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What are joints? What are the types?

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i. A point where two or more bones get articulated is called joint or articulation or athrosis. 

They are classified based on degree of flexibility or movement they permit into lastly synovial or freely movable or diarthroses type of joints.

ii. Synarthroses / fibrous joints / movable joints: 

In this joint, the articulating bones are held together by means of fibrous connective tissue. Bones do not exhibit movement. Hence, it is immovable or fixed type of joint. Synarthroses are further classified into sutures, syndesmoses and gomphoses.

  • Sutures: 

It is composed of thin layer of a dense fibrous connective tissue. Sutures are places of growth. They remain open till growth is complete. On completion of growth, they tend to ossify. Sutures may permit some moulding during childhood. Sutures are further classified into butt joint, scarf joint, lap joint and serrate joint. 

  • Syndesmoses: It is present where there is greater distance between articulating bones. At such locations, fibrous connective tissue is arranged as a sheet or bundle, 

e.g. Distal tibiofibular joint, interosseous membrane between tibia and fibula and that between radius and ulna. 

  • Gomphoses: In this type of joint, a cone shaped bone fits into a socket provided by other bone, 
  • e. g. Tooth and jaw bones.

iii. Cartilaginous / slightly movable joints / amphiarthroses: 

These joints are neither fixed nor freely movable. Articulating bones are held together by hyaline or fibrocartilages. 

They are further classified as 

  • Synchondroses: 

The two bones are held together by hyaline cartilage. They are meant for growth. On completion of growth, the joint gets ossified, e.g. Epiphyseal plate found between epiphysis and diaphysis of a long bone, Rib – Sternum junction. 

Symphysis: In this type of joint, broad flat disc of fibrocartilage connects two bones. It occurs in mid-line of the body. 

e.g. Intervertebral discs, manubrium and sternum, pubic symphysis.

v. Synovial joints / freely movable joints / diarthroses:

1. It is characterized by presence of a space called synovial cavity between articulating bones that renders free movement at the joint. 

2. The articulating surfaces of bones at a synovial joint are covered by a layer of hyaline cartilage. It reduces friction during movement and helps to absorb shock.

3. Synovial cavity is lined by synovial membrane that forms synovial capsule. Synovial membrane secretes synovial fluid.

4. Synovial fluid is a clear, viscous, straw coloured fluid similar to lymph. It is viscous due to hyaluronic acid. The synovial fluid also contains nutrients, mucous and phagocytic cells to remove microbes. Synovial fluid lubricates the joint, absorbs shocks, nourishes the hyaline cartilage and removes waste materials from hyaline cartilage cells (as cartilage is avascular). Phagocytic cells destroy microbes and cellular debris formed by wear and tear of the joint. 

5. If the joint is immobile for a while, the synovial fluid becomes viscous and as joint movement starts, it becomes less viscous.

6. The joint is provided with capsular ligament and numerous accessory ligaments. The fibrous capsule is attached to periosteum of articulating bones. The ligament helps in avoiding dislocation of joint. 

The types of synovial joints are on follows: 

1. Pivot joint: In this type of joint, the rounded or pointed surface of one bone articulates with a ring formed partly by another bone and partly by the ligament. Rotation only around its own longitudinal axis is possible.

e.g. in joint between atlas and axis vertebrae, head turns side ways to form ‘NO’ joint.

2. BaIl and socket joint: 

The ball like surface of one bone fits into cup like depression of another bone forming a movable joint. Multi-axial movements are possible. This type of joint allows movements along all three axes and in all directions.

e.g. Shoulder and hip joint.

3. Hinge joint: 

In a hinge joint, convex surface of one bone fits into concave surface of another bone. In most hinge joints one bone remains stationary and other moves. The angular opening and closing motion (like hinge) is possible. In this joint only mono-axial movement takes place like flexion and extension. 

e.g. Elbow and knee joint.

4. Condyloid joint: 

It is an ellipsoid joint. The convex oval shaped projection of one bone fits into oval shaped depression in another bone. It is a biaxial joint because it permits movement along two axes viz, flexion, extension, abduction, adduction and circumduction is possible.

e.g. Metacarpophalangeal joint.

5. Gliding joint: 

It is a planar joint, where the articulating surfaces of bones are flat or slightly curved. These joints are non-axial because the motion they allow does not occur along an axis or a plane. 

e.g. Intercarpal and intertarsal joints. 

Saddle joint: This joint is a characteristic of Homo sapiens. Here the articular surface of one bone is saddle-shaped and that of other bone fits into saddle (each bone forming this joint have both concave and convex areas). It is a modified condyloid joint in which movement is somewhat more free. It is a biaxial joint that allows flexion, extension, abduction, adduction and circumduction. 

e.g. Carpometa carpellar joint between carpal (trapezium) and metacarpal of thumb.

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