A selectively expanding spine cage has a minimized diameter in its unexpanded state that is smaller that the diameter of the neuroforamen through which it passes in the distracted spine. The cage conformably engages between the endplates of vertebrae to effectively distract the anterior disc space, stabilize the motion segments, eliminate pathologic spine motion, or effect vertebral body replacement. Angular deformities can be corrected, and natural curvatures restored and maintained. The cage enhances spinal arthrodesis by creating a rigid spine segment, or if filled with compressible substances, the cage can be used for motion preservation between vertebral bodies. Expanding selectively (anteriorly, along the vertical axis of the spine) rather than uniformly, 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. The cage shape intends to rest proximate to the anterior column cortices securing the desired spread and fixation, allowing for bone graft in, around, and through the implant for arthrodesis whereas for arthroplasty it fixes to endpoints but cushions the spine naturally.