The invention relates to a drive mechanism for a medical injection device in which a piston rod (10) has a threaded outer surface and a not circular cross-section and engages a nut assembly (2, 102, 202) having either an internal thread (41, 141, 241) mating the outer thread (11) of the piston rod (10), or a shape mating the non-circular cross section of the piston rod (10). The nut assembly (2) comprises a first part (30, 130, 230) and a second part (40, 140, 240), which are connected by a hinge allowing the first part (30, 130, 230) and the second part (40, 140, 240) to tilt relatively to each other in a non-axial plane.
