Patent ID: 8333739

Claim:
A method for expelling medication from a prefilled cartridge having an end closed by a piston wherein the cartridge is contained in an injection device having a rack and pinion gearbox assembly that couples an injection button with a piston driver; the method comprising the steps of: setting a dose to be injected from the prefilled cartridge; moving a first rack, which contains teeth, in a proximal direction, away from the cartridge and injecting end of the device during setting of a dose, the rack remaining in continuous meshed engagement with a pinion which during the setting of the dose causing the pinion to rotate in a first direction; moving the pinion in a proximal direction away from an injecting end of the device during the setting of the dose moving the first rack distally, which is toward, the injecting end of the device during injection of a dose and causing the pinion to rotate in an opposite direction and to move forward, thereby causing part of the transmission to drive the piston driver which in turn moves the piston within the cartridge and expels medication from the device.