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
Collagen - There are at least 20 known types of collagen making up the different connective tissues within biological materials and organs such as cartilage, tendons, ligaments, skin and
muscle [140]. Within cartilage, collagen type II is the most abundant type, accounting for 90
− 95 of the collagen in the matrix, and is responsible for its great tensile strength. This strength is attributed to the triple helical structure [140] see Fig. 20 consisting of three left
handed α
1II polypeptide helices wound around each other in right handed twist [141]. The oppositely twisting direction of the helices prevents the unravelling of the molecule
under tension [140]. Type XI collagen interacts with type II to form the fibril meshwork, and may control fibril diameter. Type X contains interrupted triple helices which are thought
to form bridges between the collagen and proteoglycans and is also found mineralised in the calcified zone of cartilage. Type VI collagen is microfibrillar, forming elastic fibres that
are in higher concentration around the periosteum.
Figure 20. Structure of the collagen fibre. The collagen molecule is made up of three collagen threads wound in a triple helix. These microfibril molecules are then packaged to
form strong fibrils, which appear as striated bands due to the overlap and hole zones in the packaged molecules [142].
In its healthy state, collagen is structurally characterized by an amorphous structure of fibrils of varying diameter. As the collagen fibrils extend throughout the matrix the fibrils
are occasionally observed to run in parallel bundles over long and short distances, randomly entwined [143].
In the superficial zone, the collagen fibrils run parallel to the surface [133, 144]. The study of split line direction has been used to determine the directional orientation of the
collagen fibrils in the superficial zone [132, 133, 144, 145, 146, 147]. When the surface is pierced with an Indian ink charged pin, a longitudinal split line forms along this surface
indicating the predominant direction of collagen fibrils in the surface layer.
There have been many contrasting interpretations of split line results regarding the di- rectional organization within the articular surface. Studies involving split line propagation
Can Modern Statistical Mechanics Unravel Some Practical Problems . . .
59 have reported definite alignment along split line direction [144], while others have noted
multidirectionality [132, 133]. The articular surface is most strain limiting in tension along the split line direction, and least strain limiting across the split line [132, 148, 149]. This
means that the tissue exhibits a much higher stiffness along the split line than across it, with increasing strain or deformation. Some researchers have interpreted this result as an indi-
cation of the alignment of the fibrils within the articular surface [148, 149], while others hypothesise that the split line direction provides a measure of the ability of the fibrils to
rearrange under loading [132].
The proteoglycans - The predominant proteoglycan in AC consist of a protein core of
hyaluronic acid covalently bonded by link proteins to aggrecan monomers. The aggrecan consists of a core protein, attached to two types of glycosaminoglycans, keratin sulphate
and chondroitin sulphate, giving the aggrecan its bottle-brush form as shown in Fig. 21. The primary role of the aggrecan is to provide the cartilage with compressive stiffness
[150], through its high negative charge which creates repulsive forces. This force attracts positive ions including Sodium Na
+
or Calcium Ca
2 +
and water H
2
O, causing the proteoglycan molecules to swell.
Figure 21. Partial proteoglycan molecule entangled in the collagen matrix. The proteo- glycan consists of a hyaluronic backbone, attached to many aggrecan monomers by link
proteins. The glycosaminoglycans chondroitin sulphate and keratin sulphate are the gly- cosaminoglycans that result in the bottle brush form [153].
Perlecan is a large heparin sulphate proteoglycan found prominently in the pericellular matrix surrounding the chondrocytes and it is has been suggested that it promotes cell ad-
hesion, chondrocyte differentiation, cartilage and extracellular matrix maintenance [151]. The small leucine-rich proteoglycans include fibromodulin, epiphycan, lumican, decorin
and biglycan. These are thought to contribute to cartilage maintenance through their in- teraction with the collagen network in binding growth factors and contributing to the fixed
charge density. In normal cartilage, proteoglycans are heterogeneous, varying in size and composition [152].
The lipids - Lipids are found on the surface of AC, within the extra-cellular [154], and
intra-cellular matrix, contributing to about 0.5 to 1.0 of the wet weight [155]. Extensive research has explored the role of surface lipids in providing a boundary lubricant within the
synovial joint, decreasing the frictional resistance and providing the hydrophobic properties of the AC surface [156, 157, 158]. Also, intra-cellular lipids have been shown to have an
60 A. Gadomski, I. Santamaria-Holek, N. Kruszewska et al.
effect on the stiffness of cartilage, with delipidization found to decrease the deformation of the matrix, decreasing the strain by 15
− 20 relative to the normal intact tissue [159, 160].
The chondrocytes - Chondrocytes account for between 3
−10 of the cartilage volume, and are in no physical contact with each other, which in addition to the avascular, aneural
and alymphatic properties of cartilage, results in a tissue that has a very limited and slow response to damage [161].
The primary role of the chondrocyte is to synthesise, remodel and replace the matrix of collagen and PG. During mechanical stimulation of AC, the matrix transmits signals to the
chondrocyte to maintain its normal composition. The synthetic activity of the chondrocytes also appears elevated in the presence of matrix fragments. When the joint is immobilized,
the chondrocyte responds with the secretion of proteinases which breaks down the proeto- glycans and collagen fibrils. This process of synthesis and degradation continues for many
decades, until age, disease or environmental impacts lead to an imbalance in the chondro- cytes breakdown of the matrix.
3.1.3. Biomechanics of Cartilage
