10 million cycles, 20 millions cycles, 85 million?

[lkastl]

Hi Everyone,

I have only posted a few times on this board, but have read a lot of the wonderful information here. I used to post on another board while I was pre-op and post-op for my L5-S1 Prodisc in 2003. I am happy with my surgery, although I admit I did not to a whole lot of research ahead of time. I just wanted to get out of pain and move on with my life! What brings me to this board is some new pains I am experiecing after a bit of a tramatic event about 6 weeks ago. Hoepfully it has nothing to do with my disc, but we will see.

I have been reading tons here, which is great, but leads me to a theortical question that I would like to ask of the group. Like all of you, I am very curious about the longevity of our implants. And while I know that the answer is " we really don't know" because there is not much clinical data out there yet. However, I do know from my reading that both the Charite and Prodisc have been tested to 10 Million cylces in a labratory setting. I believe the results showed the both held up well, but that the Charite only had 1.1 mg of debris for the poly core, while the Prodisc had 57 mg. My understanding is the difference is probably due to the sliding design of the Charite which creates less friction that the Prodisc design. I have also read that the are developing a tester where they can test the disc to 20 million cyles

While I know this is still all quite thoretical, I have read that docs and/or researchers are saying that the 10 million cycles equates to 10 years of average use. I have also read that docs are saying that the Charite and Prodisc lab studies suggest that they will last 40-50 years. Where does that come from if they have only been tested to 10 million cycles = 10 years? I have also read here somewhere that 85 million cylces equates to 40 years. Has any disc had that much testing yet?
I know that the patients lifestyle factors into this, but it seems like these are hugely different estimates.

Also, the Prodiscs wear at 10 million cycles is considerable more than the Charite. is that thought to be clincally significant?

If I get 40-50 years out of my disc, I will be very happy, as that should last me the rest of my life. But if the "true" lifespan is 10 years, I am concerned given my understanding that there is no way to take out and replace a Prodisc.

I know there has been lively debate on the wear and lifepan of the disc already and I dont mean to drag it all up again, it is just that the debate leads me to think about even more questions.
I am not really a very technical person , so I apologize in advance for not knowing how to link the resources where I got my information. If any of it is incorrect, please set me straight!

I would appreciate anyone's thoughts as to the questions/concerns raised.

Here's to a pain-free life that we all deserve . .

Thanks
Laurel

[Harrison]

These are all excellent points and questions. I’ll just offer my 2 cents now and chime back in soon.

Testing arthroplasty devices for wear is always a challenge; as one needs so many cross-disciplinary fields represented. Think about all the professional talents you need to assemble at the conference table: computer science, materials, tribology (the study of the effects of friction on moving parts), biochemistry, biomechanics – and many more!.

Check this interview out from last year, you’ll see what I mean:

Dr. Nauman (Purdue) -- Arthroplasty Testing
http://adrsupport.org/eve/forums/a/t...1/m/7071010281

When I return, I’d like to talk about core replacement (not all disc cores are easily accessible!) and long-term studies. In the meantime, see:

Long-term Results of One-Level Lumbar Arthroplasty: Minimum 10-Year Follow-up of the CHARITE Artificial Disc in 106 Patients.
http://adrsupport.org/eve/forums/a/t...1/m/3051084932

And:

How long will the Charite' disc last?
http://adrsupport.org/eve/forums/a/t...1/m/6251000181

Hope this helps a bit for now!

[lkastl]

Harrison,

Thanks for your response. I will be very interested in reading what else to you have to say on this subject

Thanks also for providing the links to the references. Makes it easier!! Most of them I have read already and they are what is making me ask my questions. In particular, that last link mentions the Charite has a life span of 40 years (85 million cylces). Yet I cant find anywhere were it has been testing for more that 10 million cycles (first article). How are they coming up with that lifespan? I also read Dr. Delameter (sp?), a Prodisc surgeon, saying that lab studies suggest that the Prodisc will last 40-50 years. Again, GREAT, but were is the study? Only lab study I can find tests for maybe 10 years of use.

I know the Charite has been in vivo for 17 years (yeah!) So far so good! and I want to believe that Prodisc will prove to have as much success. Yet I read on the Prodisc study report that it had considerable more wear debris than the Charite's similar study. So, that begs the question if it will truly last as long . . . and if it doesnt . . . are these prodisc cores replaceble?(or Charite for that matter) And if not yet, perhaps in the future they will develop revision strategies to address the issue?
I read the revision article on the Spine-dr.com website by Dr. Zucherman.I honestly couldnt understand if they were at the point where they could replace cores or not. It was interesting that they seem now to be placing some kind of material around the disc to make retrival easier . . however I bet they weren't doing it in 2003 when I had my surgery. (by the way, Dr. Z was my surgeon. Top notch surgeon, and I will ask him when I see him again. Any info he provides I will gladly share with the group

Laurel

[annapurna]

First part of the response: # of cycles compared with life. This comparison is one of the estimates that no engineer likes to do. You guess that an average person flexs x number of times per hour and multiply it out to get to an estimate number of movements per time period. Higher fidelity estimates could account for the fact that not every 'flex' is of the same magnitude. We do that with fatigue estimates for metal structures and it would seem likely that they'd do the same with medical devices. Obviously the estimate is pretty rough and its applicability to any given person is highly dependant on the lifestyle of the person in question. A person given to constant deep back bends isn't going to get the life that a sedentary person would; never mind the fact that the sedentary person is likely to drop dead of a heart attack and never benefit from the longer device life.

From this, you can tell that Charite's life estimate to Prodisc's estimate can't be done by simply comparing cycle count. The degree of flex could be different and the cycle count to hours of life conversion factor is likely to be different.

The wear particulate mass could be significant but you'd need to know more about the test conditions before you could say the tests were similar enough for comparison. Typically, if things are within an order of magnitude, ten times, it's really difficult to say if they really are different, but that presumes similar test conditions.

[jburdman7]

I am in my early 40's. There is no way I want a fusion. Nor do I want a disc that will fail in 20 years.

I have read plenty of info about flex cycling the artificial disk materials, and that is all good, BUT I have read ZERO NADA ZIP about good 'ol chemical degradation. Plastics often have chemicals added to increase their flexibility. I'm sure for the non-shock absorbing implants, material choices were available 'off the shelf' that have been proven for these applications. However I fear the long term integrity of the shock absorbing discs. Over time these polymers can decay, or outgass, leaving crumbs behind. This is either a settled issue with ADRs, or an issue no one wants to face.

I emailed the manufacturer of the M6 specific questions regarding the potential degradation of their polymer core. So far I haven't heard anything.

If I were in my 60's I don't think I would hesitate getting an M6. But since 'retreading' these is not a matter of a jack and 5 lug nuts I'm leaning to the plain jane ProDisc-C.

I know the M6 is treated like the unquestionable king of implants, which either means this question has been answered satisfactorily and I just haven't seen it, or I will really irritate the echo chamber and take a lot of heat

[annapurna]

Bluntly, I have the same concerns. UHMW polyethylene is a fairly well known material and it's use in Prodiscs and similar two-metal-plates-and-a-plastic-core ADRs uses the material in a fairly well understood manner. Wear is really the only open question.

The M6 expects their material to actually flex. Long term wear of a flexible material is something that materials engineers don't understand as well. There's always the standard solution: spec it for at least ten times the life expectancy you'll need, but you can't do that and fit the ADR in a normal back. I hope I've been a naysayer about the M6 for naught but I do worry that ten years or so from now there will be failures of the core material and the bondlines between the core material and the end plates.

[Hooch]

Do you really think that is likely?

It'd be nice if the company lit was freely available.

I did have a copy of their wear testing, it was all prettied up presumably for presentation outside of the company, if someone could pm it to me if they have it that would be nice, lost it when i changed computers.

Over the 20 million cycles under 120N, half in flex/ext at something like 7 degrees (not full range from the stuff i studied), the other half in lateral bend side to side, they lost up to to 2mm of height, and lost resistance to compressibility (significantly, like half the start value, but still physiologic, just).

So after they lose enough height and enough resistance, I dunno at what point you see the core actually start to break down.

I just find it hard to believe that they would create something they don't intend to last a lifetime knowing that these implants are really designed to never come out, tho in reality I imagine if they meet their internal wear testing requirements (whatever they are and however they arrived at them) it can just go out the door and to market.

You'd like to think they use all their professional knowledge so they can be just about damn sure there wont be any problems, but historically it's not always the case. The acroflex discs were a series of disasters.

For those of us with one in our back I suppose we better just hope that the technology has come far enough in time. Espescially those like me with mild scoliosis and a slightly translated adr. If I'd known that pre-op it honestly would've caused me to think twice, but what can we do, we are the meat in the grinder and I was out of options apart from stay disabled for another year waiting for another doctor.

There are a number of them being implanted now, so lets hope any device failures are 30 or 40 years off rather than 10.

Reading more about it it just comes down to how well polycarbonate urethane tolerates the load (shows how little I know lol), seen some studies there of Bryan discs recovered from cadavers at 3 years showing lost disc height, bugger else out there it seems. I would like to see some of these pcu discs like m6 and freedom tested to failure in significant bends.

That recent study on the Freedom disc (Rischke, 2011) quotes at 125 000 'significant bends' per year for an average person, then claims that one cycle of flex/ext or lat in the machine equals one sig bend, therefore 10 million cycles equals 80 years. For whatever thats worth.

[Harrison]

Notice this original post is from 2007. Pls also note these six-year old topics on plastics - we've really gotten into these details in the past.

Research for Hard Core Plastics Geeks


Technical Article on High Grade Medical Plastic

I am almost 7 years post-op with my Charite and my spine health is great. I met the first Charite recipient in Germany three years ago -- I believe he is 27 years post-op!

[srajan0929]

yea the degredation of polymer core is worrysome in the m6. but, i am assuming the sheath and the ultra high molecular weight polyethelene will protect the core from any type of degredation? also, the testing of the m6 is fine. technically, 800 Newtons on a particular disc is a lot. When lifting, we transfer the loads to our hips, legs, bones, joints and muscles, and finally our spine which distributes the load between the bones and the discs.

raj

[annapurna]

Originally Posted by Hooch

You'd like to think they use all their professional knowledge so they can be just about damn sure there wont be any problems,

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 metal 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.