1. Field of the Invention
The invention relates to a device for the injection of implants, preferably implants in the form of small rods, (called rods), which are used for the long-term treatment of diseases.
2. Description of the Related Art
The administration of such implants has recently acquired importance because there has been the development of plastics which undergo controlled degradation in vivo, and thus release, over a prolong period the active substance which is entrapped in, for example, a matrix or is chemically bonded thereto (active substance depot forms).
It is no longer possible for modern drug therapy to dispense with these delivery forms which combine a controlled rate of release of the active substances with high biocompatability of the depot. Such long-lasting controlled release of active substance is of great current interest because of the increasing importance of chronic diseases and therapeutic policies oriented to the long-term in human and veterinary medicine.
Devices for the administraction of active substance depot implants, which are mostly designed as small cylindrical rods, have already been disclosed. Most of them comprise an active substance container in which the rod is located, and an injection cannula attached thereto and a plunger to introduce the active substance composition into the subcutaneous tissue by being advanced.
German Offenlegungsschrift No. 3,419,876 decribes a mechanically relatively elaborate device for the subscutaneous administration of depot bodies, in which the depot body is located immediately before the pointed cannula orifice. On administration, the cannula is introduced into the subcutaneous tissue. A member which can be displaced is then used to withdraw the needle from the tissue, while a fixed pluger rod, which is in contact with the depot body, ensures that only the cannula is retracted concentrically around depot body and plunger rod, and the depot body remains in the tissue. The depot body is therefore not pushed into the subcutaneous tissue but is "exposed" in the tissue by the hollow cylindrical cannula being withdrawn over it.
The product information on (.RTM.) ZOLADEX Depot (ICI) discloses a device for the administration of implants, in which the active substance container contains a transparent central part (window) so that it is possible to see whether a rod is present in the device or not.
However, the injector described by ICI has the following technical disadvantages when used:
(a) Although the entire administration device is packed sterile in a blister pack, once this has been opened the device must be used as rapidly as possible, otherwise sterility is no longer guaranteed. This supposes that the cannula is still provided with a protective cap, but the plunger rod does not remain sterile in the "unpacked" state, which can be a disadvantage if the plunger rod is moved backwards and forwards several times, in which case the sterility of the active substance cylinder can no longer be guaranteed.
(b) There is the risk that during introduction of the cannula into the subcutaneous tissue there will be premature and unintentional actuation of the plunger rod.
These disadvantages are avoided by the design modifications, which are detailed hereinafter, compared with the ICI injector described above.