The place where two or more bones meet in the body is called a joint. Depending on their structure, joints permit different degrees of movement of bones. Do you know the different types of joints in our body?
Basically, there are three kinds of joints in our body. They have been grouped on the basis of the degree of movement they allow. They are: (I) Immovable or fixed joints, (II) Partially movable or cartilaginous joints, (Ill) Freely movable or synovial joints.
Immovable joints: In an immovable joint, the bones are held tightly together. Because dense bundles of tough and strong fibres hold the bones, they cannot change the position relative to each other. The elasticity essential for joint movements is almost absent. Joints in the tooth sockets and between the skulls bones are of this type. Immovable joints are also known as fibrous joints.
Partially movable joints: In a partially movable joint, the bones are linked by a cartilage. A cartilage is a very tough, spiny material. These joints have very little movement. The different vertebras of the spinal column are joined together by a cartilage. The joints of the spine are partially movable so that the spine can bend. Between the vertebrae there are disks of cartilage. They are called intervertebral disks. In addition to allowing the vertebrae to move against each other, they also act as shock absorbers. At the front of the chest where the ribs meet the breastbone are also partially movable joints.
Freely movable joints: These are the main joints of the body. These are found in the bones of the ankle, hip, wrist, elbow and knee. The ends of the bones that are the part of the movable joints are covered with caps of tough cartilage. A cartilage does not wear out easily. Its surfaces are very smooth and reduce friction between the moving bones.
In a freely movable joint, the space between the bones is filled with a special oily fluid called synovial fluid. It helps the parts to move easily. It is produced by a thin membrane that lies around the joint and is kept within the joint by this synovial membrane. Outside the membrane are tough, flexible ligaments (connecting tissues). The ligaments in addition to joining one bone to another also allow movement. The membrane and the ligaments together form what is called the joint capsule. Synovial (freely movable) joints may be of different types depending upon the nature of articulation and degree of freedom. They are explained below:
Ball and Socket joints: In this the ball of one bone fits into the socket of another. So it makes movement possible in all directions – bending and stretching from side to side and rotation. The shoulder and hip joints are of this type.
Hinge joints: These are found in the elbow, knee and fingers. The ends of the bones fit into each other in such a way that the movement is possible in one direction only. Angular joints: In joints like the wrist, movement is possible around two axes. Such joints are called angular joints.
Pivot joints: In these joints, turning or rotatory movement is possible around one axis only. Skull joint to the spine is a pivot joint.
Gliding joints: In joints between tarsal bones in the ankle, carpal bones in the wrist and between sternum and clavicle, the end of one bone glides across a certain portion of the surface of the other bone.
Injuries and diseases may affect joints. Arthritis is a painful disease that attacks cartilage in joints.
