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the calcified cartilage can reach higher but more variable mineralization and
calcium levels: 1-28 wt% in ZCC vs. 16-26 wt% in SCB [ 50 , 80 , 132 ]. The mineral
in the ZCC forms plate-like structures [ 133 ] and aligns along the direction of the
collagen fibrils [ 50 ].
The dimensions and degree of particle alignment in mineralized cartilage also
attains values similar to those in SCB [ 50 ], yet the mineral volume fraction has
been observed in multiple studies to exceed that of the underlying bone [ 20 , 21 , 41 ,
80 , 134 ]. Duer et al. have shown that the connection between the mineral particles
and collagen in calcified cartilage is less developed than that in bone, which may be a
reflection of increased cartilage hydration and the inherent difference in matrix
composition [ 121 ]. Unlike bone, a large amount of extrafibrillar space exists in
cartilage that is occupied by water [ 135 ]. This availability of space, along with the
50% decrease in proteoglycan content of the cartilage with mineralization, allows
for a high mineral content as compared to bone (~15% greater in ZCC) [ 136 , 137 ].
The mineral volume fraction throughout the ZCC varies across the tidemarks.
Interestingly, the mineral density within the ZCC is often observed to increase with
proximity to each tidemark and also with proximity to the hyaline cartilage
(Fig. 5.2 ). The notion that the mineral forms a functional gradient between
mineralized and unmineralized tissues at this interface is refuted by such
observations, where increased mineralization exacerbates the modulus mismatch
between the ZCC and the overlying hyaline cartilage [ 20 , 41 , 80 ].
5.3.3 Conclusions
The analogous structural scales in both bone and cartilage provide a framework for
examining the interface between these two tissues. Considering the continuum of
material or structures at each hierarchical level demonstrates how the composition
and organization at smaller length scales influence material behavior at the macro-
scopic level [ 36 ]. From this perspective, it becomes clear how the unique properties
of individual materials form biocomposites, and that these composite materials then
combine to form larger, macro-scale hierarchical-based structures that are ideally
suited for load transfer across dissimilar material interfaces. Examining the compo-
sition of the osteochondral interface from a hierarchical approach that extends from
the macro- to the nano-meter length scales provides insight into how these tissues
function within the healthy joint. This foundation is critical for understanding how
the osteochondral interface is altered by aging or disease and how to ultimately
recapitulate key aspects of native tissues with the goal of engineering osteochondral
tissues or novel engineered bimaterial systems.
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