Page 840 - Adams and Stashak's Lameness in Horses, 7th Edition

806 Chapter 7

Intermittent pressures created by the interaction of
Biomechanics the opposing articular surfaces pump fluid through the
As indicated previously, articular cartilage is a tissue
VetBooks.ir consisting of aggrecan that is stiff in compression, col­ cartilage for nutrition and the removal of metabolic by‐
63
products. Expressed fluid from the articular cartilage
lagen that is stiff and strong in tension, and somewhat
resembles synovial fluid except for its low protein con­
freely moving fluid carrying mobile ions (interstitial
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fluid). These components interact to provide the follow­ tent and viscosity. It has been shown that the concen­
tration of fixed charges from GAG is the prime
ing mechanical and physical characteristics when young 56
and healthy: (1) a permeable matrix that is stiff in com­ determinant of cartilage permeability. Simple diffusion
seems sufficient for nutrition of all but the deepest layers
pression, (2) a fibrous network capable of withstanding
high tensile stresses, (3) a fluid that flows under load or of articular cartilage, but joint movement probably
facilitates the process.
deformation and aids in dissipating high stresses in the
Studies have shown that proteoglycans restrict solute
tissue, and (4) a high swelling pressure that results in a movement within the tissue matrix. The proteoglycans
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matrix swollen with water. It has also been noted that
an important function of aggrecan in cartilage is to restrict the diffusion of large, uncharged solutes but do
not affect the diffusion of small, uncharged solutes. The
retard the rate of stretch and alignment when a tensile
load is suddenly applied and that this mechanism may molecular size and conformation of the solute is also an
important factor. Removal of proteoglycans increases
be useful in protecting the cartilage collagen network
during physiologic situations. 97 the influx of large molecules into the matrix, and this
suggests that proteoglycan removal may also increase
In an unloaded joint, the opposing articular surfaces 108
are not completely congruent. However, under physio­ the efflux of large molecules out of the tissue matrix.
28
It has been suggested that the marked loss of aggrecan
logic loading, because the cartilage is soft, deformation
causes an increase of contact area (reducing tissue stress and newly synthesized proteoglycan monomer from the
matrix in osteoarthritis (OA) is probably a direct result
levels) and increases joint conformity (which provides
additional stability). The adaptation of the shape of of the increased mobility of these macromolecules as the
tissue matrix components continue to degrade.
loaded cartilage may also help to form and retain bound­
It is also well accepted that mechanical forces modu­
ary lubrication (see below). As articular cartilage directly late the metabolic activity of chondrocytes; clarification
under load is compressed, the surrounding areas are sub­
jected to transverse tensile strains. These forces tend to of the specific mechanisms of mechanical signal trans­
duction in articular cartilage is emerging. One pro­
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redistribute fluid away from the compressed area and
into the stretched regions. As mentioned previously, the posed pathway is that chondrocytes may perceive
changes in their mechanical environment through cellu­
major contribution to the osmotic pressure provided by 33
the proteoglycans is derived from negatively charged sul­ lar deformation. More recently it is demonstrated that
the chondrocytes possess a primary cilium that is
fate and carboxyl groups of the GAG and the associated
cations (ionic or Gibbs–Donnan effect). Water is involved in mechanotransduction events related to carti­
108
lage extracellular matrix synthesis. This study con­
114
attracted by the high charge density, and this osmotic
pressure may contribute up to 50% of the compressive firmed that primary cilia were essential organelles for
cartilage mechanotransduction, as well as identified a
stiffness of the articular cartilage. The swelling pressure is
balanced by the tensile stress of the collagen framework. novel role for primary cilia not previously reported in
any other cell type, namely, cilia‐mediated control of
Mow and colleagues have described the extracellular
matrix as a cohesive porous composite. Their biphasic ATP reception. It is well established that the GAG con­
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tent of habitually loaded areas of cartilage is greater
model for articular cartilage considers the tissue as
an interacting mixture of two continuums: a porous than that of habitually unloaded areas. Work in sheep
stifle joints has shown that different areas of articular
permeable elastic solid and interstitial fluid. Because of
swelling pressure, the collagen network of the articular cartilage subjected to differing mechanical stresses con­
tain a phenotypically distinct chondrocyte population.
50
cartilage is under a tensile prestress even when unloaded.
During compression, the concentration of the organic Chondrocyte phenotypes were identified by the relative
biosynthesis of aggrecan, biglycan, and decorin.
material and the charge density increase because the
interstitial fluid is forced to flow from the matrix. A new Articular Cartilage Metabolism and Matrix Turnover
equilibrium is reached when the charge density, collagen
tension, and applied load are in balance (Figure 7.7). The chondrocytes synthesize all of the components of
The deformation of this cartilage in association with the cartilage matrix. At each stage of growth, develop­
fluid exudation is called creep. The compression time ment, and maturation, the relative rates of matrix syn­
curve consists of the creep phase, controlled by fluid thesis and degradation are adjusted to achieve net
exudation, and the second phase, which is related to growth, remodeling, or equilibrium. A unique interac­
the collagen proteoglycan matrix component. During tion exists between chondrocytes and the surrounding
prolonged periods of stationary loading, fluid is slowly matrix. This may be facilitated by a cilium from each
exuded and redistributed within the cartilage until an chondrocyte that extends into the matrix and acts as a
equilibrium position is reached. At this stage, the probe, sensing changes in the matrix composition such
increased concentration of the fixed charge density is as a loss of proteoglycan or collagen or an increase or
counterbalanced by the increased osmotic swelling decrease in HA concentration. This information is
pressure of the proteoglycan. When fluid motion relayed to the cell. Interaction between the pericellular
ceases, the entire external load is borne by the solid and territorial matrix and the chondrocyte cell
extracellular matrix. membrane also may include transmission of mechanical