Pre-filled safety syringe having a retractable needle

摘要

The syringe of the present invention is ideally supplied to the user in a pre-filled state, with the piston stem protruding from the syringe body and with the cannula covered with a protective cover. Once the disposable safety syringe of the present invention is unwrapped, it may be used for performing an injection in the usual way. As the last bit of fluid is injected through the cannula, the piston engages the proximal portion of the needle axial translating structure. The piston stem is then withdrawn back through the syringe body, pulling the cannula inside the syringe body. The piston is prevented from exiting the syringe body because of the presence of a flange at the proximal end of the syringe. The piston stem is then quickly removable from the piston by disengaging the large-pitch thread with which it was attached to the piston, which eliminates the possibility of inadvertent proximal motion of the piston.