A joint is a point of contact between two or more bones, or between a bone and a cartilage. Joints are important because they facilitate movement of the body parts. Adjacent bones and cartilage articulate with one another to form a joint. A joint acts as a pivot and the muscles create the necessary force needed for movement.
Classification of Joints: There are two ways of classifying joints.
Structural classification - Bones may be connected to each other by a fibrous tissue or a cartilage or they may be connected within a fluid filled cavity. There are three types of joints:
Fibrous joint - In this joint the bones are connected to each other by dense fibrous connective tissue, usually collagen. There is no joint cavity. The joint does not allow any movement so they are also called fixed or immovable joints.
Examples : Bones of skull are joined by fibrous tissue forming a cranium. These joints are called sutures. Another fibrous joint is seen in gomphoses of teeth which is a joint between teeth and bony sockets.
Cartilaginous joint - Here the bones are tied to each other with the help of connective tissue called cartilage. It allows limited movement, more than that of a fibrous joint but less than a synovial joint.
Examples : Joints between adjacent vertebrae, joint between the first rib and sternum (also called synchondrosis).
Synovial joint - In this joint there is a fluid filled cavity called synovial cavity where the two bones are connected to each other. The fluid in the cavity helps in lubrication of joints and prevents wear and tear of bones due to friction. The two bones at the joint are not directly connected to each other with a cartilage or fibrous tissue. This allows free movement of bones and helps in locomotion. These are the most common types of joints.
There is a cavity at the joint.
The cavity is filled with a fluid called synovial fluid. It lubricates the joint and also nourishes the articular cartilage.
The articular bones are covered with a sheath of cartilage called articular cartilage. This prevents friction between bones.]\]
The cavity is covered on the outside with a fibrous connective tissue forming an articular capsule.
On the inner side of capsule is a lining called synovial membrane.
There is ligament connecting the bones together outside the joint. It keeps the bones together and prevents their separation.
There is another connective tissue called tendon that attaches muscle to the bone at the joint.
There are six types of synovial joints:
Ball and socket joint
Gliding joint - This is also called plane joint as the bones are of same size and are mostly flat. The bones slide past each other in a gliding manner. It is a non-axial joint as there is no rotatory motion around any axis.
Example : The joints between the carpals of wrist and tarsals of foot.
Hinge joint - This joint can be compared to a door hinge where the movement is allowed in only one direction. It is a uniaxial joint in which cylindrical end of a bone fits into the trough of adjoining bone. This is the most common synovial joint.
Example : The joint in the elbow, knee and ankle.
Ball and socket joint - This joint allows maximum movement in all directions hence called multi axial joint. It also allows rotatory movement. The head of one bone fits into the depression of another bone like a ball fits into a socket.
Example : The joints in the hip and shoulder.
Pivot joint - In this joint, the bones rotate around a single axis. The rounded end of one bone moves within a ring like structure formed by the other bone along with a ligament at the joint. This joint is uniaxial joint as it allows movement in only one axis.
Example: The joint between the first (atlas) and second (axis) cervical vertebrae is a pivot joint. It allows the head to turn from side to side.
Ellipsoid joint - This is also called condyloid joint. This joint allows all sorts of movements like straightening, bending, side to side and rotatory. It is a biaxial joint as it allows movement in two planes. Oval end of one bone which is convex fits into the depression of concave end of another bone.
Example : Joint between radius of forearm and bones of the wrist.
Saddle joint - In this joint one bone is turned inward and the other is turned outward. The joint looks as if there is a rider on a saddle. This joint allows all sorts of movements like straightening, bending, side to side and rotation. It is also a biaxial joint.
Example : The joint between thumb and palm.
Each synovial joint allows movement in specific direction. Some allow movement in only one direction while some allow movements in multiple directions. The muscles and ligaments also play an important role in these joints. Ligaments, tendons and fluid filled sacs called bursae are also present around these joints.
Functional classification : This classification is based on the degree of movement allowed by the joint. These are further divided into three types:
Immovable joint/ synarthrosis
Slightly movable joint/ amphiarthrosis
Freely movable joint/ diarthrosis
Synarthroses - This is an immobile joint so it offers a strong bonding between the articulating bones. These joints do not allow any mobility and are mostly fibrous and cartilaginous joints. They are found at places where they provide protection to internal organs such as the brain and heart.
They are found in the fibrous joint of sutures of cranium which protect the brain.
This joint is also present in the manubriosternal joint which is a cartilaginous joint connecting the bones of sternum to manubrium. This protects the heart.
Amphiarthroses - These joints allow little mobility and are mostly cartilaginous joints. These are found in joints between vertebrae. In between two vertebrae there is a gap that is filled with a cartilage called intervertebral disc. This disc not only connects the two vertebrae but also allows some movement between the vertebral bones. Because of these joints the entire vertebral column can move.
Diarthroses - These are freely movable joints and are synovial joints. They provide maximum mobility to body parts and are mostly found in the limbs. Depending on the number of axes of motion provided by these joints, they are further classified as follows:
Uniaxial joint - This joint allows movement in only one plane.
Example : Elbow joint which allows only bending and straightening.
Biaxial joint - This joint allows motion in two planes.
Example : Joint of the knuckles where the joint allows movement in two axes; one bending and straightening of fingers and second spreading of fingers.
Multiaxial joint - This joint allows motion in many directions; along all three axes.
Example : Shoulder and hip joint where the limbs can move in forward- backward direction, sideways and can also rotate.