Technical Note
The elastic properties of trabecular and cortical bone tissues are similar: results from two microscopic measurement techniques

https://doi.org/10.1016/S0021-9290(98)00177-8Get rights and content

Abstract

Acoustic microscopy (30–60 μm resolution) and nanoindentation (1–5 μm resolution) are techniques that can be used to evaluate the elastic properties of human bone at a microstructural level. The goals of the current study were (1) to measure and compare the Young’s moduli of trabecular and cortical bone tissues from a common human donor, and (2) to compare the Young’s moduli of bone tissue measured using acoustic microscopy to those measured using nanoindentation. The Young’s modulus of cortical bone in the longitudinal direction was about 40% greater than (p<0.01) the Young’s modulus in the transverse direction. The Young’s modulus of trabecular bone tissue was slightly higher than the transverse Young’s modulus of cortical bone, but substantially lower than the longitudinal Young’s modulus of cortical bone. These findings were consistent for both measurement methods and suggest that elasticity of trabecular tissue is within the range of that of cortical bone tissue. The calculation of Young’s modulus using nanoindentation assumes that the material is elastically isotropic. The current results, i.e., the average anisotropy ratio (EL/ET) for cortical bone determined by nanoindentation was similar to that determined by the acoustic microscope, suggest that this assumption does not limit nanoindentation as a technique for measurement of Young’s modulus in anisotropic bone.

Introduction

Accurate measurement of the intrinsic stiffness or Young’s modulus of bone tissues is important now with the growing capabilities for tissue-level computational stress analysis. The reported Young’s moduli for cortical bone tissue have been shown consistently to be about 20–22 GPa along the axis of long bone and about 12–14 GPa transversely (Ashman et al., 1984; Yoon and Katz, 1976), while the reported Young’s moduli for trabecular bone tissue range from 1 to over 20 GPa (Ashman and Rho, 1988; Choi et al., 1990; Choi and Goldstein, 1992; Guo and Goldstein, 1997; Ko et al., 1995; Kuhn et al., 1989; Mente and Lewis, 1989; Rho et al., 1993; Runkle and Pugh, 1975; Ryan and Williams, 1989; Townsend et al., 1975; Williams and Lewis, 1982; Williams and Johnson, 1989). The mineral and collagen contents of trabecular and cortical tissues are similar (Gong et al., 1964), so it is unclear why there is so much discrepancy between the measured Young’s modulus values for the two tissues. Possibly the difficulty in preparing and mechanically testing specimens from trabeculae has contributed to the variation in measured elastic properties. Newer, microscopic testing techniques, used in the current study, allow measurement of bone elasticity with minimal experimental artifact and provide new information about the Young’s moduli of cortical and trabecular bone tissue.

Here we report Young’s moduli of cortical and trabecular bone tissue measured using acoustic microscopy, with a resolution of 30–60 μm (Hasegawa et al., 1995; Katz and Meunier, 1993; Shieh et al., 1995; Zimmerman et al., 1994), and using a nanoindentation technique, often with a resolution of better than 1 μm (Doerner and Nix, 1986; Oliver and Pharr, 1992; Pharr and Oliver, 1992). The results of these two techniques are compared to help resolve a technical question about the nanoindentation technique. Nanoindentation allows the calculation of Young’s modulus of a material under the assumption that the material is elastically isotropic. Bone is elastically anisotropic so the assumption used for Young’s modulus measurement by nanoindentation may produce inaccurate results.

The goals of the current study were (1) to measure and compare the Young’s moduli of trabecular and cortical bone tissues from a common human donor, and (2) to compare the Young’s moduli of bone tissue measured using acoustic microscopy to those measured using nanoindentation.

Section snippets

Methods

Samples of human trabecular bone from the distal femur and human cortical bone from the femoral midshaft from a human donor (age 65, male, no history of bone pathology) were prepared for acoustic measurements made at Indiana University and adjacent sections were sent to Oak Ridge National Laboratory for nanoindentation measurements.

Results

The Young’s modulus of cortical bone in the longitudinal/axial direction (EL) was about 40% greater than (p<0.01) the Young’s modulus in the transverse direction (ET) (Table 1). The Young’s modulus of trabecular bone tissue was slightly higher than the transverse Young’s modulus of cortical bone, but substantially lower than (p<0.01) the longitudinal Young’s modulus of cortical bone. The Young’s modulus of trabecular tissue measured acoustically was not significantly different than the average

Discussion

The Young’s modulus values for cortical bone measured using the acoustic microscope were similar to those measured by Ashman et al. (1984) using a continuous wave acoustic technique. Ashman found an average Young’s modulus of 13.4 GPa in the transverse direction and 20.0 GPa in the longitudinal direction. Young’s moduli measured using nanoindentation techniques were 10–20% higher than those found by Ashman. This discrepancy may have been caused the dehydration of the bone tissue during

Acknowledgements

This work was supported by research grants from the USPHS National Institutes of Health (#DE11291, CHT), and the Whitaker Foundation (JR). Analytical instrumentation for the nanoindentation testing was provided by the Division of Materials Sciences, US Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp., through the SHaRE Program under contract DE-AC05-76OR00033 between the US Department of Energy and Oak Ridge Associated Universities (JR).

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