10 million cycles, 20 millions cycles, 85 million?

[jburdman7]

This geeky plastics stuff is just what I was looking for - Thanks!!

It seems to me the extrapolation of the wear data from hips and knees, namely the expectation they will last 10 years +, misses a fundamental factor. Wear is going to vary by the load, and the amount of surface area the load is distributed across (as well as the amount of motion it endures.)

IF the same UHMWPE insert which lasts 10 years+ when used on a hip joint loaded at 100Lbs, is used in a ProDisc-C loaded at 20Lbs, I would expect at minimum 5X the life in the C application (actually FAR more because wear likely increases with weight and the range of motion would be less in the spinal application.)

But this is just all talk since it is likely not the same insert. What if the 1/5th the weight is distributed over 1/20th the UHMWPE material? Then it better have to deal with a faction of the motion if the longevity is to match.

Without these relationships known I do not see an ability to interpret the hip / knee data into ProDisc-L or C or any other ADR's speculated lifespan.

After reading this plastics data I am content that the minimal motion an ADR is subject to will likely result in acceptable ADR lifespans. However if anyone out there has an educated guess as to the relative (to hip /knees / ) surface area of UHMWPE these loads are spread across... do share!

[Hooch]

I'll get my old hard disk back in a couple of weeks and I'll post the data up or summarise it. I think the patient has a right to know. (I meant 1200N in the load test before btw, but I'm running off memory. From the Stuart McGill stuff I read a push up will put about 2200N through the column, and other exercises more. They did individual tests to catastrophic failure that were far ahead of a natural disc, but only for brand new adrs)

None of the disc studies I've seen have shown cycles to failure, I'm not sure if it is a matter of time running these things in the lab or it being ugly data that isn't released.

I can't imagine anyone who isn't genuinely incompetent being misled by a log scale. When you talk about cycles to failure Jim, are you talking about a computer model or an actual disc in the testing device?

There is a medical device engineer who posted recently on another spine forum, it might not be a bad idea for someone who is a member there to direct him to this topic.

Hey Banhorn has them up on his blog if you want to read them, I'm sure he doesnt mind me posting the link.

http://backup.muellhorn.ca/m6-technical-information/

[srajan0929]

Originally Posted by annapurna

My problem with this is it is my field of expertise. Neither Laura nor I are medical professionals nor have we ever claimed to be. We are, though, materials engineers. It is my professional opinion that fatigue and bonding models for flexible polymers and polymer interfaces are a little lacking for me to be happy with using them in an unexaminable location when failure has the kinds of consequences that you'd see in a failed ADR. I'd love to be wrong but, in general, cycles to failure are plotted on a log scale. 10 million, 20 million, and 85 million aren't all that far apart when you're plotting log scale graphs. When your data points are close together, it's easy to misinterpret the data trends. I really hope that I'm wrong about this. For that matter, wear data in UHMWPE is a little weak as well and Rich pointed out that neither he nor Laura who received their disks at similar times have dropped over due to wear failures. I just want to make sure that people realize that everything isn't resolved for the M6.

Raj, the sheath will help but, depending on the failure mechanism, it may not prevent internal fatigue or failure of the bondline between the flexible core and the meal end plates. Also, 800 N sounds like a lot of force but that's the resolved force placed on the spinal column, not the weight you lift. Depending on how you lift and how quickly the load is applied, you can reach very high loads on the spine for short times until your body can shift that force around or the temporary loading from a quick lift, for example, drops.

i actually agree with what u are saying. now, by resolved force...you mean by an angle correct? it says on the website...800 N applied at some angle. so that would mean around 179lbs..on an angle. so lifting up would mean, give or take much lesser vertically correct? i dont remember how to do all this calculations...179sin(8 degrees * pi / 180) = 25lbs on a particular disc? that seems so little...i am going to sleep..its almost 8 and i have been studying all night...and spent 3 hours reading on viscoelastic polymers..i hate ADR...

now i am really worried. especially after reading some articles on creep and tear etc...

not able to find any much info on viscoelastic polymer as implants...

[annapurna]

I don't have the reference off-hand but ASM International sponsors a medical device-specific materials conference once a year. The pretty pictures in the ASM journal are all of metal items but I wouldn't be surprised to find discussion of polymers in there as well.

[srajan0929]

i was just reading an article by Dr. Rosen...He even says that there are tooo many problems with the JNJ charite implant design, and the prodisc only compares to an instrumented 360 degree fusion. He does mention the polyethylene issues and he seems concerned about it. He does reiterate that he likes the maverick and at present is having very good results with this device...

raj

[laid up doc]

ya'll are making my head numb w/ the physics easily my least favorite pre-med subject!!!

unfortunately there's always a level of uncertainty w/ a newer device. the charite was relatively new in the early 2000's, and while Laura and Harrison have done very well w/ theirs, there have been many others w/ issues.

all one can do is make the best decision they can w/ the info available at the time. i didn't really have a "choice", and am fortunate i was able to have anything done at all. 2-3 yrs ago there was no non-chromium lumbar disc. so i'll count my blessings, try to take good care of myself from here forward and therefore take good care of my M6.

working on weight loss, doing lower-stress exercises, and maintaining good core strength. no more horseback riding for me, and i'm glad i'm not a runner b/c that would be out too.

[srajan0929]

Originally Posted by laid up doc

and i'm glad i'm not a runner b/c that would be out too.

dammit...cuz i am...ughh..im gonna learn swimming then...and swim my butt offffffffff

[srajan0929]

all these uncertainties...but atleast i am informed. i will bring all this up with Dr. Boeree. I am concerned though because post-op, after getting the M6...lets say it lasts for 10 years...do yall think that in 10 years we would be able to come out with better revision surgeries...that make it more easier to revise?

[srajan0929]

take it with a grain of salt...But i figure it would be hard for them to lie about the robustness of these devices rather than lie about the surgical outcomes...


....................................
All testing indicated an extremely robust device that successfully lasts the projected life of the implant. The functional kinematic testing and physiologic dynamic testing demonstrated that the M6-L passed all acceptance criteria. The assembly and all the components remained fully intact and functional. The device remained fully functional after 20 million cycles. The axial compressive stiffness of the M6-C remained in the physiologic range throughout and at completion of all cycles of testing. The results demonstrate the durability of the M6-C: despite being subjected to highly non-physiologic loading up to the limits of the test equipment, no mechanical or functional failures were achieved. The results of the creep testing and the worst case physiologic sheath retention testing provide further verification of the robustness of the M6-C.

Conclusion
The M6-C was subjected to rigorous testing which confirms the inherent robustness of the device. The disc remains fully intact and functional after functional kinematic testing to 20 Million cycles of combined motion; physiologic dynamic compression, compression shear, and torsion; creep to the equivalent of 100 years; and worst case physiologic extension over 30,000 cycles. Even when highly nonphysiologic static loads are applied, the device does not exhibit any mechanical or functional failures. The static and dynamic mechanical characterization of the M6-C lumbar disc demonstrated that the device has the structural integrity to last the life of the implant and that it exceeds the necessary criteria for device safety over the life of the patient.

[Hooch]

I suppose they key point is what does 20 million cycles equate to, but even that seems to be a subject of widespread controversy for polyethylene discs. There is questions over whether the current astm standards are appropriate for viscoelastics as they are too quick and the polymer is not allowed to reform before being recompressed, underestimating wear.

What I would like to see done is the discs at the end of 20 million cycle tests resubjected to the failure tests in compression shear etc, but I doubt they'd release the data.

The really relevant point there is the loss of axial compression, to 800 +- 300 N/mm, from the McGill stuff I read 500 N/mm could perhaps be just physiologic, by their quoted data that's half of a healthy disc.

Fyi a simple push up will put up to 3000N compressive force through the lumbar spine, and that is a non-ballistic force. A 1 armer 6000N if you can do it. Sit ups about 3000N, but your spine is flexed. The big difference between flexed and neutral spine is reduction in compression shear.

I suppose no one really knows, a lot of it is best guess and the more I look the less I find, there is no data. Still I get stuck into it as best I can and definitely put load through my spine. That's what it is there for. If I really wanted to save the prosthetic I'd lay in bed all day, or maybe not have it put in me and leave it on the shelf in the box. I suppose it is a matter of being responsible with your body, and that will be different for everyone. For me the body requires functional strength, and that requires significant spinal load. You don't have to back squat 100kg to get your body strong. And if I could run freely I would. I mean I'd cross train, but I'd still run. lol I've run a fair bit since the op actually, gimping around.

I mean, life is for living, not for baby sitting a disc replacement.

Re revision, they tend to leave it for people who are quite profoundly disabled, and I can't see the difficulties associated with cutting through scar tissue and tearing arteries ever being resolved. Perhaps in the future this will be different. In 20 or 30 years time there are going to be some very old adrs around. A lot of the charites auto fused, maybe the other adrs will do the same. Lateral access is an option for l4/l5 and above. If people have a prosthetic explanted due to some catastrophic failure like a sudden expulsion they can do quite well if there are minimal complications. Unfortunately a lot of people facing revision have severely degenerated facets or improper adr sizing/placement etc, and there is a lot of associated damage and degeneration.

I suppose if your young Raj all you can hope is that it gives you enough time and any loss of disc integrity is matched by normal degeneration in an aging body and slowing your activities up. Or get a prodisc or a maverick. In the end I went with the best surgeon I could find as I wasn't even thinking about long term stuff then, and if he or she had a disc of preference I was pretty well going to follow that. Let's just hope the M6 goes the distance in terms of its durability or there are going to be a lot of messed up people getting about. My disc is translated due to scoliosis, so it gets more of a flogging. Better hope mine wasn't one of those ones made on a Friday arvo that are more minus 300N/mm than plus 300N/mm ... lol I guess