SCUBA and Spinal Kinetics' M6

[Justin]

Ladies and Gentlemen,

We're just back from Isla Mujeres, Mexico and we had a GREAT time! Specifically to the point of this thread, I managed one 45 minute dive at a maximum depth of 28 feet. There was no decompression required at that depth, so the SCUBA problem that theoretically exists with the M6 shouldn't have been a problem at that depth. As far as I can tell my pair of M6's did fine.

Yeah! Great news, Jeff.

[MDE]

I just actually sent a preliminary answer to this to Justin today, but I'll add as your post highlighted some information that I was not aware of.


This is my original response:

M6 and Scuba diving:
The material used for the nucleus of the M6 is a polymer which is actually more similar to a solid. The outer shell that shields the internals from soft tissue encroachment MAY be air tight, and if so, it would "collapse" in towards the center of the device when under great pressure due to air that is trapped inside the shell mostly in the pseudo annulus. This outer shell is made of a flexible silicone that allows it to regain it's shape when the pressure is removed.

You stated that it is not air tight, and as such becomes filled with interstitial fluid. While your logic up until a bubble being potentially formed inside the implant is flawless, the reduced pressure (back up to the surface) at the point where the air would become gaseous again would also mean that the air could freely leave the implant through the same vents. There would not be any pressure holding the air, or fluid in place. It then couldn't put pressure on the outer sheath to force it outwards once you are back to the surface.

I would say the easiest parallel is breast implants. These are much larger than the center of the M6 and typically have a very small percentage of air that is compressible in them as well. These have been tested in hyperbaric chambers to proven that while it DOES effect them, the effect is minimal.

The polymer in the center of the M6 is specifically designed to allow "compression" but only axially. Think of that polymer as similar to a silicone, but is significantly stiffer. It will compress, but it doesn't change volume. When you push on it axially, the sides expand.

I misstated this as the polymer center is designed to take a compression force axially, but can also be compressed in other directions, however the pressure of diving (even extremely deep) will never be greater than the force caused axially through natural movement.

I can almost guarantee that Spinal Kinetics has not done this testing, but I don't believe there is cause for concern. The nucleus will be unaffected, the pseudo annulus will be unaffected, and the air space inside will allow the outer shell to collapse in slightly, but not requiring more movement than it would already go through for normal disc motion.

That said, this is a very interesting question, and one that I would take up directly with Spinal Kinetics, mostly just to see their response

Obviously for this last part it wasn't exactly something they were prepared to answer. I guarantee if you could actually get a hold of one of the design engineers instead of customer service or marketing you would have gotten a much more in depth analysis.

[Cindylou]

Jeff, AWESOME NEWS!!!! You are getting back to living your life!!!

[jss]

MDE, thank you so much for chiming in. With your very unique and knowledgeable perspective, I was hoping that you'd show up! You have alleviated any concerns I had of adverse effects to the nucleus. My paramount concern is damage to the polymer sheath. I didn't detail this concern here previously, but since you have addressed it, here specifically is my chief concern. Perhaps you could tell me if this is a legitimate concern?

You stated that it is not air tight, and as such becomes filled with interstitial fluid. While your logic up until a bubble being potentially formed inside the implant is flawless, the reduced pressure (back up to the surface) at the point where the air would become gaseous again would also mean that the air could freely leave the implant through the same vents. There would not be any pressure holding the air, or fluid in place. It then couldn't put pressure on the outer sheath to force it outwards once you are back to the surface.

Per Spinal Kinetics the polymer sheath is placed to keep wear debris confined inside the device. If both vent ports became plugged with internal debris, they might be cleared at SCUBA depths by the increased pressure of my interstitial fluid forcing its way in through the ports. As that pressure subsided during ascent, could the ports become re-obstructed with internal debris, trapping some super saturated fluid within the device? From this fluid solution gasses would leave, forming bubbles inside the now air-tight sheath. Is that an impossible scenario? If not, can enough gas be dissolved into the fluid to damage the sheath or deform it to the point that it might touch the spinal cord or root nerves at the surface? Or in the airplane home which is pressurized (I believe) to the equivalent of 8000 feet?

And of course you are correct. After surgery I was no longer given access to engineers, only marketing and sales support staff.

[jss]

Justin, Cindy, Katie, ... thanks so much for the encouragement. It has truly been a treat to return to the land of the living! Having been totally incapacitated (and I know that ya'll can relate), since my cure I've had almost the need to no longer live life, but to ATTACK it!

[MDE]

MDE, thank you so much for chiming in. With your very unique and knowledgeable perspective, I was hoping that you'd show up! You have alleviated any concerns I had of adverse effects to the nucleus. My paramount concern is damage to the polymer sheath. I didn't detail this concern here previously, but since you have addressed it, here specifically is my chief concern. Perhaps you could tell me if this is a legitimate concern?



Per Spinal Kinetics the polymer sheath is placed to keep wear debris confined inside the device. If both vent ports became plugged with internal debris, they might be cleared at SCUBA depths by the increased pressure of my interstitial fluid forcing its way in through the ports. As that pressure subsided during ascent, could the ports become re-obstructed with internal debris, trapping some super saturated fluid within the device? From this fluid solution gasses would leave, forming bubbles inside the now air-tight sheath. Is that an impossible scenario? If not, can enough gas be dissolved into the fluid to damage the sheath or deform it to the point that it might touch the spinal cord or root nerves at the surface? Or in the airplane home which is pressurized (I believe) to the equivalent of 8000 feet?

And of course you are correct. After surgery I was no longer given access to engineers, only marketing and sales support staff.

Good question. I actually considered this when writing my response, but I don't particularly think that it is a concern. there are several basic reasons for this. If you think the overall volume of "empty space" inside the sheath to be less than 10mL, the total amount of air that can be dissolved into the interstitial fluid, enter the disc, and remain inside the now airtight sheath (Assuming blockage) would be 1 mL of air at most. Returning to normal pressure a 10% expansion of the inner sheath would be almost unnoticeable to the naked eye. If you look at the distance between the sheath and the posterior aspect of the vetebral body (the closest structure to the spinal chord), there is a minimum of 1-3mm of space.

The other reason is based upon the fact that wear debris and particulate is a very well studied science. It's been proven that some wear debris is more harmful or less harmful to biological structures based upon it's size and shape, and as such devices are studied as to what wear debris is produced, on top of quantitatively how much debris (by weight) is produced. The wear debris that may potentially exist within the M6 would be produced from the fake annulus structure which is made up of fibrous PolyEthylene (PE, or UHMWPE) PE is a fairly lubricious substance (think similar to teflon) and it's wear characteristics have been studied heavily for more than two decades. The debris should not be too small, or too large. If the debris is in the "typically acceptable" size range, it would be significantly smaller than a hole designed for venting. Now, I didn't design the hole, nor do I have their wear debris analyses so this is mostly just a "gut feeling" reaction.

I can't give you a formula that completely answers your question or a 100% guarantee, but my experience and basic assumptions would lead me to believe that it poses zero risk when you follow the appropriate diving techniques. My confidence level is that if I had the M6 and were a diver that I would have no hesitation to do it. That said, I probably wouldn't try a record setting dive my first time with the ADRs in place.

[jss]

MDE,

Thank you so very much for your reply. It also contains a lot of good news; particularly in that if the worst case scenario occurred (vent ports plugging at 4 atmospheres) that the device would almost certainly not be damaged and that nearby root nerves and spinal cord would not be impinged.

I am left to wonder one thing. If the debris produced by the M6 is small enough to pass through the vent ports in the sheath that was placed there to prevent that debris from migrating, I'm wondering why the sheath was placed there in the first place?

So, "To dive or not to dive?" is no longer a question.

Thanks, Jeff

[tyler]

Hi Jeff and MDE,

Thanks so much for your helpful posts! :thumpup: I'm a candidate for ADR at C5-6 and C6-7 - other discs in generally decent shape for a 48yo - but also active like yourself. I've been trying to find out everything I can about discs and have been strongly leaning toward M6.

I've also looked into the NeoDisc a bit but I haven't been able to find much on technical experience with it - especially the implications of the design for the long-term. I just wonder about how the polyester sleeve holding the compressible silicone would hold up (1) to potential sheer stresses in sports or in an accident or (2) over time with continuing wear stress from the adjacent vertebrae. Since it seems like a perforated sleeve (as a fellow diver I think of my wet gear bag) - I also wonder whether it would tend to be more susceptible to bone ingrowth and ossification / auto-fusion?

Jeff - as far as M6, did you go with Clavel over Ritter-Lang just because of costs or were there other considerations?

Any thoughts / recommendations would be greatly appreciated!

Best,
Tyler

[jss]

... I just wonder about how the polyester sleeve holding the compressible silicone would hold up (1) to potential sheer stresses in sports or in an accident or (2) over time with continuing wear stress from the adjacent vertebrae. Since it seems like a perforated sleeve (as a fellow diver I think of my wet gear bag) - I also wonder whether it would tend to be more susceptible to bone ingrowth and ossification / auto-fusion?

I can only speculate, but when considering Stenum, I was told by two of their representatives that they had multiple cervical M6 implant patients that were marshal arts instructors that had gone back to practicing the marshal arts. I assume that sheer forces occur in marshal arts if you don't keep your guard up?

Spinal Kinetics has to my knowledge not claimed to have conquered the bone ingrowth complication; but they do claim that their design does mitigate that complication. Also, Dr Clavel told me that to inhibit that complication that he positioned the M6's as far posterior as possible and that he used the largest implant possible; the "Large Long" at C6/7 and the "Large" at C3/4.

... Jeff - as far as M6, did you go with Clavel over Ritter-Lang just because of costs or were there other considerations?

There were two main considerations; cost and surgeon. Going to a US facility would have meant dipping into my retirement funds, going to Stenum would have meant dipping into my youngest's college fund, where going to Barcelona Spine meant only dipping into his mother's new car fund; a rhetorical decision. BTW: the youngest is starting UNT in Materials Engineering/Nanotechnology this Fall; he's going to be an engineer like the old man! My research revealed that Dr Clavel had performed far fewer ADRs than the Germans (thousands each), but far more than any US surgeon. Dr Clavel had performed just over 200 ADRs at the time of my surgery eight months ago; only 10 of them were M6, a handful were Mobi and the rest were Prestige. Back then he preferred the Prestige, while today he prefers the M6.

Good luck, Jeff

[tyler]

My comments about the stress were regarding the new NeoDisc (originally developed by Pearsalls, acquired by NuVasive, U.S. cinical trial just completed)

It looks like the polyester sleeve is actually intended to promote ingrowth. The photo shown in the link describes it as "fibrous tissue ingrowth" as shown in ovine (i.e. sheep) studies.



This system is clearly very different than anything else out there - it would be interesting to know how it holds up over time.