Abstract
Biological requirements call for substantial porosities in clinical biomaterials – challenging the mechanical integrity and strength of the latter. In this study, the authors resort to quantitative engineering principles to assess the fracture safety of double-porous hydroxyapatite ceramics: micro-computed tomography scans give access to the morphology of macropores at the submillimeter scale, as well as to voxel-specific microporosities. Advanced micromechanics of porous ceramics with needle-shaped elementary units then allows for translating voxel-specific microporosities to corresponding elasticity and strength properties, as well as to macro-to-micro scale transition (‘concentration’) tensors. These mechanical properties and tensors are fed into a large-scale finite-element model of a biomaterial granule as used for mandibular tissue regeneration. Loading the granule in splitting mode, up to physiological strain, evidences stress concentrations at the loaded poles and close to internal macropores and cracks. A parallel computing-supported subvoxel analysis of needle orientations evidences that in highly loaded regions, the intravoxel ‘single crystals’ oriented perpendicular to the loading direction undergo the most unfavorable loading. Still, only 0·6% of the finite-elements show stresses indicating failure, and the mean safety factor against fracture is as high as 7. This analysis confirms, from an engineering science viewpoint, the successful use of the investigated biomaterials in clinical practice.
Highlights
Large bone defects that do not heal autonomously can be regenerated by means of tissue engineering strategies
As a complement to the aforementioned studies, the present study focuses on the strength properties of the aforementioned hydroxyapatite granules used for mandibular bone tissue regeneration: This study is based on a recently reported general theory for the poro-micromechanics of quasibrittle porous polycrystals[6,7,8] across many ceramic systems built up by randomly oriented needle- or disk-type single crystals
Details of underlying methods and results are given in the remainder of the present paper, along the following structure: The paper starts by reviewing the microCT- and micromechanicssupported, elastic finite-element analysis (FEA) of Dejaco et al.,[4] which serves as the basis for the subsequent developments
Summary
Large bone defects that do not heal autonomously can be regenerated by means of tissue engineering strategies. The corresponding microporosities are converted into FE-specific isotropic elastic properties (in terms of Young’s modulus and Poisson’s ratio), by means of the selfconsistent micromechanical model of Fritsch et al.,[7] which was developed in the framework of so-called random homogenization theory or continuum micromechanics,[11,12] for porous polycrystals built up by elongated hydroxyapatite crystal phases oriented in all space directions (see Figure 3 for the microstructure envisioned in each FE and Figure 4 for the elasticity relations applied to each and every FE) This resulted in an FE model consisting of 1 171 176 FEs. As far as loading boundary conditions are concerned, uniaxial compression on the order of magnitude of physiological strain (0·1% of the granule’s diameter) was imposed in the form of Figure 3. Resulting stress and strain fields, based on the 1·2 million element mesh, turned out to be satisfactorily converged.[4]
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