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Old 05-02-2006, 09:00 AM
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Harrison Harrison is offline
Join Date: Oct 2004
Posts: 7,011

Last year, I posted an article about the device testing at Purdue University: Machines, Software Model Helping To Create Better Spinal Implants Naturally, I was curious and called Purdue.

The professors at Purdue were kind enough to speak with me about the keen interests we patients have in their work; that is, testing all kinds of artificial disc devices. I recently interviewed Dr. Eric Nauman, a professor at Perdue whose expertise spans from stem cell treatment to software simulation of the spine. He was kind enough to answer some questions to help us better understand the complexities of device testing.
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Harrison, ADRSupport: Dr. Nauman, can you tell us about what your lab does, and how it’s equipped to test artificial disc devices?

Dr. Nauman: Our research group has diverse interests. We are exploring how adult stem cells can be used to treat musculoskeletal damage, especially in the spine, and trauma to the spinal cord. To help us in this effort, we often have to study injury mechanisms. We’ve found that understanding the cause of a fracture, herniated disc, or spinal cord contusion gives us better insights into how best to treat the injury. And, at Purdue, we have the advantage that Ben Hillberry has been studying the spine for many years. He and his graduate students built our spine simulator, the first generation spinal implant wear tester, and developed finite element models of the entire lumbar spine. Together, we’ve been improving our wear simulator, building a second generation spinal implant wear testing device, and refining our finite element models.

Harrison, ADRSupport: Is the Finite element model generally regarded as the best available testing standard? Is it accepted and/or endorsed by the FDA and professional spine associations?

Dr. Nauman: I’m afraid I don’t know if finite element analyses (FEA) are officially endorsed by the FDA or professional spine associations. But FEA is a tool that can help us better understand what is going on in the spine and a good model can suggest new experiments or inspire new implant designs.

As you know, the spine is remarkably complicated and quite elegantly designed. If you’ve ever looked at the way that the facet joints interact and the intervertebral disc transmits loads and especially the way the muscles and ligaments wrap around the bony protrusions, you appreciate the cacophony of structures. And the microstructure of each element is adapted to its physiological role. No FEA can hope to account for all of these features. Ever. But researchers like Vijay Goel, Aboulfazl Shirazi-Adl, Ian Stokes, Ben Hillberry, and many others have been working on these problems for a long time and have developed very sophisticated models.

One thing that we as researchers have to be careful about is that we don’t start believing that our finite element models are the same as actual spines. I’m guilty of putting too much stock in my computational skills and I know some of my colleagues are as well. Consequently, it is imperative that researchers, clinicians, FDA reviewers, and patients appreciate that FEA is just one tool.

Harrison, ADRSupport: Do you currently perform testing to simulate 20 years – or longer – of disc longevity? The mfr’s or the FDA may not require longevity results, but you can bet every patient who is implanted with a spinal device (or two) cares about these test results?! And are you testing for multiple device levels?

Dr. Nauman: Our second generation wear tester will be online in about a month. With that system, we will be able to test out to 20 million cycles (the current ASTM recommendation – the standard is not official yet). For most patients, this should represent at least 10 years of use and probably 20 or more. Right now we are not testing multiple device levels, but we are moving towards that with our spine simulator which can run at least 1000 cycles. Unfortunately, we don’t have concrete plans right now to test multiple device levels out to 20 million cycles. But we are working on techniques for simulating that kind of wear with computer models.

Harrison, ADRSupport: You may have seen this article a while back: Effects of Charite Artificial Disc on the Implanted and Adjacent Spinal Segments Mechanics Using a Hybrid Testing Protocol We had questions about the findings in this article, but most of us found it confusing. Can you break it down for us in laymen’s terms?

I will do my best. I should begin by noting that this is exactly the kind of work that needs to be done. It is a nice combination of experiments and modeling. The difficulty I think is that it could easily be two separate papers.

First they developed an FEA model of the lower lumbar spine (L3 – S1). I haven’t tried to reproduce their model, but their group has an excellent track record in this are. They then re-worked the model to include a simulated Charité TDR.

Second, they validated their model of the intact spine by comparing the response of instrumented lumbar spines to their FEA model (Fig. 4 in original article). This information alone would make a very strong paper by itself because it would allow them to examine the changes in stress state at the adjacent level with an implant and without. Very good stuff.

Third, they developed what they call a “hybrid testing protocol.” Personally, I think they would help themselves by coming up with a better name. But it is an excellent idea that hasn’t received much attention from the research community. Essentially, they want to get away from the standard method of testing implants and move to a more physiological model. In the past, the most common way to test spines was to apply a moment (or some combination of moment and compressive force) to an intact spine. Then put an implant in and apply the same load in order to see what happens. But, what Goel et al. argue is that a better way to test the spine is to apply a load to the intact spine and measure the motion it generates. Then put an implant in and apply whatever load is required to give you the same motion. The idea is that your body generally operates in position control – not force control. So it’s a better comparison to make.

One could argue that it would be even better to just apply the same motions to the intact spine and the spine with the implant and compare the required loads directly. And some people may take issue with the fact that most of the comparisons were made on the computer simulations and not on actual spines. But I think they did a very nice job. I hope this helps a little.

Harrison, ADRSupport: What spinal conditions can be factored into your testing model? E.g. natural aging, osteoporosis, scoliosis, etc.

Dr. Nauman: We are currently developing models of aging, osteoporosis, and scoliosis. We use clinical data to investigate the effects of scoliosis and osteoporosis and have published a couple of papers on it. And we have developed analytical models of the effects of aging on the mechanical response of bone. But, it is very difficult to incorporate osteoporosis and scoliosis into most experimental protocols because there just aren’t enough spines of these types for research. So, in general, we incorporate these factors into our analytical or computational models.

Harrison, ADRSupport: Can you envision a day when your software could be adapted to simulate an actual patient’s spinal condition – to help with the proper selection of treatment(s) – whether it be surgery or physical therapy?

Dr. Nauman: To be honest, a couple of years ago I was convinced that software would probably not ever be much use in simulating a given patient’s spinal condition. Too much of the research was trying to precisely reproduce the trabecular network and it seemed like simulating the musculature (a huge difficulty for computational and experimental models) was simply too complicated. But in the last year or two, I have come around and I think that, with some basic inputs from DXA or CT scans, we can actually build a model that is a pretty good representation of a given patient’s spine. The next step is to prove that these kind of models actually help and getting the clinicians to buy into it.

Harrison, ADRSupport: What have you learned about how arthroplasty devices, in terms of how artificial discs change the complex bio-mechanics of the spine?

Dr. Nauman: One thing that we struggle with is the fact that every individual is different. So, our measurements always have large standard deviations. Consequently, it is sometimes difficult to determine if an effect we observe is really important or perhaps masked by some deficiency in our testing protocol. For instance, to test human spines, we have to remove most of the musculature and that seriously de-stabilizes the spine.

But given all those caveats, my feeling is that most of the TDRs, if implanted properly, actually do a pretty good job of reproducing the kinematics of the spine. The difficulty is that, to put them in place, you typically have to cut the anterior ligament that runs down the front of the spine. That causes a huge change in the kinematics and kinetics of the spine. We’re hoping to fix that in the near future.

Harrison, ADRSupport: Are there certain physical activities that you would avoid if you had a lumbar artificial disc? Cervical disc? (Most of us patients don’t have luxury of receiving an owner’s manual with our fancy implant!)

Dr. Nauman: Please keep in mind that I am not an M.D. so I shouldn’t be making concrete recommendations. But, one thing that we discussed recently in a class that I teach was picking up small children. Not only should you be careful not to bend over at the waist, you should avoid picking them up if they are resisting you (unless they’re in danger of course). What happens when a small child kicks and thrashes while you’re trying to hold them is their center of mass moves around. That requires your back muscles – the major stabilizing structures – to respond very quickly and it can be very difficult on your spine.

Don’t smoke.

Also, be very careful around low hanging branches. I’ve heard of people on riding lawn mowers or people raking their yards who hit their heads on unforgiving branches and destabilize their implant. Very scary situations.

Harrison, ADRSupport: Thank you very much for your time and insights. Any parting words of wisdom for the patient community?

Dr. Nauman: Please be patient with us while we figure these things out. And make sure you stay active in groups like ADRSupport. If nothing else, reading about your experiences helps researchers like myself come up with new ideas. You may also want to publish your experiences. To be quite honest, researchers are often at the mercy of physicians in terms of coming up with research ideas. We often try to solve their problems and not always those of the patient.


Our gratitude and thanks to:

Dr. Eric A. Nauman
Assistant Professor of Mechanical Engineering and Biomedical Engineering
"Harrison" - info (at)
Fell on my ***winter 2003, Canceled fusion April 6 2004
Reborn June 25th, 2004, L5-S1 ADR Charite in Boston
Founder & moderator of ADRSupport - 2004
Founder Arthroplasty Patient Foundation a 501(c)(3) - 2006
Creator & producer, Why Am I Still Sick? - 2012
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Old 01-30-2012, 04:41 PM
jburdman7 jburdman7 is offline
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Join Date: Jan 2012
Posts: 25

Originally Posted by Harrison View Post
be very careful around low hanging branches. I’ve heard of people on riding lawn mowers or people raking their yards who hit their heads on unforgiving branches and destabilize their implant. Very scary situations.
It would be useful to know if this was this before the bone grafted into the implant or if this can happen after. "Destabilize" is such a generic term. What is going on here? It would be a scary thought if you can this innocently bump the plastic pivot of a Prodisc implant out of its socket!
3/2002 L5-S1 microdiscectomy- went well
1/2012 C6,7 arthritic, ADR recommended
BCBS will cover
UPDATE: No doc since the first surgeon has recommended an ADR. It now looks like I am up for another microdiscectomy / foraminotomy
I'm certain I am not the last for such a diagnosis change so we can all still learn from each others experiences!
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