Joints

• A Joint, also called an articulation, is a point of contact between two bones or between bone & cartilage.
• Joints are essential for all types of movements involving the bony parts of the body. 
• The movability at these joints varies depending on different factors. 
• The structural classification of joints is based on two criteria:

1. The presence or absence of a space between the articulating bones called a synovial cavity.
2. The type of connective tissue that binds the bones together.

• Joints have been classified into three major structural forms, namely, fibrous, cartilaginous and synovial.

1.0Fibrous Joints

• There is no synovial cavity.
• Bones are held together by dense irregular connective tissue, rich in collagen fibres. 
• Fibrous joints do not allow any movement. 
• This type of joint is shown by the flat skull bones which fuse end-to-end with the help of dense fibrous connective tissues in the form of sutures, to form the cranium.

Fibrous Joints

2.0Cartilaginous Joints

• There is no synovial cavity.
• In cartilaginous joints the bones involved are joined together with the help of cartilages.
• The joint between the adjacent vertebrae in the vertebral column is of this pattern and it permits limited movements. 

3.0Synovial Joints

• Synovial joints are characterized by the presence of a fluid filled synovial cavity between the articulating surfaces of the two bones. Such an arrangement allows considerable movement. 
• The bones at a synovial joint are covered by a layer of hyaline cartilage called articular cartilage. 
• Articular cartilage reduces friction between bones in the joint during movement.
• These joints help in locomotion and many other movements.

Synovial Joints

Examples of Synovial Joints

• Ball and socket joint (between humerus and glenoid cavity of pectoral girdle).
• Ball and socket joint (between femur and acetabulum of pelvic girdle).
• Hinge joint (knee joint). 
• Pivot joint (between atlas and axis). 
• Hinge joint (Between phalanges).
• Hinge joint (Elbow joint).

Types of Synovial Joints

(a) Ball and socket joint: In this joint, the rounded head (like a ball) of one bone fits into a cup-shaped cavity formed by the other bone. This allows for a great range of motion. It allows backward, forward, sideways and rotating movements.

🞂 For example, you can move your shoulder and hip in all directions.

Ball and socket joint

(b) Hinge joint: In a hinge joint, the convex surface of one bone fits into the concave surface of another bone. It allows the movement only in one direction (back and forth movement) like those of a door or the lid of a box.

🞂 Elbow joint (joints between the upper and the lower arm), knee-joint (between thigh and lower leg) and Inter phalangeal joints are examples of this type of joint.

Hinge joint

(c) Pivot joint: In a pivot joint the rounded or pointed surface of one bone articulate with a ring formed partly by another bone & partly by a ligament. Such joints allow rotational movement around its own longitudinal axis. 

🞂 For example, Atlanto – axial joint

Pivot joint

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