A bone fixation system with variable z-axis translation is provided. The bone fixation system includes an outer tulip coupled to a bone fastener. An inner tulip, which provides a seat for an elongate fixation rod, is located inside the outer tulip. The inner tulip together with a seated rod is permitted to translate along the z-axis in an unlocked position. Also in the unlocked position, the bone fastener is free to angulate relative to the outer tulip. The bone fixation system includes a locked position in which the z-axis position of the inner tulip and rod relative to the outer tulip is fixed. Also in the locked position, the bone fastener is locked with respect to the outer tulip. The system may be adjusted between the locked and unlocked positions by way of a set screw or independently by rotation of the inner tulip relative to the outer tulip.