Collagen Content and Organization Relate to Bone Nanomechanical Properties
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Collagen Content and Organization Relate to Bone Nanomechanical Properties Eve Donnelly1, Rebecca M. Williams2, Shefford P. Baker3, and Marjolein C. H. van der Meulen1,4 1 Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 2 Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 3 Department of Materials Science and Engineering, Cornell University, Ithaca, NY 4 Musculoskeletal Integrity Program, Hospital for Special Surgery, New York, NY ABSTRACT Cancellous bone plays an important load-bearing role in the skeleton, yet relatively little is known about the microstructure-mechanical property relationships of the tissue at the sub-10 µm level. Cancellous tissue is characterized by a layered microstructure with variable proportions of collagen and mineral. The lamellar material is substantially stiffer than the interlamellar material at the nanomechanical level. However, the microstructural origin of the observed differences in mechanical properties of these structures has not been investigated. In this study, second harmonic generation microscopy was used to examine collagen in human vertebral cancellous bone. At the same location in the tissue, nanoindentation was used to assess the indentation modulus of lamellar and interlamellar bone. The stiff lamellae corresponded to areas of highly ordered, collagen-rich material, while the compliant interlamellar regions corresponded to areas of unoriented or collagen-poor material. The lamellar bone was approximately 30% stiffer and contained approximately 50% more oriented collagen than the interlamellar bone. These observed differences in the mechanical properties and collagen content and organization of lamellar and interlamellar tissue are consistent with previous scanning electron microscopy studies showing greater mineral and collagen content and organization in lamellar bone. Given the well-known coupling between collagen and mineral in bone tissue, the mineral distribution may mirror that of the aligned collagen. However, similar measurements of local variations in mineral content are needed to confirm this hypothesis and may provide additional insights into the tissue nanomechanical behavior.
INTRODUCTION Cancellous bone structural integrity is essential to skeletal function. Three key factors, apparent density, microarchitecture, and material properties, determine the structural behavior of this tissue. The effect of apparent density on whole-bone mechanical properties has been investigated extensively, and high-resolution tomographic and magnetic resonance imaging have more recently enabled analysis of the relationship between microarchitecture and whole-bone structural behavor [1,2]. Although the microstructure of bone has been characterized extensively [3-5], less is known about the role of material properties in the tissue nanomechanical behavior. In previous micro-scale studies of bone tissue, systematic variations in tissue mechanical properties [6,7], mineral composition and structure [8], and
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