Diarthrosis is a joint classification used when considering joint function (degree of movement). These joints can move freely, allowing much action, such as elbows, knees, and shoulders.
The synovial joints are formed by bones connected by ligaments and separated by a joint capsule when considering the joint structure. The space between the joints is filled with a joint capsule with a lubricating synovial fluid.
The synovial joints allow a large amount of movement and are of the same type of joints as the diarthrosis joints.
They are named differently because each name is based on a different method of classification of unions, either by function or structure.
The term diarthrodial is also ubiquitously used, so “a diarthrodial joint” means the same as a “diarthrosis joint.” These are also sometimes called diarthrosis joints. These joints provide a low friction environment that can withstand considerable wear.
The diarthrodial or synovial joints are the primary joints on which movements are produced, including most of the body’s joints.
These are uniaxial, biaxial, or multi-axial joints. They are divided into six groups according to the type of movement, which is allowed by their structure.
The basic types are sliding (flat, arthrodial), hinge (ginglymus), pivot (trochoidal), condyle, chair and ball, and socket. Each of these types has a different degree of movement.
In the diarthrosis or synovial joints, the ends of the bones are covered by a thin layer of hyaline cartilage, and no cartilaginous tissue connects the bones, so they are free to move about each other.
The bones are indirectly connected by a joint capsule covering and enclosing the joint. This joint capsule is formed by fibrous material, and this capsule has a joint cavity whose internal surface is lined with synovial material.
Types of synovial joints
These are the general types, but it is essential to realize that no articulation perfectly represents any of these types. Joints are classified by the kind to which they most closely resemble each other.
Sliding (flat, irregular, arthrodial): The surfaces of this joint are small and almost flat or only slightly curved. The movement of the joint is a balanced movement of sliding or twisting.
Some examples are the joints between several bones of the wrist and ankle, such as the intercarpal joints of the wrist or ankle. Another example is the acromioclavicular joint of the shoulder girdle.
Hinge (ginglymus): The surface of the bone is a convex cylinder that articulates with the shallow concave facet of another. The movement is a uniaxial hinge movement carried out in a single plane.
This is bending and its inverse, extension. Examples are the elbow, knee, ankle, and interphalangeal joints of the fingers.
Pivot (trochoidal or trochoid): The cylindrical surface of the bone is articulated with a ring of bone and fibrous tissue over another to move into rotation around a longitudinal axis.
An example is an articulation between the proximal end (elbow) of the radius and the bones of the ulna of the forearm, which must rotate around each other, producing supination or pronation.
In addition, the articulation of the atlas and the axis, the first two cervical vertebrae of the neck, called the atlantoaxial joint, is another example that turns the head to the right or the left.
Condylar (ellipsoid, ovoid): A condyle, which is oval or egg-shaped, articulates at the end of a bone with the corresponding elliptical cavity of another bone.
These condyloid or ellipsoid joints are biaxial and produce a variety of movements in different planes, such as flexion, extension, abduction, adduction, bypass, or a combination, but are unable to have rotation.
The radiocarpal joint of the wrist and the metacarpophalangeal and metatarsophalangeal joints of the fingers and toes are examples. In addition, the occipitoatlantal joint is the joint between the head and the first cervical vertebra, the atlas.
Selar (saddle): Both articulated surfaces have concave and convex surfaces in a sellar joint. The cover of one fits on the complementary character of another like two frames rotated 90 degrees to each other and working together.
To illustrate this cup, put your hands together and place them together to turn one hand at a right angle to the other. Now slide your hands back and forth, moving your fingers forward.
This is how a saddle joint works, more or less, and you can see that it allows much movement. These joints allow flexion, extension, abduction, adduction, and bypass. An example is the carpometacarpal joint of the thumb.
Ball and socket: The head of a bone is ball-shaped and adapts to the cup-shaped socket of another bone. These tri-axial joints move in all planes and also rotation.
The spherical joints are the most freely movable in the body, which allows for flexion, extension, abduction, adduction, and internal and external rotation. The joints of the shoulder and the hip are spherical.
Examples of synovial joint
The human knee: The human knee joint is a typical synovial joint. The knee joint, which consists of several connections between four bones, becomes the largest synovial joint in the human body.
At the junction of the knee, the large femur bone of the thigh connects with the tibia and fibula of the leg and the patella.
The synovial joint in the knee is formed between the tibia and the femur. Ligaments connect the fibula to the femur. The patella serves as a protective bone to protect the synovial joint from damage or acute impacts.
The crocodile jaw: The most powerful bite in the world gets its power from the joint formed by a synovial joint.
The crocodiles have a small hinge on the back of the skull, formed by connecting the bones of the head with the jaw.
This small joint is a synovial joint and allows the jawbone to rotate with the skull to join the teeth together.
The saltwater crocodile has the most powerful jaw because of the amount of muscle and its position on the hinge. Note that the area in front of the hinge is open, with many holes and grooves for muscle attachment.
Enormous muscles populate this region and powerfully work in the synovial joint, slamming the jaw.
Unfortunately for crocodiles, the muscles have evolved to work only in one direction.