Chapter 9: Joints (Articulations) – Structure and Function

Joints (Articulations)

Introduction to Articulations

Joints, also known as articulations, are the sites where two or more bones meet in the human body. They play a crucial role in providing mobility and stability to the skeleton, allowing for a wide range of movements and supporting the body's structure.

• Definition: A joint (articulation) is a point of contact between bones, or between bone and cartilage.

• Function: Joints enable movement and provide mechanical support.

• Examples: Knee joint, shoulder joint, sutures in the skull.

Functional Characteristics of Joints

Functional Classification

Joints are classified functionally based on the amount of movement they allow. This classification helps in understanding the mobility and stability provided by different types of joints.

• Synarthrosis: Immovable joints. These provide stability and are typically found in the axial skeleton.

• Amphiarthrosis: Slightly movable joints. These allow limited movement and are often found in the axial skeleton.

• Diarthrosis: Freely movable joints. These are common in the appendicular skeleton and allow a wide range of motion.

Examples:

Type	Movement	Example
Synarthrosis	None (minimal)	Sutures, Teeth, Epiphyseal plates, 1st rib and costal cartilage
Amphiarthrosis	Slight	Distal Tibia/fibula, Intervertebral disc, Pubic symphysis
Diarthrosis	Great	Glenohumeral joint, Knee joint, TMJ

Structural Characteristics of Joints

Structural Classification

Joints are also classified structurally based on the material binding the bones together and the presence or absence of a joint cavity. This classification provides insight into the anatomical features and functional capabilities of each joint type.

• Fibrous Joints: Bones are joined by dense fibrous connective tissue; no joint cavity is present. Movement depends on the length of the connective fibers.

• Cartilaginous Joints: Articulating bones are united by cartilage; lack a joint cavity. These joints allow more movement than fibrous joints but less than synovial joints.

• Synovial Joints: Bones are separated by a fluid-filled joint cavity; most freely movable type of joint.

Fibrous Joints

Fibrous joints are connected by dense connective tissue and do not have a joint cavity. They are generally immovable or allow very limited movement.

• Sutures: Found between bones of the skull; immovable (synarthrosis).

• Syndesmoses: Bones connected by ligaments; movement varies from immovable to slightly movable (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., teeth in their sockets).

Cartilaginous Joints

Cartilaginous joints unite bones using cartilage and lack a joint cavity. They allow limited movement.

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plates in children, first rib and sternum).

• Symphyses: Bones united by fibrocartilage (e.g., intervertebral discs, pubic symphysis).

Synovial Joints

Synovial joints are the most common and most movable type of joint in the body. They feature a joint cavity filled with synovial fluid, which reduces friction and nourishes the joint.

• Articular Cartilage: Hyaline cartilage covers the ends of bones, reducing friction and absorbing shock.

• Joint (Articular) Capsule: Encloses the joint cavity; consists of two layers:

  • Fibrous Membrane: Outer layer of dense irregular connective tissue, providing strength.

  • Synovial Membrane: Inner layer of areolar connective tissue, secretes synovial fluid.

• Synovial Fluid: Lubricates the joint, nourishes articular cartilage, contains phagocytes for cleaning debris.

• Ligaments and Tendons: Provide additional support and stability to the joint.

Examples: Knee joint, shoulder joint, temporomandibular joint (TMJ).

Summary Table: Structural Classification of Joints

Structural Class	Characteristics	Types	Movement	Examples
Fibrous	Dense connective tissue; no joint cavity	Sutures, Syndesmoses, Gomphoses	Immovable or slightly movable	Skull sutures, distal tibiofibular joint, teeth in sockets
Cartilaginous	Cartilage binds bones; no joint cavity	Synchondroses, Symphyses	Slightly movable	Epiphyseal plate, intervertebral discs, pubic symphysis
Synovial	Joint cavity with synovial fluid	Various (e.g., hinge, ball-and-socket)	Freely movable	Knee, shoulder, TMJ

Key Terms and Definitions

• Articulation: The site where two bones meet.

• Synarthrosis: An immovable joint.

• Amphiarthrosis: A slightly movable joint.

• Diarthrosis: A freely movable joint.

• Fibrous Joint: A joint where bones are joined by fibrous tissue.

• Cartilaginous Joint: A joint where bones are joined by cartilage.

• Synovial Joint: A joint with a fluid-filled cavity allowing free movement.

Additional info:

• Ligaments connect bone to bone and provide joint stability.

• Tendons connect muscle to bone and assist in joint movement.

• Synovial joints are the most common type in the limbs, allowing complex movements.