Virtual Fields Methods for Soft Musculoskeletal Tissue Characterization and Model Validation

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Each year hundreds of thousands of people suffer soft tissue injuries. The mechanical properties of the tissues that are damaged are difficult to measure. It is very difficult to make a test specimen from a natural soft tissue because the pre-load in the tissue is lost when it is cut. In addition, the cutting process damages fibers that are essential to the properties. This project will create and apply a method that can measure the deformations of a soft tissue without cutting a specimen and without removing the natural preload. All of the stretch of the tissue is measured using an MRI (magnetic resonance imaging) method and the deformations are put into a mathematical model of the tissue to calculate the properties needed to understand how injury occurs. The project will be applied to the anterior cruciate ligament (or ACL) which is part of the knee. Damage or rupture of the ACL is the most common soft tissue injury in athletic individuals. Women are particularly susceptible to ACL injury with rates from 4-10 times that of men depending on which activity. This research will develop new technology for measuring soft tissue mechanical properties and apply it to the ACL. The developed methods will be applicable to other soft, preloaded tissues and also to MRI visible soft engineering materials.The mechanical properties of the anterior cruciate ligament (ACL) of the knee are particularly challenging to experimentally characterize; in its anatomically relevant state the ACL is twisted and at least one of its two bundles is partially extended regardless of knee flexion angle. This tissue is also mechanically non-linear, anisotropic, (poro)viscoelastic, and heterogeneous. Full-field displacement methods that provide two- and three- dimensional displacement information over a large surface can reduce the number of test geometries needed to characterize soft tissue, as information about anisotropy, heterogeneity, non-linearity, and even viscoelasticity is contained within the displacement fields. This work seeks to explore full-surface and full-volume imaging modalities with corresponding displacement field analyses as a means to characterize the ACL bundles under uniaxial loading. A constitutive model for the ACL based on the uniaxial loading will be implemented into an MRI-derived finite element framework for computational analysis of the ACL. This model will be validated by using it to predict the ACL response to anterior tibial translation (ATT), its primary failure deformation mode.