The natural disc is a miniature shock absorber sitting between two bones (vertebral bodies). The disc keeps the bones apart, absorb and transmit forces passed through the bones to other areas, and help keep proper alignment of the facet (spinal) joints on either side of the spine.
Anytime the disc is compressed, the facet joints are also compressed. The opposing joint surfaces get squeezed together and rub instead of sliding and gliding against each other. The result over time can be degeneration of the spinal joints. Disc replacement helps maintain the natural heighth of the spine and keep the vertebral bodies separated and supported.
Studies show that when placed with good alignment, these implants do indeed hold up better and last longer with fewer problems over time. But current artificial disc replacements on the market don’t appear to have good shock absorption. The makers of artificial disc replacements are considering ways to improve the design to help absorb compressive loading and vibration. The hope is that the implant could actually help protect the segments above and below it, rather than transfer the load to the vertebrae above and below.
Long-term studies comparing the results of artificial discs against spinal fusion are underway. In time, we will have more data to show just how these implants (and the surrounding spinal structures) hold up under normal everyday wear as well as under compressive or traumatic forces.