The junction where two bones meet is referred to as a
joint, also known as a bony articulation.
They are constructed to allow movement (except for skull,
sacral, sternal, and pelvic bones) and provide mechanical support, and are
classified structurally and functionally.
Functional Classification of Joints
Joints may be classified
functionally based upon how much movement they allow.
a)
Synarthrosis Joint- A joint that permits no movement is known as a synarthrosis.
The sutures of the skull and the gomphoses that connect the teeth to the skull
are examples of synarthroses.
b)
Amphiarthrosis Joint -
An amphiarthrosis allows a slight amount of movement at the joint. Examples of
amphiarthroses include the intervertebral disks of the spine and the pubic
symphysis of the hips.
c)
Diarthrosis Joint -
The third functional class of joints is the freely movable diarthrosis joints.
Diarthroses have the highest range of motion of any joint and include the
elbow, knee, shoulder, and wrist.
Structural Classification of Joints
Joints may also be
classified structurally based upon what kind of material is present in the
joint.
a)
Fibrous joints-
They are made of tough collagen fibers and include the sutures of the skull and
the syndesmosis joint that holds the ulna and radius of the forearm together.
b)
Cartilaginous joints- They
are made of a band of cartilage that binds bones together. Some examples of
cartilaginous joints include joints between the ribs and costal cartilage, and
the intervertebral disks of the spine.
c)
Synovial joint-
It features a fluid-filled space between smooth cartilage pads at the end of
articulating bones. Surrounding the joint is a capsule of tough dense irregular
connective tissue lined with synovial membrane. The outer layer of capsule may
extend into thick, strong bands called ligaments that reinforce the joint and
prevent undesired movements and dislocations. Synovial membrane lining the
capsule produces the oily synovial fluid that lubricates the joint and reduces
friction and wear. There are many different classes of synovial joints in the
body, including gliding, hinge, saddle, and ball and socket joints.
d)
Gliding joints,
such as the ones between the carpals of the wrist, are found where bones meet
as flat surfaces and allow for the bones to glide past one another in any
direction.
e)
Hinge joints,
such as the elbow and knee, limit movement in only one direction so that the
angle between bones can increase or decrease at the joint. The limited motion
at hinge joints provides for more strength and reinforcement from the bones,
muscles, and ligaments that make up the joint.
f)
Saddle joints,
such as the one between the first metacarpal and trapezium bone, permit 360
degree motion by allowing the bones to pivot along two axes.
g)
Ball and Socket Joint-
The shoulder and hip joints form the only ball and socket joints in the body.
These joints have the freest range of motion of any joint in the body – they
are the only joints that can move in a full circle and rotate around their
axis. However, the drawback to the ball and socket joint is that its free range
of motion makes it more susceptible to dislocation than less mobile joints.
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