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

3. METHODS
3.1 Experimental Method
The T13-L1 disc was dissected from a skeletally mature (aged 3-5 years) ovine spine
that had been wrapped in plastic filmand stored at -20˚C until use. A block of AF tissue
was taken from the anterior portion of the disc and mounted in a freezing sledging
microtome with the radial face of the tissue block facing the microtome blade. Slices
60µm thick were cut at 30˚ to the axis in plane with the one direction of the fibres and
transected the alternating direction of fibres as previously described as “in-plane” 4,5 .
The slices were floated in a bath of saline at 5˚C and tested within 2 hours of extraction
from the disc.
The slices were individually placed into a bespoke tensile testing rig previously
described in detail 6 , with the radial ends securely clamped. The slices were sandwiched
between two glass cover slips that allowed light to be transmitted through the specimen.
The rig was mounted directly into the rotating stage of a conventional light microscope
(Nikon Multizoom AZ100). The slices were incrementally strained in the radial
direction. An image of the strained tissue was captured with every 0.5mm of extension
until the specimen failed.
3.2 Computational Methods
The prior-to-loading and loaded images of one of the slices was selected for the FE
analysis. A portion was chosen that showed three lamellae: two in which the fibres were
in-plane and one with cross sectioned fibres (Figure 2a). Proprietary software (ScanIP,
version 4, Simpleware Limited, Exeter, U.K.) was used to develop a 2D model from the
preload image, specific to the specimen’s geometry captured in the micrograph. The inplane
and sectioned lamellae were segmented as separate components. Additional
features, such as the interlamellar bridging elements were also included in the models.
The model was exported to a commercially available FEA software package (ABAQUS
CAE 6.9-1, Simulia Corp, Providence, RI, USA). The model contained approximately
30,000 2D-plane stress elements. This will henceforth be referred to as the “FE model”.
Adjacent lamellae were assigned opposite material orientations to account for their
differing fibre orientations. Previously derived orthotropic material properties for the
AF 7 were assigned to the lamellae (Table 1). The ratios of the material properties in
the x, y and z directions were maintained, however the material properties were not
assigned units. No experimental loading data was acquired, hence only the relative
material properties of the constituent tissues were investigated, rather than their absolute
values.
Ex Ey Ez νxy νxz νyz Gall directions
0.0004 0.07 0.016 0.023 0.075 1.38 0.0001
Table 1 – Unit-less orthogonal material properties applied to the lamellar, based on previous material model
development 7 . Shear modulus (G) is assumed to be the same in all directions.
The process of creating a model based on a single microscopy image was repeated for
the loaded image. This will henceforth be referred to as the “experimental model”.