Identification of important interactions between subchondral bone ...

CHAPTER 2: Introduction
18
Fig. 3. Schematic drawing of a long
bone. A femur bone divided in sections:
epiphyses, metaphyses, and a diaphysis.
The different types of bone are shown;
cortex (hard) and trabecular (spongy)
bone. The proximal epiphysis (femur
head) and the distal epiphysis are
covered with cartilage to provide a
smooth friction (during movement). The
figure was produced by Madsen, S.H.
The weight bearing bones, such as the femur and tibia that comprises the knee, have two
opposite and important functions. The first is to resist loading, thus the bone must be stiff. The
second is to be flexible to withstand shock, compression and tension by energy absorption and
deformation 21 . The stiffness is achieved by mineralization and flexibility is achieved by a suitable
organic matrix composition. However, if the bone matrix is too mineralized, it will become brittle
and more prone to fractures, whereas if it is under-mineralized, the bone will be too flexible and
will crack more readily. Therefore, the functionality of the bone highly depends on the ratio of
organic and inorganic matrices 21 .
In mammalian bone, the inorganic part of the matrix (50-70%) consist of
hydroxyapatite crystals [3Ca 3(PO 4) 2·(OH) 2], which are layered on and within collagen fibres,
ensuring bone matrix stiffness (mineralization). The crystals tend to orient in the same direction
as the collagen fibres. The organic part of the matrix (20-40%) consists mainly of collagen type I
(approximately 90%), but also glycoproteins, proteoglycans, and non-collagenous proteins. The
collagen type I molecule is a triple helix (described further in section 2.3.4), which is assembled
into fibres, providing the tensile strength of the matrix. The orientation of the collagen fibres
determines the microscopic structure of the bone. During bone development or fracture healing,
where bone is formed very rapidly, there is no preferential organization of the collagen fibres; this
type of bone is called woven bone (fig. 4) 17 . Woven bone is characterized by irregular bundles of
collagen fibres and calcification, which occur in irregularly distributed patches. Woven bone is
progressively replaced by mature lamellar bone (fig. 4) during the remodelling process that