They are classified by structure and function, where
the structural classification is concerned with bonebinding materials, and if a joint cavity is present.
The three joint types mentioned already, is an
example of structural classification. The movement
of a joint is the basis of functional classification;
which are synarthroses (immovable), amphiarthroses
(slightly movable), and diarthroses (freely movable).
The functional classification (diarthroses) is the
classification synovial joints fall under, which is
almost all the joints of the limbs—basically the
majority of articulations in the body. Fibrous joints
are termed as such, because there articulations are
united by fibrous tissue, and cartilaginous joints are
binned by hyaline cartilage or fibrocartilage
respectively. Now onto the focus of our
understanding, synovial joints are characterized by a
fluid-filled cavity separating the articulating bones.
This synovial fluid serves two functions, as a
lubricant for joint surfaces and to nourish the
articular cartilage. (Keith, M. L. & others, 2015).
“Synovial joints are usually reinforced by accessory
ligaments that either are separate (extrinsic) or are a
thickened part of the joint capsule (intrinsic). Some
synovial joints have other distinguishing features,
such as cartilaginous articular discs or menisci,
which are present when the articulating surfaces of
the bones are incongruous.” Synovial joints come in
six varieties, which are pivot, ball and socket, plane,
hinge, saddle, and condyloid. They all have varying
degrees of movement, but this is a general
interpretation. These joint designs are what influence
the diarthroses (freely movements) characterized by
synovial joint.
Let’s consider the general structure of a synovial
joint as expressed in the figure titled “General
structure of a synovial joint.” There are six
distinguishing features that are identifiable in
synovial joints. They include articular cartilage, joint
(articular) cavity, nerves and blood vessels, articular
capsule, synovial fluid, and reinforcing ligaments. I
will start with articulating cartilage and work my
way to nerves and blood vessels. Articular cartilage
is a type of hyaline cartilage and has a smooth glass
like appearance. They are very thin (1mm or less)
spongy cushions, which function to absorb the forces
or compression applied to the joint. This in turn
keeps the ends of the bones from crushing together.
The joint (articular) capsule is a synovial specific
feature, just a potential space containing fluid
(synovial fluid). This synovial fluid as mentioned
earlier plays a big role in rheumatoid arthritis. It is
the first structure effected and lends itself indirectly
to the perpetuation of joint damage, for without this
lubricant, rubbing begins to wear away joint
surfaces. Which produces exorbitant friction and
destroys the joint tissues, literally cooking them! Not
to go into too much detail the reinforcing ligaments
are as the name suggest reinforcing ligaments. They
commonly are capsular ligaments (thickened parts of
the fibrous layer), and come in two varieties,
extracapsular (outside the capsule) and intracapsular
(deep to it). Although not represented in the figures;
innervating the capsule are richly supplied sensory
nerve fibers, which often detects pain sensation. As
most anybody who has experienced joint
inflammation or injury knows. Also richly supplying
synovial joints are blood vessels. This is an
important thing to understand, because mostly
supplying the synovial membrane are these capillary
beds, which are the factories that produce blood
filtrate, forming the basis of synovial fluid. This
synovial fluid largely being derived from the blood
filtrate. Might this be a site of concern in clinical
diagnoses of rheumatoid arthritis? This leads us into
the medical device(s) used in diagnosing RA, and
the normal operation of such devices.