[annapurna]

There is a hefty body of knowledge of how long term loading affects elastomeric materials. If they've done what they're supposed to, and I can't believe they wouldn't, the loading on the elastomeric disk will stay low enough that there shouldn't be any long term damage in the bulk material. It shouldn't compress or tear.

My problem, and Laura's doesn't completely agree with me so you can tell it's a pretty fine distinction, is with the bonded joints. When you design a test plan for a device, you try to mimic real life but you also make assumptions about what parts of real life gets mimicked in your test plan. We don't have a good model about how bonded joints fail in fatigue so it's difficult to make sure you've made the right assumptions. It looks like real life loading for a spinal disk is now sufficiently well understood that there's not a whole lot of assumptions left to worry about but consider me a conservative engineer. I wouldn't tell people to get M6s taken out or avoid them under all circumstances but I would suggest that the marketing claims about how good the M6 are could be a bit too ambitious.