ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Validation of Strain Mapping for the Assessment of Microdamage Initiation
and Propagation in Cortical Bone
David Christen 1 , Alina Levchuk 1 , Stefan Schori 1 , Philipp Schneider 1 ,
Steven K. Boyd 2 and Ralph Müller 1
1. ABSTRACT
The initiation and propagation of microdamage is a critical element towards a better
understanding of bone strength and fractures. Dynamic image-guided failure assessment
(DIGFA) combined with high-resolution synchrotron radiation-based computed
tomography (SR-CT) allows time-lapsed three-dimensional visualization and
quantification of microcrack initiation and progression. Here we present a novel strain
mapping technique, based on a multi-stage deformable image registration process, to
quantify local deformations and strains during microdamage accumulation in murine
cortical bone. The accuracy and precision of the displacement field, 0.0006 and 0.1750
voxel respectively, were measured from an image that was artificially displaced by 0.5
voxel in each direction. For the strain map, accuracy and precision were measured from
artificially deformed images corresponding to applied tensile strains between 0.00 and
0.05, and yielded 0.0003 and 0.0123 respectively. The spatial resolution of the strain
map was determined by the modulation transfer function (MTF). 10% modulation was
measured at 49.5 line pairs per mm, corresponding to a resolution of 10µm or 13.45
voxel. A marked drop in tensile and shear strains ahead of the segmented microcracks
was associated with the progression of the wake front of the microcrack and was
successfully quantified. In conclusion, strain mapping allows the investigation of the
propagation of microdamage caused by overloading. In particular, the relationship
between the microstructural components and microdamage dynamics in bone can now
be assessed by the quantification of local strains involved in the initiation, guidance or
arrest of microcracks.
2. INTRODUCTION
The initiation and propagation of microdamage in cortical bone can be investigated
using a combination of nondestructive time-lapsed three-dimensional (3D) imaging and
suitable biomechanical testing scenarios (1). Using SR-CT it is possible to describe
microcrack initiation and propagation (2), while the quantification of displacements and
strains has not been investigated yet.
In this study, we present a multi-stage procedure combining digital volume correlation
(DVC) (3) to align regions in the images and demons deformable image registration (4)
to increase the localization (i.e. spatial resolution) of the strain maps, which allows for
the precise analysis of the local deformations around microcracks (5). Combined with
time-lapsed imaging during the mechanical experiment, this method allows
quantification of the 3D strain and deformation fields at initiation and during
1 Institute for Biomechanics, ETH Zurich, Zurich, Switzerland;
2 Schulich School of Engineering, University of Calgary, Calgary, Canada