The Potential Biomechanical Etiology for Lumbar Disc Replacement Failures:

[Don]

Has anyone read this paper? I found it on the Spine-Health.com Back Pain Discussion board today. Brings up a few interesting points.


http://www.ispub.com/ostia/index.php?xmlFilePath=journa.../vol1n2/failures.xml

Don

[annapurna]

I read it jsut now and pondered it on and off during a walk. I'd be interested in hearing what the finite element spine model people that Richard had mentioned on this site some time ago would think about the comments in the paper. As far as I understand the assumptions in the finite element model, they should be able to answer directly whether or not their system rotates where Rosen says it does or where he says all ADR assumes it does. That doesn't give you an immediate answer on whether or not Rosen's wrong as there may be problems in the FEA model but, in my mind, it would point out whether or not Rosen has any solid basis or this is just another cloud of hot air and attitude. Rosen hasn't had the best of reputations for unbiased reporting, but it may be that he's happened into a real design problem.

[Brad]

While I can't give an opinion on the center of rotation issue I can say that he is right in that we have seen people on this forum that fit his exact description of a failed ADR. Some people go straight into the facet problem and within 8 months they are in trouble.

His pictures are the first I have seen that show in detail the degenerated facets.

What I think is positive is that he is documenting a procedure that "fixes" this problem without taking out the disc. This means that ADR is really not the end of the road with respect to surgeries. If you are one of the unlucky that have instant facet problems then you have an option. I know it is still a major surgery but the feeling that you have to live with what you get, otherwise get a life threatening surgery, after ADR surgery looks not to be the case.

Very interesting and thanks for posting.

Brad

[joeytoey]

I have read this Rosen paper as well. My wife had a CT scan done last month and her damaged facets look very similar to the pictures in the paper. As some of you know, wifey's condition has deteriated 3 years post-op of a Charite at L4-5.

Before this paper I had been confused on the role of the facets and their mechanics. Rosen's explanantion of the facets being the center of rotation at each vertabral level woke me right up and I believe he is absolutely correct. The facets are nothing more than 'bearing surfaces' to allow the vertabraes to rotate around during twisting and bending and to keep the 'spine pole' in line. When the body twists, the vertabrae rotate around the facets while the disc stretches laterally with the direction of the rotation. The rotation at each level is subtle, of course. And with the spinal cord being cradled close to the facets, it is disturbed very little during rotation because it is very near the center of rotation, unlike the disc location which is farther away.

So, with those mechanics in mind, the ADR must allow the vertabrael bodies to stretch laterally during twisting motions. Does the Charite or other ADR's allow lateral, or sideways, movement like a natural disc would? I believe the Charite allows the vertabrae to bend at angles up to 8 degrees (correct me if I'm wrong on this number) in any direction. However, I don't believe it allows the vertabrae to stretch or move sideways on a lateral plain like a natural disc seems to, imo.

If this is the case, then Rosen may be right in claiming the Charite (and other ADR's) does not imitate the natural characteristics of a natural disc. But, the big question is: Why do some people have great results from the Charite? What's the difference between the two groups? I think until we know these answers the Charite should be avoided lest more folks end up like my wife and other unfortunate folks.

We just got a radioligists report today on her CT scan. Along with facet damage, he sees a mild broad-based 2mm annular bulge at L3-4.

More things to think about.

joey

[annapurna]

There are a few other points that Rosen didn't include. The facets can't logically be the center of rotation. The way the facet joints function would argue that the spinal cord is the centere of rotation not the facets themselves. The Charite also is designed to force the center of rotation behind the disk, closer to the spinal cord. You can see this if you look at the effect of the sliding core on the rotation. Rosen's point is still worth investigation as the slight movement of the center of rotation may explain why a fraction of people can't seem to tolerate ADR, i.e. they can't tolerate the shift in loading.

I'll also pass along my wife's first thought when I mentioned the paper: Rosen has always advocated fusion and is not mentioning the effect of fusion on altering the center of rotation of adjacent segments. He mentions that it may not have an effect but doesn't elaborate.

Personally, Laura's facets are enlarging even though the cartilage is preserved and the joints show no evidence of inflamation. The best explanation based on that observation is that her facets are seeing higher loads and her body's responding to that.

[Harrison]

Interesting discussion all around. I'd like to offer my opinion as a hack researcher, patient and editor who has followed Rosen's work for several years:

1. We’ve discussed Dr. Rosen’s work throughout the years on this board. Search for his name and read all the posts – you’ll be surprised with the many comments.
2. I don’t see that the article is peer-reviewed, and this is the same data that has been shown before…nothing new here.
3. If docs go through the trouble of doing “research” in excised devices, why not ALSO do an analysis of excised tissue from the patient? E.g., send it to the pathology lab?! Now that may provide truly interesting results (e.g., pathologies that reveal viruses, cell-wall deficient bacteria, etc.).
4. Does this work also help Dr. Rosen gain other employment opportunities, e.g. as an expert witness? (I don’t know if it does; hence my question.)

I emailed Dr. Rosen (as I have before) and offered to meet with him to discuss his work. If I do obtain an appointment, I’ll ask folks here to relay their questions to me.

Jim, just to clarify, are you suggesting that Laura’s facets are getting stronger?

[annapurna]

Jim, just to clarify, are you suggesting that Laura’s facets are getting stronger?

Yes. The radiologist's comment was they looked like they were buttressing in the way an engineer would increase their size in response to higher loads. The cartilage looks good and the joint space is preserved throughout the joint and the bone is not becoming denser but not showing signs of cysts or other breakdown.

[joeytoey]

Annapurna, I just don't see how the spinal canal can be the center of rotation. No offense, it's just my opinion.

If you look at the CT pictures of lumbar facets you'll see 2 curved facet joints at each level. On each facet there are two surfaces to the joint, One surface is the upper vertabrae and the other surface is the lower vertabrae. This point is where the vertabrae of the spine 'interlock'. You can sort of compare the spine with a length of kids plastic interlocking chain links!

So, the facets are the surfaces that meet at the junction where the vertabraes interlock with each other. Because of this interlock, there's no way you can have any lateral movement without causing damage to the facet joints. This interlocking point has to be the center of rotation. It's mechanically impossible for it not to be.

It's interesting that your wifes facets are gaining strength. Was this after an ADR implantation? I wonder if any other ADR people have had this happen. Could this be the answer as to why some peoples ADR's are doing well?

[kanutta]

The lateral recess where the nerve passes on it's way to the intervertebral foramen is partly formed by the facet. If the facet get larger, I suppose you can end up with lateral recess stenosis, compressing the exiting or the traversing nerveroot.

I think hypertrophed facets are a common reason for lateral stenosis...?

[annapurna]

Hypertrophy would both be expected to show signs of breakdown within the bone. This isn't showing those signs. I can see, depending upon the person, this response leading to stenosis in some people but Laura's nerve canals are clear.

Joeytoey, if I'm reading correctly, you're imagining the center of rotation based on a turn of the upper body, like you were looking behind you. If you image the center of rotation based on bending over to touch your toes, I think that's the sense that Rosen's paper was using and I was saying should be near to the spinal canal. Also, the facets don't interlock that tightly. They do allow some movement of the vertebra before the mating facet surfaces touch, so the facets don't necessarily define the point of rotation.