A method for postoperatively compressing a bone graft between adjacent spinal vertebrae utilizing first and second plates having openings at remote ends for receiving bone screws to fix the plates to respective vertebrae on opposite sides of a bone graft-receiving site. The first and second plates have male and female parts having interlocking elements cooperable with one another enabling movement of the plates and the adjacent vertebrae to one another to progressively compress the bone graft between the adjacent vertebrae and prevent movement of the first and second plates and adjacent vertebrae away from one another.