fig1 shows a housing 1 of a drug delivery device for receiving a cartridge holder ( fig2 ) containing a medicinal product for example a medicament such as insulin . the housing 1 is of sleeve - like shape . at the distal end 2 of the housing 1 a section 3 is formed as an opening to receive a cartridge holder and to be locked therewith . on the inner surface of the receiving section 3 of the housing 1 , a set of locking elements 4 is disposed projecting radially inwardly from the inner surface of the inner wall of the housing 1 and being basically of rectangular shape with a tapered or chamfered edge facing towards the distal end and the cartridge holder . in the inner wall , there is also formed a set of spline grooves 5 extending in longitudinal direction from the distal end 2 of the housing 1 in the opposite direction toward a proximal end . the spline grooves 5 are arranged such as to establish a keyed connection with corresponding spline elements on the cartridge holder . the same applies to the locking elements 4 that are adapted to be received in respective receptacles in the cartridge holder . fig2 shows the cartridge holder 6 adapted to be connected with the housing 1 shown in fig1 . the cartridge holder 6 has a proximal end 7 and a distal end 8 . the proximal end 7 is adapted to be connected to the receiving section 3 of the housing 1 shown in fig1 . the cartridge holder 6 has an elongated tube - like shape , i . e . it is substantially designed as a cylindrical body for receiving a cartridge or an ampoule ( not shown ) and having a longitudinal axis 9 . at the distal end 8 of the cartridge holder 6 , there is a section onto which a needle arrangement can be mounted . for this purpose , the section is provided with a helical thread 10 arranged on the outside of the distal end section 8 . other suitable attachment means could be used instead of or in addition to a thread . on a substantially cylindrical section of the cartridge holder 6 , a continuous opening 11 extending an axial direction is formed into the cartridge holder . a similar opening is provided on the radially opposite side of the cartridge holder 6 , which can be best seen in fig3 . the cartridge can be inserted into the cartridge holder 6 through its proximal end . for the patient &# 39 ; s convenience and in regard to safety requirements , the filling level of the cartridge and other important information can be visually checked through the opening 11 . in addition or as an alternative to the openings 11 , the cartridge holder 6 may be made at least partly of a transparent or translucent material . the cartridge holder 6 and the housing 1 can be connected by snap fastening connection means that fasten the cartridge holder 6 to the housing 1 . for this purpose , on its proximal end 7 , the cartridge holder 6 is provided with an insertion section 12 . the insertion section 12 is separated from the cylindrical section with the openings 11 by a collar 13 , the collar being formed on the outer circumference of the holder 6 and extending radially outwardly . the collar 13 is adapted to abut the distal end of housing 1 when the insertion section is inserted into the receiving section 3 of housing 1 . on the outer surface of the insertion section 12 , four longitudinal spline elements or ribs 14 extending in axial direction from the proximal end 7 toward the collar 13 are disposed . the longitudinal ribs 14 are adapted to engage the spline grooves 5 formed in the receiving section 3 of housing 1 to establish a keyed connection in radial direction to prevent relative rotational movements between the housing 1 and the cartridge holder 6 but to allow relative axial movement e . g . for the assembly process . the insertion section 12 also has a number of irregularly spaced windows 15 close to the proximal end 7 . as the opening 11 , the windows 15 are continuous openings through which the inner section of the cartridge holder 6 can communicate with the outside . an enlargement of section 12 is shown in a side view in fig4 revealing the specific geometry of the windows 15 . the window 15 is basically of a rectangular shape as indicated by a broken line 16 . the basic shape of the window 15 conforms in its dimensions to the locking elements 4 which means that the locking elements 4 that are basically of an rectangular shape as well conform to the size of the rectangular shape in their dimensions . given the basic rectangular shape 16 of the window 15 fig4 shows that each of the four corners of the window 15 is provided with a round contour diverting from the basic rectangular shape 16 of the window 15 . the round contour is formed into the corners in a notch - like manner , which means the round contour of the edge is set back a distance from respective edge of the basic rectangle . accordingly , on one side of the window 15 in circumferential direction , there are first notches 17 a , 17 b formed into the window 15 . opposite the first notches 17 a , 17 b , there are second notches 18 a , 18 b formed into the window 15 . a distal section of the first notch 17 a is an extension 19 a of a distal side 20 a of the rectangle 16 of the window 15 . one proximal section of the first notch 17 b is an extension 19 b of an opposite proximal side 20 b of the rectangle 16 of the window 15 . one section of each of the second notches 18 a , 18 b is formed as an extension 19 c of a third side 20 c of the window 15 . the first notches 17 a , 17 b are substantially arranged in a parallel relationship so as to form an intermediate section in - between said section constituting a resilient lug 21 . in order to connect and to fasten the cartridge holder 6 to the housing 1 , the cartridge holder 6 is inserted with its insertion section 12 into the receiving section 3 of the housing 1 . the longitudinal splines 14 engage with the spline grooves 5 of the receiving section 3 and the locking elements 4 are received in the window 15 . in the window 15 , the lug 21 exerts a force toward the locking element 4 with the tendency to bias the locking element 4 toward the third side 20 c of the window 15 thereby securing a firm seat of the locking element 4 in the window 15 . in case , an inexperienced user now tries to pull the cartridge holder 6 out of the housing 1 or accidentally buckles the cartridge holder relative to the housing 1 , the connection between the cartridge holder and the housing is loaded with mechanical forces . due to the notches 17 a , 17 b , 18 a , 18 b and the way the locking element is received in the window , local stress peaks in the corners of the window 15 are avoided .
a cartridge holder for a drug delivery device is presented having snap fastening connection means to fasten the cartridge holder to a housing of the drug delivery device , wherein the fastening connection means are provided with at least on basically rectangular window for receiving a locking element of said housing . the invention is further directed to a pen - type injector equipped with a respective snap fastening connection for the cartridge holder .