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Types of Joints – Classification of Joints in the Human Body
Our skeleton is made up of hard and strong bones. Bones cannot be bent. We can bend (or move) our body parts only at those places where two (or more) bones join together. Such places are called joints. For example, we can bend our arm at the elbow because the bones of the upper arm and lower arm form a joint at the elbow. If, however, our arm were one long bone from shoulder to wrist, then we could not bend our arm. This point will become more clear from the following activity.
Straighten your right arm. Ask your friend to place a light wooden plank lengthwise under the arm in such a way that the elbow is in the centre. Tie the plank with the arm at both ends by using a string [see Figure (a)]. Let us now try to bend the arm. We will find that when wooden plank is tied to the arm, we cannot bend the arm at the elbow at all [see Figure (a)].
We now remove the wooden plank from the arm. Let us now try to bend the arm. We will find that we can now bend our arm easily at the elbow [see Figure (a)]. We can bend our arm because there is a joint at the elbow between the bones of upper arm and lower arm. When we tie the wooden plank under the arm, then the elbow joint cannot work, the whole arm behaves like a single bone and hence cannot bend. This activity shows that we can bend (or move) our arm at elbow because the bones are joined by a ‘movable joint’ at the elbow.
The place where two (or more) bones meet in the skeleton is called a joint. Several types of joints occur in the human skeleton. Most of the joints allow the bones to move. The amount of movement depends on the type of joint. Thus, joints of the bones help in body movements. We can move (or bend) our body parts only at those places where bones meet to form joints. For example, we can move our head, arms, hands, legs, and feet because all these parts have ‘bone joints’. If, however, our skeleton had no joints between various bones, then we could not move our body parts like head, arms, hands, legs and feet, etc.
In the freely movable joints (like the elbow, knee, shoulder and hip joints), the ends of bones forming the joint are held in place by strong connective tissue called ‘ligaments’. Ligaments connect the bones forming the joints and prevent the bones from falling apart during movement. Ligaments are, however, elastic and so they can stretch to let the bones move at the joints easily. Another point to be noted is that the ends of the bones at joints are covered with a soft and smooth layer of cartilage. The layers of cartilage allow the ends of the bones to move over each other smoothly without friction. This prevents damage to the ends of bones due to wear and tear caused by rubbing over each other. There is also a thick oily liquid between the ends of bones of a freely movable joint. This oily liquid lubricates the cartilage at the ends of bones and reduces friction even further. In order to keep the diagrams simple, we will not show cartilage and oily liquid in them.
There are different types of joints in the human skeleton (or human body) to help us to carry out different kinds of movements or other activities. The main types of joints in the human body are :
- Hinge joints
- Ball and Socket joints
- Pivot joints
- Fixed joints
We will now describe all these types of joints in detail, one by one. Let us start with the hinge joints.
1. Hinge Joints
If we open and close a door, we will find that the door can move only ‘forwards’ and ‘backwards’. Actually, the door is joined to its frame by small metal ‘joints’ called ‘hinges’ (kabze). These hinges allow the door to move only in one direction : forwards and backwards. So, a door is said to have a hinge joint. In our body also, we have some bone joints which move like a door on its hinges. They are called hinge joints.
A hinge joint allows the movement of bones in only one direction : forwards and backwards. In a hinge joint, the movement of bones is restricted to one direction by the shape of the ends of the bones which form the joint, and by the ligaments which hold the bones together at the joint. In our body, hinge joints occur at elbow, knee, knuckles (finger joints) and jaw. In other words, elbow, knee, knuckles (finger joints) and jaw are hinge joints. We will now describe elbow and knee joints in detail.
Let us try to bend our arm at the elbow. We will find that we can bend (or move) it in only one direction-forwards and backwards. This is because our elbow is a hinge joint (see Figure). At the elbow, the upper arm bone forms a hinge joint with the lower arm bones. This happens as follows: The lower end of upper arm bone is in the shape of a ‘knob’ and the upper end Upper leg bone of lower arm bones is in the shape of a ‘cup’ (see Figure).
The knob of upper arm bone fits into the cup of lower arm bones to form a hinge joint at the elbow (see Figure). The ligaments hold the upper arm bone and lower arm bones together at the elbow joint (so that the ends of bones do not fall apart when the elbow bends). We will now describe the knee joint.
Let us try to bend our leg at the knee. We will find that we can move it in only one direction, forwards and backwards, like a door on its hinges. So, the knee joint is a hinge joint. The knee joint is shown in Figure. At the knee, the lower end of upper leg bone (or thigh bone) is in the form of a ‘knob’ and the upper end of the lower leg bones is in the form of ‘a cup'(see Figure). The knob of upper leg bone fits into the cup of lower leg bones to form a hinge joint at the knee (see Figure). The ligaments hold the upper leg bone and lower leg bones together at the knee joint (so that they may not fall apart when the knee bends).
The joints between the bones of our fingers allow only a hinge like movement. So, the finger joints are hinge joints. The lower jaw also forms a hinge joint with the fixed bones of the skull.
2. Ball and Socket Joints
In the ball and socket joint, one end of the bone has a round shape like a ‘ball’ which fits into a ‘socket’ (hollow space) in the other bone. The ball type end of one bone can turn freely in the socket of the other bone. So, in the ball and socket joint, the bones can be turned in any direction : forwards and backwards, side to side, and even rotated. The ball and socket joints occur at the shoulder and hips in our body. In other words, the shoulder joints and hip joints are ball and socket joints. A ball and socket joint allows much more movement of bones than a hinge joint. In fact, the ball and socket joint allows the maximum variety of movements as compared to all other type of joints in the human body. We will now describe the shoulder joint and hip joint in detail.
If we stand up and move our arm around at the shoulder, we will find that we can move it forwards and backwards, and from side to side, and we can also rotate it. This is because shoulder is a ball and socket joint. The ball and socket joint at the shoulder is formed as follows : The head of upper arm bone
is round like a ball’ and the shoulder blade bone has a ‘socket’ (hollow space) in it [sec Figure (a)]. At the shoulder, the ball on the top end of upper arm bone fits into the socket in the shoulder blade bone to form a ball and socket joint [see Figure (b)]. The ball-shaped top end of the upper arm hone can rotate freely in the socket of shoulder blade so that the arm can be moved in all directions. We will now discuss the hip joint.
If we stand up and move our leg around at the hip, we will find that we can move it forwards and backwards, and from side to side, and we can also rotate it. This is because hip joint is a ball and socket
joint. The ball and socket joint at the hip is formed as follows : The upper end of thigh bone is round like a ‘ball’ whereas the hip bone has ‘sockets’ (hollow spaces) in it [see Figure (a)]. At our hip, the ball on the top end of thigh bone fits into the socket in hip bone to form a ball and socket joint [see Figure (b)]. The ball of thigh bone can rotate freely in the socket of hip bone so that the ball and socket joint at the hip allows the leg to be moved in all directions. In Figure (b) we have joined only one leg bone with hip bone. You can also join the other leg bone in the second socket of the hip bone.
3. Pivot Joints
In a pivot joint, a cylindrical bone turns in a ring-type bone. The pivot joints allow rotation around an axis. A pivot joint exists between our ‘skull’ and the ‘top vertebra’ of backbone. Now, the top vertebra of backbone is a part of our neck, so we usually say that : A pivot joint exists between our skull and neck (or a pivot joint exists between our head and neck) (see Figure). The pivot joint between the skull and neck allows our head to bend ‘up and down’ and turn from ‘side to side’.
We can also say that a pivot joint connects our head to the neck. Another pivot joint occurs in the forearm (lower arm) near the elbow. The forearm has two bones, radius and ulna. In the forearm, the pivot joint makes the radius and ulna bones to ‘twist around each other’. The pivot joint in the forearm makes us turn our forearm to hold the palm of hand up or down. From this discussion we conclude that pivot joints occur in the neck and forearm.
4. Fixed Joints
In some joints, the bones are held so tightly together that they cannot move at all. Such joints are called fixed joints. In fixed joints, the bones are held very firmly together by strong fibres. The fixed joints are immovable joints. The function of fixed joints is to provide strength and support to the body, or to protect delicate organs (like brain) which cannot withstand any kind of deformation.
Fixed joints occur in our skull (see Figure). The plate-type bones of our skull are held together by fixed joints and cannot move at all. This makes the skull very strong. The hard and strong skull protects a delicate organ of our body called ‘brain’. It is clear from this discussion that our skull is not a single bone. It is made up of a number of bony plates connected through fixed joints.
If we open our mouth wide, we will find that we can move our lower jaw away but the upper jaw does not move at all. We cannot move the upper jaw because it is connected to skull by a fixed joint. Thus, there is a fixed joint between the skull and the upper jaw. We can move the lower jaw because the lower jaw is connected to the skull by a movable joint called hinge joint. The hinge joint in lower jaw allows the chewing of food. Fixed joints also occur in the hip bone. The fixed joints between various hip bones make our hip very strong. The hip bone is also connected to backbone by a fixed joint.