Synovial joints

Synovial joints. These joints move freely, are classified from the functional point of view as diarthrosis and are characterized by the presence of articular cartilage, this covers the surfaces of the bones of the joint, it is a hyaline cartilage.

Summary

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  • 1 capsules
  • 2 Clinical application
  • 3 Types of synovial joints
    • 1 Sliding joints
    • 2 Hinge
    • 3 pivot
    • 4 Ellipsoidal
    • 5 In saddle
    • 6 Ball and catcher (sphere type)
  • 4 Joints
  • 5 Sinosvial joints
  • 6 Types of synovial unions
  • 7 Fibrous or immobile
  • 8 Cartilaginous or semi-mobile
  • 9 Sources

Capsules

The joint capsule is made up of two layers:

  1. The fibrous (outermost) capsule, consisting of dense connective tissue (collagen).
  2. The synovial membrane, made up of loose connective tissue with elastic fibers and a variable amount of adipose tissue.

The synovial joints are surrounded by a sleeve-shaped articular capsule that encloses the synovial cavity and joins the joint bones.

The flexibility of the fibrous capsule allows movement in the joint, while its great strength resists dislocation. The synovial membrane secretes synovial fluid, which lubricates the joint and provides nutrition to the articular cartilage. The amount of synovial fluid varies in the different joints of the body, with a range that goes from a thin slimy layer to a layer of almost 3.5 ml of free fluid in a long joint, as in the case of the knee. It serves to reduce friction and provide nutrients, eliminating metabolic waste from the cartilage cells of the articular cartilage. Many synovial joints also contain accessory ligaments, which are called extracapsular ligaments and intracapsular ligaments.

On one side of some synovial joints there are fibrocartilage bearings that meet between the articular surfaces of the bones and join the fibrous capsule at their margins. These bearings are called articular discs or menisci and generally divide the synovial cavity into two separate spaces, helping to maintain joint stability and directing synovial fluid to areas of increased friction.

Clinical application

Tearing of the knee joint disc (synovial joint), commonly called torn cartilage, occurs frequently among athletes. Said damaged cartilage requires surgical removal (menisectomy) or if it starts to be used it produces arthritis. At one point, knee joint surgery for torn cartilage involves removing layers of healthy tissue and removing much, if not all, cartilage. This procedure is generally painful and expensive and full recovery is not always achieved.

These problems have been resolved by arthroscopy, an examination of the inside of a joint using a light pencil-diameter instrument. (It serves to determine the nature and extent of damage after injury, to remove torn cartilage, and to repair interlocking ligaments.)

Types of synovial joints

Sliding joints

  • The articular surfaces are flat.
  • They allow movements from side to side and back to front.
  • Twisting and rotation are generally inhibited because adjacent ligaments and bones restrict range of motion.

Examples: joints between the carpal bones , tarsal bones , the sternum with the clavicle and the scapula with the clavicle.

On hinge

  • Allows limited movements.
  • The convex surface of one bone enters the concave surface of another bone.
  • It is monoaxial (extension and flexion).

Examples: elbow, ankle and interphalangeal joints.

Pivot

  • A chronic rounded or pointed surface of a bone is articulated within a ring formed in one part by a bone and in another part by a ligament.
  • Make rotation (monoaxial joint).
  • It is responsible for the supination and pronation of the palms of the hands and the rotation of the head from one side to the other.

Examples: articulation between the atlas and the axis and between the proximal ends of the radius and the ulna.

Ellipsoidal

  • An oval-shaped condyle of one bone is located within an elliptical cavity of another bone.
  • It allows movement from side to side and back to front (biaxial joint).
  • It is responsible for flexion, extension, abduction, adduction, and circumduction of the wrist.

Example: wrist joint between radius and carpal bones.

In saddle

  • The articular surface of one bone is saddle-shaped, and the articular surface of the other bone is shaped like the rider sitting in the saddle.
  • It is a reciprocal lace joint.
  • It is the most freely moving modification of an ellipsoidal joint.
  • It moves from side to side and back to front (biaxial joint).

Example: joint between the trapezius of the carpus and the metacarpus of the thumb.

Ball and catcher (sphere type)

  • It consists of a ball-like surface of one bone within a cup-shaped depression of another bone.
  • It allows triaxial movement: flexion-extension, abduction-adduction and rotation-circumduction.

Joints

If it were not for the joints, the bones could not have mobility, so thanks to them the different extremities of the body can be bent. There are three three types of joints.

  • Mobile: also called diarthrosis or synovium, they are the joints that have the greatest range of movement. They are those that connect limb bones to the trunk, shoulders or hips.
  • Semimóvil: also called amphiarthrosis, are those that make limited movements, such as the joints between the vertebrae.
  • Fixed: Also known as synarthrosis, most are located in the skull and do not need movements, because the main function is to protect the internal organs.

Sinosvial joints

The most common, the synovial, are characterized by having some elements in common, these are:

  • Bony surfaces, which are the ends of the bones involved in a given joint.
  • Articular cartilage, soft and smooth tissue, composed of collagen, which allows a good glide between the

bone ends.

  • Synovial membrane, the layer that internally covers the entire joint and that secretes the synovial fluid, lubricant

of the joint.

  • Menisci, crescent-shaped flattened structures, with the function of cushioning and protection of cartilage, among others.
  • Joining means, made up of collagen fibers, arranged as an envelope called a joint capsule and as reinforcements called ligaments.

Types of synovial unions

These according to the specific type of movement performed by the joints:

  1. Ball joint: The head of one bone fits into the concave cavity of the other. They are located on the hips and on the shoulders.
  2. Saddle joint: only exists at the base of the thumbs and allows movement in two directions (forward and backward and side to side).
  3. Hinge Joints – Like elbow and finger joints, they are less mobile and allow movement in only one direction.
  4. Pivotal joint: is one in which a bone cylinder rotates around its own axis, being in contact with another surface that forms a ring (part bone, part ligament) such as the joint, superior radio-ulnar (forearm).
  5. Elliptical joint: at the end of an ovoid (egg) shaped bone it moves in an elliptical cavity. They are located in the radius of the forearm and the scaphoid bone of the hand.
  6. Sliding joint: Some bone surfaces are almost flat and slide over each other. They are found in some joints of the hands and feet.

Fibrous or immobile

These joints are joints of bones in which fibrous tissue participates, joining them, the mobility of these joints is defined by the length of the tissue fibers. By way of example, mention may be made of the joints of the back, those of the sacrum, those of the skull, the parts of the junction between the parietal, occipital, frontal and temporal parts, some of the ankle and those of the pelvis. But the spinal joints are not entirely immobile, as they are flexible enough to allow some movement and maintain their supporting role for the spine.

There are 3 types of fibrous joints:

  1. Syndesmosis: semi-immobile junctions, where a membrane joins the bones.
  2. Sutures: can be flat, jagged, or scaly (found mainly on the skull ).
  3. Schindylesis – A type of fibrous joint found only at the junction between the vomer and the sphenoid crest.

Cartilaginous or semi-mobile

This type of joints is carried out between cartilage and bone, they do not allow as much movement as mobile ones. They can be synchondrosis when they are made of hyaline cartilage or symphysis when they are fibrocartilage, they are of two types:

  • Primary cartilaginous joints, which are passing joints between bones using cartilage as the joints between parts of the same growing bone.
  • Secondary cartilaginous joints or symphysis, which are cartilaginous junctions between two bones by a very robust cartilage, not very movable and definitive.

Joints can also be classified functionally, according to the degree of mobility they allow to perform.

 

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