A Porous Media Approach to Finite Deformation Behaviour in Soft Tissues

Investigation of Cell-Substrate Adhesion Properties of Living Chondrocyte by Measuring Adhesive Shear Force and Detachment Using AFM and Inverse FEA

It is well-known that cell adhesion is important in many biological processes such as cell migration and proliferation. A better understanding of the cell adhesion process will shed insight into these cellular biological responses as well as cell adhesion-related diseases treatment. However, there is little research which has attempted to investigate the process of cell adhesion and its mechanism. Thus, this paper aims to study the time-dependent adhesion properties of single living chondrocytes using an advanced coupled experimental-numerical approach. Atomic Force Microscopy (AFM) tips will be used to apply lateral forces to detach chondrocytes that are seeded for three different periods. An advanced Finite Element Analysis (FEA) model combining porohyperelastic (PHE) constitutive model and cohesive zone formulation is developed to explore the mechanism of adhesion. The results revealed that the cells can resist normal traction better than tangential traction in the beginning of adhesion. This is when the cell adhesion molecules establish early attachment to the substrates. After that when the cells are spreading, stress fiber bundles generate tangential traction on the substrate to form strong adhesion. Both simulation and experimental results agree well with each other, providing a powerful tool to study the cellular adhesion process.

Investigation of the Effects of Extracellular Osmotic Pressure on Morphology and Mechanical Properties of Individual Chondrocyte

It has been demonstrated that most cells of the body respond to osmotic pressure in a systematic manner. The disruption of the collagen network in the early stages of osteoarthritis causes an increase in water content of cartilage which leads to a reduction of pericellular osmolality in chondrocytes distributed within the extracellular environment. It is therefore arguable that an insight into the mechanical properties of chondrocytes under varying osmotic pressure would provide a better understanding of chondrocyte mechanotransduction and potentially contribute to knowledge on cartilage degeneration. In this present study, the chondrocyte cells were exposed to solutions with different osmolality. Changes in their dimensions and mechanical properties were measured over time. Atomic force microscopy (AFM) was used to apply load at various strain-rates and the force-time curves were logged. The thin-layer elastic model was used to extract the elastic stiffness of chondrocytes at different strain-rates and at different solution osmolality. In addition, the porohyperelastic (PHE) model was used to investigate the strain-rate-dependent responses under the loading and osmotic pressure conditions. The results revealed that the hypo-osmotic external environment increased chondrocyte dimensions and reduced Young's modulus of the cells at all strain-rates tested. In contrast, the hyper-osmotic external environment reduced dimensions and increased Young's modulus. Moreover, using the PHE model coupled with inverse FEA simulation, we established that the hydraulic permeability of chondrocytes increased with decreasing extracellular osmolality which is consistent with previous work in the literature. This could be due to a higher intracellular fluid volume fraction with lower osmolality.

Microscale consolidation analysis of relaxation behavior of single living chondrocytes subjected to varying strain-rates

Besides the elastic stiffness, the relaxation behavior of single living cells is also of interest of various researchers when studying cell mechanics. It is hypothesized that the relaxation response of the cells is governed by both intrinsic viscoelasticity of the solid phase and fluid-solid interactions mechanisms. There are a number of mechanical models have been developed to investigate the relaxation behavior of single cells. However, there is lack of model enable to accurately capture both of the mechanisms. Therefore, in this study, the porohyperelastic (PHE) model, which is an extension of the consolidation theory, combined with inverse Finite Element Analysis (FEA) technique was used at the first time to investigate the relaxation response of living chondrocytes. This model was also utilized to study the dependence of relaxation behavior of the cells on strain-rates. The stress-relaxation experiments under the various strain-rates were conducted with the Atomic Force Microscopy (AFM). The results have demonstrated that the PHE model could effectively capture the stress-relaxation behavior of the living chondrocytes, especially at intermediate to high strain-rates. Although this model gave some errors at lower strain-rates, its performance was acceptable. Therefore, the PHE model is properly a promising model for single cell mechanics studies. Moreover, it has been found that the hydraulic permeability of living chondrocytes reduced with decreasing of strain-rates. It might be due to the intracellular fluid volume fraction and the fluid pore pressure gradients of chondrocytes were higher when higher strain-rates applied.

A two-phase model of plantar tissue: a step toward prediction of diabetic foot ulceration

A new computational model, based on the thermodynamically constrained averaging theory, has been recently proposed to predict tumor initiation and proliferation. A similar mathematical approach is proposed here as an aid in diabetic ulcer prevention. The common aspects at the continuum level are the macroscopic balance equations governing the flow of the fluid phase, diffusion of chemical species, tissue mechanics, and some of the constitutive equations. The soft plantar tissue is modeled as a two-phase system: a solid phase consisting of the tissue cells and their extracellular matrix, and a fluid one (interstitial fluid and dissolved chemical species). The solid phase may become necrotic depending on the stress level and on the oxygen availability in the tissue. Actually, in diabetic patients, peripheral vascular disease impacts tissue necrosis; this is considered in the model via the introduction of an effective diffusion coefficient that governs transport of nutrients within the microvasculature. The governing equations of the mathematical model are discretized in space by the finite element method and in time domain using the θ-Wilson Method. While the full mathematical model is developed in this paper, the example is limited to the simulation of several gait cycles of a healthy foot.

Biomechanical Multiphase Approaches in Soft Tissues

Presence of fluids drives mechanical response in biological, soft hydrated tissues. A multiphase approach has been developed to allow for a fully-comprehensive, physically based view of their history-dependent response in static and dynamics. The related formulation considers both geometric and material non-linearities. In the present work, the non-linear aspects related to fluid flows are investigated. Low speed flows of liquid phase depend on the permeability of the deforming solid matrix; permeability is naturally related to void ratio, but further dependences on other variables can be explicitly added. Two different biomechanical problems are considered and the results are presented here. In the first case, a human lumbar intervertebral segment is investigated in healthy and degenerate condition: higher permeability and lower mechanical properties of disc affect the capability to withstand an imposed constant compressive load in time. The permeability is related only to void ratio and is not affected by the overpressure that dissipates with time. The second case refers to the trabecular meshwork in the eye; this tissue has a rule in the regulation of outflow of the aqueous humour from the eye: a variation in permeability determines obviously a variation of internal ocular pressure to guarantee the assumed constant outflow, and on the other side, pressure changes affect permeability distribution and thus final pressure changes. This effect is particularly significant with glaucoma, and thus the increment of intraocular pressure can be related to a progressive reduction in tissue permeability. The modelling of this effect can be improved by introducing a proper permeability-pressure relationship in addition to the permeability-void ratio dependency.

Quantification of intervertebral disc volume properties below spine fusion, using magnetic resonance imaging, in adolescent idiopathic scoliosis surgery

STUDY DESIGN

Prospective clinical study. A quantification of volume and hydration variation of the intervertebral discs, using magnetic resonance imaging (MRI), in the lumbar spine before and after surgery performed in adolescent idiopathic scoliosis (AIS).

OBJECTIVES

To evaluate an objective quantification of volume and hydration of intervertebral discs below spine fusion in scoliosis surgery.

SUMMARY AND BACKGROUND DATA

Repercussion of long spine fusion on the free lower lumbar spine is one of the major concerns of scoliosis surgery. However, the evolution of lumbar intervertebral disc below thoracolumbar fusions remains unknown.

METHODS

MRI performed in the clinical protocol, concerned 28 patients having an idiopathic scoliosis. They underwent posterior instrumentations. MRI was obtained before surgery, after surgery at 3 months and for 15 patients at 1 year. MRI data were posttreated using a custom-made image processing software to semiautomatically derive volume properties of disc, anulus fibrosus, and nucleus pulposus. The nucleus-disc volume ratio was also an indicator of the hydration level.

RESULTS

The reliability of the three-dimensional reconstruction process was initially verified using an intraoperator reproducibility test. Original preoperative data on disc volume properties were then derived. Postoperative volume variations were quantified in discs below spine fusion taking into account the level of the arthrodesis and the disc location. It showed that the postoperative volume criteria increased significantly for nucleus, disc, and nucleus-disc volume ratio and some magnitude modulation could be conditioned by the location of surgical instrumentation. Some stabilization or reduction depending on disc level and arthrodesis size between 3 months and 1 year is observed in the follow-up. It tended to prove that the recovery of balance physiologic positioning and inherent biomechanical loads could induce a restored hydration of disc, which should favor the remodeling of free segments.

CONCLUSIONS

This work was the first report dealing with consequences of scoliosis surgery on subjacent disc in term of volume and hydration properties.

Objective quantification of intervertebral disc volume properties using MRI in idiopathic scoliosis surgery

The aim of this study was to quantify from magnetic resonance imaging (MRI) the volume and hydration variation of the intervertebral disc in the lumbar spine before and after surgery in severe idiopathic scoliosis cases. MRI data were posttreated using a custom-made image processing software to semiautomatically derive volume properties of disc, annulus fibrosus and nucleus pulposus. The nucleus-disc volume ratio was also an indicator of the hydration level. The MRI that was performed in the clinical protocol concerned 28 patients with severe idiopathic scoliosis; four types of surgical instrumentations were used. The reliability of the three-dimensional reconstruction process was initially verified using an intraoperator reproducibility test. Original preoperative data on disc volume properties were then derived. Postoperative volume variations were quantified in discs below spine fusion, taking into account the level of arthrodesis and the disc location. It showed that the postoperative volume criteria increased significantly for nucleus, disc and nucleus-disc volume ratio and that some magnitude modulation could be conditioned by the location of surgical instrumentation. It tended to prove that the recovery of balanced physiological positioning and inherent biomechanical loads could induce a restored hydration of disc, which should favor the remodeling of free segments. This work was the first report to deal with the consequences of scoliosis surgery on subjacent disc in terms of volume and hydration properties. The clinical outcome will follow based on the patient cohort follow-up at 1 year after surgery.