Mobile v. Fixed Core TDA Intervertebral Displacement: SAS Journal, 2009 September

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SAS Journal, Volume 3, Issue 3, Pages 91-99 (September 2009), Full Text
© 2009 Elsevier, All Rights Reserved

VT = Intervertebral Translation
M = Mobile Core = Mobidisc; LDR Médical, Troyes, France
F = Fixed Core = ProDisc-L; Synthes Spine, West Chester, Pennsylvania


Does core mobility of lumbar total disc arthroplasty influence sagittal and frontal intervertebral displacement?
Radiologic comparison with fixed-core prosthesis

Joël Delécrin, MDa, Jérôme Allain, MDb, Jacques Beaurain, MDc, Jean-Paul Steib, MDd, Hervé Chataigner, MDe, Lucie Aubourg, PhDf, Jean Huppert, MDg, Marc Ameil, MDh, Jean-Michel Nguyen, MD, PhDi

a University Hospital – Orthopedic, CHU Hôtel Dieu, Nantes, France
b University Hospital – Orthopedic, Paris Creteil, France
c University Hospital – Neurosurgery, Dijon, France
d University Hospital – Spine, Strasbourg, France
e Private Hospital – Orthopaedic, Besancon, France
f Employee, LDR Compagny, Troyes, France
g Private Hospital – Neurosurgery, Saint Etienne, France
h Private Hospital – Orthopaedic, Reims, France
i University Hospital – Biostatistic, Nantes, France


Abstract

Background
An artificial disc prosthesis is thought to restore segmental motion in the lumbar spine.
However, it is reported that disc prosthesis can increase the intervertebral translation (VT).
The concept of the mobile-core prosthesis is to mimic the kinematic effects of the migration
of the natural nucleus and therefore core mobility should minimize the VT. This study
explored the hypothesis that core translation should influence VT and that a mobile core
prosthesis may facilitate physiological motion.

Methods
Vertebral translation (measured with a new method presented here), core translation,
range of motion (ROM), and distribution of flexion-extension were measured on
flexion-extension, neutral standing, and lateral bending films in
89 patients (63 mobile-core [M]; 33 fixed-core [F]).


Results
At L4-5 levels the VT with M was lower than with F and similar to the VT of untreated levels.
At L5-S1 levels the VT with M was lower than with F but was significantly different compared
to untreated levels. At M levels a strong correlation was found between VT and core translation;
the VT decreases as the core translation increases. At F levels the VT increases as the ROM
increases. No significant difference was found between the ROM of untreated levels and levels
implanted with either M or F. Regarding the mobility distribution with M and F we observed a
deficit in extension at L5-S1 levels and a similar distribution at L4-5 levels compared to untreated
levels.

Conclusion
The intervertebral mobility was different between M and F.
The M at L4-5 levels succeeded to replicate mobility similar to L4-5 untreated levels.
The M at L5-S1 succeeded in ROM, but failed regarding VT and mobility distribution. Nevertheless
M minimized VT at L5-S1 levels.
The F increased VT at both L4-5 and L5-S1.


Clinical Relevance
This study validates the concept that the core translation of an artificial lumbar disc prosthesis minimizes the VT.

Keywords: Lumbar segmental motion, Mobile-core prosthesis, Fixed-core prosthesis, Translation measurement method

Article Outline
• Abstract
• Introduction
• Materials and methods
• Results
• Sagittal plane
• Frontal plane
• Discussion
• Conclusion
• References
• Extended References
• Copyright


Introduction
The goal of the lumbar disc prosthesis is to restore physiological segmental motion
at the operative levels, and consequently, to prevent posterior facet degradation and
adjacent segment disease. However, it is reported that disc prosthesis placement
could provide unnatural kinematics including a decrease of sagittal range of motion,1, 2
a disturbance of flexion-extension distribution,1 and also an abnormal increase of axial
rotation.3, 4 Some authors5 reported an increase of the intervertebral translation (VT)
during flexion-extension, which consequently may increase load on the facet joints and
may generate arthritic progression. This report5 involved different prosthesis designs
providing different amounts of excess VT occurring with sagittal rotation.

There are several total disc replacements that have been designed and are in various
stages of testing and clinical use.6, 7 Among the different designs is the mobile-core
prosthesis. The concept of the mobile-core prosthesis is to mimic the kinematic effects
of the migration of the natural nucleus. The mobile-core designs do not constrain the
center of rotation to a fixed point. The locus of points that define the different
instantaneous centers of rotation can be theoretically replicated. Therefore, core mobility
should minimize the VT because the natural migration of the nucleus minimizes
the VT and core mobility replicates the natural migration of the nucleus.

For the fixed-core designs, meaning ball-and-socket configurations, the center of rotation
is fixed and corresponds to the geometric center of the ball. The amount of VT occurring
with rotation in the sagittal plane depends on the radius of curvature of the ball. In the
frontal plane (during lateral-bending), frontal rotation and axial rotation and axial VT
are coupled as the axial center of rotation is posterior to the disc space close to the
facet joints.8 With core-translation (allowing axial VT), the coupled motion of frontal
rotation and axial rotation should be facilitated

The goal of this study was to assess the concept that core translation in a lumbar artificial disc
should influence VT and may facilitate physiological intervertebral mobility replication in both
the sagittal and frontal planes. Radiologic intervertebral motion characteristics of levels
implanted with mobile-core prosthesis were compared to
(a) levels treated by fixed-core prosthesis,
(b) to adjacent untreated levels, and
(c) to normative data from literature.9

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