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The Mechanics of Bimaterial Attachment at the Tendon-to-Bone Insertion Site
Date of Award
Doctor of Philosophy (PhD)
Stress concentrations arising at the interface of dissimilar materials are a major challenge in engineering. One of the greatest material mismatches in nature occurs at the attachment of tendon to bone, the so-called tendon-to-bone "insertion site". This attachment is characterized by an interfacial zone that is more compliant than either tendon or bone. The focus of this dissertation is an understanding of this attachment system from the perspective of mechanics. We first examined the attachment schemes employed at the tendon-to-bone connection through the lens of solid mechanics, focusing on the gradual spatial variation in material composition and structure ("functional grading") of the transitional tissue at the interface. We performed optimization procedure to identify the optimal material property distribution to reduce stress concentration. Results provide a rationale for such a seemingly illogical interfacial system. We then investigated structure-function relationships to see how the specific grading interfacial system is achieved. We developed a set of linear and nonlinear bounds and estimates for different models of the stiffening of collagen by bioapatite mineral across spatial hierarchies based upon recent microscopy data that reveal the accommodation of bioapatite by collagen. Percolation of the bioapatite phase proved to be an important determinant of the degree of stiffening by bioapatite. Applying these models to study attachment at the tendon-to-bone insertion site, we found that all models combined with the angular deviation of collagen fibers predicted a region of tissue between tendon and bone that is more compliant than either tendon or bone. We further studied the implication of the insertion site development, during which a constant size of gradient region presents, and investigated the possible roles of mechanical stress in the presence of mineralized grading region in the insertion site at early postnatal time. These observations from study of mechanics at the insertion site are important for the design of engineered tissue scaffolds for repair at the attachment of tendon to bone.
Guy M Genin
Victor Birman, Guy Genin, Thomas Harmon, Jr-Shin Li