Hyaline Articular Cartilage - Articular Cartilage Tissue Engineering - page 8

1.1.2.2 Territorial Structure
In addition to the zonal organization associated with cartilage extracellular matrix, the tissue also has
a microscale structure oriented with respect to distance from the chondrocyte cell membrane (Fig-
ure 1.6). As described above, the region immediately surrounding a chondrocyte is termed the
pericellular matrix and is characterized by having fine collagen fibers, high concentrations of pro-
teoglycans, and the presence of fibronectin and collagen type VI [ 40 , 41 ]. The exact function of the
pericellular matrix is not fully understood. However, strong evidence indicates that it helps to protect
the physical integrity of articular chondrocytes during compressive loading [ 27 ].
The region immediately surrounding the pericellular matrix (the territorial matrix) is com-
posed of similar molecular constituents as the surrounding extracellular matrix, namely collagen
type II and proteoglycans. In normal cartilage, collagen type VI has also been shown to be localized
to this region [ 42 ]. The territorial matrix exhibits a higher concentration of proteoglycans than the
surrounding extracellular matrix, as well as having a finer collagen structure [ 26 ].
The interterritorial matrix, which contains large collagen type IV fibers and varying con-
centrations of Proteoglycans, comprises the bulk of articular cartilage, providing the tissue with its
mechanical properties. Loading of articular cartilage involves force transmission through the in-
terterritorial, territorial, and pericellular matrices before reaching the chondrocytes. These regions
likely assist in modulating strains seen at the cellular level [ 13 ]. The interterritorial regions are rep-
resentative of bulk extracellular matrix tissue and contain large collagen type II fibers as well as
varying concentrations of proteoglycans, dependent on depth from the surface. Therefore, structural
breakdown of any region can dramatically affect the forces experienced by individual cells.
1.1.3 FUNCTION
The primary role of articular cartilage is to provide a low-friction, wear-resistant surface that can
withstand large loads over decades of constant use. Within the body, cartilage functions to facilitate
load support and load transfer while allowing for translation and rotation between bones. The degree
of loading in an articulating joint is dependent on its location in the body. The force exerted on the
hip has been calculated to be 3.3 times a person's bodyweight. The knee experiences a load of
approximately 3.5 times bodyweight, the ankle 2.5 times bodyweight, and the shoulder 1.5 times
bodyweight [ 43 ]. Experimentally, compressive stresses in the hip routinely reach 7-10 MPa and
have been measured up to 18 MPa during more stressful activities such as standing up [ 44 ]. The
biochemical and mechanical characteristics of articular cartilage directly affect how it performs in the
joint. Changes in these characteristics can dramatically alter the loading profile, thereby beginning
a process of degradation that can eventually result in total loss of the tissue
The deformation characteristics of articular cartilage play an important role in its mechanical
functionality. The time- and rate-dependent behavior of articular cartilage stems from interstitial
fluid flow through the solid matrix and is manifested via creep, stress relaxation, and energy dissipation
or hysteresis [ 8 , 45 - 47 ]. Sudden loading is initially borne by the fluid phase of the cartilage, helping
to absorb the energy of impact that would otherwise be felt by the solid phase. The contact stress