the nut element threaded bore a ring formed piston rod drive element 230 rotationally arranged in the housing and a ring formed clutch element 240 which is in rotational engagement with the drive element see below the engagement allowing axial movement of the clutch element the clutch element is provided with outer spline elements 241 adapted to engage corresponding splines 204 see fig4 b on the housing inner surface this allowing the clutch element to be moved between a rotationally locked proximal position in which the splines are in engagement and a rotationally free distal position in which the splines are out of engagement as just mentioned in both positions the clutch element is rotationally locked to the drive element the drive element comprises a central bore with two opposed protrusions 231 in engagement with the grooves on the piston rod whereby rotation of the drive element results in rotation and thereby distal axial movement of the piston rod due to the threaded engagement between the piston rod and the nut element the drive element further comprises a pair of opposed circumferentially extending flexible ratchet arms 235 adapted to engage corresponding ratchet teeth 205 arranged on the housing inner surface the drive element and the clutch element comprise cooperating coupling structures rotationally locking them.