Artificial disc Mulholland R. Scientific basis for treatment of LBP Am Surg Royal college. 2007

The introduction of the artificial disc, producing results similar to fusion provides strong support for the hypothesis that loading is central to back pain. The artificial disc does not create a pattern of normal movement, and is not designed to do so. It does not necessarily require removal of the posterior innervated annulus, often regarded as a primary source of back pain, which continues to be moved and stressed after surgery. The one mechanical effect it has is to alter load Transmission while allowing movement.
The interface between implant and underlying vertebra is critical. It is here that load is transmitted. Bony integration must not only stop movement, but must be sufficiently extensive that the area of bone transmitting is adequate. Cages demonstrate that a small area of bony integration sufficient to stop movement, but insufficient to transfer load over a wide footprint is associated with continued pain.
With the artificial disc with a larger footprint transfer of load is thru plates resting on the vertebra…a universal feature of currently used disc replacements. This load transfer may be abnormal if the bone-plate interface is a mixture of fibrous tissue and bone, producing an irregular pattern of load.
This could be one reason the procedure is showing no better results than fusion.
The artificial disc must be focused on its role as a transmitter of load, not an enabler of movement. It seems one cause of clinical failure could be due to the failure to establish a normal loading pattern at the plate-bone interface.