A needle assembly for percutaneously infusing fluids to a body and/or withdrawing fluids from a body, comprising: a polymeric body having a passageway in a proximodistal direction; a wing assembly secured to the body, the wing assembly including first and second wings extending laterally in opposite directions from said body; and a hollow needle arranged in said passageway, such that it extends distally from said body; a shielding arm being attached to the polymeric body, such that it is pivotable from an open position to a closed position, wherein a tip of the needle is covered by the shielding arm in the closed position while the needle tip is uncovered in the open position; wherein the shielding arm is attached to the body via a hinge structure for urging said shielding arm into one of either said open position or said closed position; wherein said hinge structure has a dead-center position, such that said shielding arm is urged into said open position and said closed position, respectively, on corresponding sides of said dead-center position; and characterized in that the hinge structure is connected to said shielding arm via a mounting base, which in turn is connected to the body, said mounting base being ring-shaped and provided with an axial groove or ridge, which is configured to cooperate with a corresponding ridge or groove, respectively, on the body to prevent rotational movement of the mounting base; and wherein said dead-center position is a position in which the angle between the longitudinal extension of said needle and the longitudinal extension of said shielding arm is in the range of 45 to 135°, when said mounting base is attached to said body. a polymeric body having a passageway in a proximodistal direction; a wing assembly secured to the body, the wing assembly including first and second wings extending laterally in opposite directions from said body; and a hollow needle arranged in said passageway, such that it extends distally from said body; a shielding arm being attached to the polymeric body, such that it is pivotable from an open position to a closed position, wherein a tip of the needle is covered by the shielding arm in the closed position while the needle tip is uncovered in the open position; wherein the shielding arm is attached to the body via a hinge structure for urging said shielding arm into one of either said open position or said closed position; wherein said hinge structure has a dead-center position, such that said shielding arm is urged into said open position and said closed position, respectively, on corresponding sides of said dead-center position; and characterized in that the hinge structure is connected to said shielding arm via a mounting base, which in turn is connected to the body, said mounting base being ring-shaped and provided with an axial groove or ridge, which is configured to cooperate with a corresponding ridge or groove, respectively, on the body to prevent rotational movement of the mounting base; and wherein said dead-center position is a position in which the angle between the longitudinal extension of said needle and the longitudinal extension of said shielding arm is in the range of 45 to 135°, when said mounting base is attached to said body.