A selectively expanding spine cage has a minimized diameter in its unexpanded state that is smaller than the diameter of the neuroforamen through which it passes in the distracted spine. The cage conformably engages between the endplates of the adjacent vertebrae to effectively distract the anterior disc space, stabilize the motion segments and eliminate pathologic spine motion. The cage enhances spinal arthrodesis by creating a rigid spine segment. Expanding selectively, the cage height increases and holds the vertebrae with fixation forces greater than adjacent bone and soft tissue failure forces in natural lordosis. Stability is thus achieved immediately, enabling patient function by eliminating painful motion. Greater distraction height is achieved without an increase in implant size through the use of interfitted stages.