From Wikipedia, the free encyclopedia
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A joint (articulation) is the location at which two
bones make
contact (articulate). Joints are constructed to both allow movement and
provide mechanical support.
Classification
Structure and function of a joint are closely related.
Structural classification
Structurally, joints are classified as:
- fibrous - bones are connected by fibrous
connective tissue.
- cartilaginous - bones are connected by
cartilage.
- synovial - there is a space (synovial
cavity) between the articulating bones.
Fibrous joints
In fibrous joints bones are joined by tight and inflexible layers of
dense connective tissue, consisting mainly of
collagen
fibers. In
adults, these are not designed to allow any movement; however, in
children,
fibrous joints have not solidified and are movable. Examples of fibrous
joints are:
- Cranial sutures, joining the bones of the
cranium.
- Gomphoses, the joints between the roots of the
teeth and
their sockets (or alveoli) in
maxilla
and
mandible.
Cartilaginous joints
In cartilaginous joints (also known as synchondroses) bones are
connected entirely by cartilage. In comparison to synovial joints,
cartilaginous joints allow only slight movement. Examples of cartilaginous
joints are the
pubic symphysis, the joints between the
ribs and the
sternum,
and the cartilage connecting the
growth regions of immature long bones. Another example is in the
spinal column - the cartilaginous region between adjacent vertebrae.
Synovial joints
The term "Synovial joint" and "Diarthrosis joint" are often used
interchangeably, although the first term refers to the structure and the
second one to the function. For more details, see "Diarthrosis joints"
below.
Functional classification
Functionally, they can be classified as:
- synarthrosis - permit no movement.
- amphiarthrosis - permit little movement.
- diarthrosis - permit a variety of movements (e.g.
flexion,
adduction,
pronation). Only synovial joints are diarthrosis.
Synarthrosis joints
Synarthroses are joints with very little (if any) mobility. They can be
categorised by how the two bones are joined together:
- Syndesmoses are joints where the two bones are joined by one
of more ligaments.
- Synchondroses are joints where the two bones are joined by a
piece of cartilage.
- Synostoses are the fusion of two bones, to the point that
they are practically one bone. In humans, the plates of the cranium,
initially separate, fuse together as the child approaches adulthood.
Children whose craniums fuse too early may suffer deformities and brain
damage, as the skull does not expand properly to accommodate the growing
brain - a condition known as
craniostenosis.
- Amphiarthroses are slightly moveable joints where the two
bone surfaces at the joint - both covered in hyaline cartilage - are
joined by strands of
fibrocartilage.
Amphiarthrosis joints
Most amphiarthrosis joints are cartilaginous. See above for more
details.
Diarthroses (Synovial, Diarthroidal joints)
Diarthroses (sometimes called synovial joints and also
diarthroidal joints) are the most common and most moveable type of
joint in the body.
The whole of a diarthrosis is contained by a ligamentous sac called the
articular capsule.
The surfaces of the two bones at the joint are covered in
cartilage. The thickness of the cartilage varies with each joint, and
sometimes may be of uneven thickness. Articular cartilage is
multi-layered. A thin superficial layer provides a smooth surface for the
two bones to slide against each other. Of all the layers, it has the
highest concentration of
collagen
and the lowest concentration of
proteoglycans, making it very resistant to shear stresses. Deeper than
that is an intermediate layer, which is mechanically designed to absorb
shocks and distribute the load efficiently. The deepest layer is highly
calcified, and anchors the articular cartilage to the bone.
In joints where the two surfaces do not fit snugly together, a meniscus
or multiple folds of
fibro-cartilage within the joint correct the fit, ensuring stability
and the optimal distribution of load forces.
The
synovium is a membrane that covers all the non-cartilaginous surfaces
within the articular capsule. It secretes
synovial fluid into the joint, which nourishes and lubricates the
articular cartilage. The synovium is separated from the capsule by a layer
of celluar tissue that contains blood vessels and nerves.
Grouping by appearance
Joints:
1: Ball and Socket; 2: Condyloid (Ellipsoid); 3:
Saddle; 4 Hinge; 5: Pivot;
Synovial joints can be further grouped by their shape, which controls
the movement they allow.
- Ball and socket joints, such as the
hip joint.
These allow a wide arrange of movement.
- Condyloid (ellipsoid) joints, such as the
knee. When
the knee is extended there is no rotation, when it is flexed some
rotation is possible. A condyloid joint is where two bones fit together
with an odd shape (e.g. an
ellipse),
and one bone is concave, the other convex. Some classifications make a
distinction between condyloid and ellipsoid joints.
- Saddle joints, such as at the
thumb
(between the
metacarpal and
carpal).
Saddle joints, which resemble a
saddle, permit the same movements as the condyloid joints.
- Hinge joints, such as the
elbow
(between the
humerus
and the ulna).
These joints act like a
door
hinge,
allowing flexion and extension in just one plane.
- Pivot joints, such as the
elbow
(between the
radius and the
ulna). This
is where one bone rotates about another.
- Gliding joints, such as in the
carpals
of the wrist. These joints allow a wide variety of movement, but not
much distance.
See also
External links
