Joints

A joint is the place where two or more bones meet or articulate. Joints allow movement. The bones meeting at a joint are always held together By strong ligaments which prevent dislocation during movement.

   The degree of movement at a joint varies with the joint. The shoulder joint is the most mobile joint in the body, while the joints of the bones that form the cranium are immovable. A slightly movable joint is the one between the hip bones. Martial arts exercises help to make this joint flexible. Childbirth can also be made easy by making this joint flexible. Freely mobile joints are, however , the weak spots of skeleton. (Shoulder dislocation is common especially among athletes.)

   There are four main kinds of movable joints

• ball and socket joint,

• hinge joint,

• gliding or sliding joint, and

• pivot joint.

Ball and socket joints

 The shoulder joint and hip joint are ball and socket joints. The head of the humerus is a ball like structure which fits into the  glenoid cavity of the scapula. This allows movement in several directions. Similarly, the round head of the femur fits into the acetabulum of the pelvis to form a freely articulating joint.

Hinge Joints

These occur at the elbow and knee. Here, movement of the bodies is limited to one direction only. As the name implies, each set of bones making up this joint functions like one half of a hinge.

Gliding or sliding joints

This type of joint allows the sliding of bones over one another. It occurs at the wrist and ankle, and allows the hand and foot to be moved up and down or rotated slightly.

Pivot joints

This type of joints allow rotation of one part of the body on another. It is found between the Atlas and axis vertebrae. The odontoid process of the axis acts as a pivot which allows the rotation of the head on the vertebral column (I.e. the atlas and the skull together rotate about the odontoid process).

Structure of a joint

A joint is usually enclosed in a capsule made of ligament. The articulating surfaces of the bones are covered by smooth cartilage. This prevents the surfaces of the bones from being  worn out when they rub against one another. Between the surfaces of the articular cartilage is a sac lined by synovial membrane. It is filled with a thick, viscous fluid, the synovial fluid. This fluid serves as a lubricant during movement. This structure allows free movement of the bones meeting at the joint