The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 105

tissue sections, shear stresses reaching a critical value caused initiation of a large,
fast-moving crack of combined modes (mixed-mode I, due to out-of-plane forces
and mode II due to in-plane forces) that propagated in the osteochondral region
until it arrested. This mixed-mode crack also formed fractures vertically through
the articular cartilage. In contrast, mature tissues exhibited smaller cracks that
propagated primarily within the tidemark region without vertically oriented
damage. The mechanism of failure within the osteochondral region thus changes
with development and the consequent

increasing mineralization of the

developing ZCC.
Less is known about failure in or near the osteochondral interface within the
spine. In contrast to the synovial joint, the hyaline cartilage of the cartilaginous
endplate consists of collagen fibers that are oriented parallel to the mineralized
tissue surface (Fig. 5.1 ). Anecdotally, orthopaedic surgeons often note that
the mature endplate is easy to separate from the underlying bone during surgery.
A gap exists in understanding the transfer of mechanical loads and contribution of
tissue microstructure to the function of this complex region. Specifically, it is
unclear how the parallel orientation of the collagen fibrils sustains function of
this interface in the spinal tissues and if this specific construction may be one factor
that leads to disc damage and degeneration. Also, how the integration of mineral
within this collagen framework causes dissipation of compressive and shear forces
within the spinal tissue environment is unknown.

5.5 Concluding Remarks

This chapter both summarizes current knowledge and also highlights gaps in our
current understanding of the bone cartilage interface. Overall, the osteochondral

interface forms a complex region that is constructed of multiple tissues and cell
types that each respond differently, but not separately, to intrinsic factors including
mechanical loading and to extrinsic factors such as drug therapies. In combination
with microstructural and compositional differences across the tissues, these factors
lead to different mechanical and material properties, mechanosensory responses,
and underlying physiology [ 41 ]. This complex environment therefore necessitates
an improved understanding of the interaction between cell populations, the manner
in which mechanical loads are transferred, and how damage and/or altered loading
in one tissue may affect the overall unit.
In order to elucidate how alterations in factors such as mineralization, porosity,
and remodeling of the ZCC and SCB contribute to degeneration with aging and
disease in both synovial and cartilaginous joints, further study on the interface is
necessary. Further, an accurate representation of tissue-scale through nano-scale
material properties of the hyaline cartilage, ZCC, and SCB is needed in the contexts
of hierarchical organization and anisotropy. An improved understanding of
this complex, layered, dynamic region will enable interpretation of experimental
results from testing whole joint or osteochondral sections, development of more

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