Abstract
At its highest level of microstructural organization—the mesoscale or millimeter scale—cortical bone exhibits a heterogeneous distribution of pores (Haversian canals, resorption cavities). Multi-scale mechanical models rely on the definition of a representative volume element (RVE). Analytical homogenization techniques are usually based on an idealized RVE microstructure, while finite element homogenization using high-resolution images is based on a realistic RVE of finite size. The objective of this paper was to quantify the size and content of possible cortical bone mesoscale RVEs. RVE size was defined as the minimum size: (1) for which the apparent (homogenized) stiffness tensor becomes independent of the applied boundary conditions or (2) for which the variance of elastic properties for a set of microstructure realizations is sufficiently small. The field of elastic coefficients and microstructure in RVEs was derived from one acoustic microscopy image of a human femur cortical bone sample with an overall porosity of 8.5%. The homogenized properties of RVEs were computed with a finite element technique. It was found that the size of the RVE representative of the overall tissue is about 1.5 mm. Smaller RVEs (~0.5 mm) can also be considered to estimate local mesoscopic properties that strongly depend on the local pores volume fraction. This result provides a sound basis for the application of homogenization techniques to model the heterogeneity of cortical microstructures. An application of the findings to estimate elastic properties in the case of a porosity gradient is briefly presented.
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- (x, y, z):
-
Orthogonal frame. Bone cross-section is assumed to be in the plane (x, y)
- Z = Z(x, y):
-
Surface acoustic impedance value measured at point (x, y)
- \({\hat Z}\) :
-
Mineralized matrix impedance averaged over the mesodomain matrix area
- p :
-
Porosity
- N (i) :
-
Number of mesodomains in each size group i = 1, . . . , 4
- L (i) :
-
Edge size of the square cross-section of mesodomains in the plane (x, y) in each group i = 1, . . . , 4
- θ :
-
Denotes one random realization of a mesodomain
- L z :
-
Dimension of one mesodomain in direction z
- C(θ):
-
Apparent stiffness tensor for one mesodomain realization
- C t (θ):
-
Lower bound of the apparent stiffness tensor calculated with SUBC for one mesodomain realization
- C d (θ):
-
Upper bound of the apparent stiffness tensor calculated with KUBC for one mesodomain realization
- h :
-
Characteristic size of the finite element mesh
- E, G and ν:
-
Apparent engineering elastic moduli, respectively, Young modulus, shear modulus and Poisson ratio
- W :
-
Denotes any of the apparent engineering moduli
- W t , W d :
-
Denotes any of the apparent engineering moduli obtained with either SUBC or KUBC, respectively
- δ(θ), d W (θ):
-
Measures of the interval between the bounds of the apparent elastic behavior for one mesodomain realization
- \({\hat \delta_i, \hat d_{W ;i}}\) :
-
Mean values of δ and d W in each quartile i = 1, . . . , 4
- \({\hat W (L)}\) :
-
Mean of the engineering modulus W of mesodomains of size L
- D W (L):
-
Standard deviation of the engineering modulus W of mesodomains of size L
- \({\epsilon_W}\) :
-
Absolute uncertainty on the mean value of W
- \({L = L_{{\rm RVE}} (\epsilon_W; W)}\) :
-
Size of RVE (Definition 2) for modulus W for a prescribed precision of \({\epsilon_W}\)
- W 0;L (p):
-
Linear regression model for property W as a function of porosity, calculated for mesodomains of size L
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Grimal, Q., Raum, K., Gerisch, A. et al. A determination of the minimum sizes of representative volume elements for the prediction of cortical bone elastic properties. Biomech Model Mechanobiol 10, 925–937 (2011). https://doi.org/10.1007/s10237-010-0284-9
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DOI: https://doi.org/10.1007/s10237-010-0284-9
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