Daniel H. Cortes

Bio: Daniel H. Cortes is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Tendon & Achilles tendon. The author has an hindex of 19, co-authored 59 publications receiving 1163 citations. Previous affiliations of Daniel H. Cortes include West Virginia University & University of Pennsylvania.

A mechanistic model for transverse damage initiation, evolution, and stiffness reduction in laminated composites

Abstract: A constitutive model to predict stiffness reduction due to transverse matrix cracking is derived for laminae with arbitrary orientation, subject to in-plane stress, embedded in laminates with symmetric but otherwise arbitrary laminate stacking sequence. The moduli of the damaged laminate are a function of the crack densities in the damaging laminae, which are analyzed one by one. The evolution of crack density in each lamina is derived in terms of the calculated strain energy release rate and predicted as function of the applied load using a fracture mechanics approach. Unlike plasticity-inspired formulations, the proposed model does not postulate damage evolution functions and thus there is no need for additional experimental data to adjust material parameters. All that it is needed are the elastic moduli and critical energy release rates for the laminae. The reduction of lamina stiffness is an integral part of the model, allowing for stress redistribution among laminae. Comparisons with experimental data and some results from the literature are presented.

Continuous Shear Wave Elastography: A New Method to Measure Viscoelastic Properties of Tendons in Vivo.

Abstract: Viscoelastic mechanical properties are frequently altered after tendon injuries and during recovery. Therefore, non-invasive measurements of shear viscoelastic properties may help evaluate tendon recovery and compare the effectiveness of different therapies. The objectives of this study were to describe an elastography method for measuring localized viscoelastic properties of tendons and to discuss the initial results in healthy and injured human Achilles and semitendinosus tendons. The technique used an external actuator to generate the shear waves in the tendon at different frequencies and plane wave imaging to measure shear wave displacements. For each of the excitation frequencies, maps of direction-specific wave speeds were calculated using local frequency estimation. Maps of viscoelastic properties were obtained using a pixel-wise curve fit of wave speed and frequency. The method was validated by comparing measurements of wave speed in agarose gels with those obtained using magnetic resonance elastography. Measurements in human healthy Achilles tendons revealed a pronounced increase in wave speed as a function of frequency, which highlights the importance of tendon viscoelasticity. Additionally, the viscoelastic properties of the Achilles tendon were larger than those reported for other tissues. Measurements in a tendinopathic Achilles tendon indicated that it is feasible to quantify local viscoelastic properties. Similarly, measurement in the semitendinosus tendon revealed substantial differences in viscoelastic properties between the healthy and contralateral tendons. Consequently, this technique has the potential to evaluate localized changes in tendon viscoelastic properties caused by injury and during recovery in a clinical setting.

Characterizing the mechanical contribution of fiber angular distribution in connective tissue: comparison of two modeling approaches

Abstract: Modeling of connective tissues often includes collagen fibers explicitly as one of the components. These fibers can be oriented in many directions; therefore, several studies have considered statistical distributions to describe the fiber arrangement. One approach to formulate a constitutive framework for distributed fibers is to express the mechanical parameters, such as strain energy and stresses, in terms of angular integrals. These integrals represent the addition of the contribution of infinitesimal fractions of fibers oriented in a given direction. This approach leads to accurate results; however, it requires lengthy calculations. Recently, the use of generalized structure tensors has been proposed to represent the angular distribution in the constitutive equations of the fibers. Although this formulation is much simpler and fewer calculations are required, such structure tensors can only be used when all the fibers are in tension and the angular distribution is small. However, the amount of error introduced in these cases of non-tensile fiber loading and large angular distributions have not been quantified. Therefore, the objective of this study is to determine the range of values of angular distribution for which acceptable differences (less than 10%) between these two formulations are obtained. It was found, analytically and numerically, that both formulations are equivalent for planar distributions under equal-biaxial stretch. The comparison also showed, for other loading conditions, that the differences decrease when the fiber distribution is very small. Differences of less than 10% were usually obtained when the fiber distribution was very low (κ ≈ 0.03; κ ranges between 0 and 1/3, for aligned and isotropic distributed fibers, respectively). This range of angular distribution greatly limits the types of tissue that can be accurately analyzed using generalized structure tensors. It is expected that the results from this study guide the selection of a proper approach to analyze a particular tissue under a particular loading condition.

Role of cytokines in intervertebral disc degeneration: pain and disc content.

Abstract: Degeneration of the intervertebral discs (IVDs) is a major contributor to back, neck and radicular pain. IVD degeneration is characterized by increases in levels of the proinflammatory cytokines TNF, IL-1α, IL-1β, IL-6 and IL-17 secreted by the IVD cells; these cytokines promote extracellular matrix degradation, chemokine production and changes in IVD cell phenotype. The resulting imbalance in catabolic and anabolic responses leads to the degeneration of IVD tissues, as well as disc herniation and radicular pain. The release of chemokines from degenerating discs promotes the infiltration and activation of immune cells, further amplifying the inflammatory cascade. Leukocyte migration into the IVD is accompanied by the appearance of microvasculature tissue and nerve fibres. Furthermore, neurogenic factors, generated by both disc and immune cells, induce expression of pain-associated cation channels in the dorsal root ganglion. Depolarization of these ion channels is likely to promote discogenic and radicular pain, and reinforce the cytokine-mediated degenerative cascade. Taken together, an enhanced understanding of the contribution of cytokines and immune cells to these catabolic, angiogenic and nociceptive processes could provide new targets for the treatment of symptomatic disc disease. In this Review, the role of key inflammatory cytokines during each of the individual phases of degenerative disc disease, as well as the outcomes of major clinical studies aimed at blocking cytokine function, are discussed.

Constitutive modelling of arteries

Abstract: This review article is concerned with the mathematical modelling of the mechanical properties of the soft biological tissues that constitute the walls of arteries. Many important aspects of the mechanical behaviour of arterial tissue can be treated on the basis of elasticity theory, and the focus of the article is therefore on the constitutive modelling of the anisotropic and highly nonlinear elastic properties of the artery wall. The discussion focuses primarily on developments over the last decade based on the theory of deformation invariants, in particular invariants that in part capture structural aspects of the tissue, specifically the orientation of collagen fibres, the dispersion in the orientation, and the associated anisotropy of the material properties. The main features of the relevant theory are summarized briefly and particular forms of the elastic strain-energy function are discussed and then applied to an artery considered as a thick-walled circular cylindrical tube in order to illustrate its extension–inflation behaviour. The wide range of applications of the constitutive modelling framework to artery walls in both health and disease and to the other fibrous soft tissues is discussed in detail. Since the main modelling effort in the literature has been on the passive response of arteries, this is also the concern of the major part of this article. A section is nevertheless devoted to reviewing the limited literature within the continuum mechanics framework on the active response of artery walls, i.e. the mechanical behaviour associated with the activation of smooth muscle, a very important but also very challenging topic that requires substantial further development. A final section provides a brief summary of the current state of arterial wall mechanical modelling and points to key areas that need further modelling effort in order to improve understanding of the biomechanics and mechanobiology of arteries and other soft tissues, from the molecular, to the cellular, tissue and organ levels.

Mechanical confinement regulates cartilage matrix formation by chondrocytes.

Abstract: Cartilage tissue equivalents formed from hydrogels containing chondrocytes could provide a solution for replacing damaged cartilage. Previous approaches have often utilized elastic hydrogels. However, elastic stresses may restrict cartilage matrix formation and alter the chondrocyte phenotype. Here we investigated the use of viscoelastic hydrogels, in which stresses are relaxed over time and which exhibit creep, for three-dimensional (3D) culture of chondrocytes. We found that faster relaxation promoted a striking increase in the volume of interconnected cartilage matrix formed by chondrocytes. In slower relaxing gels, restriction of cell volume expansion by elastic stresses led to increased secretion of IL-1β, which in turn drove strong up-regulation of genes associated with cartilage degradation and cell death. As no cell-adhesion ligands are presented by the hydrogels, these results reveal cell sensing of cell volume confinement as an adhesion-independent mechanism of mechanotransduction in 3D culture, and highlight stress relaxation as a key design parameter for cartilage tissue engineering.

Shear-Wave Elastography: Basic Physics and Musculoskeletal Applications.

Abstract: Shear wave elastography is a rapidly evolving US imaging technique that allows quantification of mechanical and elastic tissue properties and serves as an adjunct to conventional US techniques, aiding in initial characterization and treatment follow-up of various traumatic and pathologic conditions of the musculoskeletal system.

Inflammation in intervertebral disc degeneration and regeneration

Abstract: Intervertebral disc (IVD) degeneration is one of the major causes of low back pain, a problem with a heavy economic burden, which has been increasing in prevalence as populations age. Deeper knowledge of the complex spatial and temporal orchestration of cellular interactions and extracellular matrix remodelling is critical to improve current IVD therapies, which have so far proved unsatisfactory. Inflammation has been correlated with degenerative disc disease but its role in discogenic pain and hernia regression remains controversial. The inflammatory response may be involved in the onset of disease, but it is also crucial in maintaining tissue homeostasis. Furthermore, if properly balanced it may contribute to tissue repair/regeneration as has already been demonstrated in other tissues. In this review, we focus on how inflammation has been associated with IVD degeneration by describing observational and in vitro studies as well as in vivo animal models. Finally, we provide an overview of IVD regenerative therapies that target key inflammatory players.