The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 96

region can only be accomplished by understanding the differences and interactions
between each of the component tissues that make up this complex interface region.

5.3 Materials Within the Bone-Cartilage Interface

The tissues that make up the osteochondral interface are comprised of collagen
fibrils, proteoglycans, water, and cells. Additionally, bone and ZCC are dynamic
biological composite materials that are strengthened and stiffened with increasing
levels of mineralization. This section considers how the composition and
microstructural organization within the osteochondral tissues influence the mechan-
ics of the larger joint structure.
Cartilage and bone differ greatly in the organization of their collagenous matrix,
type of collagen, and space that is available for infilling by mineral [ 20 , 50 ].
However, the basic materials that make up these two tissues serve similar bio-
mechanical roles. Tissue organization at the ultra- and micro-scales contributes to a
smooth transition between cartilage and bone (Table 5.1 ). This includes collagen
fibril orientation and organization and structural differences between mineralized
tissues. It is also at these same length scales that load transfer is predominantly
accomplished.

5.3.1 The Organic Phase

Soft tissues within human joints are usually in the form of hyaline cartilage
(containing predominately collagen types II and V) or fibrocartilage (collagen
types I and II). Both collagen types I and II are fiber forming and possess similar
amino acid sequences, yet these proteins possess a different combination of the

a

-

helix strands (type I contains 2 1(II)) [ 116 ]. In

addition, cartilage tissues contain a high concentration of hydrophilic, charged
proteoglycans that provide cushioning and resistance to compression. Other
materials include water, inorganic salts, and small amounts of other matrix proteins,
glycoproteins, and lipids [ 45 ]. The proteoglycans intermingle with collagen fibrils.
Proteoglycans form a bottlebrush-like structure via a protein core to which glycosa-
minoglycans (GAGs) attach. Significant amounts of water inflate this hydrophilic
matrix and interact with charged sites on the glycoproteins to contribute to
cartilage's osmotic pressure and time-dependent behavior [ 45 ]. Cartilage is thus a
biocomposite material where, rather than primarily contributing as a matrix com-
ponent (as occurs in bone), collagen fibers and their cross-links provide fiber
reinforcement to the proteoglycan network [ 37 ] and align in tension along primary
loading directions.
From an engineering perspective, bone can be considered as a particle-
reinforced composite composed of a type I collagen fiber matrix, ground substance

a 1(I) and 1 a 2(I), type II contains 3 a