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Research Papers

Mechanisms of Bimaterial Attachment at the Interface of Tendon to Bone

[+] Author and Article Information
Yanxin Liu

Department of Mechanical, Aerospace, and Structural Engineering, Washington University, St. Louis, MO 63130

Victor Birman1

Engineering Education Center, Missouri University of Science and Technology, St. Louis, MO 63121vbirman@mst.edu

Changqing Chen

Department of Engineering Mechanics, AML, Tsinghua University, Beijing 100084, China

Stavros Thomopoulos2

Department of Orthopaedic Surgery, Washington University School of Medicine, and Center for Materials Innovation, Washington University, St. Louis, MO 63110

Guy M. Genin2

Department of Mechanical, Aerospace, and Structural Engineering, and Center for Materials Innovation, Washington University, St. Louis, MO 63130

1

Corresponding author.

2

Both authors contributed equally.

J. Eng. Mater. Technol 133(1), 011006 (Dec 01, 2010) (8 pages) doi:10.1115/1.4002641 History: Received February 14, 2010; Revised July 26, 2010; Published December 01, 2010; Online December 01, 2010

The material mismatch at the attachment of tendon to bone is among the most severe for any tensile connection in nature. Attaching dissimilar materials is a major challenge in engineering, and has proven to be a challenge in surgical practice as well. Here, we examine the material attachment schemes employed at this connection through the lens of solid mechanics. We identify four strategies that the body adopts to achieve effective load transfer between tendon and bone: (1) a shallow attachment angle at the insertion of transitional tissue and bone, (2) shaping of gross tissue morphology of the transitional tissue, (3) interdigitation of bone with the transitional tissue, and (4) functional grading of transitional tissue between tendon and bone. We provide solutions to model problems that highlight the first two mechanisms: discuss the third qualitatively in the context of engineering practice and provide a review of our earlier work on the fourth. We study these strategies both in terms of ways that biomimetic attachment might benefit engineering practice and of ways that engineering experience might serve to improve surgical healing outcomes.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

The natural tendon-to-bone insertion site (left) and the site after healing (right). The tendon, which is nominally orthotropic, attaches to bone, here presented as nominally isotropic, through a tissue region that presents gradients in mineral volume fraction (represented by shading) and in collagen fiber orientation (represented by lines). Scar tissue at the tendon-to-bone interface following healing presents neither of these features, resulting in a highly vulnerable attachment.

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Figure 2

Geometry in the vicinity of the free edge at the attachment between two dissimilar anisotropic materials (the z-axis is perpendicular to the plane of the drawing)

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Figure 3

The order of the singularity drops near zero for specific elastic property mismatches. Pictured here is the order of the singularity for the case of attachment of an isotropic material of elastic modulus E to an orthotropic material whose mechanical properties approximate those of tendon. (a) Cases for which Poisson’s ratio of the isotropic material ν≥0; (b) cases for which ν≤0.

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Figure 4

The singularity disappears only for very shallow attachment angles between the orthotropic tendon and an isotropic material (ν=0.3) that is relatively stiff. The relationship is nonmonotonic for attachment to an isotropic material that is relatively compliant.

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Figure 5

Idealized plane stress model of the attachment of tendon to bone at the healing supraspinatus insertion site. This configuration served as a starting configuration for the shape optimization studies performed.

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Figure 6

Selected geometries representing the evolution of the outer boundary of a scar region during optimization to reduce the peak principal stress associated with uniform axial loading of the tendon. Contours represent peak principal stress normalized by the applied load.

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Figure 7

Interdigitation between tendon/mineralized fibrocartilage and bone at the rat tendon-to-bone insertion site. This image comes from a paraformaldehyde fixed, paraffin embedded, 5 μm thin section of a rat supraspinatus tendon-humeral head bone insertion stained with Toluidine blue.

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Figure 8

The interdigitation in the natural tendon-to-bone insertion site is reminiscent of z-pinning of composite plates.

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