A screw extension assembly for use in minimally invasive spinal surgery, the assembly has an inner slotted shaft and an outer shaft, a rod reducer and a removable nut. The combination when assembled is configured to move a spinal fixation rod into a slotted rod receiving spinal implant where it is seated and affixed thereto. The screw extension assembly further has a locking knob rotationally coupled to a proximal end of the outer shaft, wherein the inner shaft has one or more cam grooves and the locking knob has a pin extending into and guided by said cam groove causing the outer shaft to translate longitudinally upon rotation of the locking knob relative to the inner shaft toward an engaged position locking the deflectable legs in the coupled position to the slotted rod receiving implant.