Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the sites where two or more bones meet. Understanding the nature of joints is essential for diagnosing and treating injuries related to movement and skeletal stability.

• Functions of Joints: Provide mobility to the skeleton and hold the skeleton together.

• Clinical Relevance: Knowledge of joints is crucial for treating joint injuries and disorders.

Classification of Joints

Structural and Functional Classification

Joints are classified based on their structure (the material binding the bones and the presence or absence of a joint cavity) and their function (the amount of movement they allow).

• Structural Classifications:

  • Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity.

  • Cartilaginous Joints: Bones joined by cartilage; no joint cavity.

  • Synovial Joints: Bones separated by a fluid-filled joint cavity.

• Functional Classifications:

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

Fibrous Joints

General Features

Fibrous joints are connected by dense fibrous connective tissue. They are mostly immovable, and the amount of movement depends on the length of the connective tissue fibers.

• Types of Fibrous Joints:

  • Sutures

  • Gomphoses

  • Syndesmoses

Sutures

Sutures are rigid, interlocking joints found only in the skull. During youth, the short connective tissue fibers allow for growth. In adulthood, sutures ossify and fuse, forming a single unit that protects the brain. Immovable sutures are referred to as synostoses after fusion.

• Example: Lambdoid and squamous sutures of the skull.

Gomphoses

Gomphoses are peg-in-socket fibrous joints. The only example in the human body is the articulation of a tooth with its alveolar socket. The fibrous connection is the periodontal ligament.

• Example: Teeth anchored in the jawbone.

Syndesmoses

Syndesmoses are joints where bones are connected by ligaments, bands of fibrous tissue. The amount of movement allowed depends on the length of the connecting fibers.

• Little to no movement: Distal tibiofibular joint.

• More movement: Interosseous membrane connecting the radius and ulna in the forearm.

Cartilaginous Joints

General Features

Cartilaginous joints are united by cartilage and lack a joint cavity. They allow little to no movement.

• Types of Cartilaginous Joints:

  • Synchondroses

  • Symphyses (not shown in images, but commonly included)

Synchondroses

Synchondroses are joints where a bar or plate of hyaline cartilage unites the bones. These joints are synarthrotic (immovable).

• Examples:

  • Epiphyseal plate (temporary joint in growing children; becomes synostosis after plate closure).

  • Joint between the first rib and the manubrium of the sternum.

Summary Table: Structural and Functional Classification of Joints

Additional info: Symphyses (such as the pubic symphysis and intervertebral discs) are another type of cartilaginous joint, united by fibrocartilage and allowing slight movement.