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Old 10-08-2008, 07:37 PM
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Harrison Harrison is offline
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Default Footprint Mismatch in Lumbar Total Disc Arthroplasty

We caught up with one of the authors of a recent paper which examines the criticality of endplate selection criteria for patients receiving artificial disc replacement. Christian Bach, M.D., M.Sc., one of the authors in this interesting paper, was kind enough to address my questions by email. See the referring abstract here, or you can email me to request the complete version.

Q & A Column with Dr. Christian Bach, M.D., M.Sc.
Commentary on “Footprint Mismatch in Lumbar Total Disc Arthroplasty”
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ADRSupport: What is the genesis of this project? What were the questions that led you to initiate the research?
Christian Bach M.D., M.Sc.: The paper was triggered by our own experiences with different implants (we noticed subsidence and migration in some patients with large vertebrae in post-operative follow up) and by other publications from other centers reporting on the same problem. So we realized that this is a major problem and we are not the only ones with that concern.

ADRSupport: Who sponsored the research?
Christian Bach M.D., M.Sc.: there was no sponsorship.

ADRSupport: What is a footprint mismatch? E.g., at what point is a mismatch significant? What degree of mismatch will lead to subsidence or other complications?
Christian Bach M.D., M.Sc.: Footprint mismatch means that the size of the prosthesis does not fit to the size of the vertebra correctly. Biomechanical studies have shown that implants should be as large as possible to reach the periphery of the vertebra -- because this is the strongest part of bone (cortical bone). If the implant does not reach the strong, lateral area of the vertebra, subsidence and migration may result. But this cannot be predicted in advance; i.e. we do not know which prosthesis will migrate and which will stay stable in place, even if there is a mismatch. But mismatch is a RISK FRACTOR for subsidence.

ADRSupport: Can you tell us the details of the patient selection process? How were these 22 patients selected? What were the inclusion and exclusion criteria? Were they screened for osteoporosis of the spine or any other diseases? How many of these patients had artificial discs that subsided into their vertebrae? Was this measured?
Christian Bach M.D., M.Sc.: We selected lumbar CT scans from patients treated at our outpatient clinic because of non-radicular chronic low back pain. CT scans were performed between March 2006 and January 2007. Age was 27–65 years. Tumors, infection and things like that were excluded. However these patients were “low back pain patients” and did not have any type of surgery. Therefore, there was no need to screen them for osteoporosis. None of these patients had/or were scheduled to receive artificial disc replacement.

ADRSupport: Is the sample size of 22 patients statistically relevant? What conclusions can be made of the patient populations in non-German countries, like the U.S.? Does the patient country of origin effect the conclusions we can draw? E.g., there are variations in patient selection and surgical training between countries, so how do these differences effect tentative conclusions we may be tempted to make?
Christian Bach M.D., M.Sc.: Of course! The sample size is adequate. As you can read in the paper, we measured several vertebrae in each of these patients -- a total of 240 endplates which were measured. I don’t think that there is a relevant difference between US and Europeans regarding the endplate size; however, there are no studies proving that. I don’t think that the US population has smaller endplates than Europeans; on the other hand if US patients would have large endplates, the results of measurement would even be worse (even higher percentage of mismatch) because the prosthesis (various artificial disc implants) are distributed with the same sizes all over the world. Things may be different in Asia, because we know that the population there is significantly smaller. If the same study would be performed which “Asian endplates” results may be better (better matching).

Surgical training is always relevant. However, if the prosthesis’s are too small, migration may occur even if the surgeon is experienced. This topic of the study is technical. The surgeon, no matter how experienced he/she is, should be provided with all the implants sizes possible.

ADRSupport: Given the tens of thousands of patients that had disc replacements since the 1980s, we might expect to have read about huge numbers of catastrophic outcomes. And that’s not been the case. But to your point, on a smaller scale, we’ve read about several of these events in our own patient community. Yet, is it statistically safe to extrapolate from your results and make clinical conclusions about a global population of patients?
Christian Bach M.D., M.Sc.: First, I don’t think that subsidence and migration will cause “catastrophic outcomes” in every patient. But it is not the (design) intention of a prosthesis to subside and therefore this complication should be avoided. We don`t know exactly what will happen in the long term with these subsided implants (pain, fusion, loss of range of motion, osteolysis,, etc.). Second, since we do not know which implant will subside, we cannot predict the “global” rate of subsided discs. We know the rate of mismatches and we know the rate of subsidence from different clinical studies (which ranges significantly). We do not know which prosthesis will migrate and which will stay stable in place, even if there is a mismatch. But mismatch is a risk factor for subsidence.

ADRSupport: Interesting. My anecdotal data points are based on my experiences: what I’ve read in studies (and tried to understand), my success with my Charite in 2004, the good health of the first patient in the world to receive an artificial disc implant 24 years ago (met him in Berlin last year) and what you stated about “forgiveness” of the unconstrained design of artificial discs. Can you talk a bit more about what you meant in your paper about this notion of “forgiveness” in terms of compensatory errors of implant placement? If a device design is “forgiving,” doesn’t that translate to less stress (literally) on all spinal structures – and less particle wear?
Christian Bach M.D., M.Sc.: No. A less constrained design may be less sensitive to misplacement within the motion segment. A correct placement regarding the center of rotation is mandatory to achieve movement of the implant. In a more constrained design, the implant has to be placed in more precisely (in the posterior region of the segment). A more anteriorly placed prosthesis would not move because the center of rotation is not restored. However, it is thought that a constrained design may provide a protective effect for the facets; and therefore it may have advantages in the long term.

ADRSupport: Is there enough evidence presented in the paper to safely assume, “the bigger the endplate used -- the better?” If larger endplate selection creates too much of a technical challenge for the surgeon population, what can be done to manage this issue?
Christian Bach M.D., M.Sc.: As already mentioned above there are biomechanical/cadaver studies showing that the periphery of the endplate is the strongest part of the endplate. This is beyond controversy. Implantation of larger endplates is no challenge for an experienced surgeon; however, the lateral parts of the annulus must be preserved to provide adequate stability of the motion segment. This was taken into account in the current study.

ADRSupport:The authors offer some interesting conclusions about the advantages of unconstrained devices (Charite’ and Mobi) relative to errors in surgical placement (“forgiveness”). We’ve discussed this topic this for several years now and patients can’t help to wonder -- is their a consensus on this issue in the medical community? What evidence is there for the basis of this conclusion? Is this based solely on finite element analysis?
Christian Bach M.D., M.Sc.: I am sorry but there is no consensus at all on this issue. However in the meantime, there are several publications on failure of unconstrained and less constrained prosthesis’s showing significant wear of the polyethylene inlay. This may indicate that the less constrained concept does not work so well in the long term but there is no level 1 evidence on that.

ADRSupport:What does all this mean for patients? What can patients do about this new information?
Christian Bach M.D., M.Sc.: That’s a difficult question. I think that first of all, spine surgeons should be aware of this fact. They should screen their patients prior to surgery. They should know the sizes of the endplates of the segment that is planned for an arthroplasty to see if the available prosthesis is matching the individual anatomy. In total hip replacement, such a preoperative planning is common and state of the art. Maybe this procedure of preoperative measurement will become a standard in TDR in the future.

Currently, based on the results of the current investigation, this should cause a high rejection rate (patients with inadequate endplate sizes). In the next step, companies should offer more different footprints for their implants to provide the surgeon and patient with appropriate implants to reduce the rate of patients which cannot be operated because of footprint mismatch!

ADRSupport: Vielen Dank, Dr. Bach.
Christian Bach M.D., M.Sc.: Kein Problem. Ich freue mich!
__________________________________________________ ________________

Note: This interview is copyrighted material.
"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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artificial disc subsidence, device design, device migration, disc replacement, endplate sizing, migration, prosthesis design, prosthesis endplate, prosthesis endplate sizing, subsidence, vertebral body size

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