I see they have disc replacements now. I had a spinal fusion years ago before these things were on the market. Does it ever happen that the spine fuses itself anyway when an artificial disc is used?

Studies are slowly trickling in on the use of artificial disc replacements (ADR). In Germany and France they've been used for at least the last 10 years--long enough for some analysis of the results.

X-rays taken right after the operation are compared to X-rays taken up to 10 or 11 years later. Doctors look for places where the ADR has moved or migrated forward or back.
Sometimes the implant sinks down into the end plates of the vertebra. The end plate is a layer of cartilage between the disc and the vertebral bone. The implant can even sink so far as to go through the end-plate into the vertebral body.

Fusion is not the goal with disc replacements. If fusion occurs, the body does so spontaneously. A recent study from France reported interbody fusion at the level of the ADR in one patient. The ADR penetrated the end-plate and bony fusion occurred at that level.

It looks like fusion after ADR occurs in rare cases.