fig1 and 2 show an auto - injector 1 in an initial state prior to an injection . fig2 a and 2 b are two longitudinal sections in different section planes of the auto - injector 1 , the different section planes approximately 90 ° rotated to each other . the auto - injector comprises an elongate outer casing 2 . a syringe 3 with a hollow needle 4 is arranged in a proximal part of the auto - injector 1 . when the auto - injector 1 is assembled a protective needle shield 5 is attached to the needle 4 and protruding through an orifice 6 at the proximal end p . a finger guard 7 in the shape of a sheet metal spring is arranged near the protective needle shield 5 . the finger guard 7 is shown in detail in fig4 and 5 . the finger guard 7 comprises two spring arms 8 ( cf . fig4 and 5 ) which are inwardly biased so they bear against the protective needle shield 5 as long as it is still in place . a respective locking arm 9 is assigned to each spring arm 8 . the locking arms 9 are biased in distal direction d so they bear against a part of the spring arms 8 when the protective needle shield 5 is in place . as the protective needle shield 5 is pulled away from the needle 4 ( see fig5 ) the spring arms 8 move inwards and relax leaving a small gap between them just wide enough to let the needle 4 pass without touching it . this allows the locking arms 9 to come clear of the spring arms 8 and move distally into a position where they prevent the spring arms 8 from being pushed outward again so despite the rather big orifice 6 the user cannot touch the tip of the needle 4 . the tips of the spring arms 8 where the spring arms 8 bear against the protective needle shield 5 are rounded off in order to facilitate removal of the protective needle shield 5 . in alternative embodiments the spring arms 8 and / or the locking arms 9 may be made of or comprise spring wire and / or plastic instead of sheet metal . the spring arms 8 and locking arms 9 may be integrally formed as illustrated . they may also be separate parts , e . g . attached to inner walls of the proximal part of the auto - injector 1 . referring now to fig4 and 5 , the spring arms 8 are essentially s - shaped with a longitudinal leg 8 . 1 in the middle and two transversal legs 8 . 2 , 8 . 3 adjoining the longitudinal leg 8 . 1 . when the spring arm 8 is relaxed , the transversal legs 8 . 2 , 8 . 3 are essentially parallel to each other . an outer transversal leg 8 . 2 of each spring arm 8 adjoins a wall portion 7 . 1 of the sheet metal spring 7 . the other , inner transversal 8 . 3 leg of each spring arm 8 is intended to bear against the protective needle shield 5 . when the protective needle shield 5 is removed , a small gap is defined between the two inner transversal legs 8 . 3 of the spring arms 8 . the locking arm 9 is a short arm with an outer end 9 . 1 adjoining a front portion 7 . 2 of the sheet metal spring 7 and with an inner end 9 . 2 bearing against the inner transversal leg 8 . 3 in distal direction d when the protective needle shield 5 is in place . when the protective needle shield 5 is removed the spring arms 8 move together and the locking arms 9 come clear of the inner transversal leg 8 . 3 when the joint between the inner transversal leg 8 . 3 and the longitudinal leg 8 . 1 passes the inner end 9 . 2 . the inner end 9 . 2 locks behind the longitudinal leg 8 . 1 thus preventing the spring arm 8 from being pushed outward again . the tips of the spring arms &# 39 ; 8 inner transversal legs 8 . 3 where the spring arms 8 bear against the protective needle shield 5 are rounded off in order to facilitate removal of the protective needle shield 5 . at the distal end d of the auto - injector 1 a trigger button 10 for releasing a torsion spring 11 is arranged . the torsion spring 11 is arranged inside the outer casing 2 and grounded with its distal end 11 . 1 in a ring - shaped locking slider 12 arranged in the outer casing 2 near the distal end d of the auto - injector 1 . the proximal end 11 . 2 of the torsion spring 11 is grounded in a follower tube 13 arranged inside the torsion spring 11 and rotatable with respect to the outer casing 2 . in the initial state the locking slider 12 is in a splined engagement with the follower tube 13 preventing rotation of the follower tube 13 relative to the locking slider 12 and hence preventing release of the torsion spring 11 ( see fig3 ). the locking slider 12 is arranged to be translated in the proximal direction p by the trigger button 10 for disengaging its splined engagement to the follower tube 13 but is splined into the outer case 2 to statically resolve any torque from the torsion spring 11 . in the initial state , full depression of the trigger button 10 is prevented by a skin interlock mechanism described below . if the trigger button 10 is depressed , a beam element 10 . 1 on the trigger button 10 is forced to deflect inwards through ramped interference with a first rib 34 in the outer case 2 . when deflected , the beam element 10 . 1 is located such that it interferes with a first shoulder 13 . 2 on the distal end of the follower tube 13 preventing further depression of the trigger button 10 and thus initiation of the auto - injector 1 ( see fig6 ). the skin interlock is arranged to change the position of the follower tube 13 such that the first shoulder 13 . 2 is located distally from the beam element 10 . 1 so the beam element 10 . 1 no longer interferes with the follower tube 13 . hence the trigger button 10 can be fully depressed for starting an injection cycle . ( see fig9 ) the follower tube 13 is telescoped with a lead screw tube 16 . the lead screw tube 16 is supported and guided in a retraction slider tube 17 arranged in the proximal part of the outer casing 2 in a manner to prevent the lead screw tube 16 from rotating while allowing it to be moved axially in proximal direction p . the retraction slider tube 17 in turn is engaged with the outer casing 2 by flats 18 ( cf . fig2 and latches 19 in a manner to prevent both rotation and translation with respect to the outer casing 2 at least in the initial situation shown in fig1 and 2 . it will be shown in the following how the retraction slider tube 17 is disengaged from the latches 19 for being axially moved . the retraction slider tube 17 and the follower tube 13 are provided with respective second and third shoulders 17 . 1 , 13 . 1 held together by a coupling ring 20 for allowing relative rotation but preventing them from being independently axially moved . the lead screw tube 16 has an external lead screw which is engaged with the follower tube 13 by one or more ball bearings 21 . rotation of the follower tube 13 therefore results in translative movement of the lead screw tube 16 . in the initial situation shown in fig1 and 2 the retraction slider tube 17 cannot rotate but move axially in the distal direction d , the follower tube 13 is prevented from rotating by the spline engagement with the locking slider 12 and the lead screw tube 16 is prevented from rotation by its engagement with refraction slider tube 17 . a number of skin contact elements 17 . 2 arranged proximally on the retraction slider tube 17 protrude through recesses in the proximal end of the outer case 2 . a sequence of operation of the auto - injector 1 is as follows : the user removes the protective needle shield 5 from the needle 4 . for this purpose a device cap ( not shown ) may be attached to the protective needle shield 5 . when the protective needle shield 5 is removed the finger guard 7 locks into place to protect the user from accidental needle - stick injuries . when ready to do so , the user pushes the auto - injector 1 against the injection site . the user presses the proximal end p of the auto - injector 1 against the injection site . this causes the skin contact elements 17 . 2 of the retraction slider tube 17 to be depressed inside the outer casing 2 ( see fig7 ). the follower tube 13 is axially fixed to the retraction slider tube 17 through the coupling ring 20 and thus the whole assembly of the refraction slider tube 17 and the follower tube 13 translate within the outer casing 2 in the distal direction d with depression of the skin contact element 17 . 2 . this motion is opposed by a button spring 35 . once translated , the first shoulder 13 . 2 on the follower tube 13 no longer interferes with the beam element 10 . 1 on the trigger button 10 . the beam element 10 . 1 may deflect inwards proximally behind the first shoulder 13 . 2 . hence , the trigger button 10 can now be fully depressed . the button spring 35 may be arranged as a metal compression spring as illustrated , but it could equally be embodied as an integrally moulded flexible beam feature on either the trigger button 10 or the locking slider 12 . when the trigger button 10 is depressed it comes into contact with the locking slider 12 translating it in proximal direction p when fully depressed . with axial movement of the locking slider 12 its splined coupling with the follower tube 13 is disengaged so load from the proximal end of the torsion spring 11 is no longer statically resolved . the torque from the torsion spring 11 is released causing the follower tube 13 to rotate and drive the lead screw tube 16 forward . when the trigger button 10 is fully depressed the resilient beam element 10 . 1 flexes outward again behind the first rib 34 thus locking the trigger button 10 in this depressed position . ( see fig1 ). this could likewise be achieved by a separate locking feature . the rotation of the follower tube 13 causes translative movement of the lead screw tube 16 in proximal direction p . inside the lead screw tube 16 a two part plunger with a plunger rear 22 and a plunger front 23 is arranged , the plunger rear 22 telescoped into the hollow plunger front 23 . in the plunger front 23 a plunger spring 24 in the shape of a compression spring is arranged which bears against the plunger rear 22 when the plunger rear 22 pushed in proximal direction p . the plunger front 23 in turn pushes against a stopper 25 arranged for sealing the syringe 3 distally and for displacing a liquid medicament m through the hollow needle 4 . the syringe 3 is held in a tubular syringe carrier 26 and supported at its proximal end therein . the plunger rear 22 is coupled for joined axial movement to the lead screw tube 16 by a plunger ball 27 arranged in a recess in the lead screw tube 16 and guided in a circumferential notch 28 of the plunger rear 22 . in the initial position shown in fig1 and 2 , the plunger ball 27 is held in position by the follower tube 13 in order to keep the plunger rear 22 and lead screw tube 16 from disengaging . consequently , when the lead screw tube 16 is advanced in proximal direction p the syringe 3 is driven forward by the plunger pushing on the stopper 25 . the external lead screw of the lead screw tube 16 has a variable pitch . in the embodiment shown in the figures the pitch is steeper in the proximal part of the external lead screw . this allows for a rapid insertion of the hollow needle 4 into the patient &# 39 ; s skin in order to avoid unnecessary pain for the patient . the load required to insert a siliconized fine gauge needle is thought to be in the region of 5 n , which is relatively low so a steep screw pitch can be used with little risk of the screw engagement locking fig1 shows the auto - injector 1 with the hollow needle 4 fully advanced . in case the screw engagement between the follower tube 13 and the lead screw tube 16 comprises more than one ball bearing 21 each ball 21 may be engaged with a respective screw thread so the lead screw tube 16 would have a multi - start thread . in fig1 the syringe carrier 26 has bottomed out at the proximal end p of the outer casing 2 thus defining an injection depth , e . g . for a subcutaneous injection . as the torsion spring 11 continues rotating the lead screw tube 16 , and plunger rear 22 are further forwarded . due to friction effective between the stopper 25 and the inner wall of the syringe 3 and due to the thin fluid channel inside the hollow needle 4 opposing the displacement of the medicament m the stopper 25 exerts a load against the forward movement of the plunger front 23 . thus , the plunger spring 24 is slightly compressed ( see fig1 ). the thrust load is reacted through the coupling ring 20 into the retraction slider tube 17 which is coupled to the outer casing 2 by the latches 19 . thus the follower tube 13 is kept from moving further in distal direction d . with continued forward movement of the plunger the stopper 25 is advanced and injects the medicament m from the syringe 3 into the injection site ( see fig1 ). during injection of the dose of medicament m the pitch of the lead screw is slightly reduced compared to the needle insertion in order to give a greater mechanical advantage to the lead screw engagement and avoid it stalling due to the increased load . in fig1 the auto - injector 1 is shown towards the end of the dose , i . e . just before the stopper 25 bottoms out in the syringe 3 . in this situation viscous dampers 29 contained in pockets in the proximal end of the lead screw tube 16 contact small second ribs 30 in the proximal end p of the outer casing 2 . thus load from the torsion spring 11 is shared between the stopper 25 and the contact between the second ribs 30 and the viscous dampers 29 , so the plunger spring 24 is allowed to extend and complete the dose by fully advancing the stopper 25 . this allows for fully emptying the syringe 3 before starting to retract the needle 4 . the viscous damper 29 has a speed dependent load characteristic . in this instance the load from the torsion spring 11 is almost constant over the small axial travel of the viscous damper 29 so the speed can be tuned so that the plunger spring 24 has enough time to fully expel the residual contents of the syringe 3 . the material of the viscous damper 29 may be viscoelastic foam or a fluid forced through a small orifice . a change in the lead screw pitch at this point allows a controlled increase in the mechanical advantage to apply sufficient force to the mechanism . in fig1 the stopper 25 has bottomed out in the syringe 3 and the lead screw tube 16 reaches the end of travel . the plunger ball 27 disengages the plunger rear 22 from the lead screw tube 16 by dropping out of its recess into a pocket 31 in the follower tube 13 . just after this the latches 19 are released by ramp features 32 of the lead screw tube 16 pushing them outward so the retraction slider tube 17 and the follower tube 13 are released from the outer casing 2 for translation . since the lead screw tube 16 has bottomed out at the proximal end p of the outer casing continued rotation of the torsion spring results in a backward movement of the retraction slider tube 17 and the follower tube 13 which is still rotating . the retraction slider tube 17 takes along the syringe carrier 26 and syringe 3 with the needle 4 and retracts them into the auto - injector 1 until the needle 4 is fully covered ( see fig1 ). for this purpose the retraction slider tube 17 has one or more dog features 33 ( see fig2 ) extending inwardly through recesses in the lead screw tube 16 and engaging the syringe carrier 26 . the auto - injector 1 may preferably be used for subcutaneous or intra - muscular injection , particularly for delivering one of an analgetic , an anticoagulant , insulin , an insulin derivate , heparin , lovenox , a vaccine , a growth hormone , a peptide hormone , a protein , antibodies and complex carbohydrates .
the invention refers to an auto - injector for administering a dose of a liquid medicament , comprising : an outer casing , a syringe with a hollow needle and a stopper a torsion spring and a gear arrangement for inserting the needle , delivering the medicament , and retracting the syringe after injection . at least one skin contact trigger element is arranged at a proximal end of the outer case , the skin contact trigger element translatable in longitudinal direction between a proximal position and a distal position and biased in proximal direction in a manner to protrude from the outer casing in the proximal position , wherein in its proximal position the skin contact trigger element is arranged to prevent the spring means from being released and wherein the skin contact trigger element in its distal position is arranged to allow release of the spring means .