The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 100

enable high resolution mechanical imaging across very small tissue features due to the
small size of most nanoindentation or AFM probes. Testing at such small scales
measures the properties of small volumes of material (where regions containing
defects are discarded) and may be more sensitive to the nano- or micro-scale con-
struction of the tissue than at the macro-scale, where contributions to the modulus may
result from different physical phenomena. Second, probing techniques produce a
modulus that represents a multiaxial measurement, unlike what is observed in uniaxial
or bending of the bulk specimens. Anisotropy can be observed using multiaxial tests,
but its effect is muted and produces modulus values that represent all directions, but
with a primary influence from the direction of loading.
Within the literature, most reports cite properties of the ZCC and SCB from a
single investigation performed by Mente and Lewis in 1994 [ 64 ]. In this study,
modulus values were collected from bending tests of beams machined from the
ZCC and/or SCB. An analytical approach was then used to determine modulus
values for the two individual tissues. This heavily cited study is often used to bolster
the argument that the ZCC is less stiff than the SCB, and so must functionally grade
properties between bone and cartilage. However, this analysis failed to explicitly
consider the anisotropic nature of the material within the ZCC and that the ZCC
varies greatly in mineral volume fraction. While directional properties of the ZCC
have not been fully characterized, basic composites theory and existing knowledge
of other mineralized tissues imply that the modulus would be the greatest in the
direction perpendicular to the ZCC surface and the least in the transverse directions
(i.e., radially around the joint surface). The modulus that is produced by bending
beams machined from transverse sections of the ZCC thus represents the weakest
direction of loading where failure would likely occur through detachment of
neighboring mineralized collagen fibrils. From this perspective, the modulus that
is commonly reported for transverse beams of ZCC does not represent the aniso-
tropic nature of this material. Nor do the low values of modulus, as compared to the
adjacent SCB, imply that the ZCC serves to functionally grade loads from the stiff
bone to compliant cartilage.
Other studies reporting ZCC properties also fail to capture the anisotropic nature of
this mineralized tissue. Nanoindentation measurements provide a glimpse into how
the ZCC properties relate to those of the SCB; however, these results are relative and
scale-dependent and lack a directional perspective. More advanced testing of the
materials in this interface region needs to be performed, perhaps via testing of
small, machined specimens to elucidate the material properties at relevant length
scales. However, current data in the literature are valuable and provide a starting point
to interpret tissue function and identify gaps that can be filled by future studies.
5.4.1 Function of Cartilaginous Tissues
In the deep zone of cartilage, collagen fibrils increase in packing density and
perpendicular alignment as they penetrate through the depth of cartilage and into
the ZCC [ 1 ]. Like fibers that combine to form a rope, the increased packing density