Biomedical Engineering Reference
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Fig. 5.4 Quantitative backscattered electron (qBSE) image of sagittal hemisection of an ovine
lumbar vertebral endplate showing the ZCC (marked with arrows ) that connects the SCB to the
annulus fibrosis and nucleus pulposus tissues (approximate locations shown in top panel as “AF”
and “NP,” respectively). The ZCC is thinnest in the regions that are primarily compressively
loaded in vivo in regions overlying the NP ( } ) and the inner AF ( { ); also described in [ 42 ]. The
ZCC is thickest in the outer AF region where joint-stabilizing fibrils insert into the outer AF ( { )
and apply significant tensile and shear loads; multiple tidemarks exist in this region and indicate
that the vertebral ZCC may be dynamic in its response to loading in a manner that is similar to that
in articular joints. The thick SCB may transition loads into the underlying struts within the
subchondral trabecular bone (STB). Marrow space is shown as black ( gray level ¼ 0) and
increasing mineral content shows as shades of gray (up to 255)
deepest zone of unmineralized hyaline cartilage [ 1 , 62 , 118 ]. These same studies
note that deformations of ZCC were not observed during compressive or shear
loading of excised osteochondral sections. The role of the ZCC in transitioning
loads from bone to cartilage remains understudied, yet this tissue is undoubtedly
critical in the mechanics and mechanobiology of this interfacial region.
The leading tidemark, through which the continuum of collagen fibrils
penetrates, forms an abrupt edge between the mineralized material and the adjacent
deep zone of the unmineralized hyaline cartilage. BSE and contact resonance-force
microscopy images show the interface width, or the distance between high modulus
material, ~20 GPa, within the ZCC and the comparably low modulus values,
~10 GPa, in the (PMMA-embedded) hyaline cartilage, is only on the order of
5-10
m (Figs. 5.2 and 5.4 )[ 22 , 23 ]. The tidemark has sometimes been observed
to be of a higher calcium concentration [ 143 ] and mineral volume fraction than
deeper regions of the ZCC [ 20 , 21 , 80 ]. This abrupt interface and pattern of
mineralization can also be visualized at sub-micron resolution in backscattered
electron images that are calibrated to exact mineral volume fraction (Figs. 5.2 and
5.4 ). Such mineralization patterns may exacerbate the modulus mismatch and the
abrupt edge at the tissue interface.
m
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