Self-retracting syringe needle

Syringe with a self-retracting needle, comprising a body (1) carrying at one end (2) injection needle support means (3), a piston (4) mobile in said body (1), a sheath (5) in which said body (1) can slide, a spring (6) disposed between the sheath (5) and the body (1), at least one flexible lug (7) cooperating abutment-fashion with a portion (18) of said body (1) to define a first position of immobilization of the body (1) relative to the sheath (5) in a first direction, the spring (6) being compressed and the needle (8) deployed from the sheath (5) in this first position, and immobilizing means for immobilizing the body (1) in the sheath (5) in a second immobilizing position, the spring (6) being at least partially relaxed and the needle (8) being inside the sheath (5) in this second position, the change from the first position to the second position being effected by cooperation of the piston (4) with said flexible lug (7) to release the lug (7) from said portion (18) of the body (1) substantially at the end of the travel of the piston (4) and thereby to cause the spring (6) to retract the body (1) into the sheath (5) until it is immobilized in the second position, characterized in that the immobilizing means comprise a circlip (9) mounted around said body (1) in a compressed configuration between the sleeve (5) and the body (1) in said first immobilizing position and in a relaxed configuration in said second immobilizing position, the sheath (5) having at one end a sleeve part (11), the circlip (9) having at least one part (9a) housed inside the sleeve part (11) in said second immobilizing position between said end of the sheath and at least one shoulder (12) of the sleeve part (11) adapted to cooperate abutment-fashion with the circlip (9) in said second immobilizing position to immobilize the body (1) in the sheath (5) in said first direction.