Biomedical Engineering Reference
In-Depth Information
1998). The transmission of information is possible between osteocytes
themselves and between osteocytes and osteoblasts on the bone surface
through rapid fluxes of bone calcium across the gap junctions. In addition
to the gap junctions, the interstitial fluid that surrounds the osteocytes and
their processes provides an additional route for the diffusion of nutrients
and waste products. This information network is also linked to the activity
of osteoclasts, giant cells with 50 or more nuclei which have the function
of removing bone matrix.
Bone can be viewed as a composite material made of solid and liquid
phases. The solid phase is represented by the mineral phase whereas the
liquid phase is represented by the interstitial fluid. The solid phase is made
up of collagen type I produced by osteoblast bone cells. Apart from collagen
type I, osteoblast produces a variety of non-collagenous proteins, including
osteocalcin, osteopontin, osteonectin, and proteoglycans; regulatory factors,
such as cytokines, growth factors and prostaglandins; and neutral proteases,
alkaline phosphatase, and other enzymes that degrade the extracellular matrix
and prepare it for calcification.
Bone collagen is constructed in the form of a triple helix of two identical
a1(I) chains and one unique a2 chain stabilized by hydrogen bonding between
hydroxyproline and other charged residues. This configuration gives a fairly
rigid linear molecule of 300 nm long. Each molecule is aligned with the
next in a parallel fashion in a quarter-staggered array to produce a collagen
fibril. The collagen fibrils are then grouped in bundles to form the collagen
fiber.
4.1.2 Bone structure
There are two types of bone: trabecular (spongy or cancellous) bone and
cortical (dense or compact) bone (Fig. 4.1). Trabecular bone is found in
the epiphysis and metaphysis of long bones and inside flat or small bones.
Trabecular bone has an extensive network of small and interconnected
plates and rods of individual trabeculae oriented according to the external
loading. Cortical bone consists of layers with vascular channels surrounded
by lamellar bone. This arrangement is called the osteon or Haversian system.
The central canal of an osteon contains cells, vessels and nerves and the
canals connecting osteons are called Volkmann's canals.
at the microscopic level, trabecular and cortical bones consist of two
forms: woven and lamellar. Woven bone is considered to be immature
bone or primitive bone and is found in the embryo, the newly born and in
fracture healing. Woven bone has a matrix of interwoven coarse collagen
fibers with osteocytes distributed more or less at random. It is less organized
than lamellar bone. Woven bone is replaced by lamellar bone at age 2 or 3
years. Lamellar bone is a mature bone that results from the remodeling of
