229 research outputs found
Significance of the collagen criss-cross angle distributions in lumbar annuli fibrosi as revealed by finite element simulations
In the human lumbar spine, annulus fibr
osus (AF) fibres la
rgely contribute to
intervertebral disc (IVD) stability, and deta
iled annulus models are required to obtain
reliable predictions of lumbar spine biom
echanics by finite element (FE) modelling.
However, different definitions of collagen
orientation coexist in the literature for
healthy human lumbar AFs and are indiscrimi
nately used in mode
lling. Therefore, four
AF fibre-induced anisotropy models were bu
ilt from reported anatomical descriptions
and inserted in a L3-L5 lumbar bi-segment
FE model. AF models were respectively
characterized by radial, tange
ntial, radial and tangentia
l, and no fibre orientation
gradients. IVD local biomechanics was
studied under axial rotation and axial
compression. A new parameter, i.e. the Fi
bre Contribution Qual
ity parameter, was
computed in the anterior, lateral, postero-l
ateral and posterior AFs of each model, in
function of fibre stresses, load distributions,
and matrix shear strains. Locally, each AF
model behaved differently, affecting the
IVD biomechanics. The Fibre Contribution
Quality (FCQ) parameter established a direct
link between local AF fibre organization
and loading, while other biomechanical data
did not. It was conc
luded that local AF
fibre orientations should be modelled in rela
tion to other segment characteristics. The
proposed FCQ parameter could be used to examine such relations, being, therefore
particularly relevant to patient-specifi
c models or artificial disc designs.Postprint (published version
Regional annulus fibre orientations used as a tool for the calibration of lumbar intervertebral disc finite element models
The highly organized collagen
network of human lumbar a
nnulus fibrosus (AF) is
fundamental to preserve the mechanical inte
grity of the interverte
bral discs. In the
healthy AF, fibres are embedded in a hydrated
matrix and arranged in a criss-cross
fashion, giving an anisotropic structure capab
le to undergo large st
rains. Quantitative
anatomical examinations revealed particular
fibre orientation patterns, possibly coming
from regional adaptations of the AF mechan
ics. Based on such hypothesis, this study
aimed to show that the regional differen
ces in AF mechanical behaviour can be
reproduced by considering only fibre orientatio
n changes. Using the finite element (FE)
method, AF matrix was modelled as a poro-hy
perelastic material, where the porous
solid was treated as a comp
ressible continuum following
a Neo-Hookean constitutive
law. Strain-dependent permeability was assumed and all material parameters were taken
from the literature. Fibre reinforcement wa
s accounted for by adding an extra-term to
the porous matrix strain energy density func
tion, only active along th
e fibre directions.
Through such term, fibre orientations were then adjusted, to reproduce AF tensile
behaviours measured for four different regi
ons: posterior outer (PO), anterior outer
(AO), posterior inner (PI) and anterior inne
r (AI). Curve calibrations resulted in the
following optimal angles, calculated with respect to the circumferential axis: 28º for PO,
23º for AO, 43º for PI and
31º for AI. In average, we
obtained fibres 30% more
transversal in the inner than in the outer
AF against 38% as measured by Cassidy et al.
(1989). Fibres more axial in the posterior than
in the anterior AF were also measured by
Holzapfel et al. (2005), with
angle values comparable to
our computed average values.
Since all the hyperelastic and fluid-phase material parameters remained unchanged
throughout the AF, calibration based only on
fibre patterns variations may be an
effective tool to calibrate the regional AF mechanics in a realistic way.Postprint (published version
A micro-macro evaluation of the vertebral bony endplate permeability based on computational fluid dynamics
The intrinsic permeability is an important parameter that
describes the resistance
of a porous structure
to fluid flo
w. It
has a key role in poroelastic finite element models of spinal
segments, especially at the
vertebral endplate, i.e. the
interface
between intervertebral disc and vertebra. In the understanding
of the properties of the complex endplate system, an expli
cit
evaluation for permeability of subchondral bone is missing.
Thus, a
new method
wa
s
proposed to evaluate the intrinsic
permeability of the bony endplate.
CT
-
based reconstruction
s
of the bony endplate from a lumbar vertebra were analyzed
using computational fluid dynamics
, and
the i
ntrinsic
permeability and porosity
of the structure
were calculated.
Results showed that
the
permeability
did
not depend on
the
fluid flow direction,
and was statistically similar for both the
superior and inferior endplates
. Permeability values varied
within the range of trabecular bone, while porosity
values
w
ere
lower than trabecular bone characteristic values.
Finally,
i
ntrins
ic permeability correlated well with porosity
through the
Kozeny
-
Karman model, which
offer
s
perspectives for
parametric studies involving degenerative or age
-
related
changes at the disc
-
bone interface.Postprint (published version
Clavos intramedulares vs. placas de osteosíntesis para fracturas de fémur. Análisis por elementos finitos
La estabilización interna de fracturas de la diáfisis femoral se realiza mediante la implantación de clavos intramedulares o placas de osteosíntesis. El objetivo de este estudio es realizar una comparación biomecánica por elementos finitos de ambos implantes y así desarrollar una herramienta preclínica para guiar a los cirujanos en la elección del método de estabilización más adecuado para cada fractura.
Se malló un fémur entero en el cual se practicó una fractura de 1mm ó 3mm en la diáfisis. Los modelos de placa y clavo se mallaron a partir de la geometría de los implantes, para el clavo se realizaron dos modelos con diámetros distintos (11 y 13 mm). El estudio incluye simulaciones en dos etapas de regeneración de la fractura (con modulo de elasticidad en la fractura de 1 MPa ó 10 MPa), con implantes de acero inoxidable y titanio. Las cargas aplicadas simulan el apoyo monopodal e incluyen la actuación de la cadera, y los músculos abductores, psoas ilíaco e iliotibial.
Los resultados muestran que la tensión en los implantes aumenta con el tamaño de la fractura y disminuye con su módulo. Los clavos tienden a inducir más deformación en la fractura que las placas. El diámetro de los clavos influye en la tensión inducida en el hueso y en la deformación en la fractura, los resultados de los clavos más anchos tienden a los resultados de las placas
Clavos intramedulares vs. placas de osteosíntesis para fracturas de fémur: Análisis por elementos finitos.
La estabilización interna de fracturas de la diáfisis femoral se realiza mediante la implantación
de clavos intramedulares o placas de osteosíntesis. El objetivo de este estudio es realizar una comparación
biomecánica por elementos finitos de ambos implantes y así desarrollar una herramienta
preclínica para guiar a los cirujanos en la elección del método de estabilización más adecuado para
cada fractura.
Se malló un fémur entero en el cual se practicó una fractura de 1mm ó 3mm en la diáfisis. Los
modelos de placa y clavo se mallaron a partir de la geometría de los implantes, para el clavo se
realizaron dos modelos con diámetros distintos (11 y 13 mm). El estudio incluye simulaciones en dos
etapas de regeneración de la fractura (con modulo de elasticidad en la fractura de 1 MPa ó 10
MPa), con implantes de acero inoxidable y titanio. Las cargas aplicadas simulan el apoyo monopodal
e incluyen la actuación de la cadera, y los músculos abductores, psoas ilíaco e iliotibial.
Los resultados muestran que la tensión en los implantes aumenta con el tamaño de la fractura y
disminuye con su módulo. Los clavos tienden a inducir más deformación en la fractura que las
placas. El diámetro de los clavos influye en la tensión inducida en el hueso y en la deformación en la
fractura, los resultados de los clavos más anchos tienden a los resultados de las placas
Focused ultrasound excites neurons via mechanosensitive calcium accumulation and ion channel amplification
Ultrasonic neuromodulation has the unique potential to provide non-invasive control of neural activity in deep brain regions with high spatial precision and without chemical or genetic modification. However, the biomolecular and cellular mechanisms by which focused ultrasound excites mammalian neurons have remained unclear, posing significant challenges for the use of this technology in research and potential clinical applications. Here, we show that focused ultrasound excites neurons through a primarily mechanical mechanism mediated by specific calcium-selective mechanosensitive ion channels. The activation of these channels results in a gradual build-up of calcium, which is amplified by calcium- and voltage-gated channels, generating a burst firing response. Cavitation, temperature changes, large-scale deformation, and synaptic transmission are not required for this excitation to occur. Pharmacological and genetic inhibition of specific ion channels leads to reduced responses to ultrasound, while over-expressing these channels results in stronger ultrasonic stimulation. These findings provide a critical missing explanation for the effect of ultrasound on neurons and facilitate the further development of ultrasonic neuromodulation and sonogenetics as unique tools for neuroscience research
Nanotechnology in regenerative medicine: the materials side
Regenerative medicine is an emerging multidisciplinary field that aims to restore, maintain or enhance tissues and hence organ functions. Regeneration of tissues can be achieved by the combination of living cells, which will provide biological functionality, and materials, which act as scaffolds to support cell proliferation. Mammalian cells behave in vivo in response to the biological signals they receive from the surrounding environment, which is structured by nanometre-scaled components. Therefore, materials used in repairing the human body have to reproduce the correct signals that guide the cellstowards a desirable behaviour. Nanotechnology is not only an excellent tool to produce material structures that mimic the biological ones but also holds the promise of providing efficient delivery systems. The application of nanotechnology to regenerative medicine is a wide issue and this short review will only focus on aspects of nanotechnology relevant to biomaterials science.
Specifically, the fabrication of materials, such as nanoparticles and scaffolds for tissue engineering, and the
nanopatterning of surfaces aimed at eliciting specific biological responses from the host tissue will be addressed.Postprint (published version
Molecular mechanism for depolarization-induced modulation of Kv channel closure
Voltage-dependent potassium (Kv) channels provide the repolarizing power that shapes the action potential duration and helps control the firing frequency of neurons. The K(+) permeation through the channel pore is controlled by an intracellularly located bundle-crossing (BC) gate that communicates with the voltage-sensing domains (VSDs). During prolonged membrane depolarizations, most Kv channels display C-type inactivation that halts K(+) conduction through constriction of the K(+) selectivity filter. Besides triggering C-type inactivation, we show that in Shaker and Kv1.2 channels (expressed in Xenopus laevis oocytes), prolonged membrane depolarizations also slow down the kinetics of VSD deactivation and BC gate closure during the subsequent membrane repolarization. Measurements of deactivating gating currents (reporting VSD movement) and ionic currents (BC gate status) showed that the kinetics of both slowed down in two distinct phases with increasing duration of the depolarizing prepulse. The biphasic slowing in VSD deactivation and BC gate closure was strongly correlated in time and magnitude. Simultaneous recordings of ionic currents and fluorescence from a probe tracking VSD movement in Shaker directly demonstrated that both processes were synchronized. Whereas the first slowing originates from a stabilization imposed by BC gate opening, the subsequent slowing reflects the rearrangement of the VSD toward its relaxed state (relaxation). The VSD relaxation was observed in the Ciona intestinalis voltage-sensitive phosphatase and in its isolated VSD. Collectively, our results show that the VSD relaxation is not kinetically related to C-type inactivation and is an intrinsic property of the VSD. We propose VSD relaxation as a general mechanism for depolarization-induced slowing of BC gate closure that may enable Kv1.2 channels to modulate the firing frequency of neurons based on the depolarization history
Molecular mechanism for depolarization-induced modulation of Kv channel closure
Voltage-dependent potassium (Kv) channels provide the repolarizing power that shapes the action potential duration and helps control the firing frequency of neurons. The K+ permeation through the channel pore is controlled by an intracellularly located bundle-crossing (BC) gate that communicates with the voltage-sensing domains (VSDs). During prolonged membrane depolarizations, most Kv channels display C-type inactivation that halts K+ conduction through constriction of the K+ selectivity filter. Besides triggering C-type inactivation, we show that in Shaker and Kv1.2 channels (expressed in Xenopus laevis oocytes), prolonged membrane depolarizations also slow down the kinetics of VSD deactivation and BC gate closure during the subsequent membrane repolarization. Measurements of deactivating gating currents (reporting VSD movement) and ionic currents (BC gate status) showed that the kinetics of both slowed down in two distinct phases with increasing duration of the depolarizing prepulse. The biphasic slowing in VSD deactivation and BC gate closure was strongly correlated in time and magnitude. Simultaneous recordings of ionic currents and fluorescence from a probe tracking VSD movement in Shaker directly demonstrated that both processes were synchronized. Whereas the first slowing originates from a stabilization imposed by BC gate opening, the subsequent slowing reflects the rearrangement of the VSD toward its relaxed state (relaxation). The VSD relaxation was observed in the Ciona intestinalis voltage-sensitive phosphatase and in its isolated VSD. Collectively, our results show that the VSD relaxation is not kinetically related to C-type inactivation and is an intrinsic property of the VSD. We propose VSD relaxation as a general mechanism for depolarization-induced slowing of BC gate closure that may enable Kv1.2 channels to modulate the firing frequency of neurons based on the depolarization history
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