Human Skeleton: Different types of joints
What is a joint?
A joint is the structure of human body where two or more bones are held together in order to allow various types of movements and moldings in the rigid bony human skeleton. A joint is not exclusively for bones, there can be three different types of joints on the basis of what structures are involved in making it.
A joint can exist between
1. Two bones (for example the shoulder joint that exists between the scapula and the humerus).
2. A cartilage and a bone (for example the joint that exists between the ribs and the costal cartilages).
3. A cartilage and a cartilage (for example the joint between the 6th and the 7th costal cartilage.
Types of joints
Joints occur at numerous places in the body and they differ in structure and function. They are classified as Immovable joints and Movable joints.
They are also known as fixed joints. They are those joints in which the relative movements of the bones forming the joint do not occur. In such joints the bones are in actual contact with one another without any type of cartilage in between them for example the joints of the bones of cranium as well as those of the face which fix teeth into jaws.
Movable joints are sometimes called synovial joints. These are the joints in which the bones forming the joints are capable of movements with one another. The opposing surface of the bones of a moveable joint is separated by a piece of cartilage called the articular cartilage. According to the range of movement the movable joints are further divided into two kinds; partially moveable joints and freely moveable joints.
Partially moveable joints: These joints have a very limited range of movement for example the joint between the vertebrae and the joints of the cranium and hip bones.
Freely moveable joints: These joints allow the free movements between the articulating bones. In such joints the articular surfaces are often clothed with cartilage which reduces the friction between the two surfaces of bones making the joint. It is covered by a synovial membrane. This membrane often constitutes a closed sac. The sac contains lubricating synovial fluid which also reduces friction in the joint where it is found. Such freely moveable joints are classified further into three classes according the degree of movement that they allow. These types are Hinge joints, Ball and Socket joints and the Pivot joints.
Hinge joints: This type of joints allows the backward and forward movement in only one plane.
Ball and socket joints: In these types of joints the movement is vast virtually occurring in every plane.
Pivot joints: In these joints rotation is the only possible movement.