n., plural: diarthrodial joints; diarthroses
What is a diarthrodial joint?
A diarthrosis joint is a freely moving joint characterized by its mobility and joint cavity within a synovial membrane encased in the joint capsule. Diarthrodial joints are also known as true joint or movable joint, or synovial joint, that allows free movement of the joints.
To better understand the diarthrodial joints, we must understand the joints and the classification of the joints. Joints or articulations are the connecting point between the two bones that provide the required mobility and stability to the skeletal system. Different types of joints in the body can be classified in two ways:
A. Functionality – based on the amount of mobility, there are three types of joints:
- Immovable joints, i.e., synarthrosis joints. Example: skull sutures
- Slightly movable joints, i.e., amphiarthrodial joints. Example: tibia and the fibula joint
- Freely movable joints or mobile joints, i.e., diarthrosis. Example: elbow joints
B. Histologically or structurally – based on the type of connective tissue in the joint, there are three types of joints:
Correlating these two classifications leads to the following conclusions:
- Amphiarthroses are cartilaginous joints
- Synarthroses are fibrous joints
- Diarthroses are synovial joints
From the above classification, it can be concluded that all the synovial joints of the body are the diarthrodial joints and are involved in the majority of the body movements. Diarthrodial joints are the most common joint found in the body of mammals.
Diarthrodial joint is the most common and movable type of joint characterized by the presence of a layer of fibrocartilage or hyaline cartilage that lines the opposing bony surfaces, as well as a lubricating synovial fluid within the synovial cavity. Etymology: “dia”, meaning “between” and “arthroun”, meaning to fasten by a joint. Synonym: synovial joint, diarthrosis. Compare: cartilaginous joint
Structure of diarthrodial joint or synovial joint
Diarthrodial joints are freely moving joints wherein the joint is encased in an articular capsule, and the bones connect with each other in a fluid-filled cavity known as the synovial cavity. The articular capsule is made up of cartilage and fibrous tissues. Within the articular capsule, the synovial membrane creates a fluid-filled sac or the joint cavity. The fluid present in the joint cavity, known as synovial fluid, provides the required smoothness and lubrication to the bones for motion. Additionally, the ends of the articular bones are encased in a protective cartilaginous layer known as articular cartilage.
Accordingly, the essential component of the diarthrodial or synovial joints are:
- The articular capsule encloses the diarthrodial joint and is in continuation of the periosteum of articulating bones. The articular capsule is made up of two layers-
- The outer fibrous layer comprises white fibrous tissue that holds the joint together and supports the synovium.
- The inner layer (or the synovial layer) is richly supplied with blood vessels and made up of the serous connective tissue. The synovial layer or synovium secretes and absorbs the synovial fluid and continuously enriches it with nutrients.
- Articular cartilage: this is a cartilaginous layer covering the surface of the articulating bones to reduce the friction between the bone surfaces and absorb the shock. The cartilaginous layer is made up of hyaline cartilage and is non-vascular.
- Ligaments: ligaments are the bands of white fibrous tissue that reinforce and support the diarthrodial joint. Ligaments are essential to limit the movement of the diarthrodial joint.
- Articular discs and meniscus: some of the diarthrodial joints have a fibrocartilage structure between articulating bones. When this fibrocartilage structure is a small oval-shaped disc-shaped, it is known as an articular disc, while when this fibrocartilage structure is a large c-shaped structure known as meniscus.
The distinguishing feature of the diarthrodial joint from the fibrous or cartilaginous joints is the presence of a joint cavity in it. Unlike fibrous or cartilaginous joints, articulating surfaces of the bones in a diarthrodial joint connect with each other in a fluid-filled space. The presence of fluid between the two surfaces of the bones further aids in the smooth and free movement of the diarthrodial joint.
Types of Diarthrodial Joints
Diarthrodial joints may be classified based on the axis of the movement or on the shape and structure of the joint.
A. Based on the axis of the movement
Body movements occur in three different axes or planes, i.e., transverse, frontal, and sagittal. Accordingly, diarthrodial joints can also be classified into three types: based on the axis or the plane of their movement.
- Uniaxial diarthrodial joints – These joints move only in one plane or axis. Bending and straightening of the elbow joint is an example of the uniaxial diarthrodial joint.
- Biaxial diarthrodial joints – These joints move in two planes or axes. Bending and straightening in one axis and spreading and closing fingers in the second axis by the knuckle joint or the metacarpophalangeal joint is an example of the biaxial diarthrodial joint.
- Multiaxial diarthrodial joints – These joints allow movement in multiple directions and are also known as triaxial or polyaxial joints. Movement in the anterior-posterior direction, medial-lateral direction, and rotation along the long axis by the hip joint and shoulder joint are examples of the multiaxial diarthrodial joints.
Types of movement in a Diarthrodial or Synovial joint
Diarthrodial joints carry out several movements, which can be classified into four broad categories:
- Gliding movement: Joints of the carpal and tarsal bones are involved in the gliding movement. Gliding movement to a larger extent does not result in rotation or angular movement of the bones; it occurs when relatively flat bones glide or move past each other.
- Angular movement: The joint movement that results in alteration in the joint angle is known as an angular movement. Flexion, abduction, adduction extension, hyperextension, and Circumduction are some of the angular movements.
- Flexion, or bending angular movement, occurs when the angle between the bones is reduced, e.g., upward movement of the Elbow requires the forward movement of the forearm.
- Extension is the reverse to that of flexion angular movement, wherein the angle between the bones is increased for the resultant movement, for example, straightening the limb post flexion movement.
- Abduction angular movement happens due to the movement of the bone away from the midline of the body; for example, lateral movement of the arms or legs to lift the load straight out to the side.
- Adduction angular movement is the opposite of abduction, wherein the movement of the bone occurs toward the midline of the body, e.g., inward movement of the limbs inward post-abduction.
While the circular limb movement is known as circumduction, e.g., circular motion of the arm.
- Rotational movement results from the rotational movement of the bones around the longitudinal axis of the body, for e.g., side to side movement of the head. The rotational movement that occurs towards the longitudinal axis or the body’s midline is known as medial rotation. While the movement that occurs away from the longitudinal axis or the midline of the body is known as lateral rotation.
- Special movement includes the movement that can not be classified as gliding/angular/rotational. Inversion, Eversion, Protraction, Retraction, Elevation, Depression, Dorsiflexion, Plantar flexion, Supination, Pronation, and Opposition are special joint movements.
B. Based on the shape and structure of the joint
Shape and structure of the joint play a critical role in determining the joint movement. Based on shape and structure, diarthrodial joints can be as follows:
- The planar joint is formed between reasonably flat or slightly curved bones. This kind of joint exhibits limited movement, and movement occurs due to the gliding movement of the bones. Hence, these joints are sometimes also referred to as gliding joints.
Example: Carpal bones and the tarsal bones of the hand and foot, respectively, are examples of the planar joint.
- The hinge joint is formed by the bones wherein one of the bones has a rounded end while the other bone has a concave surface that houses the rounded end of the other bone. Herein, only one bone moves while the other bone remains static.
Example: Elbow joint and Knee joint.
- The pivot joint is formed by the bones wherein the rounded end of one of the bones fits into the ring or the circular cavity formed by the other bone. The pivot joint is involved in the rotational movement.
Example: Side to side and back and forth movement of the human head due to the joint of the first and second vertebrae of the neck.
- A condyloid joint or ellipsoidal joint is formed by an oval ended bone that fits into the other bone’s oval-shaped hollow cavity. This joint is responsible for the angular movement along two axes.
Example: Metacarpophalangeal joint or the Wrist joint that results in up and down & side to side movement.
· The saddle joint is similar to the saddle shape wherein one convex bone surface fits into the concave bone surface. However, the saddle joint also exhibits angular movement to a larger extent compared to the condyloid joint.
Example: Thumb joint or carpometacarpal joint that causes its up and down & back and forth movement.
- The ball-and-socket joint is formed by a ball-shaped bone end that fits into the cup-shaped cavity of the other bone. The ball and socket joint results in movement in all directions and are the joint that can result in movement in a diverse range of the axis. Therefore, Ball and socket joint can also be classified as the multiaxial diarthrodial joint.
Example- Shoulder and Hip joint.
Diarthrodial Joint Examples
- Gliding joints (or planar joints) – e.g. the carpals of the wrist
- Hinge joints – e.g. the elbow (between the humerus and the ulna) and knee
- Pivot joints – e.g the wrist
- Condyloid joints (or ellipsoidal joints) – e.g. the thumb (between the metacarpal and carpal)
- Saddle joints – e.g. the shoulder and hip joints
- Ball and socket joints
These joints are the most movable type of joints in the human body in contrast to cartilaginous joints (such as synchondroses and symphyses) and fibrous joints (such as sutures, gomphoses, and syndesmoses).
Try to answer the quiz below to check what you have learned so far about diarthrodial joints.
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