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

known about the local stresses experienced inside the tissue. Due to its complex
extracellular matrix structure, simple uni-axial compression of the tissue may induce a
variety of stresses such as shear stress, pore pressure, shear fluid stress in the
pericellular region different to that assumed by platen displacement and load cell
measurements. Therefore, the magnitude and type of loading which promotes cartilage
growth and constitutes as healthy loading is still unknown.
Finite element constitutive models for predicting local stresses and strains experienced
inside the tissue strive to accurately describe the intricate mechanical, electrical and
chemical interactions between the various constituents of the tissue. They range from
biphasic transversely isotropic models 4 , biphasic poroviscoelastic models 5 , fibrilreinforced
6 hyperelastic models 7, to swelling models 8,9,10 and those that include the
tissues depth varying mechanical properties 11 . Nonetheless, these models assume the
cartilage explants are perfect axis-symmetric flat surface cylinders. These constitutive
models often require optimisation of the finite element predictions to experimental data
for determination of material parameters, as well as validation of the constitutive
models and the flat surface assumption can cause inaccuracies. To eliminate possible
errors, the top and bottom of the articular cartilage explants are sometimes sliced using
a microtome 9 . As gene expression studies are looking to study cell response to
mechanical stimuli, it is essential that the cells are kept viable. To prevent apoptosis,
cell death, cutting of the surface must be avoided. Results from gene expression studies
on the uncut surfaces have been found to be variable. This further supports the
hypothesis that flat surface cylinder simplification can cause inaccurate predictions of
the local stresses and strains experienced inside the tissue during loading.
The aim is to investigate the significance of an accurate surface geometry representation
of the articular cartilage explants in finite element predictions. In addition, to establish a
method that will allow gene expression studies and finite element modelling to be
carried out for the same explant while keeping the risks of apoptosis to the minimum.
3. METHODS
3.1. Three dimensional reconstruction of the condyle surface
Excised human femoral condyles were obtained from TKR surgery under the centre
wide ethical approval for Arthritis Research UK Biomechanics and Bioengineering
Centre. The femoral condyles were kept in physiological solution until testing. Dental
putty was used to take moulds of the condyle surface geometry pre and post explants
extraction with a 9mm diameter biopsy punch. To avoid contamination from the dental
putty, the femoral condyles were wrapped in Polyethylene cling film.
Digital Image Correlation was used to extract the condyle surface geometry from the
dental putty moulds. The dental moulds were first covered in a coat of uniform white
paint. A fine uniquely random speckle pattern was then affixed to the surface of the
moulds using black paint. Two high-resolution cameras (Limess Messtechnik &
Software GmbH, Pforzheim, Germany) with lenses of 2.8mm focal length were used to
image the moulds. Using a Digital Image Correlation analysis software, VIC-3D
(Correlated Solutions, Inc., Columbia, USA), a three-dimensional representation of the
moulds was reconstructed.