Remodelling
Bone remodeling
occurs throughout life. It is estimated
that in the first year of life 80-100% of bone is replaced by the remodeling
process. In adults, this slows to a
rate of 1-5% per year. The remodeling
process is a dynamic process that responds to outside forces such as load
demands and muscular attachments. In
1868, Julius Wolf first described the process by which bone can adapt its
internal structure and external form to accommodate specific load demands. The process would later be called Wolf’s
Law. In regions where load increases,
the remodeling process vigorously lays down new bone to more effectively carry
the load. In regions where load
decreases, the bone is resorbed. These
processes in conjunction can affect the overall shape of the bone to its most
efficient configuration capable of meeting demands. To this day little is understood about the mechanisms that drive
Wolf’s Law, but the remodeling process has been clearly elucidated.
Remodeling follows
a very specific sequence of events that invoke the bone multicellular unit
(BMU). The process begins with
resorption as a sheet of osteoclasts form the head of a “cutting cone” (Figure 21) that borrows a hole
into formed bone. Directly behind this,
new blood vessels infiltrate the new space.
This allows the immigration of osteoblasts that immediately begin replacing
the dissolved bone. This process
ultimately results in the formation of a new osteon.
Figure 21: Cutting Cone. Note the leading osteoclasts followed by a vascular
channel with peripheral osteocytes that have begun to lay down new bone.
The osteon (figure 22), also known as the
Haversian System, was first described in the 17th century by Clopton
Havers. The osteon is a long narrow
tube that is 200 micrometers in diameter of variable length. Centrally, the osteon consists of a small
channel that is 50-100 micrometers wide where one or two blood vessels, small
nerve, and lymphatic channels reside.
Layers of collagen and bone radiate from this central region in four to
twenty concentric circles. Embedded
within the layers are osteoblasts that have become encased in bony lacunae
thereafter called osteocytes. The
osteocytes lose their bone formation capacity and take up the role of
architectural maintenance, which is thought to last 25 years. The osteocytes communicate with each other
thru small channels called canaliculi.
The outer limit of the osteon is encased in a special layer called a
cement line. Osteons communicate with
each other through the cement lines with lateral passages called Volmann’s
canals. Ultimately this forms a complex
interconnected system with a maximized volume of bone surface area and
strength. This continuous process of
replacement is important as a means of constantly repairing the strain damage
done to the bone at the cement line interfaces that occur as a consequence of everyday
loading. See complete diagram in
Figure 23.
Figure 22
