Many of the new artificial disc replacements (ADRs) have an internal constraint mechanism to limit their motion. The manufacturer-suggested limit is two millimeters of motion. That's a very slight amount. Just enough to allow the ADR to go with the flow of motion and not create a blockage or rigid stop.
Some ADRs have what look like teeth on the part that goes next to the bone. This helps anchor the implant in place. The bone grows in and around each spike to give it the strength it needs to keep from tilting or pushing one way or the other.
Even with these mechanisms there are times that the ADRs shift, tilt, or sink into the bone. Manufacturers are continuing to look for ways to improve the current devices.