Reusable torsion spring driven injection device

A reusable torsion spring driven injection device for expelling doses of a liquid drug from a loaded cartridge, comprising:

• a piston rod having an outer surface with an outer thread extending helically in a longitudinal direction and which outer surface further is provided with a longitudinal extending engagement surface such that the outer surface of the piston rod has a non-circular cross section,
• a housing structure comprising:
• a base part comprising a torsion spring driven expelling mechanism for driving the piston rod axially, and
• a removable cartridge holder part holding the cartridge and releasable connectable to the base part,
• wherein the torsion spring driven expelling mechanism comprises:
• a rotatable piston rod driver engaging the engagement surface of the piston rod such that rotation of the rotatable piston rod driver is transmitted to a similar rotation of the piston rod,
• an internal thread associated with the housing structure and engaging the outer thread on the piston rod such that the piston rod is moved helically when rotated, and
• a strained torsion spring holding a torque for rotating the rotatable piston rod driver during dose expelling, and
• wherein, the rotatable piston rod driver comprises an outer element and an inner element releasable coupled together,
• the outer element being maintained stationary during dose setting and rotated by the torque of the torsion spring during dose expelling, and
• the inner element being axially movable relatively to the outer element between a first uncoupled position and a second coupled position,
• the first uncoupled position being a position wherein the inner element is uncoupled from the outer element such that the inner element is rotatable independently of the outer element, and
• the second coupled position being a position wherein the inner element is coupled to the outer element to follow rotation of the outer element,
• and wherein
• a movable element operable connected to the inner element is axially movable to move the inner element axially from the first uncoupled position to the second coupled position.