The joints in the human body are points of contact between bones or between bones and cartilage. Joints, or articulations, are the points where two or more bones connect or meet in the body. Joints vary in structure, from immovable to highly mobile, depending on their location and function in the body.
Joints can be classified into different categories based on their structure and the function they perform.
- Based on structure, the joint is classified into Fibrous joint, Cartilaginous joint, and Synovial joint
- Based on function, the joint is classified into synarthrosis, amphiarthrosis, and diarthrosis.
The importance of joints lies in their ability to facilitate movement, provide stability and support, distribute forces, enable weight-bearing, and allow a range of motion.
Based on Structure
Based on structure, i.e., based on the presence or absence of any supporting tissues or joint cavity, joints are classified into 3 types of joints:
1. Fibrous Joint
A fibrous joint is a type of joint where the bones are connected by fibrous connective tissue. The fibrous connective tissue in fibrous joints is composed of dense collagen fibers that bind the bones tightly together. Although the fibrous joint is known as an "immovable joint," as it allows minimal to no movement, it is important for maintaining the integrity and stability of the skeletal system. Fibrous joints examples include flat skull bones, rib cage, upper jaw, backbone, etc.
2. Cartilaginous Joint
A cartilaginous joint is a type of joint where the bones are connected by cartilage. The cartilage present between the bones in the cartilaginous joint is either hyaline cartilage or fibrocartilage. Although it provides less movement compared to synovial joints, its primary function is to provide structural support and shock absorption. These joints are essential in areas where flexibility is needed, such as the spine. Cartilaginous joint examples include the intervertebral discs between the vertebrae and the pubic symphysis in the pelvic region.
There are two types of cartilaginous joints
- Synchondroses: Temporary joints connected by hyaline cartilage, found in growth plates
- Symphyses: Permanent joints with fibrocartilage. Examples include the pubic symphysis
3. Synovial joint
Synovial joints are the most common type of joint and are responsible for the majority of body movements. The joint is characterized by the presence of a synovial cavity, synovial fluid, and a joint capsule. Synovial fluid present in the synovial cavity acts as a lubricant, reducing friction and facilitating smooth movement. Synovial joints are surrounded by a joint capsule composed of fibrous connective tissue. Synovial joints enable a wide range of movements, such as bending, straightening, rotation, gliding, etc. Examples of synovial joints includes elbow, knee, hip, shoulder, wrist, ankle joint, etc.
Synovial joints are of six main types based on their specific structure and range of motion:
- Ball and socket
- Pivot
- Saddle
- Condyloid or Ellipsoid joint
- Gliding joint or Plane joint
- Hinge joint
Ball and Socket joint
A ball and socket joint is a type of synovial joint in which a rounded, ball-shaped end of one bone fits into a cup-like socket of another bone. This structure allows for a wide range of movement in multiple directions, including rotation, flexion, extension, abduction, adduction, and circumduction. The hip joint and the shoulder joint are examples of ball-and-socket joints in the human body.
Pivotal joint
A pivotal joint allows rotational movement around a single axis. In a joint, one bone rotates or pivots around another bone, resulting in movement. An example of a pivotal joint in the body is the joint between the atlas (C1 vertebra) and the axis (C2 vertebra) in the neck.
Hinge joint
A hinge joint allows movement primarily in one plane, similar to the action of a hinge on a door. It permits flexion (bending) and extension (straightening) along a single axis. The movement at a hinge joint resembles the opening and closing of a door. Hinge joints play an important role in various daily activities such as walking, running, and gripping objects. An example includes the elbow and knee joint.
Saddle joint
A saddle joint allows movement in two planes, flexion/extension and abduction/adduction, similar to the movement of a rider in a saddle. Saddle joints are characterized by the presence of two saddle-shaped surfaces, one convex and the other concave. An example includes the joint at the base of the thumb.
Condyloid Joint / Ellipsoid Joint
Condyloid joints are also known as ellipsoid joints, as they allow movement in an elliptical manner. They allow movement in two directions, where an oval-shaped condyle of one bone fits into an elliptical cavity of another bone. Condyloid joints permit flexion and extension, as well as adduction and abduction, but they do not allow rotational movement. The condyloid joint example includes the base of the index finger and the carpals of the wrist.
Gliding Joint
A gliding joint, also known as a plane joint, is where the articulating surfaces of the bones are relatively flat or slightly curved. Gliding joints allow for gliding or sliding movements between the bones along various planes without significant angular or rotational movement. An example includes the wrists, ankles, and the space between the flat bones of the spine (vertebrae).
Based on Function
The joints can also be classified into three types based on the degree of movement that the joints are allowed to show.
1. Synarthrosis/Immovable Joints
The joints that are immovable or nearly immovable are called synarthroses. These joints are held together by dense fibrous connective tissue, such as collagen fibers that help provide stability and structural support to the body and are held.
Examples of Types of Joints: Sutures of the skull, gomphosis of the maxilla and mandible, and synchondrosis of the epiphyseal plate in growing long bones.
2. Amphiarthrosis/Slightly Movable Joints
The joints that show limited movement are called amphiarthrosis. These joints are more flexible as compared to synarthrosis. They are connected by fibrous or cartilaginous tissues that provide them a certain amount of flexibility to move.
Examples of Types of Joints: Syndesmosis of the tibiofibular joint and symphysis of the pubic region.
3. Diarthrosis/Freely Movable Joints
The joints that are freely movable are called diarthrosis. These joints are characterized by the presence of a synovial cavity filled with synovial fluid that helps reduce friction for smooth movement of the joints. They are capable of showing complex, multidirectional movements.
Examples of Types of Joints: hinge joints of the elbow, ball-and-socket joints of the shoulder and hip, pivot joints of the neck, saddle joints of the thumb, condyloid joints of the wrist, and gliding joints of the wrist and ankle.
Functions of Joints
- They help in the facilitation of movement in the body.
- Provide stability to the skeletal system.
- They help absorb shocks to protect internal organs.
- Support to bear weight.
- They enable flexibility in the body for movement.