[Artificial Disc Replacement] Comparison of Cervical Spine Biomechanics After Fixed- and Mobile-Core Artificial Disc Replacement: A Finite Element Analysis

[Justin]

Spine. Volume 36(9), 20 April 2011, p 700–708

Comparison of Cervical Spine Biomechanics After Fixed- and Mobile-Core Artificial Disc Replacement: A Finite Element Analysis

Lee, Sang-Hun MD*; Im, Yang-Jin MD*; Kim, Ki-Tack MD*; Kim, Yoon-Hyuk PhD†; Park, Won-Man PhD†; Kim, Kyungsoo PhD‡
Author Information
*Department of Orthopaedic Surgery, Spine Center, Kyung Hee University Hospital at Gangdong, School of Medicine
†Department of Mechanical Engineering and Center of Biomechanical System, ILR Institute, Kyung Hee University, Seoul, Korea
‡Department of Mathematics, Kyonggi University, Suwon, Korea
© 2011 Lippincott Williams & Wilkins, Inc.

Study Design. A biomechanical comparison between the intact C2–C7 segments and the C5–C6 segments implanted with two different constrained types (fixed and mobile core) of artificial disc replacement (ADR) using a three-dimensional nonlinear finite element (FE) model.

Objective. To analyze the biomechanical changes in subaxial cervical spine after ADR and the differences between fixed- and mobile-core prostheses.

Summary of Background Data. Few studies have investigated the changes in kinematics after cervical ADR, particularly in relation to the influence of constrain types.

Methods. A FE model of intact C2–C7 segments was developed and validated. Fixed-core (Prodisc-C, Synthes) and mobile-core (Mobi-C, LDR Spine) artificial disc prostheses were integrated at the C5–C6 segment into the validated FE model. All models were subjected to a follower load of 50 N and a moment of 1 Nm in flexion-extension, lateral bending, and axial torsion. The range of segmental motion (ROM), facet joint force, tension on major ligaments, and stress on the polyethylene (PE) cores were analyzed.

Results. The ROM in the intact segments after ADR was not significantly different from those of the normal cervical spine model. The ROM in the implanted segment (C5–C6) increased during flexion (19% for fixed and 33% for mobile core), extension (48% for fixed and 56% for mobile core), lateral bending (28% for fixed and 35% for mobile core) and axial torsion (45% for fixed and 105% for mobile core). The facet joint force increased by 210% in both fixed and mobile core models during extension and the tension increased (range, 66%–166%) in all ligaments during flexion. The peak stress on a PE core was greater than the yield stress (51 MPa for fixed and 36 MPa for mobile core).

Conclusion. The results of our study presented an increase in ROM, facet joint force, and ligament tension at the ADR segments. The mobile-core model showed a higher increase in segmental motion, facet force, and ligament tension, but lower stress on the PE core than the fixed-core model.

[brookscw]

Have you seen any studies that compare the M6 with the ProDisc? I was surprised to read in the above study that the facet joint pressure was the same for both devices, especially since the ProDisc has unconstrained rotation and the Mobi-C claims to restrict its rotation. Or do I misunderstand something?

[Justin]

Have you seen any studies that compare the M6 with the ProDisc? I was surprised to read in the above study that the facet joint pressure was the same for both devices, especially since the ProDisc has unconstrained rotation and the Mobi-C claims to restrict its rotation. Or do I misunderstand something?

I'm not aware of any studies comparing the M6 and the ProDisc. However, I do know that the Freedom Lumbar Artificial Disc Clinical Trial has the ProDisc as the control device.

To address your second question: I believe you are reading it right. It seems counterintuitive, but that's what the data shows. Interesting for sure.

[KanRunMo]

Justin,
A lot of people mention facets. I'm not sure what that is. My MRI report does not have anything about facets. Would you tell me what that is?
Thanks.

[brookscw]

I'm not Justin, but here is the information you are looking for. MRI reports likely wouldn't mention facet joints unless there was an order specifically looking for that type of information, or if they were really abnormal. I think they usually show up better on X-rays, but I'm not sure about that.

Facet Joints

[spinches]

I am not well-informed about bio-mechanics and I don't understand the implications of the research results. It sounds like the artificial discs caused a large undesirable increase in various kinds of stress on the cervical spine. I would be interested in hearing how other, more mechanically minded members interpret these findings. I haven't had surgery yet and am still debating fusion versus artificial discs, so this study is important to me.

[brookscw]

There just isn't much information on this yet unfortunately because degeneration of facet joints takes time, and even the oldest artificial discs aren't really that old. I hear anecdotal evidence that patients who have had older generation discs like the ProDisc for many years are starting to see arthritic changes due to additional stresses being places on the joints, but nothing even approaching the scientific level of the above study. And even the above study doesn't tell us anything about what those additional stresses would mean in an actual human over time.

I will say though that the newer discs, like the M6, are making a concerted effort to try and match the kinematics of the natural disc as closely as possible. The M6 gets close...the tilting, rotation and lateral motion are all equal to, or slightly less than a natural disc, but the flexion is more than what is natural, and not by an insignificant amount. My guess is the M6's loads on facet joints and the like will be much closer to a natural disc's, but it still won't be perfect.

Unfortunately, fusion is far from a perfect solution too, especially if you're talking about the fusion of multiple levels. I think there's a decent majority of doctors that will tell you that a fusion puts additional stress on the adjacent discs, but it's hard to say that's definitive. It could be the fusion, or it could be that the patient just has bad discs and they were going to fail eventually anyway. None of the studies that I've seen are able to differentiate that. However, from what I can tell, there is a significantly higher rate of additional operations for fusion patients vs. the patients who are receiving the newer discs like the M6, not to mention faster recovery times, less pain, etc. How that patient will fare 10 or 20 years from now is really hard to say. In one case, they might need surgery at an adjacent level (fusion), but in another case, the adjacent levels are fine, but they might have so much arthritic change at the operative level, they'll need a revision of the artificial disc (which means fusion just about every time). Or, in a worst-case scenario with an artificial disc, they have arthritic changes and adjacent level failures. With the newer discs though, I'm just not seeing a lot of evidence that this worst-case is very likely.

For my money (and situation: 2-level surgery, just like yours actually), if I end up needing surgery, I'm pretty convinced that I'll roll the dice with the artificial discs and hope that it's everything they promise. I already know, with a decent amount of confidence, what my future will be with a 2-level fusion (an almost-guaranteed additional operation in 5-10 years time, less range of motion, restricted activities) and the promise of a better life with artificial discs (possibility of never having surgery again, full range of motion, almost no activity restrictions) is worth the risk to me. But every person is different, and in some cases, fusion is definitely the better option. More importantly though, the surgeons you speak with are much smarter than your average forum contributor (read: me), so I would encourage you to get at least 5 or 6 opinions if you are definitely looking at surgery. And feel free to contact any of us privately too with any questions you wouldn't want to put on the forums...that's what this community is here for.

[spinches]

Thank you so much for the detailed and informative post, brookscw. I shall probably take my chances with ADR rather than fusion. I like the way the research looks so far, but beyond that, I think the choice is partly a matter of the personality style of the patient and the surgeon. I am attracted to new and different ways of solving problems, and can accept a moderate amount of risk in the process. Most of the surgeons I have consulted so far have described themselves as "conservative" with an obvious sense of satisfaction, which is not at all how I would describe myself. They emphasize the dangers involved in any kind of spine surgery, but are especially wary of ADR, because the long-range outcomes are still unknown. I have listened carefully to their concerns, but I am much more fearful about their recommendations that I do nothing, and wait until I have a lot more pain and myelopathy before undertaking surgery. I am starting to understand that it is very important to work with a surgeon whose general attitude toward innovation and risk-taking is more similar to my own.

[brookscw]

A lot of times surgeons are viewed as scalpel-cowboys, so in some circles, being considered conservative is a good thing. Certainly, if there is an option of not having surgery and still preserving your quality of life, then I think it would be irresponsible for a surgeon to recommend going under the knife, but unfortunately more than a few surgeons see surgery as the solution to all problems. But when you actually press them on it, none of them can give you a 100% guarantee.

The question in most of the cases you'll find here on these forums is how you define quality of life? For some people, just being able to work and managing the pain with medication is enough (and I think most conservative surgeons would agree with that), but for others that just isn't enough. For my part, I'm willing to give conservative treatment a chance to see if I can delay surgery for a few years (maybe forever if I'm really, really lucky), but not being able to ever run or lift things again just isn't acceptable, so if at the end of my treatment I'm still not able to do the active things I enjoy, even if I can control the pain with meds, that's just not a good quality of life for me.

Also, if you are in the States, remember that in your case, a 2-level problem, the only option available to surgeons is fusion, which is a most decidedly one-way street. That might have something to do with their recommendation, or it might not, but it's something to keep in mind.

[spinches]

Thank you for all your responses to my post, Brook. The point you make about quality of life is very interesting, and I would like to pursue the subject in another thread. I would like to ask another question on this thread relevant to the biomechanics subject and ADR. I have seen a number of comments on this site and others that refer to "the problems with the ProDisc (and other older AD's)." The contexts of these comments convey that the ProDisc allows a greater than normal range of movement, and this caused pain and other issues. If there are members well-versed in biomechanics reading this post, I would appreciate more information about the disadvantages of ProDisc, Bryan, and Prestige for cervical ADR. Have the mechanical problems been corrected in newer versions of the ProDisc, Bryan, etc., or are these still concerns for cervical surgery patients?