A device for insertion into a spinal (intervertebral or intravertebral) space is expandable from a first circumference to a second circumference through axial compression of segments of the device, particularly once the device has been properly situated within a vertebral space. The interbody/intravertebral body device is characterized by a plurality of axially stacked, individual segments that are provided on a central insertion and deployment rod. Each segment includes a central plate or body to which are pivotally attached plate or leaf structures. Pivoting of the structures provides a collapsed or unexpanded position of the first circumference and an open or expanded position of the second circumference.
