The Function of Joints
Skeletal system
Our musculoskeletal system is made of muscles, tendons, ligaments, cartilage, bones, and bursae. When impulse stimulates, the muscles contract and these muscles are attached to the ligaments. Most of the skeletal system is made of bones and cartilage. The cartilage or ligaments are further attached to bones. To protect the ligaments and tendons from friction, bursae are present, which are small fluid filled sacs.
What is a joint?
A joint is a place where two bones or a bone and cartilage meet. Both based on the structure and function, joints have been classified. This is based on whether by fibrous connective tissue or cartilage the bones are anchored or within a joint cavity the bones meet, structurally the joints are classified. Depending on whether the joint is immobile, or slightly mobile or freely movable and based on the degree or movement functionally the joints are classified. The joints that are slightly movable or immobile protect the internal organs and make the body stable. They permit limited body movement on the other hand, joints that are freely movable allow the body and the limbs to move to a larger extent.
Types of movement
The main purpose of joints is to enable limited or a wider range of motion. The following are the types of movements:
- One broad and one flat surface slide against each other. This kind of movement is found in the ankle or wrist.
- The angle between the two bones is increased or decreased. In the long bones of the body this is found. This takes place when the arm is extended or it is bent.
- Allowing circular movement, this kind of movement is seen in the shoulder and is brought about by the ball and socket joint.
- Enable rotation of the bones without displacing them. This is seen when from the head is turned side to side.
Types of joints
Based on how much movement is enabled, the joints have been described. Broadly joints are classified as follows:
- Immovable - there is a close contact between the two bones but they do not allow any movement. This is seen in the bones of the skull that are joined by suture.
- Slightly movable - there is a tight or strong connection between the two bones, thus limited movement is allowed. This can be seen in the vertebrae of the spine.
- Freely movable - majority of the joints are of this type. They enable movement of the bones.
Structural classification
This is based on whether there is a joint cavity between the surfaces or they meet with the help of fibrous connective tissue or cartilage. Based on this the joints are classified into three types. When fibrous connective tissue unites with adjacent bones then it is a fibrous joint. When hyaline cartilage or fibrocartilage unites two bones, then the joint is a cartilagenous joint. Within a joint cavity when the two bones are in contact with each other then it is a synovial joint. Bones are able to move freely with the help of the synovial joint.
Functional classification
Based on the mobility between the bones this classification is made. The joints are classified as follows:
- Immobile joint or synarthrosis
- Slightly movable or amphiarthrosis
- Freely move able or diarthrosis
- Synarthrosis - this joint is immobile or nearly immobile. The immobility helps nearby bones to strongly unite with each other. At places where the internal organs need to be protected this kind of joint is important. One example of such joint where it protects the brain is the bones of the skull that meet with fibrous joints. Another example is the joint between the manubrium and sternum, they meet with the help of manubriosternal joint and cartilaginous joint that protects the heart.
- Amphiarthrosis - this kind of joint has limited movement. For example, the adjacent vertebrae bodies meet with the help of cartilagenous joints. Between the vertebrae is an interverterbral disc that allows limited movement of the vertebrae. However, a larger range of body movement is enabled when the small movements of the adjacent vertebrae sum up together. Pubic symphysis of the pelvis is another example of this joint articulation between the left hip bones and right pubic region. Very little mobility is allowed by this kind of joint. Weight bearing stability is enabled by the strength of the pubic symphysis.
- Diarthrosis - This joint is freely movable. They allow most of the body movements. In the appendicular skeleton, this kind of joint is found due to which a wide range of motion is enabled. Based on the number of axis of motion, the joint provides they have been classified. The three types are uniaxial, biaxial or multiaxial. The uniaxial joint enables bending or straightening of the elbow or knee. The metacarpophalanegal joint of the hand is an example of biaxial joint. In several directions, movement is allowed by the multiaxial joint. An example of this is the shoulder and hip joint, they enable movement of the upper or lower limb in the anterior, posterior direction, and medial lateral direction. Also, around the long axis the limb can rotate.
There are six types:
- Ball and socket joint - the head of one bone and the cup of another bone fit into each other. This allows movement of the shoulder and the hip in all directions.
- Saddle joint - this is seen in the base of the thumb. It allows side to side and back and forth movement but does not allow the bones to rotate.
- Hinge joint - in one plane only the two bones open and close like a door. This can be seen in the elbow joint and knee joint.
- Condyloid joint - this is seen in the finger joint and jaw joint. There is no rotation only movement is permitted.
- Pivot joint - there is a ring-like structure of one bone in which the other bone swivels. This can be seen in the first and second vertebrae of the neck region.
- Gliding joint - it allows limited movement since the surfaces of the bone are smooth and they slip over each other. This can be seen in the wrist joint.
Importance of joint
The purpose of a joint is to be able to bear one's body weight and during movement. When the muscles involved contract then the joint moves. Based on the range of motion, joints have been classified. Synarthroses are joints that are found in the gomphoses, the place where the skull and teeth connect with each other. Amphiarthrosis are joints that allow slight movement. This is found in the pubic symphysis of the hip and intervertebral disc. The largest range of motion is enabled by diarthrosis which is found in the elbow joint, knee joint, wrist joint, and shoulder joint.
Other than range of motion, joints are also classified based on the material present. The most common ones are fibrous and synovial. Also, another material is cartilaginous. Synovial joints can be made of various types. Some examples of synovial joints are the ball and socket joint, hinge joint, and saddle joint. Different ranges of motions are enabled by different types of joints. The greatest range of motion is enabled by the ball and socket joint. The gliding joint allows the bone to move in any direction whereas the hinge joint allows movement in only one direction. The range of motion of the saddle joint is 360 degrees.
Tough bands of connective tissue hold the joints together. These are known as ligaments. As the bones move against one another, friction is prevented by the smooth cartilage. In joints that are freely movable, a membrane filled with synovial fluid encloses the entire joint. Extra cushioning is provided by this lubricating fluid. Thick bands of connective tissue attach the muscles and bones together. The area where the tendons are close to the bones bursae is present between them in order to reduce the friction. Synovial fluid is present in these bursae.
If the joints do not function properly or due to any reason they are no longer able to function properly, it can result in these joint conditions:
- Arthritis - the joints become stiff and painful due to inflammation or the joints become degenerated.
- Bursitis - the bursae becomes inflamed.
- Infection - the joint may become infected.
- Tendonitis - the tendon that is attached to the joint may get inflamed, irritated and swollen.
- Injury - the ligament may get strained or sprained or nearby muscle or bone may get fractured.
A study has found that as the space between the joints becomes narrow, the bone mineral density varies.
Location of various joints
- Synovial joints
In the human body, this is the most common type of joint. They are also known as diarthrosis. This kind of joint can be of various types and these are saddle joint, hinge joint, ball and socket joint, pivot joint, plane joint, and condylar joint. Spaces separate the bones of the synovial joint so that in different directions movement can take place.
- Synovial ball and socket joint, saddle joint and condylar joint
Most of the wide range of motion is enabled by the ball and socket joint. This kind of joint is seen in the hip and shoulder. In the condylar joint the rounded end of one bone and the curved surface of other bone fit into each other. Condylar joints connect the fingers to the hand and the thigh bone to the lower leg bone. In the saddle joint, one bone has a part like saddle and the other bone has a complementary shape and these both fit into each other. They allow back and forth motion. The saddle joint can be seen in the connection between the breastbone and the collar bone and between the base of the thumb.
- Synovial plane, hinge joint and pivot joint
Between the small bones in the wrist and feet is the plane joint which is also known as the gliding joint. They enable sliding and rotation of the bones. The hinge joint works the same way as a door opens and closes. It can be seen in the elbow and the small joints; they bend and straighten due to the hinge joint. Rotation around a single axis is enabled by the pivot joint. It works the way a doorknob works. This joint can be seen in the two spine bones on top and between the forearm bones.
- Cartilaginous joint
This joint is also known as amphiarthrosis. In this joint, the cartilage connects the two bones. Between the ends of the bones an open space is lacking in this kind of joint. The cartilaginous joint can be two different types, which are synchondroses and symphyses. Joints that are connected with hyaline cartilage are known as synchondroses cartilaginous joints. In adults this kind of joint is found between the first rib and breastbone and this joint is also found in growing children. When these joints mature no movement occurs between them.
Cartilage also holds the symphyses together but more flexibility is provided by the tissue. An example of the symphysis joint is the joint between the pubic bones. At this joint there is some flexibility. During childbirth this plays an important role.
- Fibrous joint
These are also known as synarthroses. In the body, this is the sturdiest joint. The three types of fibrous joints are the suture, syndesmosis, and gomphosis. Between the bones in the skull the suture joint is present. Between the tooth socket in the jaw and the teeth the gomphosis is present, and these joints do not allow any movement. The most flexible joint is syndesmosis. Between the forearm bones this joint can be found. The longer connective tissue fibers connects the bones.
Gliding joints have various types:
- Zygapophyseal joint - between the processes of the vertebrae these joints are found. With the help of tiny filaments these processes are connected to the processes of the above and below vertebrae. Thus between the vertebral arches gliding joints are formed.
- Wrist joint - there are eight small carpal bones, the first layer is closer to the finger bones and the second layer is closer to the forearm bones. Gliding joints occurs between the bones which are held together by ligaments.
- Ankle joint - this is found in the tibia, fibula, and the tarsal bone of the foot. Between the other tarsal bones gliding joint occurs as well. The three cuneiform bones are included in the tarsal bones.
Article resources:
https://courses.lumenlearning.com/boundless-biology/chapter/joints-and-skeletal-movement/
https://www.researchgate.net/publication/226788729_Structure_and_Function_of_Joints
https://www.reference.com/science/function-joints-996463d0fc74a5f
