An expandable intervertebral cage apparatus includes a pair of cage components, each generally in the shape of a half cylinder. Each of the cage components provides a corresponding, abutting conically shaped recess that cooperates with a conical end portion of an expansion screw. One of the cage components carries a fitting with an internally threaded bore that receives external threads of the expansion screw. As the expansion screw advances into the cage, the conically shaped end portion of the expansion screw engages the conically shaped recesses on the cage components to expand the two cage components apart until they are in contact with the vertebral plates of adjacent vertebrae. The cage components have passageways for packing with bone-inducing-growth material allowing the cage to become part of a spinal fusion. A surgical method for installing the expandable intervertebral cage is also disclosed.