Auto-injector

An auto-injector for delivering a liquid medicament, comprising an elongate housing arranged to contain a syringe with a hollow injection needle and a stopper for sealing the syringe and displacing the medicament, the housing having a distal end and a proximal end with an orifice intended to be applied against an injection site, wherein the syringe is slidably arranged with respect to the housing,

• spring means capable of, upon activation:
• pushing the needle from a covered position inside the housing into an advanced position through the orifice and past the proximal end for insertion into an injection site,
• operating the syringe to inject a dose of medicament, and
• retracting the syringe with the needle into the covered position after at least partially delivering the medicament,
• activating means arranged to lock the spring means in a pressurized state prior to manual operation and capable of, upon manual operation, releasing the spring means for injection,
• wherein the spring means is a compression spring grounded distally in the housing and proximally bearing against a thrust tube arranged to transmit load from the spring means via a plunger to the syringe and/or the stopper,
• wherein a tubular syringe carrier is arranged for holding the syringe and supporting it at a proximal end, the syringe and the syringe carrier arranged for joint axial translation,
• wherein the thrust tube is arranged to be rotationally constrained relative to the housing, wherein the thrust tube and the plunger exhibit corresponding first and second threads having a first direction arranged to be engaged at least in an initial state,
• wherein the plunger is rotationally constrained in the initial state, during needle insertion and during injection, wherein the plunger is arranged to be rotationally released upon removal of the auto-injector from the injection site resulting in rotation of the plunger and consequently disengagement of the corresponding first and second threads,
• wherein the thrust tube is arranged to consequently translate further under load of the compression spring in such a manner that the second thread engages a corresponding third thread of a retract collar which is engaged to the syringe carrier by corresponding fourth and fifth threads having an opposite direction of the first direction so that continued translation of the thrust tube in proximal direction results in rotation of the retract collar and translation of the syringe carrier in distal direction for needle retraction.