The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 99

5.4 The Mechanical Properties of Tissues
in the Bone-Cartilage Interface
It is generally understood that the tidemark and the ZCC play critical roles in the
biomechanical integrity of anchoring cartilage and bone. However, assessing
the nature of this role in controlled experiments is complicated by the mismatched
material interface. Standard sample processing methods that are typically employed
for bone or cartilage samples often fail to address the needs of the entire
osteochondral region. Processing methods also cause significant environmental
alterations from in vivo conditions that introduce bias into native tissue property
measurements [ 1 , 20 , 21 , 62 , 80 , 138 ]. Despite these limitations, some work has
provided insight into the properties and function of the osteochondral tissues
through determination of material properties of the individual tissues and via
functional assessment of mechanical interactions between these tissues.
While empirical and observational data on the osteochondral interface tissues
reveal information about their collective function, an equal importance lies in
quantifying the material properties (e.g., modulus), function, and mechanisms of
failure for individual tissues and for the combined osteochondral unit. An accurate
representation of material properties of the hyaline cartilage, ZCC, and SCB is
needed in order to interpret experimental results from testing whole joint or
osteochondral sections, to develop more accurate computational models, to inter-
pret tissue changes that are involved in aging and disease, and to inform processes
and strategies for tissue engineering osteochondral replacement materials.
However, due to challenges associated with specimen preparation, the heterogene-
ity and intermingling of tissues, and the small length scales of interface components
(e.g., the ZCC), little data exist that accurately characterize the material properties
of the tissues in the osteochondral region. Further, the hierarchical construction of
these tissues complicates interpretation of results, where data collected at the nano-
or micro-scale may only represent the properties of the tissue and compare poorly to
the macro-scale. This challenge is consistent with similar challenges in interpreting
bone material properties, where properties vary across length scales [ 82 , 139 ].
Of the osteochondral tissues, the hyaline articular cartilage and the SCB are the
most thoroughly characterized. Hyaline cartilage properties vary throughout its depth,
but are generally several orders of magnitude below that of the neighboring
mineralized tissues (Table 5.2 ). Determination of SCB properties has been performed
bymachining and testing a shell, of controlled thickness, fromweight-bearing regions
of the femoral head [ 83 ], machining and testing SCB beams in bending [ 64 , 82 ], and
by performing nanoindentation or AFM on SCB in plastic-embedded sections
[ 20 - 23 , 80 , 81 ]. Similarly, modulus of ZCC has been determined by machining
beams of SCB and the ZCC, and then using an analytical approach to determine
properties of bulk sections of the ZCC [ 64 ], or using nanoindentation and AFM to
determine properties at nano- to micro-meter length scales [ 20 , 21 , 23 , 80 , 81 ].
Modulus values for the bulk SCB and ZCC specimens are lower than the values
obtained from nanoindentation or AFM (Table 5.2 ). Two explanations may exist for
this discrepancy. First, modulus values collected at nano- or micro-meter length scales