[annapurna]

Drewrad,
Before I answer your question, I have to say that neither Laura nor I are medical professional in the slightest. If a doctor told you your foot was connected to your wrist and we told you it wasn't, I'd trust them and not us.

Strictly from an engineering standpoint, the Activ-L design is simpler. It fails to mimic the loading in a natural spine and tends to put more load on the facets and the connective tissue and musculature. On the other hand, there are relatively few ways it can fail, outside of poor selection, poor placement and subsidence, which can take down any ADR. Wear of plastics is fairly well understood and we have pretty good models to go from bench tests to reality. That doesn't mean we've got a good handle on x bench test cycles yielded y grams of material loss which means z many years of life but experience is much clearer to put those piece together into what to expect for long-term performance.

That said, just because the disk is less likely to fail doesn't mean that it's the best thing for someone's spine. I'm speaking out against the M6 because I'm concerned that the rush to the new design is going to land people in trouble if they fail to check on it regularly. I can't really say if I'd prefer to have the Activ-L in my back or get the M6 and plan on regular flexion-extension x-rays for the rest of my life. Both approaches are reasonable. I wouldn't accept an M6, though, if I didn't plan for some kind of regular monitoring.

Richard and I have spoken about the M6 and he brings up a completely different, but valid concern. The simpler ADR designs have fewer pieces and a tendency to "scrub" those surfaces during movement. Biofilms can still form but there would seem to be much less tendency on an Active-L design compared to a M6 design. I simply can't answer to the degree that this should factor into your decision. That's well beyond my knowledge and I'd effectively be parroting what I learned from a trusted (and possibly trustworthy) source. It is something for you to be aware of, though.