Synovial joints


Synovial joints are free moving joints that are encapsulated in synovial fluid. They can be categorized by the movements which they permit as well as the shapr of the anterior surface. The synovial joints are split up into categories including ball and socket, hinge, gliding, pivot, saddle, and condyloid. Synovial joints provide the most noticeable bodily movements such as the bending of a knee. The primary responsibility of the synovial joints is to allow the body to experience smooth motion and freedom of movement through ample range of motion. They also work in conjunction with each other and other various joints to help the body maintain stability, balance, strength, and when necessary, rigidity. Out of all the joints, the synovial joints are not only the most complex, they are also the most widely varied joints in the human body.


The range of motion that is possible via a synovial joint is determined by 3 basic factors. The bones structure related to the two bones being conjoined can determine not only the range of motion, but the direction of motion. The joint capsule’s strength along with the joint’s adhering ligament and tendon strength can also determine direction and range of motion. The muscles (their size and span) are a contributing factor to either allowing great motion or a little bit of motion.

Range of motion can also be determined by the overall condition of the body. A small body with weak muscles is less likely to maximize the range of motion as a larger body with strong muscles. Obesity can interfere with the range of motion. While at the same time, the feats that some people can perform based on “double jointed limbs” does not refer to the joints, but the ligaments ability to be flexible and safely stretch beyond ordinary perceived human limitations.

Synovial joints are protectively enclosed in a capsule filled with synovial fluid which is comprised of regular connective tissue but is dense and tough to withstand the constant need for extreme motion. The fluid inside the joint capsule lubricates the joint for ease of motion. This fluid appears to have the consistency and coloring of egg whites, which is where the term was derived from, the Greek meaning for egg white being synovial. A synovial membrane which lines the interior of the capsule is responsible for secreting the fluid and maintaining fluid levels in the capsule. Synovial fluid is not that remarkably different from the fluid which is found between cells known as interstitial fluid. Synovial fluid has a high concentration of hyaluronic acid and albumin, and lesser concentrations of phagocytic cells which are responsible for cleansing the joint of any debris from the cartilage.


Synovial joints
Image: Synovial Joints

The bones which conjoin to create the synovial joint are encased in a layering of hyaline cartilage. This smooth cartilage is also called articular cartilage. This unique cartilage measures approximately 2 millimeters in thickness and is devoid of nourishing blood vessels which means it must receive nourishment from the synovial fluid. This nourishment only happens when the joint is experiencing activity.

Ligaments assist in the connective process by binding the bones of the synovial joint. These ligaments are constructed from durable connective tissue and are quite tough. This dense form of connective tissue may be located either outside the joint or in the joint cavity. The menisci are cartilage pads which provide a little extra protection from wear, shock, and trauma, however they are only found within the knee joint. Articluar disks, which can be found within a few alternate synovial joints, are not quite the same as menisci, but do a very similar job in comparison.

Flattened sacs of synovial fluid known as bursae can be found throughout some synovial joints. Still closely related to the menisci, they provide the same support but are not as dense or tough. These sacs are found most often as cushions for the passage of a tendon over a bone or between muscles to provide cushioning and impact control. A tendon sheath is very similar to the bursa which is designed for the protection and lubrication of the ligament. They are especially helpful in providing cushioning where a ligament passes over a bone.


The structure of a synovial joint as well as the functionality it permits is how it is decisively segregated into one of six various categories. Gliding joints permit a side to side motion, or alternitavely a back and forth motion. As it is the least complex of joint motion, the bones which are adjoining for this type of movement are often flat or nearly flat with perhaps a bit of concave or convex appearance. The best example of this type of movement associated with the joint is the motion facilitated by the articular processes of the vertebrates.


Hinged joints are, as one would expect, permitted to flex and extend to certain degrees, like the hings of a door. Considered monaxial joints, the surface of at least one bone must be concave. The opposing bone must be convex in order to allow this process of movement. Hinge joints are by far the most frequently occurring type of synovial joint. The knees and the joints in between the phalanges of either the feet or the hands are perfect examples of hinged joints.

A pivot joint has a great radial swing, but motion is limited to the circular motion of the adjoined axis. For these types of joints, one bone contains a depression while the other is equipped with a conical protrusion in order to fit together and maintain motion. The radius and the ulna experience this motion when twisting, like as in opening a doorknob, as does the head when shaking from side to side on the axis.

A condyloid joint is fitted to permit up and down motions as well as side to side motions and requires the structure of the bones to be a concave depression that will accept the oval protrusion of the secondary bone which permits a snug fit. Condyloid joints are biaxial joints. The most obvious example of this joint is the radiocarpal joint located in the wrist.

The saddle joint is comprised of one bone with a concave surface and one bone with a convex surface in the opposite direction. The saddle joint is very much like a condyloid joint with the exception of permitting a much wider range of motion. There are two saddle joint junctures within the human body. One is where the carpus meets the trapezium at the first metacarpal bone and the other is where the malleus and the incus meet within the middle ear. The former creates the manipulation abilities of the thumb while the latter contribute directly to the human’s ability to hear.

Ball and socket joints are created when the articulation of a deep depression in one bone meets a rounded convex protrusion. The depression can be considered more of a cup or almost a hole rather than just a crevice. A multi-axial joint, this type of joint provides the greatest ranges of motion in various directions. The shoulder is the most obvious ball and socket joint in the human body. The coxal joints, or hip joints, are also examples of ball and socket joints.
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