Injection syringe with a safeguard against reuse

Claim 1

An injection syringe comprising (a) a cylinder having a longitudinal axis extending between opposite ends of the cylinder, (b) a piston guided in the cylinder and dividing the cylinder into two cylinder chambers respectively facing the opposite cylinder ends, (c) a piston rod coupled to the piston and extending in one of the cylinder chambers and through one cylinder end, (d) a coupling arrangement at a cylinder end opposite to the one end for coupling an injection needle held by the coupling arrangement to the cylinder, the piston and the opposite cylinder end defining the other cylinder chamber, and (e) a safeguard against reuse of the injection syringe, the safeguard comprising (1) an inner piston connected to the piston rod for movement therewith and guided in a bore in the piston, the bore extending parallel to the longitudinal cylinder axis, (2) respective stops delimiting the piston bore towards the other cylinder chamber and the piston rod whereby the inner piston is coupled to the piston, the stops projecting inwardly into the path of the movement of the inner piston in the bore for engagement with the inner piston, (3) a frangible sealing element at an end of the bore remote from the piston rod for sealing the bore from the other cylinder chamber, (4) a piercing spike projecting from an end of the inner piston facing the sealing element for opening the frangible sealing element upon movement of the inner piston towards the remote bore end, and (5) a venting aperture in the piston spaced from the remote bore end and connecting the piston bore with the ambient atmosphere, the venting aperture being positioned between the inner piston and the sealing element when the inner piston engages the stops delimiting the piston bore towards the piston rod.