Biomedical Engineering Reference
In-Depth Information
The surface of bone is rough and the coefficient of friction between two bones is very
high. If bones were able to come into contact with each other, to rotate about a joint, the
bone itself would become damaged due to the high frictional forces. To prevent bone from
coming into contact with another bone, a thin layer of cartilage is present on the ends of
the bone. This cartilage is termed articular cartilage and it partially acts to reduce the coef-
ficient of friction between two bones. Under normal conditions, the two cartilage surfaces
do not come into contact with each other either. Instead, there is a small fluid layer
between the cartilage surfaces. The fluid is termed synovial fluid, and we will discuss this
fluid in the following two sections. The cartilage acts as a safety mechanism for bone-bone
contact but it also helps to maintain a layer of synovial fluid within the joint.
Ligaments provide support and strength to joints, by helping to restrict bone movement
and to directly connect the bones to one another. Ligaments are typically made of collagen
fibers. It takes a great deal of force to tear a ligament because typically they are stiffer than
the bones. Therefore, the bone will fracture prior to ligament rupture. However, in a
sprain there is some damage to collagen fibers within the ligament. Tendons are not part
of the joints themselves, but pass over the joint. Tendons connect bone and muscle
together. In relation to the joint, tendons help to hold the joint in place but may restrict the
joint motion. However, tendons do provide mechanical support for the joints by connect-
ing bones to other muscles. For instance, many of the muscles that move the humerus are
located within the torso and are connected to the axial skeleton. Therefore, the shoulder
joint maintains the appropriate orientation because of this connection between the axial
skeleton, the muscles, and the humerus.
11.2 FORMATION OF SYNOVIAL FLUID
As we have stated in the previous section, synovial fluid fills the space between joints
to reduce the coefficient of friction between the articular cartilage adhered to the ends of
the bones that compose the joint. Synovial fluid is produced by the synovial membrane
which is located on the interior side of the fibrous capsule within the joint. The synovial
membrane is composed of many interwoven matrix proteins, especially proteoglycans and
collagens. Within the joints, this membrane also includes macrophages and fibroblasts to
prevent foreign cellular matter from entering the joint space. The combined actions of the
woven matrix fibers and the inflammatory cells within the membrane regulate the compo-
sition of synovial fluid.
Synovial fluid is produced as a filtrate of interstitial fluid. The composition of synovial
fluid is very similar to interstitial fluid, except that there is a significantly higher concen-
tration of proteoglycans, within synovial fluid. The proteoglycans are secreted by fibro-
blasts within the synovial membrane. The purpose of the proteoglycans is to increase the
organization of water within synovial fluid, so that it can withstand forces better than un-
organized water (i.e., the zeta potential and the electric double layer increase, as discussed
in Chapter 7). The water becomes organized due to hydrogen bonding between water
molecules and the amino acids that make up the protein portion of the proteoglycans. Due
to this increased arrangement, there are more bonds present within the fluid phase, which
Search WWH ::
Custom Search