The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 95

trabecular network [ 41 ]. Few, or no, collagen fibers continue across the interface
from ZCC to SCB in either synovial or cartilaginous joints [ 42 , 108 ]. However, the
interdigitated interface between the ZCC and SCB serves to anchor the two tissues.
A subchondral trabecular network buttresses and transfers loads from the SCB plate
into the larger bone structure [ 41 , 113 ]. The trabeculae in this region are anisotropic
and adapt to directions bearing the greatest mechanical loads by remodeling and
reorganizing over time. While these two tissues, the SCB plate and the underlying
STB, are not well distinguished from each other in the literature, they possess
different organization, mechanical properties, and adaptations to mechanical loads
[ 20 , 21 , 64 , 80 , 82 , 114 ].
The SCB plate and STB structure are highly sensitive to altered loading states.
In exercise or with the disruption of loading patterns observed in osteoarthritis, the
marrow spaces within the trabecular bone become infilled with a structurally weak,
rapidly deposited woven bone material that possesses low mineral content [ 115 ].
Within the spine, regions of SCB that experience high tensile loading due to annular
insertion are generally thicker and show more extensive cartilage calcification
than other regions of the cartilaginous endplate [ 43 ]. The ZCC, SCB, and STB
are dynamic structures that change with loading state via normal bone remodeling
processes.
5.2.3 Conclusions
While the synovial and cartilaginous joints possess vastly different functions
(articulating motion and stability, respectively), the tissues that make up these
two joint types possess functional and morphological similarities. For example,
the avascular cartilaginous joints that form the IVD between two adjacent spinal
vertebral bodies include the cartilaginous endplate, the annulus fibrosis, and the
nucleus pulposis. These three structures are generally analogous to the synovial
joint's articular cartilage, joint capsule, and synovial fluid, respectively. It is not
surprising, then, to find similarities in patterns of interface structure and aging or
disease across these two joint types.
The common structural features that define material behavior at the microscopic
scale within both hard and soft tissues include the organization, mineralization, and
type of collagen fibrils present. The geometry of the calcification front and porosity
of the underlying bone also contribute to the structural integrity of the
osteochondral interface. The underlying structure and continuity are major factors
in predicting and determining behavior of the interface and load transfer.
The response of bone to the stresses from the overlying hyaline cartilage in the
synovial joint or in the IVD highlights the interplay between bone and cartilage and
mechanisms that regulate remodeling. Further, each tissue within this interface
possesses different microstructural and compositional organization and is differen-
tially affected by loading conditions. A complete understanding of the structure,
physiology, and function of the individual tissues that make up the osteochondral