Mobile vs. Fixed Core (Semiconstrained vs constrained) Disc Designs

[Harrison]

Hi all.

A few minutes ago, I posted an article based on the research from Doctors’ Moumene and Geisler (I also hinted at this work last week). The article is found here and I think you will find it interesting for many different reasons:

- we’ve been discussing design differences among artificial discs for three years now, with patients offering interesting opinions (pros & cons) of certain discs;
- there always seems to be articles or studies to support either “side;”
- this is a frequently discussed and sometimes contentious debate.

It need not be construed as anything more than what it is – more information to consider when researching your artificial disc options. That said, if the conclusions of the study are true, (based on finite element modeling; see previous Q & A with Perdue University) then mobile core designs may be better suited for a mass market. The logic is that the Charite' disc placement is more "forgiving," as a "less than perfect" surgical placement would not be as severe (because of less facet loading)as it might have been in a constrained device. These seems logical to me, but please realize that I am merely the editor and a layman!

In the meantime, I’ll see if I can dig up more information on this work and will post it here. Hope this helps.

[tmont]

The article also reflects some of the same points I've heard from LDR reps concerning the Mobi, a semi-constrained device for those who aren't familiar with it:

* better distribution of biological loads

* more forgiving in placement due to self-centering capacity

* longer-lasting core due to less stress (friction):

"Additionally our study predicted much less stress on a mobile core, which may increase its longevity, compared to a fixed core."

I posted this somewhere already about a year ago, but the LDR rep I had on the phone used the analogy of pushing an orange across the floor. If you push down hard on it (constrain) and push, then the skin is going to wear out more quickly than if it's allowed to roll along under your hand without undue pressure.

A counter-argument has been the purported risk of hypermobility (creating too much mobility) in a spine that is no longer accustomed to it due to ingrained pathology, thus creating the potential for 'new' stress on surrounding structures and making them pain generators independent of the corrective surgery itself. I have yet to see a study suggesting this is the case, and in our own dealings with the cervical prosthesis thus far, no complaints have yet been noted.

It should also be noted that for at least the Mobi-C (don't know about the Mobi-L ), two degrees of core mobility are now possible. This allows the surgeon to judge, level by level, how much mobility is to be restored. Especially in multi-level cases, this theoretically enables more control over and precision regarding restored mobility, to be established according to pre-operative mobility and the desired result.

Only time is going to give us the answer on constrained vs semi-constrained, but for my part--and as a layman just applying basic logic at this point-- I like the idea of two degrees of semi-constrained cores in the hands of a skilled diagnostician and surgeon.

[Slackwater]

Hi all.

A few minutes ago, I posted an article based on the research from Doctors’ Moumene and Geisler (I also hinted at this work last week). The article is found here and I think you will find it interesting for many different reasons:

Charité and ProDisc research in "Spine" Aug 1, 2007
Comparison of Biomechanical Function at Ideal and Varied Surgical Placement for Two Lumbar Artificial Disc Implant Designs: Mobile-Core Versus Fixed-Core
Methods.
A 3D nonlinear finite element model of an intact ligamentous L4-L5
motion segment was developed and validated in all 6 df based on
previous experiments conducted on human cadavers. Facet loading
of a mobile-core TDR and a fixed-core TDR were estimated with 4
different prosthesis placements for 3 different ranges of motion.


Charité and ProDisc research from "The Spine Journal", May-June 2006
Disc arthroplasty design influences intervertebral kinematics and facet forces

Methods
Twelve radiographically normal human cadaveric L5/S1 joints (age range 45–64 years)
were tested before and after disc replacement using
Prodisc II implants (Spine Solutions, Paoli, PA) in six specimens and
SB Charité III (Johnson & Johnson, New Brunswick, NJ) in six other specimens.
Semiconstrained fixtures in combination with a servo-hydraulic materials testing
system subjected the test specimens to a physiologic combination of compression
and anterior shear. ... The IAR was calculated for every 3-degree intervals, and the force
through the facet joints was simultaneously measured using flexible intra-articular sensors.
Data were analyzed using repeated-measures analysis of variance.

Conclusions
The degree of constraint affects post-implantation kinematics and load transfer. With the
Prodisc (3 DOF), the facets were partially unloaded, though the IAR
did not match the fixed geometrical center of the UHMWPE. The latter observation
suggests joint surface incongruence is developed during movement.

With the Charité (5 DOF), the IAR was less variable, yet the facet forces tended to increase,
particularly during lateral bending.

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Limited thinking from S: Different Levels, different results, looking for the next report. Either solution
for ADR is probably better than without ADR.