scholarly journals Peripapillary and Posterior Scleral Mechanics—Part I: Development of an Anisotropic Hyperelastic Constitutive Model

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Michaël J. A. Girard ◽  
J. Crawford Downs ◽  
Claude F. Burgoyne ◽  
J.-K. Francis Suh
Keyword(s):  
Mechanical Properties ◽  
Finite Element ◽  
Optic Nerve ◽  
Constitutive Model ◽  
Optic Nerve Head ◽  
Collagen Fiber ◽  
Fiber Concentration ◽  
Fiber Alignment ◽  
Fiber Organization ◽  
Collagen Fiber Alignment

The sclera is the white outer shell and principal load-bearing tissue of the eye as it sustains the intraocular pressure. We have hypothesized that the mechanical properties of the posterior sclera play a significant role in and are altered by the development of glaucoma—an ocular disease manifested by structural damage to the optic nerve head. An anisotropic hyperelastic constitutive model is presented to simulate the mechanical behavior of the posterior sclera under acute elevations of intraocular pressure. The constitutive model is derived from fiber-reinforced composite theory, and incorporates stretch-induced stiffening of the reinforcing collagen fibers. Collagen fiber alignment was assumed to be multidirectional at local material points, confined within the plane tangent to the scleral surface, and described by the semicircular von Mises distribution. The introduction of a model parameter, namely, the fiber concentration factor, was used to control collagen fiber alignment along a preferred fiber orientation. To investigate the effects of scleral collagen fiber alignment on the overall behaviors of the posterior sclera and optic nerve head, finite element simulations of an idealized eye were performed. The four output quantities analyzed were the scleral canal expansion, the scleral canal twist, the posterior scleral canal deformation, and the posterior laminar deformation. A circumferential fiber organization in the sclera restrained scleral canal expansion but created posterior laminar deformation, whereas the opposite was observed with a meridional fiber organization. Additionally, the fiber concentration factor acted as an amplifying parameter on the considered outputs. The present model simulation suggests that the posterior sclera has a large impact on the overall behavior of the optic nerve head. It is therefore primordial to provide accurate mechanical properties for this tissue. In a companion paper (Girard, Downs, Bottlang, Burgoyne, and Suh, 2009, “Peripapillary and Posterior Scleral Mechanics—Part II: Experimental and Inverse Finite Element Characterization,” ASME J. Biomech. Eng., 131, p. 051012), we present a method to measure the 3D deformations of monkey posterior sclera and extract mechanical properties based on the proposed constitutive model with an inverse finite element method.

Effect of Preconditioning on Collagen Fiber Recruitment: Inhomogeneous Properties of Rat Supraspinatus Tendon

2010 ◽  
Author(s):  
Kristin S. Miller ◽  
Lena Edelstein ◽  
Louis J. Soslowsky
Keyword(s):  
Mechanical Properties ◽  
Collagen Fiber ◽  
Insertion Site ◽  
Supraspinatus Tendon ◽  
Fiber Alignment ◽  
Rigorous Evaluation ◽  
Structural Alterations ◽  
Testing Protocol ◽  
Inhomogeneous Properties ◽  
Collagen Fiber Alignment

Cyclic preconditioning is a commonly accepted initial component of any tendon testing protocol. Preconditioning provides tendons with a consistent “history” and stress-strain results become repeatable allowing for rigorous evaluation and comparison. While it is widely accepted that preconditioning is important, changes that occur during preconditioning are not well understood. Micro-structural alterations, such as re-arrangement of collagen fibers, is one proposed mechanism of preconditioning [1,4]. However, this mechanism has not been examined. Therefore, the objective of this study is to locally measure: 1) fiber re-alignment during preconditioning, stress relaxation and tensile testing and 2) corresponding mechanical properties, to address mechanisms of preconditioning as well as tissue nonlinearity and inhomogeneity in the rat supraspinatus tendon. We hypothesize that 1) fiber re-alignment will be greatest in the toe region, but will also occur during preconditioning and 2) mechanical properties and initial collagen fiber alignment will be greater in the midsubstance location of the tendon compared to the tendon-to-bone insertion site.

Collagen Fiber Re-Alignment and Mechanical Properties in a Mouse Supraspinatus Tendon Model: Examining Changes With Age and Location

2012 ◽  
Author(s):  
Kristin S. Miller ◽  
Brianne K. Connizzo ◽  
Elizabeth Feeney ◽  
Louis J. Soslowsky
Keyword(s):  
Mechanical Properties ◽  
Collagen Fiber ◽  
Mechanical Load ◽  
Mechanical Test ◽  
Insertion Site ◽  
Supraspinatus Tendon ◽  
Tissue Response ◽  
Mechanical Stimuli ◽  
Fiber Alignment ◽  
Collagen Fiber Alignment

One postulated mechanism of tendon structural response to mechanical load is collagen fiber re-alignment. Recently, where collagen fiber re-alignment occurs during a tensile mechanical test has been shown to vary by tendon age and location in a postnatal developmental mouse supraspinatus tendon (SST) model [1]. It is thought that as the tendon matures and its collagen fibril network, collagen cross-links and collagen-matrix interactions develop, its ability to respond quickly to mechanical stimuli hastens [1]. Additionally, the insertion site and midsubstance of postnatal SST may develop differently and at different rates, providing a potential explanation for differences in fiber re-alignment behaviors at the insertion site and midsubstance at postnatal developmental time points [1]. However, collagen fiber re-alignment behavior, in response to mechanical load at a mature age and in comparison to developmental ages, have not been examined. Therefore, the objectives of this study are to locally measure: 1) fiber re-alignment during preconditioning and tensile mechanical testing and 2) to compare local differences in collagen fiber alignment and corresponding mechanical properties to address tissue response to mechanical load in the mature and postnatal developmental mouse SST. We hypothesize that 1) 90 day tendons will demonstrate the largest shift in fiber re-alignment during preconditioning, but will also re-align during the toe- and linear-regions. Additionally, we hypothesize that 2) mechanical properties and initial collagen fiber alignment will be greater in the midsubstance of the tendon compared to the tendon-to-bone insertion site at 90 days, 3) that mechanical properties will increase with age, and that 4) collagen fiber organization at the insertion site will decrease with age.

Mechanical properties of tissue formed in vivo are affected by 3D-bioplotted scaffold microarchitecture and correlate with ECM collagen fiber alignment

2019 ◽  
Vol 61 (2) ◽  
pp. 190-204 ◽  
Author(s):  
Pedro Huebner ◽  
Paul B. Warren ◽  
Daniel Chester ◽  
Jeffrey T. Spang ◽  
Ashley C. Brown ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Collagen Fiber ◽  
Fiber Alignment ◽  
Collagen Fiber Alignment

scholarly journals Microstructural and Mechanical Properties of the Anterolateral Ligament (ALL) of the Knee

2020 ◽  
Vol 8 (7_suppl6) ◽  
pp. 2325967120S0044
Author(s):  
Ryan Castile ◽  
Spencer Lake ◽  
Robert Brophy ◽  
Ronak Patel
Keyword(s):  
Mechanical Properties ◽  
Elastic Moduli ◽  
Collagen Fiber ◽  
Microstructural Analysis ◽  
Microstructural Properties ◽  
Fiber Alignment ◽  
Data Points ◽  
Mechanical And Microstructural Properties ◽  
Collagen Alignment ◽  
Collagen Fiber Alignment

Objectives: The anterolateral ligament (ALL) of the knee has recently emerged as a potential contributor to rotational stability of the knee, with growing interest in ALL reconstruction as a supplement to anterior cruciate ligament reconstruction. The prevalence of the ALL in the knee has varied in anatomic dissection and imaging studies, raising questions about its importance as a knee stabilizer. The purpose of this study was to assess the microstructural and mechanical properties of the anterolateral knee, to better understand the ALL structure compared to the surrounding anterolateral capsule (ALC) and lateral collateral ligament (LCL). A polarized light imaging technique was used to quantify collagen fiber alignment simultaneously with measurement of tensile mechanical properties. Our primary hypothesis was that there is no difference in the microstructural and mechanical properties between the ALL and ALC. Our secondary hypothesis was that the properties of the LCL are different from the ALL and ALC. Methods: Twenty-five knee specimens from sixteen donors (five males, eleven females; mean age 45.6 +/- 6.4; age range 35-59 years; mean BMI 26.5 +/- 8.4) were obtained as determined by a priori power analysis. The anatomic technique to dissect the anterolateral knee structures was performed as described previously. Three tissue samples (LCL, ALL, and ALC) were harvested (Fig. 1). The ALL was taken as a quadrilateral piece of tissue starting posterior/proximal from the lateral femoral epicondyle and ending at the lateral border of Gerdy’s tubercle. During gross dissection, the knee was assessed for the presence or absence of a distinct visible and palpable structure within the area defined as the ALL. Harvested samples were thinned to approximately 1-mm thick using a freezing-stage sliding microtome. Cross-sectional area was measured using a 3D laser scanning system. Four 0.8-mm diameter aluminum beads were attached to the sample surface to enable strain measurement. Mechanical testing was performed with preconditioning followed by both a stress-relaxation test and a quasi-static ramp to failure. Microstructural analysis was performed using transmitted circularly-polarized incident light and a high-resolution, division-of-focal-plane polarization camera. The average degree of linear polarization (AVG DoLP; i.e., mean strength of collagen alignment) and standard deviation of the angle of polarization (STD AoP; i.e., degree of variation in collagen angle orientation) were calculated for the region of interest of each sample. Statistical analysis was performed using Kruskal-Wallis test (assuming nonparametric data) with Dunn’s correction for multiple comparisons. Results: Mechanical analysis of elastic moduli for the toe- and linear-region of the stress-strain curves showed no difference between the ALL and ALC but were significantly higher for the LCL (p<0.0001; Fig. 2). Microstructural analysis of the ALL and ALC during quasi-static ramp to failure showed no difference in AVG DoLP and STD AoP values at all strain levels (Fig. 3). Larger DoLP values (i.e., stronger collagen fiber alignment) were observed for the LCL than both the ALL and ALC (p<0.0001). Larger STD AoP values (i.e., more variation in collagen orientation) were observed for the ALL and ALC compared to the LCL (p<0.0001; Fig. 3). When looking at correlations between mechanical and microstructural properties (Fig. 4), we found clustering of the LCL data points at high linear modulus and AVG DoLP while the ALL and ALC data points were clustered together. Similarly, we found clustering of the LCL at high linear modulus and low STD AoP while the ALL and ALC were clustered together. Only three of 25 knee specimens (12%) were observed to have a distinct, ligamentous structure in the region of the ALL. Interestingly, these distinct ALL samples (outlined in black on figures) showed relatively larger elastic moduli, higher AVG DoLP, and lower STD AoP (i.e., uniform and organized collagen alignment) across the stress-strain curve compared to samples harvested from knees without a distinct ALL. The distinct ALL tissues were also seen clustered near the LCL data points in the correlation plots. Conclusions: Overall, there were no differences in the mechanical and microstructural properties between the ALL and ALC, while the LCL demonstrated different properties compared to both the ALL and ALC. Both the ALC and ALL show significantly weaker collagen fiber alignment and more variation in the direction of collagen fiber alignment compared to the LCL. These findings suggest that the ALL has similar properties to capsule (i.e., ALC). However, when a distinct ALL was present at dissection (12%), the data indicates stronger and more uniform collagen alignment suggestive of more ligament-type qualities. Further research is needed to more precisely define the prevalence and properties of distinct ALLs in the knee.

scholarly journals Assessment of the Viscoelastic Mechanical Properties of the Porcine Optic Nerve Head using Micromechanical Testing and Finite Element Modeling

2021 ◽  
Author(s):  
Babak N. Safa ◽  
A. Thomas Read ◽  
C. Ross Ethier
Keyword(s):  
Mechanical Properties ◽  
Finite Element ◽  
Optic Nerve ◽  
Finite Element Modeling ◽  
Optic Nerve Head ◽  
Mechanical Response ◽  
Solid Matrix ◽  
Hydraulic Permeability ◽  
Element Modeling ◽  
Micromechanical Testing

AbstractOptic nerve head (ONH) biomechanics is centrally involved in the pathogenesis of glaucoma, a blinding ocular condition often characterized by elevation and fluctuation of the intraocular pressure and resulting loads on the ONH. Further, tissue viscoelasticity is expected to strongly influence the mechanical response of the ONH to mechanical loading, yet the viscoelastic mechanical properties of the ONH remain unknown. To determine these properties, we conducted micromechanical testing on porcine ONH tissue samples, coupled with finite element modeling based on a mixture model consisting of a biphasic material with a viscoelastic solid matrix. Our results provide a detailed description of the viscoelastic properties of the porcine ONH at each of its four anatomical quadrants (i.e., nasal, superior, temporal, and inferior). We showed that the ONH’s viscoelastic mechanical response can be explained by a dual mechanism of fluid flow and solid matrix viscoelasticity, as is common in other soft tissues. We obtained porcine ONH properties as follows: matrix Young’s modulus E=1.895 [1.056,2 .391] kPa (median [min., max.]), Poisson’s ratio ν=0.142 [0.060,0 .312], kinetic time-constant τ=214 [89,921] sec, and hydraulic permeability k=3.854 × 10−1 [3.457 × 10−2,9.994 × 10−1] mm4/(N sec). These values can be used to design and fabricate physiologically appropriate ex vivo test environments (e.g., 3D cell culture) to further understand glaucoma pathophysiology.

scholarly journals Peripapillary and Posterior Scleral Mechanics—Part II: Experimental and Inverse Finite Element Characterization

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Michaël J. A. Girard ◽  
J. Crawford Downs ◽  
Michael Bottlang ◽  
Claude F. Burgoyne ◽  
J.-K. Francis Suh
Keyword(s):  
Finite Element ◽  
Optic Nerve ◽  
Constitutive Model ◽  
Optic Nerve Head ◽  
Speckle Pattern ◽  
Companion Paper ◽  
Tangent Modulus ◽  
Electronic Speckle Pattern Interferometry ◽  
Model Parameters ◽  
Scleral Shell

The posterior sclera likely plays an important role in the development of glaucoma, and accurate characterization of its mechanical properties is needed to understand its impact on the more delicate optic nerve head—the primary site of damage in the disease. The posterior scleral shells from both eyes of one rhesus monkey were individually mounted on a custom-built pressurization apparatus. Intraocular pressure was incrementally increased from 5 mm Hg to 45 mm Hg, and the 3D displacements were measured using electronic speckle pattern interferometry. Finite element meshes of each posterior scleral shell were reconstructed from data generated by a 3D digitizer arm (shape) and a 20 MHz ultrasound transducer (thickness). An anisotropic hyperelastic constitutive model described in a companion paper (Girard, Downs, Burgoyne, and Suh, 2009, “Peripapillary and Posterior Scleral Mechanics—Part I: Development of an Anisotropic Hyperelastic Constitutive Model,” ASME J. Biomech. Eng., 131, p. 051011), which includes stretch-induced stiffening and multidirectional alignment of the collagen fibers, was applied to each reconstructed mesh. Surface node displacements of each model were fitted to the experimental displacements using an inverse finite element method, which estimated a unique set of 13 model parameters. The predictions of the proposed constitutive model matched the 3D experimental displacements well. In both eyes, the tangent modulus increased dramatically with IOP, which indicates that the sclera is mechanically nonlinear. The sclera adjacent to the optic nerve head, known as the peripapillary sclera, was thickest and exhibited the lowest tangent modulus, which might have contributed to the uniform distribution of the structural stiffness for each entire scleral shell. Posterior scleral deformation following acute IOP elevations appears to be nonlinear and governed by the underlying scleral collagen microstructure as predicted by finite element modeling. The method is currently being used to characterize posterior scleral mechanics in normal (young and old), early, and moderately glaucomatous monkey eyes.

Preconditioning is Correlated With Altered Collagen Fiber Alignment in Ligament

2011 ◽  
Vol 133 (6) ◽  
Author(s):  
Kyle P. Quinn ◽  
Beth A. Winkelstein
Keyword(s):  
Collagen Fiber ◽  
Polarized Light ◽  
Peak Force ◽  
Principal Strain ◽  
Fiber Direction ◽  
Fiber Alignment ◽  
Tangent Stiffness ◽  
Vector Correlation ◽  
Fiber Organization ◽  
Collagen Fiber Alignment

Although the mechanical phenomena associated with preconditioning are well-established, the underlying mechanisms responsible for this behavior are still not fully understood. Using quantitative polarized light imaging, this study assessed whether preconditioning alters the collagen fiber alignment of ligament tissue, and determined whether changes in fiber organization are associated with the reduced force and stiffness observed during loading. Collagen fiber alignment maps of facet capsular ligaments (n = 8) were generated before and after 30 cycles of cyclic tensile loading, and alignment vectors were correlated between the maps to identify altered fiber organization. The change in peak force and tangent stiffness between the 1st and 30th cycle were determined from the force-displacement response, and the principal strain field of the capsular ligament after preconditioning was calculated from the fiber alignment images. The decreases in peak ligament force and tangent stiffness between the 1st and 30th cycles of preconditioning were significantly correlated (R ≥ 0.976, p < 0.0001) with the change in correlation of fiber alignment vectors between maps. Furthermore, the decrease in ligament force was correlated with a rotation of the average fiber direction toward the direction of loading (R = −0.730; p = 0.0396). Decreases in peak force during loading and changes in fiber alignment after loading were correlated (p ≤ 0.0157) with the average principal strain of the unloaded ligament after preconditioning. Through the use of a vector correlation algorithm, this study quantifies detectable changes to the internal microstructure of soft tissue produced by preconditioning and demonstrates that the reorganization of the capsular ligament’s collagen fiber network, in addition to the viscoelasticity of its components, contribute to how the mechanical properties of the tissue change during its preconditioning.

scholarly journals Quantitative mapping of collagen fiber alignment in thick tissue samples using transmission polarized-light microscopy

2016 ◽  
Vol 21 (7) ◽  
pp. 071111 ◽  
Author(s):  
Dmitry D. Yakovlev ◽  
Marina E. Shvachkina ◽  
Maria M. Sherman ◽  
Andrey V. Spivak ◽  
Alexander B. Pravdin ◽  
...  
Keyword(s):  
Light Microscopy ◽  
Collagen Fiber ◽  
Polarized Light ◽  
Polarized Light Microscopy ◽  
Tissue Samples ◽  
Fiber Alignment ◽  
Quantitative Mapping ◽  
Collagen Fiber Alignment
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