An artificial intervertebral disc having a pair of opposing plate members for seating against opposing vertebral bone surfaces, separated by a ball joint coupling. One plate member has a centrally located ball-shaped protrusion that is rotatably coupled in a central socket in a cone-shaped protrusion on the other plate member. Thereby, the plate members are angulatable with respect to one another. Further, the disc assembly will not come apart under tension loads applied to the plate members, and the location of the ball joint coupling provides the disc assembly with a stationary centroid of motion that is centrally located between the vertebral bone surfaces, both causing the disc assembly to behave similarly in these respects to a healthy natural intervertebral disc.