The Skeletal System and Its Movements 13to the situation with fibrous joints, there is no space
between the adjacent bones. Like bone, cartilage
cells are surrounded by an extracellular matrix con-
taining collagen fibers. However, unlike bone, this
matrix is not calcified and is more like a firm gel in
consistency. This gives cartilage less rigidity and more
shock-absorbing capacity. Examples of cartilaginous
joints (fibrocartilage type) include the joints between
the bodies of the vertebrae (the intervertebral discs)
and the pubes (pubic symphysis). This fibrocartilage
type of joint involves a pad or disc of fibrocartilage,
a design that allows slight movement as well as
shock absorption. The epiphyseal plate, previously
discussed with long bones, is also a cartilaginous
joint, only of the hyaline cartilage type. The latter
joint allows no true movement but adds a “give” to
the associated bones.
Synovial Joints
Synovial joints differ from fibrous and cartilaginous
joints in that adjacent bones are not directly con-
nected to each other and there is actually a space
between the articulating bones. This space is called
the joint cavity. Although this space is very small, it
generally allows a large degree of motion. Synovial
joints are the most common type of joint in the human
body, and almost all of the joints found in the limbs
are synovial in nature. Synovial joints are particularly
important for the study of human movement, and
thus are the focus of this book. Examples of synovial
joints include the shoulder, elbow, wrist, hip, knee,
and ankle. The typical structure of a synovial joint is
described next and is depicted in figure 1.5.
A characteristic of synovial joints is that the ends
of the bones that come together to form the joint
are covered with articular cartilage. Articular carti-
lage is a thin layer of hyaline cartilage covering the
joint surfaces that helps decrease friction and aids
in shock absorbency. The extracellular matrix of
hyaline cartilage has characteristics between those
of a solid and a liquid and has the ability to adapt to
stress—actually exuding some of its fluid in response
to loading (Whiting and Zernicke, 1998), spread-
ing the load and reducing the stress at any contact
point by 50% or more (Hall, 1999). According to
one estimate, it also reduces friction at joints to only
approximately 17% to 33% of the friction of a skate
on ice under the same load. Articular cartilage does
not contain its own blood supply and is dependent on
nourishment from the synovial fluid and underlying
vascular bone. In some areas where this cartilage is
thick (such as the backside of the patella), nourish-
ment may not be adequate and degeneration can
occur. In general, once growth has ceased, cartilage
cell division is infrequent, and damage is generally
repaired by fibrous tissue.
Synovial joints are surrounded by a sleevelike
structure made of fibrous tissue called the articular
or joint capsule. This capsule varies markedly in thick-
ness and composition between joints to favor either
mobility or stability. The fibrous tissue composing the
capsule contains irregularly arranged collagen fibers
and some elastic fibers in its matrix (dense irregular
connective tissue), which give it strength and allow it
to withstand tension applied in many directions. The
capsule generally attaches to the bones, via the perios-
teum, at the margins of the articular cartilage.
The capsule is lined on the inside with synovial
membrane. This membrane is a vascular, fragile,
smooth tissue (loose connective tissue) that produces
synovial fluid. This synovial fluid (G. syn, together +
L. ovum, egg) has a consistency similar to egg white
and helps to lubricate the joint and decrease wear
and tear. Studies indicate that synovial fluid will
change its characteristics (viscosity) such that when
either the temperature or the velocity of joint move-
ment is low, it will offer more resistance to movement
(Levangie and Norkin, 2001). Conversely, when the
temperature is higher (such as after warm-up) or
the velocity of movement is higher, less resistance to
movement is provided. Synovial fluid also is important
for nourishment of articular cartilage, and it contains
cells that respond to the presence of a foreign object
or infection. When injury or irritation occurs, an
abundant secretion of synovial fluid follows, which can
produce noticeable swelling. It is the presence of theFIGURE 1.5 Structure of a typical synovial joint
(longitudinal section).