Estimation of local anisotropy of plexiform bone: Comparison between depth sensing micro-indentation and Reference Point Indentation

E Dall'Ara,P Grabowski,P Zioupos,M Viceconti

doi:10.1016/j.jbiomech.2015.10.001

E Dall'Ara, P Grabowski + Show 2 more

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https://doi.org/10.1016/j.jbiomech.2015.10.001

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Abstract

The recently developed Reference Point Indentation (RPI) allows the measurements of bone properties at the tissue level in vivo. The goal of this study was to compare the local anisotropic behaviour of bovine plexiform bone measured with depth sensing micro-indentation tests and with RPI. Fifteen plexiform bone specimens were extracted from a bovine femur and polished down to 0.05µm alumina paste for indentations along the axial, radial and circumferential directions (N=5 per group). Twenty-four micro-indentations (2.5µm in depth, 10% of them were excluded for testing problems) and four RPI-indentations (~50µm in depth) were performed on each sample. The local indentation modulus Eind was found to be highest for the axial direction (24.3±2.5GPa) compared to the one for the circumferential indentations (19% less stiff) and for the radial direction (30% less stiff). RPI measurements were also found to be dependent on indentation direction (p<0.001) with the exception of the Indentation Distance Increase (IDI) (p=0.173). In particular, the unloading slope US1 followed similar trends compared to the Eind: 0.47±0.03N/µm for axial, 11% lower for circumferential and 17% lower for radial. Significant correlations were found between US1 and Eind (p=0.001; R2=0.58), while no significant relationship was found between IDI and any of the micro-indentation measurements (p>0.157). In conclusion some of the RPI measurements can provide information about local anisotropy but IDI cannot. Moreover, there is a linear relationship between most local mechanical properties measured with RPI and with micro-indentations, but IDI does not correlate with any micro-indentation measurements.

Highlights

Bone is a complex hierarchical material with mechanical properties that depend on the investigated dimensional scale, from the cell up to the body levels
The aim of this study was to investigate if the Reference Point Indentation (RPI) approach is able to discriminate the anisotropic properties of plexiform bone, in comparison with a well-established technique such as depth sensing micro-indentation
The micro-indentation tests confirmed the hypothesis that plexiform bone is orthotropic at the tissue level

Summary

Introduction

Bone is a complex hierarchical material with mechanical properties that depend on the investigated dimensional scale, from the cell up to the body levels. Due to the distribution of their BSUs, both cortical and trabecular bone have exhibited anisotropic mechanical properties at the tissue level as shown by micro-indentation (Dall'Ara et al, 2013; Franzoso and Zysset, 2009; Reisinger et al, 2011; Roy et al, 1999; Wolfram et al, 2010) and ultrasound (Turner et al, 1995, 1999); and at the macro-level as shown by mechanical testing The long bones of quickly growing large animals reveal one further sub-classification into laminar and plexiform tissue, formed of parallel-fibred bone and lamellar bone around macroporosities. The better understanding of the anisotropic properties of this structure at the micro-level is important in order to understand why it appears only in large quickly growing animals and not in more active species such as humans or small mammals

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