Joe, I disagree with the concept that the M6 provides stability that is not present with the ball and socket style ADR's. I presume that the theory is that the woven 'annulus' provides additional help here. I've seen the M6 and it's a very interesting design. In theory, I do like what the pseudo annulus does... presumably providing some dampening of the motion at the ADR levels. However, I do not see this helping the 'out of control' concept of multi-level ADR.
The M6 is a mobile core device and, in practice, I see it having the same problem as other mobile core devices. If something provides some off-kilter loading, the core gets pushed to one side and it stays there. Search this forum for the word tiddly-winks and you'll find what I've written about mobile core devices. I have seen several M6 configurations that have this problem. It is a problem in single-level cases too but gets exaggerated in multi-level cases, especially if there is incorrect placement. (I have 3 clients who have had m6's explanted and know more.)
The ball and socket designs that have mobile cores (including the M6) will have this problem more than the stationary core designs. Having said that, I spoke to a friend tonight. He is the first patient in the world with a 4-level Mobi-C (highly mobile core) and is several years post-op with no stability issues at all.
I'm going to COPY the M6 posts to the M6 vs. ProDisc thread (
M6 Cervical vs ProDisc) that was started a few days ago. This thread is very long and involved and devoted to Steve's ordeal.
All the best.
Mark