An implantable assembly includes an anchor member and a spacer. The anchor member has a hub portion and a shank portion. The spacer includes a central opening to receive bone fusion material and at least one passage for receiving the anchor member. In one embodiment, the passage includes first and second means for locking the anchor member in the passage against backout. The second means for locking the anchor member in the passage against backout includes means for centering the anchor member in the passage and means for stabilizing the anchor member in the passage. In addition, the second means for locking the anchor member in the passage against backout provides a mechanical advantage to assist in driving the screw into bone and mechanical advantage to assist in reversing the screw out of the spacer.