[annapurna]

There isn't long-term data on anything beyond the Charite lumbar ADR. The Prodisc-C is just getting old enough for realistic long-term data to be coming out. To tackle some of your other questions though:

1) Does the motion remain? First, you have to assume that you will not fuse around or have the ADR subside. With that assumption, Prodiscs, Charites, Mavericks, and pretty all mobile core ADRs will retain motion so long as the wear doesn't cause problems. The M6 doesn't have a mobile core so it will depend on the long-term behavior of the polymer and that's been discussed elsewhere. Without belaboring a discussion held elsewhere, I do not trust that long-term behavior.

2) Does it prevent adjacent level overload? Well, depending on who you believe ADR either does or it doesn't and fusion either does or doesn't either. Basically, pick something that mimics natural loading, not motion, and hope for the best. It might be worth pointing out that loading and motion are similar but bones respond to loads. The M6 ideal is actually better in that the loads seem to be intended to match the loads placed on the boney structures and not use the facets, for instance, as hard stops the way the Prodisc does.

3) Fusion? First, get a fusion if you need it. You can seriously screw up your life if you insist on an ADR when your damage is such that a fusion is necessary. Second, a lot might be dependent on why you need spinal surgery at all. A single failed disk due to discrete trauma might work well with fusion. General DDD at multiple levels with the disks failing in an ever-hastening cascade probably needs ADR and fusion hybrid solutions. I've never seen a study to this effect but I'd consider how well my adjacent disks and facets are doing before settling the fusion vs. ADR question.

4) Robustness of the M6? This is actually my greatest complaint with the M6. It works great on paper. It works great at the start of its life. It has so many ways in which it can fail, I worry that it's going to fail easily. That was also discussed in another thread.

5) Non-destructive evaluation of an aged M6? An MRI isn't going to answer the questions you want answered. There are techniques we use for NDE of materials and structure; they just happen to be so nasty they'd kill the patient we'd use them on. Best bet is flexion/extension x-rays and check for changes in the way your neck bends over time. Sharply increased bend angles suggest problems with the ADR.

6) Studies? No study that I know exists for comparing across multiple cervical ADRs or for longer than about 10 to 15 years. Cervical ADR simply hasn't existed that long.