The
sites where two skeletal elements come together are termed joints. The
two general categories of joints (Fig. 1.19) are those in which:
- the skeletal elements are separated by a cavity (i.e., synovial joints); and
- there is no cavity and the components are held together by connective tissue (i.e., solid joints).
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| Blood
vessels that cross a joint and nerves that innervate muscles acting on a
joint usually contribute articular branches to that joint.
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| Synovial
joints are connections between skeletal components where the elements
involved are separated by a narrow articular cavity (Fig. 1.20). In
addition to containing an articular cavity, these joints have a number
of characteristic features.
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First, a layer of cartilage, usually
hyaline cartilage,
covers the articulating surfaces of the skeletal elements. In other
words, bony surfaces do not normally contact one another directly. As a
consequence, when these joints are viewed in normal radiographs, a wide
gap seems to separate the adjacent bones because the cartilage that
covers the articulating surfaces is more transparent to X-rays than
bone.
Figure 1.19 Joints.
A. Synovial joint.
B. Solid joint.
A second characteristic feature of synovial joints is the presence of a joint capsule consisting of an inner synovial membrane and an outer fibrous membrane.
- The synovial membrane attaches to the margins of the joint
surfaces at the interface between the cartilage and bone and encloses
the articular cavity. The synovial membrane is highly vascular and
produces synovial fluid, which percolates into the articular cavity and
lubricates the articulating surfaces. Closed sacs of synovial membrane
also occur outside joints where they form synovial bursae or tendon
sheaths. Bursae often intervene between structures, such as tendons and
bone, tendons and joints, or skin and bone, and reduce the friction of
one structure moving over the other. Tendon sheaths surround tendons and
also reduce friction.
- The fibrous membrane is formed by
dense connective tissue and surrounds and stabilizes the joint. Parts of
the fibrous membrane may thicken to form ligaments, which further
stabilize the joint. Ligaments outside the capsule usually provide
additional reinforcement.
Figure 1.20 Synovial joints.
A. Major features of a synovial joint.
B. Accessory structures associated with synovial joints.
Another
common but not universal feature of synovial joints is the presence of
additional structures within the area enclosed by the capsule or
synovial membrane, such as
articular discs (usually composed of fibrocartilage),
fat pads, and
tendons.
Articular discs absorb compression forces, adjust to changes in the
contours of joint surfaces during movements, and increase the range of
movements
that can occur at joints. Fat pads usually occur between the synovial
membrane and the capsule and move into and out of regions as joint
contours change during movement. Redundant regions of the synovial
membrane and fibrous membrane allow for large movements at joints.
| Descriptions of synovial joints based on shape and movement
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Synovial joints are described based on shape and movement:
- based on the shape of their articular surfaces, synovial joints
are described as plane (flat), hinge, pivot, bicondylar (two sets of
contact points), condylar (ellipsoid), saddle, and ball and socket;
- based
on movement, synovial joints are described as uniaxial (movement in one
plane), biaxial (movement in two planes), and multi-axial (movement in
three planes).
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| Hinge joints are uniaxial, whereas ball and socket joints are multi-axial.
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| Specific types of synovial joints (Fig. 1.21)
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- Plane
joints-allow sliding or gliding movements when one bone moves across
the surface of another (e.g., acromioclavicular joint)
- Hinge
joints-allow movement around one axis that passes transversely through
the joint; permit flexion and extension (e.g., elbow [humeroulnar]
joint)
- Pivot joints-allow movement around one axis that passes
longitudinally along the shaft of the bone; permit rotation (e.g.,
atlanto-axial joint)
- Bicondylar joints-allow movement mostly in
one axis with limited rotation around a second axis; formed by two
convex condyles that articulate with concave or flat surfaces (e.g.,
knee joint)
- Condylar (ellipsoid) joints-allow movement around
two axes that are at right angles to each other; permit flexion,
extension, abduction, adduction, and circumduction (limited) (e.g.,
wrist joint)
- Saddle joints-allow movement around two axes that
are at right angles to each other; the articular surfaces are saddle
shaped; permit flexion, extension, abduction, adduction, and
circumduction (e.g., carpometacarpal joint of the thumb)
- Ball
and socket joints-allow movement around multiple axes; permit flexion,
extension, abduction, adduction, circumduction, and rotation (e.g., hip
joint)
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Solid joints
Figure 1.21 Various types of synovial joints.
A. Condylar (wrist).
B. Gliding (radioulnar).
C. Hinge or ginglymus (elbow).
D. Ball and socket (hip).
E. Saddle (carpometacarpal of thumb).
F. Pivot (atlanto-axial).
Solid
joints are connections between skeletal elements where the adjacent
surfaces are linked together either by
fibrous connective tissue or by cartilage, usually fibrocartilage (Fig.
1.22). Movements at these joints are more restricted than at synovial
joints.
Fibrous joints include sutures, gomphoses, and syndesmoses.
- Sutures occur only in the skull where adjacent bones are linked by a thin layer of connective tissue termed a sutural ligament.
- Gomphoses
occur only between the teeth and adjacent bone. In these joints, short
collagen tissue fibers in the periodontal ligament run between the root
of the tooth and the bony socket.
- Syndesmoses are joints
in which two adjacent bones are linked by a ligament. Examples are the
ligamentum flavum, which connects adjacent vertebral laminae, and an
interosseous membrane, which links, for example, the radius and ulna in
the forearm.
Cartilaginous joints include synchondroses and symphyses.
- Synchondroses occur where two ossification centers in a
developing bone remain separated by a layer of cartilage, for example
the growth plate that occurs between the head and shaft of developing
long bones. These joints allow bone growth and eventually become
completely ossified.
- Symphyses occur where two separate
bones are interconnected by cartilage. Most of these types of joints
occur in the midline and include the pubic symphysis between the two
pelvic bones, and intervertebral discs between adjacent vertebrae.
Figure 1.22 Solid joints.
In the clinic
| Degenerative joint disease |
| Degenerative joint
disease is commonly known as osteoarthritis or osteoarthrosis. The
disorder is related to aging but not caused by aging. Typically there
are decreases in water and proteoglycan content within the cartilage.
The cartilage becomes more fragile and more susceptible to mechanical
disruption. As the cartilage wears, the underlying bone becomes fissured
and also thickens. Synovial fluid may be forced into small cracks that
appear in the bone's surface, which produces large cysts. Furthermore,
reactive juxta-articular bony nodules are formed (osteophytes). As these
processes occur, there is slight deformation, which alters the
biomechanical forces through the joint. This in turn creates abnormal
stresses, which further disrupt the joint (Figs. 1.23 and 1.24). |
| In the United
States, osteoarthritis accounts for up to one-quarter of primary health
care visits and is regarded as a significant problem. |
| The etiology of
osteoarthritis is not clear; however, osteoarthritis can occur secondary
to other joint diseases, such as rheumatoid arthritis and infection.
Overuse of joints and abnormal strains, such as those experienced by
people who play sports, often cause one to be more susceptible to
chronic joint osteoarthritis. |
| Various treatments
are available, including weight reduction, proper exercise,
anti-inflammatory drug treatment, and joint replacement (Fig. 1.25). |
| Arthroscopy is a
technique of visualizing the inside of a joint using a small telescope
placed through a tiny incision in the skin. Arthroscopy can be performed
in most joints. However, it is most commonly performed in the knee,
shoulder, ankle, and hip joints. The elbow joint and wrist joint can
also be viewed through the arthroscope. |
Arthroscopy allows
the surgeon to view the inside of the joint and its contents. Notably,
in the knee, the menisci and the ligaments are easily seen, and it is
possible using separate puncture sites and specific instruments to
remove the menisci and replace the cruciate ligaments. The advantages of
arthroscopy are that it is performed through small incisions, it
enables patients to quickly recover and return to normal activity, and
it only requires either a light anesthetic or regional anesthesia during
the procedure.
Figure 1.23 This radiograph demonstrates the loss
of joint space in the medial compartment and presence of small spiky
osteophytic regions at the medial lateral aspect of the joint.
Figure 1.25 Post-knee replacement. This radiograph shows the position of the prosthesis.
Figure 1.25 Post-knee replacement. This radiograph shows the position of the prosthesis.
Joint replacement
| Joint replacement
is undertaken for a variety of reasons. These predominantly include
degenerative joint disease and joint destruction. Joints that have
severely degenerated or lack their normal function are painful, which
can be life limiting, and in otherwise fit and healthy individuals can
restrict activities of daily living. In some patients the pain may be so
severe that it prevents them from leaving the house and undertaking
even the smallest of activities without discomfort. |
| Large joints are
commonly affected, including the hip, knee, and shoulder. However, with
ongoing developments in joint replacement materials and surgical
techniques, even small joints of the fingers can be replaced. |
| Typically, both
sides of the joint are replaced; in the hip joint the acetabulum will be
reamed, and a plastic or metal cup will be introduced. The femoral
component will be fitted precisely to the femur and cemented in place (Fig. 1.26). |
| Most patients derive significant benefit from joint replacement and continue to lead an active life afterward. |
Figure 1.26 This is a radiograph,
anterior-posterior view, of the pelvis after a right total hip
replacement. There are additional significant degenerative changes in
the left hip joint, which will also need to be replaced.
| Determination of skeletal age |
| Throughout life the
bones develop in a predictable way to form the skeletally mature adult
at the end of puberty. In western countries skeletal maturity tends to
occur between the ages of 20 and 25 years. However, this may well vary
according to geography and socioeconomic conditions. Skeletal maturity
will also be determined by genetic factors and disease states. |
| Up until the age of
skeletal maturity, bony growth and development follows a typically
predictable ordered state, which can be measured through either
ultrasound, plain radiographs, or MRI scanning. Typically, the
nondominant (left hand) is radiographed and is compared to a series of
standard radiographs. From these images the bone age can be determined (Fig. 1.13). |
| In certain disease
states, such as malnutrition and hypothyroidism, bony maturity may be
slow. If the skeletal bone age is significantly reduced from the
patient's true age, treatment may be required. |
| In the healthy
individual the bone age accurately represents the true age of the
patient. This is important in determining the true age of the subject.
This may also have medicolegal importance. |
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