56 A. Gadomski, I. Santamaria-Holek, N. Kruszewska et al.
physico-chemical properties, accounts for its stress dispersing properties.
Figure 17. The AC matrix largely contains collagen fibres, proteoglycans and water. The properties and relationship of these components determines the mechanical response of the
cartilage [130].
3.1.1. Anatomical Classification in AC
Zonal classification - Based on the development of joints and the distribution of chon- drocytes and matrix components throughout the tissue, AC can be divided, ideally, into
four parallel zones [131]. The superficial or tangential zone of between 150 and 250 µm 10
− 20 of the total thickness, constitutes the surface of cartilage, adjacent to the joint cavity. An abundance of collagen fibrils arranged in a multidirectional plane [132], parallel
to the surface [133] makes up most of the matrix in this layer, with only a small amount of proteoglycans visible. In this layer, the chondrocytes are small and squashed in appearance,
and are concentrated throughout the layer in an undefined pattern as shown in Fig. 18.
As the tissue merges into the intermediate, transitional or midzone 40 − 60 of the to-
tal thickness, the collagen fibrils begin to change from a lateral to a more radial orientation, and become less dense. The architecture of the collagen is commonly described by the Ben-
ninghoff arcade [135] see Fig. 19. Conversely to the collagen, the proteoglycans increase in number towards the subchondral bone. The chondrocytes become larger and spherical,
and more equally spaced throughout the matrix, while the population of the cells decreases dramatically between the articular surface and intermediate zone [136] see Fig. 18.
In the third, deep or radiate layer 30 of the total thickness, the collagen fibres are ar- ranged in a radial orientation [133], encapsulating the numerous proteoglycans. The chon-
drocytes are arranged into columnar groups of 4 − 8 cells [126] in this region see Fig. 18.
The fourth layer or calcified zone connects the cartilage to the subchondral bone and is distinct by its calcification, compared to the uncalcified layers above it. This zone is marked
by a basophilic line, designated the tidemark, which is proposed to help prevent the shear
Can Modern Statistical Mechanics Unravel Some Practical Problems . . .
57
Figure 18. AC stained with Haematoxylin and Eosin stain, showing the cell distribution and size in each of the four zones. Cells are flattened and small in the articular surface
tangential layer, and increase in size towards the deeper zones, while forming horizontal clusters [134].
Figure 19. The Benninghoff arcade describes the arrangement of the collagen network throughout the 4 zones, denoted by I, II, III and IV. The line indicated by the arrow repre-
sents the tidemark [137].
fatigue fracture of collagen [133, 138]. It contains few cells, and the region is concentrated with crystals of calcium salts.
Compartmental classification - Within the different zones of the tissue, cartilage can
further be classified into compartments depending on proximity to the chondrocytes. The tissue closest to and surrounding the cells is the pericellular matrix, and is devoid of reg-
ular collagen fibrils and contains only finely textured filaments. The pericellular matrix is encapsulated by a fine meshwork of collagen fibres. During histological preparation, the
cells are often lost leaving behind holes in the tissue called lacunae. Histological studies show dark staining in the cell lacunae walls [133], suggesting that collagen is densely orga-
nized around this area to present what looks like a cell protection mechanism [136]. Further away from the cell is the intercellular or territorial and inter-territorial matrix, which forms
the bulk component of cartilage. The fibres are considerably larger and coarser within this matrix, and account for the mechanical properties of the tissue.
The properties of the different compartments are not uniform across the different zones,
58 A. Gadomski, I. Santamaria-Holek, N. Kruszewska et al.
but instead change depending on the depth from the surface and therefore the zone with which they are located. For example, the fibres found in the superficial zone have a diameter
that is only a quarter of those found in the deep zone [139]. The number of cells within lacunae also varies with respect to the zones, increasing in number and aligning radially
towards the deeper zones.
3.1.2. Matrix Components
