ABSTRACT
ABSTRACT Soft tissue instability can cause or accelerate joint tissue degeneration. This is especially relevant to dynamic loading during falls, sports, and trauma-related events. The associated degenerative sequelae can result in signicant loss of function and general reduction in one’s quality of life. Thus, understanding the dynamic viscoelastic mechanical behavior of the involved soft tissue structures is a crucial rst step in developing treatment modalities for joint instability. Current soft tissue viscoelastic characterization paradigms utilize quasi-linear viscoelastic formulations that inherently do not
CONTENTS
5.1 Introduction .......................................................................................................................... 96 5.1.1 Ligament and Tendon: Morphology and Function ............................................. 96
5.1.1.1 Tendon ........................................................................................................ 97 5.1.1.2 Ligament ..................................................................................................... 97
5.2 Mechanical Behavior of Orthopedic Connective Tissues .............................................. 98 5.2.1 Elastic Behavior ........................................................................................................ 98 5.2.2 Viscoelastic Behavior ...............................................................................................99 5.2.3 Viscoelastic Theory ................................................................................................ 100
5.2.3.1 Linear Viscoelasticity ............................................................................. 100 5.2.3.2 Quasi-Linear Viscoelasticity .................................................................. 104 5.2.3.3 Fully Nonlinear Viscoelasticity ............................................................ 104
5.3 Numerical Implementation of Constitutive Viscoelastic Formulae ........................... 105 5.4 Comprehensive Viscoelastic Characterization Methodology ..................................... 108 5.5 Finite Element Implementation of Fully Nonlinear Viscoelasticity ........................... 113
5.5.1 Experimental Data Acquisition ........................................................................... 113 5.5.2 Nonlinear Viscoelastic Formulations ................................................................. 113
5.5.2.1 Analytical Formulation .......................................................................... 113 5.5.2.2 Finite Element Formulation ................................................................... 114
5.5.3 Finite Element Model ............................................................................................ 115 5.5.3.1 Finite Element Model Predictions ........................................................ 116
5.6 Future Directions ............................................................................................................... 117 References ..................................................................................................................................... 119
allow for a single, inseparable elastic and viscous behavior description. Further, available nonlinear viscoelastic formulations lack a description of relaxation manifested during dynamic loading events. Consequently, nite element implementation of nonlinear soft tissue viscoelastic behavior has not been widespread. Thus, to address these shortcomings, the following work describes the development of a novel, nonlinear viscoelastic constitutive formulation and a corresponding experimental characterization technique that is computationally tractable and implementable in state-of-the-art nite element algorithms. Implementation of the important nonlinear viscoelastic behavior of orthopedic soft tissues into computational models will greatly accelerate our ability to understand the functional role of soft connective tissues in whole-joint mechanics and facilitate future treatment options.
