Lumbar TDA, Charité®, Slide-Disc® ROM, Facet Forces, IAR: Eur.SpineJ 2009 Sep Epub

[Slackwater]

PMID: 19763638

Eur Spine J. 2009 Sep 4. [Epub ahead of print]

The effect of different design concepts in lumbar total disc arthroplasty
on the range of motion, facet joint forces and
instantaneous center of rotation of a L4-5 segment.

Schmidt H, Midderhoff S, Adkins K, Wilke HJ.

Institute of Orthopaedic Research and Biomechanics, University of Ulm, Helmholtzstrasse 14, 89081, Ulm, Germany


Although both unconstrained and constrained core lumbar artificial disc designs are in
clinical use, the effect of their design on the range of motion, center of rotations,
and facet joint forces is not well understood. It is assumed that the constrained
configuration causes a fixed center of rotation with high facet forces, while the
unconstrained configuration leads to a moving center of rotation with lower loaded facets.

The authors disagree with both assumptions and hypothesized that the two different designs
do not lead to substantial differences in the results.

For the different implant designs, a three-dimensional finite element model was created
and subsequently inserted into a validated model of a L4-5 lumbar spinal segment. The
unconstrained design was represented by two implants,
+ the Charité® disc and
+ a newly developed disc prosthesis: Slide-Disc®.

The constrained design was obtained by a modification of the Slide-Disc((R))
whereby the inner core was rigidly connected to the lower metallic endplate.
The models were exposed to an axial compression preload of 1,000 N. Pure
unconstrained moments of 7.5 Nm were subsequently applied to the three
anatomical main planes. Except for extension, the models predicted only
small and moderate inter-implant differences. The calculated values were
close to those of the intact segment. For extension, a large difference
of about 45% was calculated between both Slide-Disc designs and the Charité® disc.

The models predicted higher facet forces for the implants with an unconstrained core
compared to an implant with a constrained core. All implants caused a moving center of
rotation. Except for axial rotation, the unconstrained and constrained configurations
mimicked the intact situation. In axial rotation, only the Slide-Disc((R)) with mobile core
reproduced the intact behavior.

Results partially support our hypothesis and imply that different implant designs do not
lead to strong differences in the range of motion and the location of center of rotations.

In contrast, facet forces appeared to be strongly dependent on the implant design.
However, due to the great variability in facet forces reported in the literature,
together with our results, we could speculate that these forces may be more dependent
on the individual spine geometry rather than a specific implant design.


Keywords Mobile artificial discs - Finite element analysis - Arthroplasty devices - Back pain – Implants



Slide-Disc® Weber Instrumente GmbH) slidedisc.org
SB Charité III (DePuy).

Finite element analysis evaluating different non-fusion implant systems (Projekt: 537), page 58 of 92
http://ufbweb.medizin.uni-ulm.de/ufb/Publikationen/Jahresberichte/Jahresbericht-2007.pdf



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slackwater_sf

[Slackwater]

Originally Posted by Slackwater

PMID: 19763638

Eur Spine J. 2009 Sep 4. [Epub ahead of print]

The effect of different design concepts in lumbar total disc arthroplasty
on the range of motion, facet joint forces and
instantaneous center of rotation of a L4-5 segment.

Schmidt H, Midderhoff S, Adkins K, Wilke HJ.

Institute of Orthopaedic Research and Biomechanics, University of Ulm, Helmholtzstrasse 14, 89081, Ulm, Germany


Although both unconstrained and constrained core lumbar artificial disc designs are in
clinical use, the effect of their design on the range of motion, center of rotations,
and facet joint forces is not well understood. It is assumed that the constrained
configuration causes a fixed center of rotation with high facet forces, while the
unconstrained configuration leads to a moving center of rotation with lower loaded facets.

The authors disagree with both assumptions and hypothesized that the two different designs
do not lead to substantial differences in the results.

For the different implant designs, a three-dimensional finite element model was created
and subsequently inserted into a validated model of a L4-5 lumbar spinal segment. The
unconstrained design was represented by two implants,
+ the Charité® disc and
+ a newly developed disc prosthesis: Slide-Disc®.

European Spine Journal, 2009 Nov;18(11):1695-1705.
(c) 2009 Springer

"Conclusions
Total disc arthroplasty inherently alters the kinematics at
implant level for all clinical used prostheses. Compared to
these alterations, our finite element model predicted that
inter-implant differences are small regarding the RoM and
ICR.

In contrast, facet forces appeared to be strongly dependent
on the implant design. However, due to the great variability
in facet forces reported in the literature, together with our results,
we could speculate that these forces may be more dependent
on the individual spine geometry rather than a specific implant design.

In order to be able to make a suggestion, it is necessary to know
which load directions of the spine and which parameter is most significant.

If it is, for example, flexion and the RoM, it does not
make any difference which implant design is used.

If it is in contrast axial rotation and the ICR, our results suggest the
need of a mobile core design."