The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 102

Water is displaced as the matrix consolidates [ 141 ]. Fluid that is forced out of the
region underneath an applied compressive load first flows in the lateral direction,
where the rate of fluid flow depends on the magnitude of the applied stress and the
compressive resistance of the cartilage. The intrinsic compressive stiffness of
the hyaline cartilage results from the physiochemical linkages between the collagen
fibrils and the proteoglycans, which create a matrix that is highly resistant to fluid
flow. Water thus serves as an incompressible continuum that communicates com-
pressive strain of the matrix into volumetric dilatation and subsequent
poroviscoelastic fluid flow [ 62 ]. Excised sections that contain full thickness carti-
lage and SCB enable visualization of deformation patterns within the collagen
matrix with compressive loading of the articular surface [ 62 ]. Collagen fibrils are
generally aligned with the long axes of chondrocytes, so the cells are used to
delineate fibril orientation [ 142 ]. Compression of healthy joint tissues shows
deformation patterns indicating that strains within the matrix are limited by the
radially oriented collagen fibrils in the STZ and the mineralized anchor that is
formed by the ZCC. These strain-limiting regions thus control the laterally directed
volumetric dilatation within the central regions of the cartilage and resist tissue
damage in healthy cartilage. Under compressive loads, collagen fibril orientation at
the interface with the ZCC is perpendicular directly under the applied load. In
lateral regions, however the collagen fibrils bend to resist shear and maintain the
robust anchoring of the hyaline cartilage matrix with the ZCC (Fig. 5.3 ).
5.4.2 Function of Mineralized Tissues
A notion persists that the ZCC serves to functionally grade properties from the
underlying bone to the overlying hyaline cartilage (Table 5.2 )[ 20 , 80 , 140 ];
however, the literature lacks substantial evidence to support this claim. As
previously mentioned, Mente and Lewis demonstrated that the modulus of
calcified cartilage is an order of magnitude less than that of the underlying SCB
[ 64 ]. Nanoindentation studies of the material property transition in regions span-
ning bone and calcified cartilage have shown mixed results. Ferguson et al. and
Gupta et al. have shown the moduli of calcified cartilage to be lower than adjacent
bone in normal and pathologic tissues (i.e., osteoarthritic human or exercised
horse tissues) [ 20 , 21 , 80 ]. However, the calcified cartilage modulus is greater
than that of bone in other samples in these same studies. The limited number of
samples examined in these studies prevents one from making broad statements
about the functional role of the ZCC.
The property gradient across the osteochondral interface may depend on many
factors, including loading conditions, age of the tissue, activity or exercise level, and
disease presence. Indeed, some dispute the role of the ZCC in transitioning, or
functionally grading, properties between bone and cartilage [ 1 ]. Several functional
studies of the osteochondral region, performed by a single research group, show that
collagen fiber deformations are constrained by the rigid, high-energy state in the