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Musculoskeletal Function
Fibrous Joints (Synarthroses)
Fibrous joints, or synarthroses, are joints in which the
bones are joined by dense fibrous tissue. They have little
or no movement, depending on the length of the fibers
connecting the articulating bone. The sutures of the skull
are examples of fibrous joints. In children and young
adults the bones of the skull are joined by dense connec-
tive tissue; and in older persons, by bone. Another type of
fibrous joint that is partially moveable unites bones with
a sheet of fibrous tissue, either a ligament or fibrous mem-
brane. The interosseous membrane that joins the radius
and the ulna of the forearm is an example of this type of
joint. A unique type of fibrous joint is the one between
the root of a tooth and the alveolar processes of the jaw.
The fibrous tissue between the tooth and alveolar bone of
the tooth socket is called the periodental ligament.
Cartilaginous Joints (Amphiarthoses)
Cartilaginous joints, or amphiarthroses, are connected by
hyaline cartilage or fibrocartilage and have limited motion.
In primary cartilaginous joints, the bones are joined by
hyaline cartilage, which permits slight bending during
early childhood. Primary cartilaginous joints are usually
temporary articulations, such as those present during
early development, where the epiphysis and the shaft are
joined by cartilage or growth plate. Secondary cartilagi-
nous joints are strong, slightly moveable joints united by
fibrocartilage. The fibrocartilaginous intervertebral disks
between the vertebrae consist of connective tissue that
holds the vertebrae together. These joints provide strength
and shock absorption as well as considerable flexibility.
Synovial Joints (Diarthroses)
Synovial joints, or
diarthroses
, are freely moveable
joints in which the articulating bone ends are separated
by a joint cavity containing synovial fluid. Most joints in
the body are of this type. Although as a group they are
classified as freely moveable, synovial joints include pla-
nar joints, which allow almost no movement (e.g., ver-
tebrocostal joint), and hinge joints, which allow angular
movement in one plane (e.g., interphalangeal, knee, and
ankle joints). Only the
ball-and-socket joints
permit
movement in all directions (e.g., shoulder and hip joint).
All synovial joints are covered with articular cartilage,
a feature that allows the surfaces of these joints to slide
freely past each other during movement. The articular
cartilage, which is typically hyaline cartilage, is unique
in that its free surface is not covered with perichon-
drium. It has only a peripheral rim of perichondrium,
and calcification of the portion of cartilage abutting the
bone may limit or preclude diffusion from blood vessels
supplying the subchondral bone. Regeneration of most
cartilage is slow; it is accomplished primarily by growth
that requires the activity of perichondrium cells.
In synovial joints, the articulating ends of the bones
are not connected directly but are indirectly linked by
a strong fibrous capsule (i.e., joint capsule) that sur-
rounds the joint and is continuous with the periosteum
(Fig. 42-7). The joint capsule consists of two layers: an
outer fibrous layer and an inner synovial membrane.
The outer fibrous layer of the capsule supports the joint
and helps to hold the bones in place. Additional sup-
port is provided by ligaments that extend between the
bones and by tendons that attach to muscles supplying
the joint. The synovium secretes a slippery fluid with the
consistency of egg white called
synovial fluid.
In addition
to supplying nutrients and oxygen to the chondrocytes
in the articular cartilage, this fluid serves as a lubricant
for the joint. Moreover, macrophages in the synovial
fluid act to phagocytize debris in the joint space.
The collagen fibers in the articular surface cartilage
are arranged to evenly distribute the forces generated by
pressure to this tissue. The proteoglycan molecules of
the cartilage, found isolated or aggregated in a network,
contain large amounts of water. These matrix compo-
nents, rich in hydrophylic glycosaminoglycans, function
as a mechanical spring. When pressure is applied, water
is forced out of the cartilage matrix into the synovial
fluid. When the pressure is released, water is attracted
back into the collagen matrix. These water movements,
which are brought about by the use of a joint, are essen-
tial for the delivery of nutrients and the exchange of car-
bon dioxide, oxygen, and other molecules between the
synovial fluid and articular cartilage.
Ligaments andTendons
Ligaments and tendons are dense connective tissue
structures that connect muscles and bone and support
synovial joint structures. The dense connective tissue
found in tendons and ligaments has a limited blood sup-
ply and is composed largely of intercellular bundles of
collagen fibers arranged in the same direction and plane.
Collagen is an inelastic and insoluble fibrous protein.
Because of its molecular configuration, collagen has
great tensile strength; the breaking point of collagenous
fibers found in human tendons is reached with a force of
several hundred kilograms per square centimeter.
Articular cartilage
Fibrous capsule
Joint cavity
Synovial
membrane
Articular
cartilage
FIGURE 42-7.
Synovial (diarthrodial) joint, showing the
articular cartilage, fibrous joint capsule, joint cavity, and
synovial membrane.
