Diarthrosis is a joint classification that is used when considering joint function (degree of movement). These are joints that can move freely, which means they allow a lot of movement, such as elbows, knees and shoulders.
When considering the structure of the joint, the synovial joints are formed by bones connected by ligaments and separated by a joint capsule. 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 joints of the diarthrosis.
The reason why they are named differently is because each name is based on a different method of classification of unions, either by function or structure.
The term diarthrodial is also very common, used, so that “a diarthrodial joint” means the same as a “diarthrosis joint”. These are also sometimes called diarthrophic joints. These joints provide a low friction environment that can withstand a large amount of wear.
The diarthrodial or synovial joints are the main joints on which movements are produced, which include most of the joints of the body.
These are uniaxial, biaxial or multi-axial joints. They are further divided into six groups, according to the type of movement that occurs in them, which is allowed by its structure.
The basic types are sliding (flat, artrodial), 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 to each other, so they are free to move in relation to each other.
The bones are indirectly connected by a joint capsule that covers and encloses the joint. This joint capsule is formed by fibrous material and this capsule encloses the joint cavity, whose internal surface is lined with synovial material.
Types of synovial joints
These are the general types, but it is important to realize that no articulation is a perfect representation of any of these types. Joints are classified by the type 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 flat 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 basically 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 produce a movement of rotation around a longitudinal axis.
An example is the 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 to the left.
Condylar (ellipsoid, ovoid): A condyle, which is oval or egg-shaped, at the end of a bone articulates with the corresponding oval cavity of another bone.
These condiloid 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 produce 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, which is the joint between the head and the first cervical vertebra, the atlas.
Selar (saddle): In a sellar joint, both articulated surfaces have concave and convex surfaces and the surface of one fits on the complementary surface of another, like two frames rotated 90 degrees to each other and fitted together.
To illustrate this cup, put your hands together and place them together so that one hand is turned at right angles 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 a lot of movement. These joints allow for 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 produce movement in all planes and also rotation.
The spherical joints are the most freely movable joints 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 joints.
Examples of synovial joint
The human knee: The human knee joint is a typical synovial joint. The knee joint, which actually consists of several different 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, as well as with the patella.
The synovial joint in the knee is formed between the tibia and the femur. The fibula is connected by ligaments 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 the connection of the bones of the skull 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 reason why the saltwater crocodile has the most powerful jaw is 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 work in the synovial joint in a powerful way, slamming the jaw.
Unfortunately for crocodiles, the muscles have evolved to work only in one direction.
While a bite of a crocodile could easily break your arm, your jaws are also closed once they have bitten. The shape of your head and the way your muscles join do not allow you to open your mouth again.