Biomedical Engineering Reference
In-Depth Information
strengthens the fluid. The other major function of the proteoglycans is to increase the over-
all viscosity of the synovial fluid.
As we have discussed briefly before, one of the major functions of synovial fluid is the
lubrication of the joint. The articular cartilage becomes filled with synovial fluid and the
cartilage holds the fluid in place. When the joint is under compression, the two articular
cartilage surfaces come into contact with each other, compress each other, and force some
of the synovial fluid out of the cartilage into the joint space. The “free” synovial fluid
lubricates the two articular cartilage surfaces and significantly reduces the coefficient of
friction between these surfaces. When the compressive forces are removed from the joint,
the articular cartilage expands again to its normal size. At this point, there is some free
space within the cartilage and the synovial fluid that was forced out of this space re-enters
the cartilage. Therefore, the cartilage effectively holds the synovial fluid within the joint
and releases it as needed. It is very devastating when cartilage is torn off the bone or
degenerates significantly. When this occurs, the joint can no longer hold significant quanti-
ties of synovial fluid, and therefore, there is no added lubrication. Also, the coefficient of
friction increases significantly because now the contact surfaces become bone-bone or
bone-cartilage.
Another mechanical property of the synovial fluid is that it acts as a shock absorber
during loading. Because synovial fluid is highly viscous, during compressive loading,
there is a time-dependent strain that tends to distribute the load over a larger-surface area
and to diminish the overall stress within the joint. Synovial fluid is also a dilatant fluid,
meaning that for a large shear stress, the amount of strain needed to move the fluid
becomes extremely large. This helps to prevent an over-loading condition on the bone,
which would effectively damage the cartilage and possibly the bone itself.
The last major function of synovial fluid is to bring nutrients into the joint space. The
joints themselves are avascular, but require nutrients so that chondrocytes (the cells that
produce cartilage) can maintain their proper functions. To perform this task, the synovial
fluid must continually circulate around the joint space. This provides nutrients to all of the
cells and removes the cellular wastes. During each joint compression, the synovial fluid is
circulated throughout the joint space. As the fluid comes into contact with the synovial
membrane, wastes are removed and fresh nutrients diffuse into the fluid from the intersti-
tial fluid. The removed wastes enter the interstitial joint space and then diffuse into
capillaries that are located outside of the joint capsule. From these three functions, it
should be apparent that synovial fluid is critical for the proper functioning of movable
joints.
11.3 SYNOVIAL FLUID FLOW
In order to enter the joint space, synovial fluid traverses a fairly complex path starting
from blood vessels and ending with re-entering the vascular system. Water, ions, and
other small solutes have the ability to cross the endothelial cell boundary that comprises
capillaries, as described in Chapter 6. This forms the interstitial fluid, which has a slightly
different composition depending on the particular tissue that it is formed within. Synovial
fluid is then formed as a filtrate of the interstitial fluid within the joint capsule space. As
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