Laura,
My views on some of your concerns are a bit different, so I'll go ahead and throw them out there for you to chew on...
- On TBI being a surgical mill. I almost had a ProDisc-C implanted by Dr Zigler and know a few people that have been treated there. I never got the impression that they were a surgery mill. Some months ago someone on this site posted that their impression was that TBI was a surgery mill. As far as I can tell TBI has a reputation on this site as a surgery mill because that one statement grew legs. I don't know if they're a mill or not, but haven't had that impression from my numerous interactions with them.
- On Dr Zigler's multilevel experience... I don't know how many he's done, but he is well published on his multilevel experience with the ProDisc, and there is one poster on this site in whom Dr Zigler implanted four contiguous ProDisc-L's. As of his last posting, he's doing great. His handle is david???? It starts with david and has some numbers after it.
- On concerns about the M6... I am an engineer and share the concerns that it may be more prone to fail because it has more parts. I don't doubt that such failures will (and do) occur, but since in its 7+ years of service no one's posted on this site either of experiencing or reading of such a failure, my concern is not great. Compared with the ProDisc where there are documented cases of device failure; specifically the polyethelene sleeve sliding out of the bottom plate. My biggest concern about the M6 is its potential to house an infection.
- On the M6 keels being too large... I don't share Harrison's concern. Yes, the M6 requires three keel cuts to the ProDisc's one. But the M6's keels are neither as deep nor wide as that required by the ProDisc. Because the quantity of bone removed for the ProDisc keel is so great, there are documented cases of the vertebral body splitting both during surgery and years after implantation. I presume this is why Synthes came out with the Nova; its smaller keel should fix that problem. Because I don't know the dimensions of the keels in question, I've never calculated the total volume of bone removal required for either device; but would bet that the ProDisc requires an order of magnitude more to be removed.
When I was selecting a disc, with two prior fusions and two pending ADRs, my biggest need was to have an ADR that would best preserve my two remaining natural discs. Because the ProDisc preserves motion in only unnatural ways, and was documented to cause facet problems, I didn't have a great deal of faith that it would not add to the stress of the two natural discs I had left The ProDisc had significant documented problems, and the M6 was new enough that its problems may have not yet had time to surface. (that's still true of the M6 BTW)
Given all that, I don't know which device you should receive. If you do get the ProDisc, Dr Bertagnoli is probably a very good choice of surgeon. As has been observed, there is no incontrovertible evidence that one device is better than the other. I chose a pair of M6's because, in my judgement, they gave me the best chance of preserving my remaining anatomy and avoiding future surgeries. So far it looks like it was the right call. Will they fail in 10-20 years? ... I sure hope not.
Good luck, Jeff