Ligaments are short bands of fibrous connective tissue that guide normal joint motion and restrict abnormal joint movement. Excessive stretching or disruption can result in gross joint instability causing altered joint kinematics, load distribution and increased vulnerability to injury of other ligaments and musculoskeletal tissues. Although the injury and healing of ligaments have been topics of extensive study, fundamental information about the relationship between the ultrastructure of the tissue, structural components of the extracellular matrix and the continuum-level material behavior is severely lacking. The overall aim of this Bioengineering Research Grant focuses on elucidating the individual function and interaction between structural components of the extracellular matrix of ligaments across physical scales. The research specifically focuses on the small leucine-rich proteoglycans (decorin and biglycan), elastin and the multiscale material properties of ligaments. The hypotheses to be addressed are 1) Decorin and biglycan modulate the material properties of connective tissues containing Type I collagen by regulating the assembly and organization of collagen fibrils;2) Elastin contributes to the multiscale mechanical integrity of ligament by directly resisting applied forces and stabilizing collagen crimp in intact tissue and isolated fascicles;3) The volumetric material behavior of ligament varies between structural levels, is nonlinear, time-dependent and intrinsically coupled to the uniaxial viscoelastic behavior. Elastin modulates the Poisson's ratio by coupling fibers and fascicle between structural levels;4A) A continuum based hyperelastic strain energy that models nonlinear volumetric behavior and the time/rate dependent material behavior with poroviscoelasticity can both describe and predict the experimental results in Aim 3;4B) The deviatoric and volumetric elastic material behavior of intact ligament and fascicles can be described and predicted by a multi-scale elastic micromechanical model with helical fiber structure and an explicit representation of the elastin network. These hypotheses will be addressed through a series of aims that combine experimental measurements from the molecular level to the continuum level. By integrating molecular, structural and compositional characteristics of ligament into structural mechanical models at different levels, results of this study will have important implications for understanding the fundamental role of the small proteoglycans, elastin and fluid flow in the material behavior of fibrous connective tissues. This will aid the interpretation of the phenotypes associated with disease states that are related to alterations in expression of the small proteoglycans and elastin. The micromechanical model will provide a framework to interpret the mechanical effects of alterations to components of the ground substance and structural organization that occur due to growth, aging and different disease states, and can guide the design of engineered biomaterials.
The results of this research will elucidate the mechanistic function of the ground substance components as they relate to the hierarchical structure of ligaments and tendons. The micromechanical model will provide a framework to interpret the mechanical effects of alterations to components of the ground substance and structural organization that occur due to growth, aging and different disease states, and can guide the design of engineered biomaterials.
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