There are four different artificial disc replacements (ADRs) today. They can be divided into two groups based on the materials they're made of. One group has metal-on-plastic surfaces. The second group is metal-on-metal.

The second way to classify ADRs is by their movement. The normal, healthy spine has six separate motions. Compression and load are two other forces put on the spine with everyday movement. ADRs can be "constrained" or "unconstrained" depending on any restrictions in normal motion.

As to the actual device, some ADRs are made up of a single piece press-fitted together. Others have two or three separate pieces with a ball and socket design. The end plates are secured to the bone by spikes sticking out of the top and bottom pieces.

It's likely that ADRs will continue to evolve and change as research finds out what works best for patients.