Diarthrosis, Diarthrodial, or Synovial Joints: Diarthrosis is a joint classification used when considering joint function (degree of movement). These are joints which are freely moveable, meaning they allow a great deal of movement, such as the elbows, knees, and shoulders. In contrast, joints which allow only a slight degree of movement are called amphiarthrodial and those which allow no movement are called synarthrodial.
Recommended Resource: Structure and Function of the Musculoskeletal System by James Watkins.
When considering joint structure, synovial joints are formed by bones connected by ligaments and separated by a joint capsule. The space between the joints is filled by a joint capsule with a lubricating synovial fluid. Synovial joints allow a great deal of movement and are the same kind of joints as diarthrosis joints. The reason they are named differently is because each name is based on a different method of classifying joints, either by function or structure. The term diarthrodial is also very common, used, so that "a diarthrodial joint" means the same thing as a "diarthrosis joint." These are also sometimes called diarthrotic joints. These joints provide a low friction environment that can withstand a great amount of wear and tear.
Diarthrodial or synovial joints are the main joints about which movements occur, which includes the majority of joints in the body. These are either uniaxial, biaxial, or multiaxial joints. They are further divided into six groups, according to the type of movement that occurs in them, which is allowed by their structure. The basic types are gliding (plane; arthrodial), hinge (ginglymus), pivot (screw; trochoidal), condyloid, saddle, and ball and socket. Each of these types has a different degree of movement.
In diarthrosis or synovial joints, the ends of the bones are covered with a thin layer of hyaline cartilage and no cartilaginous tissue connects the bones together, so they are free to move in relation to one another. 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 enclose the joint cavity, the inner surface of which is lined with synovial material.
Diarthroses, Diarthrotic or Synovial Joint
Image by Madhero88 via Wikipedia
Types of Diarthrotic or Synovial Joints
These are the general types but it is important to realize that no joint is a perfect representation of any of these types. Joints are classified by which type they most closely resemble.
- Gliding: (plane, irregular, arthrodial) The articulating surfaces are small and nearly flat or only slightly curved. The movement of the joint is a planar movement of sliding or twisting. Examples are the joints between various bones of the wrist and ankle such as the intercarpal joints of the wrist or the intertarsals of the ankle. Another example is the acromioclavicular joint of the shoulder girdle.
- Hinge: (ginglymus) Surface of one bone is convex cylinder which articulates with the shallow concave trough-like facet of another. Movement is a uni-axial hinge-like motion carried out in one plane only. This is basically flexion, and its reverse, extension. Examples are the elbow, knee, ankle, and the interphalangeal joints of the fingers.
- Pivot: (screw, trochoidal or trochoid) The cylindrical surface, of one bone articulates with a ring of bone and fibrous tissue on another to produce a rotation movement about a longitudinal axis. An example is the joint between the proximal (elbow) end of the radius and ulna bones of the forearm, which must rotate around one another, producing supination or pronation. Also, the articulation of the atlas and axis, the first two cervical vertebra of the neck, called the atlantoaxial joint, is another example which rotates the head to the right or left.
- Condylod (ellipsoid, ovoid): A condyle, which is oval or egg-shaped, at the end of one bone articulates with the matching oval cavity of another bone. These condyloaid or ellipsoid joints are bi-axial and produce a variety of movements in different planes such as flexion, extension, abduction, adduction, circumduction or a combination, but incapable of producing rotation. The radiocarpal joint of the wrist and metacarpophalangeal and metatarsophalangeal joints of the fingers and toes are examples. Also, the occipitoatlantal joint, which is the joint between the head and the first cervical vertebra, the atlas.
- Sellar: (saddle) In a sellar joint, both articulating surfaces have concave and convex surfaces and the surface of one fits into the complementary surface of another, much like two saddles turned 90 degrees to each other and fit together. To illustrate this cup your hands and place them together so that one hand is turned at a right angle to the other. Now slide your hands back and forth by moving the fingers forward. This is how a saddle joint works, more or less, and you can see that it allows a great deal of movement. These joints allow flexion, extension, abduction, adduction, and circumduction. An example is the carpometacarpal joint of the thumb.
- Ball and Socket: (enarthrodial, spheroidial, cotyloidial) Head of one bone is ball-shaped and this fits into the cup-shaped socket of another bone. These tri-axial joints produce movement in all planes and also roation. Ball and socket joints are the most freely moveable joints in the body, allowing flexion, extension, abduction, adduction, and internal and external rotation. The shoulder and hip joints are ball and socket joints.
This page created 03 Oct 2011 20:48
Last updated 18 Apr 2017 20:07