Fiber Realignment Research Articles

Tensile Mechanical Properties and Dynamic Collagen Fiber Re-Alignment of the Murine Cervix are Dramatically Altered Throughout Pregnancy.

The cervix is a unique organ able to dramatically change its shape and function by serving as a physical barrier for the growing fetus and then undergoing dramatic dilation allowing for delivery of a term infant. As a result, the cervix endures changing mechanical forces from the growing fetus. There is an emerging concept that the cervix may change or remodel "early" in many cases of spontaneous preterm birth (sPTB). However, the mechanical role of the cervix in both normal and preterm birth remains unclear. Therefore, the primary objective of this study was to determine the mechanical and structural responses of murine cervical tissue throughout a normal gestational time course. In this study, both tissue structural and material properties were determined via a quasi-static tensile load-to-failure test, while simultaneously obtaining dynamic collagen fiber re-alignment via cross-polarization imaging. This study demonstrated that the majority of the mechanical properties evaluated decreased at midgestation and not just at term, while collagen fiber re-alignment occurred earlier in the loading curve for cervices at term. This suggests that although structural changes in the cervix occur throughout gestation, the differences in material properties function in combination with collagen fiber re-alignment as mechanical precursors to regulate term gestation. This work lays a foundation for investigating cervical biomechanics and the role of the cervix in preterm birth.

Effects of cyclic compression on the mechanical properties and calcification process of immature chick bone tissue in culture

Contribution of mechanical loading to tissue growth during both the development and post-natal maturation is of a particular interest, as its understanding would be important to strategies in bone tissue engineering and regenerative medicine. The present study has been performed to investigate how immature bone responds to mechanical loading using an ex vivo culture system. A slice of the tibia, with the thickness of 3mm, was obtained from 0-day-old chick. For the ex vivo culture experiment in conjunction with cyclic compressive loading, we developed a custom-made, bioreactor system where both the load and the deformation applied to the specimen was recorded. Cyclic compression, with an amplitude of 0.3N corresponding to 1 to 2% compressive strain, was applied to immature bone specimen during a 3-day culture period at an overall loading rate 3–4cycles/min, in the presence of β-glycerol phosphate and dexamethasone in culture medium. The stress-strain relationship was obtained at the beginning and the end of the culture experiment. In addition, analyses for alkaline phosphate release, cell viability and tissue calcification were also performed. It was exhibited that elastic moduli of bone slices were significantly elevated at the end of the 3-day culture in the presence of cyclic compression, which was a similar phenomenon to significant elevation of the elastic moduli of bone tissue by the maturation from 0-day old to 3-day old. By contrast, no significant changes in the moduli were observed in the absence of cyclic compression or in deactivated, cell-free samples. The increases in the moduli were coincided with the increase in calcified area in the bone samples. It was confirmed that immature bone can respond to compressive loading in vitro and demonstrate the growth of bone matrix, similar to natural, in vivo maturation. The elevation of the elastic moduli was attributable to the increased calcified area and the realignment of collagen fibers parallel to the loading direction. The ex vivo loading system established here can be further applied to study responses to mechanical loading in osteogenesis as well as callus maturation for better understanding of factors to consider in successful bone regeneration with mechanical factors.

Experimental evaluation and modified Weibull characterization of the tensile behavior of tri-component elastic-conductive composite yarn

In order to clarify the effects of the extension rate and gauge length upon the distribution of tensile strength of tri-component elastic-conductive composite yarn (t-ECCY), experimental as well as theoretical studies have been performed in this paper. Influences of the extension rate and gauge length are highlighted. The yarn exhibits an extension-rate strengthening effect, and the higher extension rate results in a higher strength and fracture strain, irrespective of the gauge length considered, and vice versa. Expressed in terms of gauge length, yarn tenacity shows a drop for a longer testing length at all extension rates, based on the weakest-link theory. A modified two-parameter Weibull strength distribution model, taking into account the effects of extension rate and gauge length, can be reasonably used to quantify the degree of variability in tensile strength and to obtain the individual Weibull parameters for practical applications. Different fracture mechanisms of the t-ECCY are demonstrated at lower and higher extension rates. A “cascade-like” break happens at lower extension rates due to inner sliding, weaker interactive transverse force of individual fibers, and sufficient time available for the fiber realignment. Nevertheless, a “chimney-like” break dominates at higher extension rates by virtue of the reduced reorientation of some disordered fibers and intensive instant impact force of the stainless steel filament component along the load direction.

Modeling the two-way feedback between contractility and matrix realignment reveals a nonlinear mode of cancer cell invasion.

Cancer cell invasion from primary tumors is mediated by a complex interplay between cellular adhesions, actomyosin-driven contractility, and the physical characteristics of the extracellular matrix (ECM). Here, we incorporate a mechanochemical free-energy-based approach to elucidate how the two-way feedback loop between cell contractility (induced by the activity of chemomechanical interactions such as Ca2+ and Rho signaling pathways) and matrix fiber realignment and strain stiffening enables the cells to polarize and develop contractile forces to break free from the tumor spheroids and invade into the ECM. Interestingly, through this computational model, we are able to identify a critical stiffness that is required by the matrix to break intercellular adhesions and initiate cell invasion. Also, by considering the kinetics of the cell movement, our model predicts a biphasic invasiveness with respect to the stiffness of the matrix. These predictions are validated by analyzing the invasion of melanoma cells in collagen matrices of varying concentration. Our model also predicts a positive correlation between the elongated morphology of the invading cells and the alignment of fibers in the matrix, suggesting that cell polarization is directly proportional to the stiffness and alignment of the matrix. In contrast, cells in nonfibrous matrices are found to be rounded and not polarized, underscoring the key role played by the nonlinear mechanics of fibrous matrices. Importantly, our model shows that mechanical principles mediated by the contractility of the cells and the nonlinearity of the ECM behavior play a crucial role in determining the phenotype of the cell invasion.

Extracellular matrix fibronectin mediates an endothelial cell response to shear stress via the heparin-binding, matricryptic RWRPK sequence of FNIII1H.

Endothelial cells (EC) respond to mechanical forces such as shear stress in a variety of ways, one of which is cytoskeletal realignment in the direction of flow. Our earlier studies implicated the extracellular matrix protein fibronectin in mechanosensory signaling to ECs in intact arterioles, via a signaling pathway dependent on the heparin-binding region of the first type III repeat of fibrillar fibronectin (FNIII1H). Here we test the hypothesis that FNIII1H is required for EC stress fiber realignment under flow. Human umbilical vein ECs (HUVECs) exposed to defined flow conditions were used as a well-characterized model of this stress fiber alignment response. Our results directly implicate FNIII1H in realignment of stress fibers in HUVECs and, importantly, show that the matricryptic heparin-binding RWRPK sequence located in FNIII1 is required for the response. Furthermore, we show that flow-mediated stress fiber realignment in ECs adhered via α5β1-integrin-specific ligands does not occur in the absence of FHIII1H, whereas, in contrast, αvβ3-integrin-mediated stress fiber realignment under flow does not require FNIII1H. Our findings thus indicate that there are two separate mechanosignaling pathways mediating the alignment of stress fibers after exposure of ECs to flow, one dependent on αvβ3-integrins and one dependent on FNIII1H. This study strongly supports the conclusion that the RWRPK region of FNIII1H may have broad capability as a mechanosensory signaling site.

Pressure therapy for post-burn scars: does it work?

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Strain Stiffening of Fibrillar Collagen during Individual and Collective Cell Migration Identified by AFM Nanoindentation.

The multistep process of cell migration requires cells to dynamically couple to extracellular interfaces and generate traction force or friction for displacement of the cell body. When deformed, biopolymer networks, including fibrillar collagen and fibrin, undergo a nonlinear elasticity change that is termed strain stiffening and is commonly measured by bulk rheology. It remains poorly characterized, however, whether forces generated by moving cells suffice to induce strain stiffening. To detect strain stiffening at the leading edge of normal and tumor cells moving across fibrillar type I collagen, we combined AFM nanoindentation and differential field probing with confocal reflection microscopy. In different cell models, gradient-like fiber realignment, densification, and elevation of Young's modulus ahead of the leading edge were observed, with peak increases of up to 1.15 kPa near the leading edge. Moving fibroblasts generated a larger anterograde strain field with a higher amplitude and up to 6-fold increased cumulative strain stiffening (52 kPa) compared with mesenchymal HT1080 fibrosarcoma cells (8.8 kPa) and epithelial SCC38 cancer cells (9.8 kPa). Collectively moving SCC38 cells produced 4-fold increased cumulative strain stiffening (38 kPa) compared with individually moving SCC38 cells in a β1 integrin- and actomyosin-dependent manner. This indicates that the extent of strain stiffening by the leading edge of moving cells scales with cell type, multicellular cooperativity, integrin availability, and contractility. By straining, migrating cells realign and densify fibrillar extracellular matrix and thus adopt an autonomous strategy to move on a "traveling wave" of stiffened substrate, which reaches levels sufficient for mechanosensory activation and self-steering of migration.

Collagen V-heterozygous and -null supraspinatus tendons exhibit altered dynamic mechanical behaviour at multiple hierarchical scales.

Tendons function using a unique set of mechanical properties governed by the extracellular matrix and its ability to respond to varied multi-axial loads. Reduction of collagen V expression, such as in classic Ehlers-Danlos syndrome, results in altered fibril morphology and altered macroscale mechanical function in both clinical and animal studies, yet the mechanism by which changes at the fibril level lead to macroscale functional changes has not yet been investigated. This study addresses this by defining the multiscale mechanical response of wild-type, collagen V-heterozygous and -null supraspinatus tendons. Tendons were subjected to mechanical testing and analysed for macroscale properties, as well as microscale (fibre re-alignment) and nanoscale (fibril deformation and sliding) responses. In many macroscale parameters, results showed a dose-dependent response with severely decreased properties in the null group. In addition, both heterozygous and null groups responded to load faster than in wild-type tendons via earlier fibre re-alignment and fibril stretch. However, the heterozygous group exhibited increased fibril sliding, while the null group exhibited no fibril sliding. These studies demonstrate that dynamic responses play an important role in determining overall function and that collagen V is a critical regulator in the development of these relationships.

Tissue Strain Reorganizes Collagen With a Switchlike Response That Regulates Neuronal Extracellular Signal-Regulated Kinase Phosphorylation In Vitro: Implications for Ligamentous Injury and Mechanotransduction.

Excessive loading of ligaments can activate the neural afferents that innervate the collagenous tissue, leading to a host of pathologies including pain. An integrated experimental and modeling approach was used to define the responses of neurons and the surrounding collagen fibers to the ligamentous matrix loading and to begin to understand how macroscopic deformation is translated to neuronal loading and signaling. A neuron-collagen construct (NCC) developed to mimic innervation of collagenous tissue underwent tension to strains simulating nonpainful (8%) or painful ligament loading (16%). Both neuronal phosphorylation of extracellular signal-regulated kinase (ERK), which is related to neuroplasticity (R2 ≥ 0.041; p ≤ 0.0171) and neuronal aspect ratio (AR) (R2 ≥ 0.250; p < 0.0001), were significantly correlated with tissue-level strains. As NCC strains increased during a slowly applied loading (1%/s), a "switchlike" fiber realignment response was detected with collagen reorganization occurring only above a transition point of 11.3% strain. A finite-element based discrete fiber network (DFN) model predicted that at bulk strains above the transition point, heterogeneous fiber strains were both tensile and compressive and increased, with strains in some fibers along the loading direction exceeding the applied bulk strain. The transition point identified for changes in collagen fiber realignment was consistent with the measured strain threshold (11.7% with a 95% confidence interval of 10.2-13.4%) for elevating ERK phosphorylation after loading. As with collagen fiber realignment, the greatest degree of neuronal reorientation toward the loading direction was observed at the NCC distraction corresponding to painful loading. Because activation of neuronal ERK occurred only at strains that produced evident collagen fiber realignment, findings suggest that tissue strain-induced changes in the micromechanical environment, especially altered local collagen fiber kinematics, may be associated with mechanotransduction signaling in neurons.

Production of a Self-Aligned Scaffold, Free of Exogenous Material, from Dermal Fibroblasts Using the Self-Assembly Technique.

Many pathologies of skin, especially ageing and cancer, involve modifications in the matrix alignment. Such tissue reorganization could have impact on cell behaviour and/or more global biological processes. Tissue engineering provides accurate study model by mimicking the skin and it allows the construction of versatile tridimensional models using human cells. It also avoids the use of animals, which gave sometimes nontranslatable results. Among the various techniques existing, the self-assembly method allows production of a near native skin, free of exogenous material. After cultivating human dermal fibroblasts in the presence of ascorbate during two weeks, a reseeding of these cells takes place after elevation of the resulting stroma on a permeable ring and culture pursued for another two weeks. This protocol induces a clear realignment of matrix fibres and cells parallel to the horizon. The thickness of this stretched reconstructed tissue is reduced compared to the stroma produced by the standard technique. Cell count is also reduced. In conclusion, a new, easy, and inexpensive method to produce aligned tissue free of exogenous material could be used for fundamental research applications in dermatology.

Multiscale modelling of solid tumour growth: the effect of collagen micromechanics.

Here we introduce a model of solid tumour growth coupled with a multiscale biomechanical description of the tumour microenvironment, which facilitates the explicit simulation of fibre-fibre and tumour-fibre interactions. We hypothesise that such a model, which provides a purely mechanical description of tumour-host interactions, can be used to explain experimental observations of the effect of collagen micromechanics on solid tumour growth. The model was specified to mouse tumour data, and numerical simulations were performed. The multiscale model produced lower stresses than an equivalent continuum-like approach, due to a more realistic remodelling of the collagen microstructure. Furthermore, solid tumour growth was found to cause a passive mechanical realignment of fibres at the tumour boundary from a random to a circumferential orientation. This is in accordance with experimental observations, thus demonstrating that such a response can be explained as purely mechanical. Finally, peritumoural fibre network anisotropy was found to produce anisotropic tumour morphology. The dependency of tumour morphology on the peritumoural microstructure was reduced by adding a load-bearing non-collagenous component to the fibre network constitutive equation.

Abstract 16534: Remodeling of Intimal Matrix by C. Pneumoniae-infected Macrophages

Introduction: Chlamydia pneumoniae, an intracellular pulmonary pathogen, is considered an important risk factor for atherosclerosis through the mechanisms is poorly understood. Infected monocytes from the lung enter atherosclerotic foci, and create a pro-inflammatory environment in the intima. Here, we tested the hypothesis that C. pneumoniae infection contributes to intimal matrix remodeling using a 3D model. Methods: Primary human monocytes infected with C. pneumoniae (either live of heat-killed organisms) at an MOI-1 were embedded in type I collagen matrix. Between 1 to 9 days, we analyzed the kinetics of MMP and cytokine release, cell migration, and structural and mechanical characteristics of the matrix by ELISA, microscopy and rheometry, respectively – with or without specific inhibitors of molecular mechanisms. All experiments were performed with at least 3 donors in triplicates; and statistical significance was assessed by Student’s t -test or ANOVA. Results: We observed that the infection of macrophages encapsulated in a 3D collagen matrix promote: (i) the production of MMP and pro-inflammatory cytokines; (ii) the remodeling process including degradation, aggregation and realignment of fibers (Fig. A); (iii) an increase in macrophage podosomal extensions and microenvironmental interactions; (iv) the collective, coordinated migration of macrophages to form focal clusters (Fig. B); and (v) an increase in the elastic modulus (Fig. C). We observed that these changes were more pronounced in live than in heat-killed infection. Further, inhibition of MMP, chemotactic stimuli and cell migratory signaling attenuates matrix remodeling. Significance: Our results provide novel insights into the role of infection and the mechanisms in modulating atherosclerotic progression, which may provide an attractive new approach to not only test the infectious burden hypothesis but also for the prevention and/or treatment of cardiovascular diseases.

Cell-mediated fibre recruitment drives extracellular matrix mechanosensing inengineered fibrillar microenvironments.

To investigate how cells sense stiffness in settings structurally similar to native extracellular matrices (ECM), we designed a synthetic fibrous material with tunable mechanics and user-defined architecture. In contrast to flat hydrogel surfaces, these fibrous materials recapitulated cell-matrix interactions observed with collagen matrices including stellate cell morphologies, cell-mediated realignment of fibers, and bulk contraction of the material. While increasing the stiffness of flat hydrogel surfaces induced mesenchymal stem cell spreading and proliferation, increasing fiber stiffness instead suppressed spreading and proliferation depending on network architecture. Lower fiber stiffness permitted active cellular forces to recruit nearby fibers, dynamically increasing ligand density at the cell surface and promoting the formation of focal adhesions and related signaling. These studies demonstrate a departure from the well-described relationship between material stiffness and spreading established with hydrogel surfaces, and introduce fiber recruitment as a novel mechanism by which cells probe and respond to mechanics in fibrillar matrices.

An investigation of the influence of extracellular matrix anisotropy and cell–matrix interactions on tissue architecture

Mechanical interactions between cells and the fibrous extracellular matrix (ECM) in which they reside play a key role in tissue development. Mechanical cues from the environment (such as stress, strain and fibre orientation) regulate a range of cell behaviours, including proliferation, differentiation and motility. In turn, the ECM structure is affected by cells exerting forces on the matrix which result in deformation and fibre realignment. In this paper we develop a mathematical model to investigate this mechanical feedback between cells and the ECM. We consider a three-phase mixture of collagen, culture medium and cells, and formulate a system of partial differential equations which represents conservation of mass and momentum for each phase. This modelling framework takes into account the anisotropic mechanical properties of the collagen gel arising from its fibrous microstructure. We also propose a cell-collagen interaction force which depends upon fibre orientation and collagen density. We use a combination of numerical and analytical techniques to study the influence of cell-ECM interactions on pattern formation in tissues. Our results illustrate the wide range of structures which may be formed, and how those that emerge depend upon the importance of cell-ECM interactions.

Biomechanical and Histological Effects of Augmented Soft Tissue Mobilization Therapy on Achilles Tendinopathy in a Rabbit Model

ObjectiveAugmented soft tissue mobilization (ASTM) has been used to treat Achilles tendinopathy and is thought to promote collagen fiber realignment and hasten tendon regeneration. The objective of this study was to evaluate the biomechanical and histological effects of ASTM therapy on rabbit Achilles tendons after enzymatically induced injury. MethodsThis study was a non-human bench controlled research study using a rabbit model. Both Achilles tendons of 12 rabbits were injected with collagenase to produce tendon injury simulating Achilles tendinopathy. One side was then randomly allocated to receive ASTM, while the other received no treatment (control). ASTM was performed on the Achilles tendon on postoperative days 21, 24, 28, 31, 35, and 38. Tendons were harvested 10 days after treatment and examined with dynamic viscoelasticity and light microscopy. ResultsCross-sectional area in the treated tendons was significantly greater than in controls. Storage modulus tended to be lower in the treated tendons but elasticity was not significantly increased. Loss modulus was significantly lower in the treated tendons. There was no significant difference found in tangent delta (loss modulus/storage modulus). Microscopy of control tendons showed that the tendon fibers were wavy and type III collagen was well stained. The tendon fibers of the augmented soft tissue mobilization treated tendons were not wavy and type III collagen was not prevalent. ConclusionBiomechanical and histological findings showed that the Achilles tendons treated with ASTM had better recovery of biomechanical function than did control tendons.

Abstract SY21-02: Extracellular matrix density, alignment, and tumor progression

Abstract An increase in the mammographic density of breast tissue is correlated to a four to six-fold increased risk of developing breast carcinoma, making it the single biggest risk factor for breast carcinoma. Increased breast density is associated with a significant increase in the deposition of ECM components, most notably collagen, which leads to an increase in matrix stiffness. In a mouse model of increased collagen deposition, we find that mammary tumor incidence, invasion, and metastasis increased three-fold. Mammary tumors arising in FAK-/- mammary glands were non-invasive and non-metastatic. Gene expression profiling of these tumors identified a metastasis signature regulated by FAK, and further identify novel mechanisms and signaling pathways governing invasion into collagen matrices. Using state-of-the-art non-linear optical imaging techniques (FLIM and SHG), we have imaged collagen and cells at the tumor-stromal boundary in live, unfixed mammary tumors. We find that during tumor progression, the structure and alignment of collagen fibers is altered, such that the perpendicular realignment of collagen fibers is completely coincident with areas of local invasion. A recent analysis of 207 human samples demonstrates that the presence of perpendicular aligned collagen was a novel significant and independent prognostic indicator of poor patient survival, with a risk factor of 4.3. To determine whether fibril alignment facilitates invasion, we engineered aligned matrices, and found that human breast carcinoma cells preferentially invaded along perpendicularly aligned vs. random or parallel collagen fibers. Our results demonstrate collagen fibers serve as a highway to facilitate local invasion, and promote distal metastasis. Citation Format: Patricia J. Keely. Extracellular matrix density, alignment, and tumor progression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr SY21-02. doi:10.1158/1538-7445.AM2014-SY21-02

Sutured tendon repair; a multi-scale finite element model.

Following rupture, tendons are sutured to reapproximate the severed ends and permit healing. Several repair techniques are employed clinically, with recent focus towards high-strength sutures, permitting early active mobilisation thus improving resultant joint mobility. However, the arrangement of suture repairs locally alters the loading environment experienced by the tendon. The extent of the augmented stress distribution and its effect on the tissue is unknown. Stress distribution cannot be established using traditional tensile testing, in vivo, or ex vivo study of suture repairs. We have developed a 3D finite element model of a Kessler suture repair employing multiscale modelling to represent tendon microstructure and incorporate its highly orthotropic behaviour into the tissue description. This was informed by ex vivo tensile testing of porcine flexor digitorum profundus tendon. The transverse modulus of the tendon was 0.2551 pm 0.0818 MPa and 0.1035 pm 0.0454 MPa in proximal and distal tendon samples, respectively, and the interfibrillar tissue modulus ranged from 0.1021 to 0.0416 MPa. We observed an elliptically shaped region of high stress around the suture anchor, consistent with a known region of acellularity which develop 72 h post-operatively and remain for at least a year. We also observed a stress shielded region close to the severed tendon ends, which may impair collagen fibre realignment during the remodelling stage of repair due to the lack of tensile stress.

Diabetes alters mechanical properties and collagen fiber re-alignment in multiple mouse tendons.

Tendons function to transfer load from muscle to bone through their complex composition and hierarchical structure, consisting mainly of type I collagen. Recent evidence suggests that type II diabetes may cause alterations in collagen structure, such as irregular fibril morphology and density, which could play a role in the mechanical function of tendons. Using the db/db mouse model of type II diabetes, the diabetic skin was found to have impaired biomechanical properties when compared to the non-diabetic group. The purpose of this study was to assess the effect of diabetes on biomechanics, collagen fiber re-alignment, and biochemistry in three functionally different tendons (Achilles, supraspinatus, patellar) using the db/db mouse model. Results showed that cross-sectional area and stiffness, but not modulus, were significantly reduced in all three tendons. However, the tendon response to load (transition strain, collagen fiber re-alignment) occurred earlier in the mechanical test, contrary to expectations. In addition, the patellar tendon had an altered response to diabetes when compared to the other two tendons, with no changes in fiber re-alignment and decreased collagen content at the midsubstance of the tendon. Overall, type II diabetes alters tendon mechanical properties and the dynamic response to load.

Validation of the modified rule of mixtures using a combination of fibre orientation and fibre length measurements

The goal of this study was to investigate the fibre orientation distribution (FOD), and subsequent mechanical properties, of an injection moulded plate with two different number averaged fibre lengths, termed in this paper medium (1.35mm) and long (2.40mm). Fibre orientation measurements (FOD) were made using the 2D elliptical section method and an in-house developed image analyser. The samples were injected from a pin gate located at the centre and top of the plate. Expansion flow on the divergent flow front from this pin gate resulted in a core region with circumferential alignment, while through thickness shear resulted in the usual realignment of fibres in the flow direction either side of the core, termed the shell layers. Two interesting aspects were discovered from these measurements. First, and most importantly, the FOD was found to be independent of the two fibre lengths in this study, and so predominantly controlled by the mould shape and the interaction with the flow front. Second, the fibres in the core region were found to be much closer packed than those in the shell regions.The interaction between the flow front and the mould shape resulted in a range of FOD across the moulded plate, from equal in-plane orientation at the centre of the plate, to highly aligned at the plate edge. This gave a very useful set of samples from which to test out the well known modified rule of mixtures (MROM). Often the fibre orientation distribution cannot be measured directly, but indirectly using the modified rule of mixtures model in reverse. The samples from this moulding (at two different average fibre lengths) gave an excellent opportunity to validate this often used approach.Both the tensile modulus and strength (measured parallel to the injection direction) were found to show a strong correlation with the measured fibre orientation, with a significant increase in both measures between the centre and the edge of both plates. The increased length of the ‘long’ fibre plate was found to give only a small increase in tensile modulus but a much larger increase in tensile strength.The tensile modulus showed a linear dependence with the measured fourth order orientation tensor average, 〈cos4θ〉, with respect to the injection direction of the plate, as predicted by the modified rule of mixtures. Excellent agreement was found between the measured modulus and the predictions from the modified rule of mixtures, based only on measured quantities (matrix modulus, fibre fraction and average fibre length) for both plates.

A Method for Predicting Collagen Fiber Realignment in Non-Planar Tissue Surfaces as Applied to Glenohumeral Capsule During Clinically Relevant Deformation

Previously developed experimental methods to characterize micro-structural tissue changes under planar mechanical loading may not be applicable for clinically relevant cases. Such limitation stems from the fact that soft tissues, represented by two-dimensional surfaces, generally do not undergo planar deformations in vivo. To address the problem, a method was developed to directly predict changes in the collagen fiber distribution of nonplanar tissue surfaces following 3D deformation. Assuming that the collagen fiber distribution was known in the un-deformed configuration via experimental methods, changes in the fiber distribution were predicted using 3D deformation. As this method was solely based on kinematics and did not require solving the stress balance equations, the computational efforts were much reduced. In other words, with the assumption of affine deformation, the deformed collagen fiber distribution was calculated using only the deformation gradient tensor (obtained via an in-plane convective curvilinear coordinate system) and the associated un-deformed collagen fiber distribution. The new method was then applied to the glenohumeral capsule during simulated clinical exams. To quantify deformation, positional markers were attached to the capsule and their 3D coordinates were recorded in the reference position and three clinically relevant joint positions. Our results showed that at 60deg of external rotation, the glenoid side of the posterior axillary pouch had significant changes in fiber distribution in comparison to the other sub-regions. The larger degree of collagen fiber alignment on the glenoid side suggests that this region is more prone to injury. It also compares well with previous experimental and clinical studies indicating maximum principle strains to be greater on the glenoid compared to the humeral side. An advantage of the new method is that it can also be easily applied to map experimentally measured collagen fiber distribution (obtained via methods that require flattening of tissue) to their in vivo nonplanar configuration. Thus, the new method could be applied to many other nonplanar fibrous tissues such as the ocular shell, heart valves, and blood vessels.