Spinal Kinetics M6 and SCUBA
 

I had a double cervical ADR with two M6s in Nov'09. This July we're spending nine days on Isla Mujeres, Mexico. I was planning to make a couple of dives, and then remembered that these M6's are soft and will change shape with increased pressure (which is why I selected them). In addition to the flexible core, there is also "space" in between the outer cover and the core. If Spinal Kinetics filled that space with an inert gas rather than a liquid, then that area can be compressed by the underwater pressures also.

Obviously I can't dive without assurance from Spinal Kinetics that my M6's have been tested under the pressure of at least three atmospheres.

I've tried since the week before Christmas to get this information from Spinal Kinetics. When I was a prospective customer I was given access to any engineer in the company it seemed. Now that they've got my money, I am allowed to speak only with the receptionist and to leave phone messages for a sales person that won't call me back. I've even written the CEO; all to no avail.

To anyone considering an M6, the next time you speak with a Spinal Kinetics engineer, would you ask them about SCUBA and then get back to me with what they say?

If it would help, a previous poster on this board, since left, had photos of his scuba diving trip after his Charite. Three atmospheres of pressure isn't all that impressive from a materials standpoint. If you work out the pressure you put on your disk by simply standing up, you're quite likely to find out that 45 psi, more or less, is almost nothing.

annapurna,

Thank you so much for your thoughts. And I agree that the up and down pressures from my six mile run at lunch yesterday puts far more stress than SCUBA on the discs; at least where the stresses of axial compression are concerned.

My concern has to do with pressures pushing on the disc from the sides. Specifically, our intervertebral discs don't incur in daily activity the kind of stress they would from a hyperbaric chamber, someone choking us, or SCUBA diving; that is, there is no lateral stress into the center of the horizontal structure of the disc (as far as I am aware). My concern is, when under that type of increased pressure, will the polymer sheath that holds wear debris inside the disc separate from the top and bottom plates?

Spinal Kinetics has demonstrated in their testing that that structure won't fail under the stress of 3,000,000 axial compressions where each compression is equal to the magnitude of that experienced when running; (which makes me think that the empty space inside the sealed disc was filled with an inert gas). I want them to tell me how hard you have to push on the polymer sheath from the outside to induce a failure.

Make sense?

Yes, I didn't know much about the design of the M6 but if that sheath is filled with an inert gas and sealed, scuba would be a pretty stupid idea. Using the inert gas, seems to be a pretty bad idea on first glance though; what do you do with a person who likes to climb mountains? The pressure differentials are lower than what you'd see with scuba but you'd lead to sheath bulges that could press on the cord. Might they have filled it with liquid instead?

Both great questions! Perhaps if someone that is considering an M6 asks the engineers (since they won't talk to me now that they have my money), they can get the answers back to us?

They assured me that flying would not be an issue, though I didn't think to ask what the space was filled with, or about SCUBA or mountain climbing (which I also enjoy). I've read that airliners are pressurized to the equivalent of standing at 8000 feet; which is well below many mountain peaks that are easily scaled.

Please excuse me for interjecting with what may be a dumb question--since I am totally ignorant when it comes to engineering, etc: I have a Prestige ST (metal on metal) and am scheduled to go to Quito next month. Quito is over 9,000 feet. We are also taking a small plane down to the Ecuadoran coast and I'm not sure if or how well it is pressurized. Is my ADR at risk???

sahuaro, the Prestige is constructed of two solid pieces, so it won't suffer from the potential problems that are the topic of this thread. If I had Prestige discs, I wouldn't be pursuing this topic.

Enjoy your trip!

Thanks, jss. Good to know it was a stupid question!
I hope you get your questions answered.

The problem posed by the M6 that jss is asking about is the shroud or covering that seals around the mobile pieces. If it's sealed and the internal volume filled with some kind of inert gas, the shroud will flex as the ADR, and the person it's in, go up an down in elevation or dive to depth. I'd be surprised if it was filled with a gas rather than something like sterile saline but beats me what Spinal Kinetics actually did.

After three months of trying I was finally allowed to speak with someone within Spinal Kinetics that was about to help with the question of SCUBA diving with the M6.

The short answer is that they have not tested it under those conditions, but think that it should be Ok provided that you don't do any nitrox diving where you're at depths greater than 135 feet, or at 135 foot depth for extended periods.

The polymer sheath around the core and weave of the M6 is NOT an air tight seal. There are two vent ports that allow fluids to travel in and out of the core of the M6 (which is exactly how an automotive shock absorber works). When the sheath is installed, whatever fraction of the atmosphere that is present is what is inside the M6 when it is implanted in the host (me). Over time, the hosts extracellular fluid (ECF) displaces the air within the M6 sheath as the M6 fills with ECF.

There are two obvious problems here for SCUBA. As we descend, pressure increases, more and more gases become soluble in our blood and other tissues. As we ascend, the gases become less soluble and come out of the solution of our blood or other tissues. When we ascend too fast, bubbles form in our tissues (this is called "The Bends"). This leaves many questions...

How long does it take for all of the gas in the M6 to be replaced with ECF? At what rate is gas saturated ECF ported into the M6? Is enough ported in that there is still enough to form a bubble during a normal ascent? If a bubble does form, is it possible that it can be large enough to deform the M6 in a way that was not intended? Since the sheath was intended to contain wear debris from the internal components, what happens when one or both ports become clogged?

These are all questions on which we can speculate and hypothesize, but Spinal Kinetics currently has no plans do the testing that would allow them to be answered. It sounds like the M6 should not contra-indicate SCUBA. I may do a shallow dive of not more than 33 feet.