265
CHAPTER EIGHT
Joints of the Skeletal System
8.4
TYPES OF SYNOVIAL JOINTS
The articulating bones of synovial joints have a variety of shapes
that allow different types of movement. Based upon their shapes
and the movements they permit, these joints can be classiF ed
into six major types—ball-and-socket joints, condylar joints,
plane joints, hinge joints, pivot joints, and saddle joints.
1. A
ball-and-socket joint,
or
spheroidal joint,
consists of
a bone with a globular or slightly egg-shaped head that
articulates with the cup-shaped cavity of another bone.
Such a joint allows a wider range of motion than does
any other type, permitting movements in all planes, as
well as rotational movement around a central axis. The
hip and shoulder have joints of this type
(f g. 8.9a)
.
2. In a
condylar joint,
or
ellipsoidal joint,
the ovoid
condyle of one bone F
ts into the elliptical cavity of
another bone, as in the joints between the metacarpals
a joint cavity is injured or infected. Synovial fl
uid contains
stem cells, which may function in ligament regeneration
following injury.
Synovial fluid has a consistency similar to uncooked
egg white, and it moistens and lubricates the smooth carti-
laginous surfaces of the joint. It also helps supply articular
cartilage with nutrients obtained from blood vessels of the
synovial membrane. The volume of synovial fl uid in a joint
cavity is usually just enough to cover the articulating sur-
faces with a thin F
lm of fl
uid. The volume of synovial fl uid
in the cavity of the knee is 0.5 mL or less.
A physician can determine the cause of joint inflammation or
degeneration (arthritis) by aspirating a sample of synovial F
uid from
the a±
ected joint using a procedure called arthrocentesis. Bloody
F
uid with lipid on top indicates a fracture extending into the joint.
Clear F
uid and an increase in stem cell number is found in osteoar-
thritis, a degeneration of collagen in the joint. Cloudy, yellowish
F
uid may indicate rheumatoid arthritis, and crystals in the synovial
F
uid signal gout. If the F
uid is cloudy but red-tinged and contain-
ing pus, a bacterial infection may be present that requires prompt
treatment. Normal synovial F
uid has 180 or fewer leukocytes (white
blood cells) per mL. If the fluid is infected, the leukocyte count
exceeds 2,000.
Some synovial joints are partially or completely divided
into two compartments by discs of fibrocartilage called
menisci
(me-nis
ke) (sing.,
meniscus
) between the articular
surfaces. Each meniscus attaches to the F brous layer of the
joint capsule peripherally, and its free surface projects into
the joint cavity. In the knee joint, crescent-shaped menisci
cushion the articulating surfaces and help distribute body
weight onto these surfaces
(f
g. 8.8)
.
±luid-F
lled sacs called
bursae
(ber
se) are associated
with certain synovial joints. Each bursa has an inner lin-
ing of synovial membrane, which may be continuous with
the synovial membrane of a nearby joint cavity. These sacs
contain synovial fl
uid and are commonly located between
the skin and underlying bony prominences, as in the case
of the patella of the knee or the olecranon process of the
elbow. Bursae cushion and aid the movement of tendons
that glide over bony parts or over other tendons. The
names of bursae indicate their locations. ±igure 8.8 shows
a
suprapatellar bursa,
a
prepatellar bursa,
and an
infrapa-
tellar bursa.
PRACTICE
5
Describe two types of cartilaginous joints.
6
What is the function of an intervertebral disc?
7
Describe the structure of a synovial joint.
8
What is the function of the synovial F
uid?
Femur
Synovial membrane
Suprapatellar bursa
Patella
Prepatellar bursa
Subpatellar fat
Articular cartilage
Menisci
Infrapatellar bursa
Tibia
FIGURE 8.8
Menisci separate the articulating surfaces of the femur
and tibia. Several bursae are associated with the knee joint.
Articular cartilage, like other cartilaginous structures, lacks a direct
blood supply (see chapter 5, p. 158). Surrounding synovial F
uid supplies
oxygen, nutrients, and other vital chemicals. Normal body movements
force these substances into the joint cartilage. When a joint is immobi-
lized or is not used for a long time, inactivity may cause degeneration of
the articular cartilage. The degeneration may reverse when joint move-
ments resume. However, it is important to avoid exercises that greatly
compress the tissue during the period of regeneration. Otherwise,
chondrocytes in the thinned cartilage may be injured, hindering repair.
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