the user depresses an injection button ( not shown ) similar in structure to the one depicted on fig1 . the injection device 400 comprises a tubular housing 420 , a cartridge 485 holding a volume of liquid drug . the injection button ( not shown ) is protruding axially from a proximal end of the housing 420 . an injection needle ( not shown ) is attached to a distal portion of the cartridge 485 for subcutaneous injection of a predetermined dose of liquid drug in accordance with the user &# 39 ; s setting of a dose size . the toothed elongate piston rod 430 is rigidly attached to a movable piston 470 . the movable piston 470 is arranged within an interior volume of the cartridge 485 . consequently , advancing the piston rod 430 a predetermined distance axially in distal direction will cause a corresponding axial displacement of the piston 470 and cause the liquid drug to be expelled via the injection needle ( not shown ). a dose setting structure is responsive to mounting of a removable cap 480 to place the injection device 400 in the illustrated prepared or loaded state with a dose of liquid drug of a first size . the dose setting structure comprises a pusher 450 configured for engagement with the removable cap 480 and axially displaceable by mounting of the removable cap 480 . the pusher 450 is configured for engaging a sliding element 435 and axially displaces the sliding element 435 in proximal direction , i . e . the direction indicated by arrow 490 which is towards the injection button . the displacement of the sliding element 435 in proximal direction leads to the loading of the injection device 400 with a dose of liquid drug of a first size as explained in further detail below . the sliding element 435 comprises teeth engaging mating teeth of the piston rod 430 . the mating teeth of the sliding element and piston rod are configured to solely allow unidirectional displacement in the proximal direction of the sliding element 435 relative to the piston rod 430 . consequently , the piston rod 430 is advanced together with the sliding element 435 when the latter is advanced in the opposite direction , i . e . a distal direction towards a second or distal position defined by the fixed distal clamping structure or end - stop as explained below in connection with fig4 b ). the sliding element 435 is coupled to , or engages , a helical compression spring 425 co - axially arranged around a tubular portion or neck of the sliding element 435 . an opposite end of the helical compression spring 425 is operatively coupled to the housing 420 in similar manner to the first embodiment of the injection device . the injection device 400 furthermore comprises a user operable dose adjustment structure actuated by the dose dial 455 and configured to , in the prepared or load state , increase or decrease the dose of the first size to set a dose of a second size . the dose dial 455 is configured to adjust a position of an axially translatable proximal clamping structure in form of a proximal adjustable shelf 460 so as to vary the set dose size in accordance with the user &# 39 ; s adjustment of the dose dial 455 as explained in further detail below in connection with fig4 b ). fig4 b ) is a perspective view of the user operable dose adjustment structure of the injection device 400 in partial cross - section . as previously mentioned , the injection device 400 is placed in its prepared state where the sliding element 435 rests on the proximal adjustable shelf 460 and the helical compression spring 425 is axially compressed . the sliding element 435 comprises an axially extending finger 437 which rests on an upper plane surface 462 of the adjustable proximal shelf 460 so as to define a first or proximal position of the piston rod 430 . the piston rod 430 comprises a first axially extending segment of teeth 434 extending across a first predetermined circumferential surface of the toothed piston rod 430 . the teeth have a first radial height . another axially extending segment of teeth 432 is placed adjacently to the first axially extending segment of teeth 434 so as to occupy a second predetermined circumferential surface of the piston rod 430 . the teeth of the second segment 432 have a radial height which is smaller than the first radial height . a radially protruding tooth 439 of the sliding element 430 is configured for engagement with individual teeth of the first axially extending segment of teeth 434 . however , in the illustrated state the radially protruding tooth 439 is placed at the teeth of the second segment 432 which have a radial height sufficiently small to avoid engagement with the radially protruding tooth 439 of the sliding element 435 . consequently , the sliding element 435 is decoupled from the piston rod 430 and rendered axially translatable by movement or adjustment of the axial position of the upper plane surface 462 of the adjustable proximal shelf 460 . the axial position of the sliding element 435 can accordingly be adjusted without adjusting the axial position of the piston rod 430 and the moveable piston 470 so as to avoid drug spillage during dose size adjustment . during firing of the injection device 400 , the sliding element 435 is firstly rotated about the longitudinal housing axis 403 of the injection device which causes the axially extending finger 437 or finger to travel across the upper plane surface 462 of the adjustable proximal shelf 460 in rotary movement until the finger 437 reaches a slot or aperture 436 in the upper plane surface 462 . during this rotary movement of the sliding element 435 , the radially protruding tooth 439 is rotated as well until it is placed at the first axially extending segment of teeth 434 or first segment of teeth of the toothed piston rod 430 . due to the larger radial height of the teeth of the first segment of teeth 434 , the radially protruding tooth 439 of the sliding element is now brought into a locked engagement with the teeth of the first segment of teeth 434 . consequently , the sliding element 435 is now coupled to the piston rod 430 such that piston rod will translate axially together with the sliding element 435 in the distal direction towards the second position of the piston rod and piston . the skilled person will appreciate that an integrally formed clutch mechanism resides in the described cooperation between the sliding element 435 and the piston rod 430 . this integrally formed clutch mechanism operates by rotational engagement and disengagement of the mating teeth structures 432 , 434 , 439 formed in respective ones of the toothed piston rod 430 and the slider element 435 . once the finger 437 has reached the slot or aperture 436 in the adjustable upper shelf 460 , the spring force or energy stored in the axially compressed helical compression spring 425 will advance the sliding element 435 and the piston rod 430 ( now brought into engagement by the clutch mechanism ) in axial direction . the sliding element 435 and the piston rod 430 will advance together until the finger 437 contacts or engages a non - adjustable or fixed lower shelf 464 which blocks further axial advancement of the sliding element 435 and the piston rod 430 . the fixed lower shelf 464 therefore defines an end - of - dose or the second position of the piston 470 and corresponding second or distal position of the piston rod 430 after delivery of the set dose size . the depicted user operable dose adjustment structure of the injection device 400 allows the user to increase or decrease a dose of a first size to set a dose of a second size by axially moving the adjustable proximal shelf 460 either distally or proximally to respectively decrease or increase the dose size . the adjustable proximal shelf 460 is translatable inside the housing 420 . the tubular dose dial 455 is configured for rotation about the housing 420 but unable to move axially relative to the housing 420 . the dose dial 455 comprises an internal thread which mates to a corresponding circumferential end structure of the adjustable shelf 460 in a manner similar to the above - described dose dial 55 ( refer to fig3 b )) of the first embodiment . the adjustable shelf 460 is accordingly forced to move axially in response to rotation of the dose dial 455 . even though the sliding element 435 rests on the upper plane surface 462 of the adjustable shelf 460 as illustrated on fig4 b ), the sliding element 435 is decoupled from the piston rod 430 by the operation of the clutch mechanism as described above . therefore , the position of the adjustable shelf 460 can be adjusted axially without inducing any corresponding movement of the piston rod 430 and movable piston 470 . the position of the adjustable shelf 460 , and therefore the dose size , can accordingly be adjusted without spillage of liquid drug . furthermore , the adjustment of the axial position of the adjustable shelf 460 leads to the desired dose size adjustment because the positional change alters the axial distance of travel of the piston rod 430 and piston movable piston 470 . the loading or preparation of the injection device 400 is generally similar to the one for the first injection device 1 described above in connection with fig2 a )- d ) albeit with a different operation of the clutch mechanism . the helical compression spring 425 is torsionally pre - tensioned and compressible by proximal displacement of the sliding element 435 during the loading sequence of the injection device 400 . the torque obtained from the torsionally pre - tensioned helical compression spring 425 is used to rotate the the sliding element 435 once the adjustable proximal shelf 460 is reached and bring the finger 437 into engagement with the upper plane surface 462 in connection with the mounting of removable cap 480 by helical twisting . once the removable cap 480 has been mounted , the injection device 400 is automatically rendered in a prepared state with a dose of the first size where the finger 437 of the sliding element 435 rests safely on the upper plane surface 462 of the adjustable upper shelf 460 . fig5 a ) and 5 b ) are respective central axial cross - sectional views of an injection device 501 in accordance with a third embodiment of the invention wherein the depicted cross - sectional views are shown at an angular separation of 90 degrees by rotation of the injection device 501 about a central longitudinal axis 503 . the injection device 501 has many features in common with the above - described first embodiment of the injection device 1 on fig1 . the dose adjustment structure of the first embodiment utilized axial movement of a distal clamping structure ( the adjustable shelf 60 ) to adjust the desired dose size in the prepared state of the injection device 1 . in contrast , the dose adjustment structure of the present injection device 501 is configured to vary an axially extending geometry of a toothed sliding element 535 to adjust the dose size in the prepared state . in the present injection device 501 , the respective axial positions of a distal clamping structure and a proximal clamping structure remain fixed . furthermore , a user operable dose dial is integrated with an injection button of the injection device 501 as explained in further details below . the injection device 501 is illustrated in an unprepared or unloaded state after delivery of a dose of liquid drug to a user or patient by self - administration . the injection device 501 comprises a tubular housing 520 , a cartridge 585 holding a volume of liquid drug and an injection button 505 protruding axially from the housing 520 . an injection needle ( not shown ) is attached to a distal portion of the cartridge 585 for subcutaneous injection of a predetermined dose of liquid drug in accordance with the user &# 39 ; s setting of a dose size . a toothed elongate piston rod 530 is rigidly attached to a movable piston 570 via a piston foot 565 . the movable piston 570 is arranged within an interior volume of the cartridge 585 . consequently , advancing the toothed piston rod 530 a predetermined distance axially in distal direction will cause a corresponding axial displacement of the piston 570 and cause a dose of the liquid drug to be expelled via the injection needle . a dose setting structure is responsive to the mounting of a removable cap 580 to place the injection device 501 in a prepared or loaded state with a dose of liquid drug of a first size . the dose setting structure comprises a pusher 550 configured for engagement with the removable cap 580 and axially displaceable by mounting of the removable cap 580 . the pusher 550 is configured for engaging a sliding element 535 and axially displaces the sliding element 535 in proximal direction , i . e . towards the injection button 505 . the displacement of the sliding element 535 leads to a loading or preparation of the injection device 501 with a dose of liquid drug of the first size as explained in further detail below . the sliding element 535 comprises teeth engaging mating teeth of the toothed elongate piston rod 530 or piston rod . the mating teeth of the sliding element 35 and the piston rod 530 are configured to solely allow unidirectional displacement in proximal direction of the sliding element 535 relative to the toothed elongate piston rod 530 or piston rod . consequently , the piston rod 530 is advanced together with the sliding element 535 when the latter moves in an opposite direction , i . e . a distal direction towards the second or distal position defined by the fixed distal clamping structure formed as a cut - out or shelf in the housing 520 . the sliding element 535 is coupled to a helical compression spring 525 co - axially arranged around a tubular portion of the sliding element 535 . the compression spring 525 is torsionally pre - tensioned and compressible by proximal displacement of the sliding element 535 during a loading sequence of the injection device 501 . the loading sequences therefore causes potential energy or compression force to be stored in the helical compression spring 525 for release in connection with forward firing or advancement of the piston rod 530 and movable piston 570 during injection of the liquid drug . one end portion of the compression spring 525 engages the sliding element 535 and an opposing end portion engages a spring base 515 rigidly attached to the housing 520 . the injection device 501 furthermore comprises user operable dose adjustment structure or dose dial 555 configured to , in the prepared or load state , increase or decrease the dose of the first size to set a dose of a second size . the dose dial 555 is configured to adjust an axial position of an axially translatable finger ( refer to item 537 on fig7 a )- b ) so as to vary the dose size in accordance with the user &# 39 ; s adjustment of the dose dial 555 as explained in further detail below . a fixed position distal clamping structure 560 or distal shelf is formed in the housing 520 and defines an end - stop for advancement of the axially translatable finger of the sliding element 535 . the user operable dose adjustment structure additionally comprises a clutch mechanism configured to decouple the dose setting structure from the injection structure in the prepared state . the clutch mechanism comprises a pusher 550 configured to selectively engage or disengage a toothed nut 540 operatively coupled to the piston rod 530 as as explained in further detail below in connection with fig6 b )- 6 f ). the injection button 505 is configured to noticeably project from the housing 520 in the prepared state of the injection device 501 as depicted on fig6 d ) to indicate a current state of the injection device 501 to the user or patient . by depression of the injection button 505 in the prepared state , a firing sequence is initiated where the sliding element 535 is released from the proximal clamping structure and the piston rod 530 advanced from a first or proximal position relative to the housing 501 to the second or distal position in an unprepared or unloaded state of the injection device 501 . a predetermined axial distance between the first and second positions accordingly corresponds to the delivery of the dose of the second size . the clutch mechanism comprises a toothed nut 540 , a nut spring 545 and a toothed inner peripheral surface of the pusher 550 . the clutch mechanism is configured to decouple the dose setting structure from the injection structure in the prepared state of the injection device so as to allow dose adjustment in the prepared state by actuation of the dose dial 555 without advancing the toothed piston rod 530 and piston 570 and spillage of liquid drug as explained in further detail below . fig6 a ) illustrates a first step of a loading and firing sequence of the injection device 501 where the device is loaded or prepared . in connection with the first step , loading or preparation is initiated by the user by twisting the replaceable cap 580 onto the injection device 501 following a helical trajectory as indicated by arrow 672 . fig6 b ) illustrates a second step of a loading and firing sequence of the injection device 501 where the device is loaded or prepared . the pusher 550 is the first portion of the dose setting structure to move in response to mounting of the removable cap 580 . as previously explained , the piston rod 530 can only move axially in one direction , a distal direction , relative to the housing 520 of the injection device . this effect is created by a pair of one way snaps 552 mounted in the housing 520 and engaging the teeth on piston rod 530 . the pusher 550 is rotationally locked to the housing 520 . the pusher 550 is axially displaced by the twisting operation of the removable cap 580 , but the toothed nut 540 , which is rotatably mounted on the piston rod 530 , stands still due to an inner thread ( not shown ) that engages a mating thread on the piston rod 530 . the disengagement between the toothed nut 540 and the pusher 550 allows the toothed nut 540 to rotate as it is pushed proximally / upwards by the pusher 550 and the nut spring 545 . the toothed nut 540 will start to rotate about the piston rod 530 due to the threaded non - locking interface with the mating teeth on the piston rod 530 . the teeth of the toothed nut 540 are arranged around a circumferential outer perimeter of the toothed nut 540 . the teeth of the pusher 550 , which are arranged on an inner tubular surface of the pusher 550 as explained above , are forced to disengage the mating teeth on the toothed nut 540 by this translation due to an axially directed biasing force supplied by the nut spring 45 . the toothed nut 540 can now rotate freely about the piston rod 530 . in effect , the dose setting structure has been decoupled from the injection structure because the piston rod 530 is no longer operatively coupled to the sliding element 535 . fig6 c ) illustrates third step of the loading and firing sequence of the injection device 501 where the device is loaded or prepared . in this step , the helical compression spring 525 is compressed and loaded with axial force and a torque . the axial force is later on used to supply dose delivery force or energy during a user initiated firing or dose delivery sequence as explained below . the torque is obtained by torsionally pre - tensioning the helical compression spring 525 and using this torque to rotate the sliding element 535 about the axis of the housing 501 into engagement with a proximal clamping structure at a first or proximal position of the piston rod 530 . the helical twisting of the removable cap 580 is configured to axially translate the pusher 550 and the sliding element 535 to a first position guided by an axial slot 532 ( depicted on fig6 d )) in a annular wall section of the housing 520 . at the first position , the circumferentially extending slot or channel 532 in the tubular wall section guides rotary movement of the sliding element 535 about the longitudinal housing axis 503 . the combination of the axial slot and the circumferentially extending slot 532 forms an l - shaped slot in the housing 520 . the toothed nut 540 is free to rotate in the non - self locking thread engaging the piston rod 530 when the pusher 550 and the sliding element 535 translate . fig6 d ) illustrates the fourth step of the loading and firing sequence where the injection device 501 is loaded or prepared . the sliding element 535 will rotate because of the freedom in the housing and the torque generated by the helical compression spring 525 . furthermore , the injection button 505 is rotated and axially translated , in response to the axial displacement and rotation of the sliding element 535 , from an unloaded or unprepared state indicated by its non - protruding placement inside the housing 520 of injection device to a loaded or prepared state indicated by the protruding placement depicted in fig6 d ). consequently , after completion of step 4 , the injection device 501 is rendered in its prepared or loaded state with the removable cap mounted on the injection device 501 . the sliding element 535 rests in the circumferentially extending slot 532 in the housing 520 with the sliding element 535 decoupled from the pusher 550 . it is now possible to adjust an axial position of the axially translatable finger ( refer to item 537 on fig7 a )- b ) movably mounted in the sliding element 535 . the adjustment of the axial position of the finger leads to an adjustment of the size of the dose as explained in further detail below . the adjustment of the dose size is accomplished by actuating the dose dial 555 by the user or patient as explained in further detail below in connection with fig7 a )- b ). fig6 e ) illustrates the fifth step of the loading and firing sequence of the injection device 501 where the device 501 fired or unloaded . the sliding element 535 rests in the circumferentially extending slot 532 in the housing 520 when the removable cap 580 is removed by the user as explained above . when the injection button 505 is depressed as indicated by the arrow adjacent to the button 505 , a first movement of the injection button 505 will disengage mating teeth structures arranged on the injection button 505 and dose adjustment structure . the injection button 505 comprises a radially and inwardly projecting toothed annular structure coupled to mating teeth extending radially outwardly from a tubular proximal end section 539 of the sliding element 535 . after the first movement , the sliding element 535 is able to rotate freely . fig6 f ) illustrates the sixth step of the loading and firing sequence where the injection device 501 is fired or unloaded . in connection with the first movement of the injection button 505 , the pusher 550 will translate a small distance axially and engage with the toothed nut 450 so as to rotationally lock to , or engage , the toothed nut 540 by virtue of the mating sets of teeth arranged on the pusher 550 and the toothed nut 540 as explained above . when the injection button 505 is depressed further as indicated by the arrow adjacent to the button 505 , the sliding element 535 is also forced to rotate due to a helical spiralling movement of the injection button 505 under engagement with the end surface of the sliding element 535 . the rotary movement of the sliding element 535 is guided by the circumferentially extending slot 532 and continues until the sliding element 535 reaches the axial slot ( not shown ) in the housing 520 . fig6 g ) illustrates the sixth step of the loading and firing sequence where the injection device 501 is fired or unloaded . when the sliding element 535 reaches the axial slot in the housing 520 , the sliding element 535 is translated axially in distal direction because of the axial force generated by the compressed helical compression spring 525 . the toothed nut 540 will translate axially in a corresponding manner because of the locked engagement with the pusher 550 . the toothed nut 540 will subsequently advance the piston rod 530 axially and distally since these components are axially locked to each other . the advancement of the piston rod 530 will lead to a corresponding advancement of the movable piston 570 inside the cartridge 585 so as to make a dosing in accordance with the user selected dose size . the depicted end - of - dose or second position of the piston rod 530 is reached once the axially translatable finger ( not shown ) reaches the distal shelf or end - stop 560 engraved into the housing 520 as explained below in further detail in connection with fig7 a ). fig7 a ) is a central cross - sectional view of a user operable dose adjustment structure of the injection device 501 depicted on fig5 . as previously explained , the dose dial 555 is integrated with the injection button 505 . adjustment of an already set first dose size , i . e . set during the above - described loading steps of the injection device , is achieved by rotation of the dose dial 555 . as previously explained , a radially and inwardly projecting toothed annular structure of the injection button is coupled to the radially and outwardly projecting teeth arranged on the tubular proximal end section 539 of the sliding element 535 . rotation of the dose dial 555 causes axial translation or movement of the position of the finger 537 as indicated by the axially pointing arrows 538 on fig7 b ) due to a threaded interface 536 between a lower tubular portion of the sliding element 535 and the finger 537 in connection with the allocated space in the housing for axial movement of the latter as illustrated . axial movement of the finger 537 changes the axial distance ad between the finger 537 and the fixed position distal shelf 560 , which defines an end - stop for the finger 537 at engagement as explained above . the end - stop also functions as an end - stop for the residual part of the sliding element 535 and therefore defines a second or distal position of the piston rod 530 after dose delivery due to the interlocked engagement between the sliding element 535 and piston rod 530 during distal advancement in connection with the above - described firing or delivery sequence . in the illustrated situation , the axial distance travelled by the piston rod 530 from the first or proximal position in the prepared state to the second position in the unprepared state is ad and corresponds to the delivery of the set dose after a possible user adjustment of an initially set first dose size by manipulation of the dose dial 555 . therefore , adjustment of the axial position of the finger 537 will adjust the travel distance ad of the piston rod 530 in a corresponding manner and adjust the size of the delivered dose of liquid drug . fig7 c ) is a central cross - sectional view of an end - of - content feature of the injection device depicted on fig5 a )- b ) under normal operating conditions . under the normal operating conditions , the piston rod 530 is sequentially advanced in axial direction for each new dose delivery . as explained above , the sliding 535 element rotates about the central axis when it reaches the circumferentially extending slot or channel in the tubular wall section of the housing in connection with the loading sequence and the firing sequence . however , in the end of content mode depicted on fig7 d ), the sliding element 535 is prevented from further rotation . a projection 531 arranged in an end portion of the piston rod 530 engages a mating cut out in the finger 537 of the sliding element 535 and locks the piston rod 530 for rotation . if the removable cap is mounted on the injection device by the user in this end of content mode , the sliding element 535 will translate and seek to rotate when it is possible . however , the finger 537 forms part of the sliding element 535 and is rotationally locked thereto . if the injection device is in the end of content mode , the sliding element 535 is blocked for rotation because the finger 537 and the piston rod 530 are unable to rotate . since the injection button 505 is advanced to its projecting position , indicating a prepared or loaded state of the injection device , by rotation of the sliding element 535 , the injection button 505 will stay in the illustrated depressed state ( not protruding from the housing 520 ) and indicate to the user that the injection device has been emptied . while the above - described injection devices have been designed as disposable devices , the skilled person will understand that the each of the disclosed injection devices by suitable modifications could be provided with suitable means for cartridge replacement to provide a durable injection device .
the present invention relates to an injection device for administering doses of liquid drug . the injection device comprises a user operable dose adjustment structure configured to adjust , in a prepared state of the device , a dose of liquid drug of a first size to set a dose of a second size . the present injection device is particularly suitable for self - injection of liquid drugs such as insulin for treating diabetes by the user or patient .